Day 5: Waking in the Descent Orbit	Journal Home Page	Apollo Lunar Surface Journal.	Solo Orbital Operations - 1

Apollo 15

Day 5: Preparations for Landing

Corrected Transcript and Commentary Copyright © 1999 - 2008 by W. David Woods and Frank O'Brien. All rights reserved.

[Very long comm break.]

[Apollo 15 has just disappeared around the Moon's western limb. The primary task for this far-side pass, during which the spacecraft will begin its tenth revolution, is to accomplish the DOI (Descent Orbit Insertion) trim maneuver, which occurs at 095:56:42.5.]

[Astronaut Ed Mitchell will take over from Robert Parker as the CapCom for the next ten hours, including the landing. Ed flew as Lunar Module Pilot on Apollo 14 and is therefore highly experienced in this stage of the mission. It has become normal practise for the LMP from the previous mission to act as CapCom during the landing shift, making best use of their recent knowledge and experience. Then, after the landing has been accomplished, there will be separate flight control teams and CapComs for the CSM and the LM.]

[Flight Plan page 3-104.]

096:25:36 Irwin (onboard): ...

096:25:39 (Clanking)

096:26:20 Scott (onboard): ... Is this your ..., Jim?

096:26:29 Irwin (onboard): No, I've got it....

096:26:33 Scott (onboard): ...

096:26:35 (Clanking)

096:27:00 (Clanking)

096:27:29 Scott (onboard): Okay, I guess the - next thing we ought to do - when you get ready - is go through the tunnel. No, I don't need that. System Checklist.

096:27:58 Irwin (onboard): I think you had the Systems Checklist, didn't you? Oh, here it is. Okay. Well, that's your job, because - Say again?

096:28:15 Worden (onboard): ...

096:28:18 Irwin (onboard): Yes.

096:28:34 (Clanking)

096:28:42 Scott (onboard): Think we ought to try and get them on the high gain? Hey, Al, you think we ought to try to get them on the high gain?

096:29:36 Scott (onboard): Well...

096:29:37 Irwin (onboard): I mean - Okay. I could easily go through it, because I could tell which ones to check - skip through .... get.

096:29:50 Scott (onboard): Turn around here, Jim.

096:30:00 Scott (onboard): Hey, we're getting them. How about that. Yes. There, got a lock.

[Flight Plan page 3-105.]

096:30:13 Scott: Roger, Houston; 15 here. We had a good burn and have a burn report for you.

096:30:18 Mitchell: Roger. Ready to copy.

096:30:23 Scott: Okay; [the burn was] on time - burn time was about 18 or 19 seconds. The residuals were minus .1, plus .2, plus .2, Delta-V_c was plus .8, and the G&N has us in a 59.4 by 10.3 [nautical mile orbit or 109.9 by 19.1 km].

096:30:46 Mitchell: And copy, Dave.

[Note that Ed Mitchell dispenses with pleasantries and gets straight down to business.]

[The burn has raised the pericynthion slightly higher than the preferred value of 9.5 nautical miles (17.6 km). By the time of Powered Descent Initiation, the perturbation of the orbit by the irregular lunar gravity field should have brought it back down to the ideal.]

096:31:03 Mitchell: And, 15, I have a landing site observation PAD for you when ready.

096:31:10 Scott: Roger. Go ahead.

096:31:13 Mitchell: T-horizon: 96:57:10. Stand by. [Long pause.]

[This is the expected time when Mission Control expects the landing site to appear on the horizon, as seen from the spacecraft. According to the Flight Plan, this should be an observation of the J-1 landmark in Mare Serenitatis, for which coordinates are given of 25.958°N, 11.3278°E, essentially a repeat of the observation at 083:42:36. However, Mitchell has just described this as a landing site observation PAD and Dave's comm just after 97 hours will confirm that he will use this opportunity to get his first look at the Hadley terrain now that the Sun has risen over it.]

096:31:50 Scott: Roger. We're there, and T-horizon: 96:57:10. Standing by for TCA minus 20.

096:31:56 Mitchell: Roger. It's at 96:59:17.

096:32:08 Scott: 96:59:17.

096:32:11 Mitchell: Looks good.

[The TCA (Time of Closest Approach) is 096:59:37. Mission Control gives them a time twenty seconds earlier to allow the observer to be at the optics, ready to acquire a successful sighting of the landmark. The observation is part of a sequence to be carried out over the next few orbits to allow the knowledge of the landing site's position and height to be refined before Dave and Jim have to attempt their approach and landing.]

096:32:35 Scott: P00 and Accept.

096:32:59 Mitchell: And, 15, Houston. I have a couple of words for Al on the system test meter if he can listen.

096:33:09 Scott: Sorry, he - he's busy right now. What do you need? I'll tell him later.

096:33:13 Mitchell: Hey, it - we'll get it to him later. It's simply that the systems test meter is okay to use for the LM checkout on the LM [electrical] current.

096:33:25 Scott: Okay; thank you.

[Comm break.]

[The problem with the switch on the systems test meter was first experienced at 081:41:29. Mission Control are happy that the engineers in the backrooms understand the nature of the problem so are prepared to allow the crew to continue using it.]

096:34:52 Mitchell: We copy. [Long pause.]

[A valve and adjoining gauge near the forward hatch are used to alter and monitor the air pressure in the tunnel and hence in the LM cabin. Despite what the PAO announcer says, Dave is beginning to equalise the pressures in both spacecraft more than an hour early.]

[As the difference between the two is reading less than 2.7 psi, the LM can be pressurised from the air within the CM cabin without the need to raise its pressure first.]

[Mission Control have uplinked a new state vector and REFSMMAT as requested three minutes earlier. The computer is again available for the crew's use.]

[Very long comm break.]

[As the PAO will report, the crew is ahead of their Flight Plan. According to the time line, Jim ought to be donning his suit, less the helmet and gloves, about now; he is the first of the crew to do so in preparation for undocking and landing. Some events in the Flight Plan, such as the landing site observation soon to occur, cannot be brought ahead in time, but like many crewmembers, Dave has his crew adopt the maxim, "Get ahead and stay ahead."]

096:48:26 Mitchell: 15, Houston.

096:48:32 Worden: Houston, 15. Go ahead.

096:48:35 Mitchell: Al, we didn't get your last torquing angles and torquing time. Could you read them down to us, please?

096:48:43 Worden: Okay, Ed - Just a minute.

096:48:46 Mitchell: If you're busy, we'll get them later.

[CapCom Ed Mitchell is referring to the P52 platform realignment which was rescheduled to 95:28, prior to the DOI trim burn.]

["Four balls, one" is the crew's shorthand for the computer's display showing 00001. Noun 05 compares the known angle between the stars, and the same angle as measured by the crewman using the sextant. As well as giving the crewman an idea of his sighting accuracy it also gives him a check that he sighted on the correct stars. If the value of Noun 05 were large, it would most likely be an indication that the sighting procedure was wrong. Five balls (all zeros) is the ideal but 000.01 degrees ("four balls, one") is still very good.]

[The orientation of the platform has been torqued (moved) by 0.022°, 0.032° and 0.038° in the x, y and z axes respectively, to bring it into proper alignment. This set of values are given by calling up Noun 93 on one of the DSKYs (Display and Keyboard).]

[The Landing site REFSMMAT has been the reference orientation for the IMU (Inertial Measurement Unit) platform since it was realigned at 80:10, shortly after Apollo 15 entered lunar orbit. To recap, the landing site REFSMMAT is defined as the orientation of the landing site, with respect to the stars, at the time of landing; and is chosen so that the FDAI or "8-ball" in the LM will display 0° in all three axes at the ideal time and place of landing, assuming it lands in a fully upright attitude. The X-axis runs from the centre of the Moon, out through the landing site position; the Z-axis is tangential to the landing site and is parallel with CSM's orbital plane and therefore with the LM's approach path.]

[The actual orientation of the landing site is, of course, continually changing as the Moon rotates on its axis and, as stated above, will only match the landing site REFSMMAT at one moment in time. This coincidence of the two is the "REFSMMAT 00 time" and is therefore the intended time of landing. The recent burn to raise their pericynthion has slightly altered the REFSMMAT 00 time so the next P52 realignment will take this new time into account. There is a box on the Flight Plan at 097:25 for Al to write this time in.]

096:49:42 Mitchell: Roger, Al. 104:41:43.00.

096:49:54 Worden: Understand. REFSMMAT time is good for 104:41:43.00.

096:50:01 Mitchell: Good readback.

[Long comm break.]

096:56:22 Worden: Roger, Ed.

[Long comm break.]

[The landmark will appear on their horizon in one minute. Strictly speaking, it will not be visible until after that because it is hidden from view by the elevated land to its east, the Swann Range.]

[Like other landmarks in the Hadley area, these explorers chose to name this range of hills after one of the geologists, Gordon Swann, who had trained them for the scientific content of their mission. Another example is a

prominent outcrop visible from the LM after they landed called Silver Spur. The HBO miniseries for television, From the Earth to the Moon, gives an account of how Lee Silver was brought from Caltech to train the crews as geologists as well as pilots. His inspiration was rewarded with this extraordinary, layered feature bearing his name.]

[While the crew slept, the Sun rose over the Hadley plain. Now Dave is going to use this pass across the landing site to get his first direct look at conditions on the surface with the aid of the sextant. In particular, he is looking to see whether there are any features which could prove to be a hazard for a Lunar Module coming in to land. This observation is particularly important in light of the poorer imagery available to the planning teams before the flight. The robotic Lunar Orbiter spacecraft had concentrated on acquiring high resolution imagery of the equatorial sites that the first landing missions were limited to reaching. Only medium resolution imagery of Hadley was available so this sighting is valuable in double checking that there are no obstacles like large single boulders or great boulder fields awaiting them.]

[At this point in the mission, the Flight Plan splits, with separate pages for the LM and the CSM. Indeed, the two LM crewmembers have individual time lines within their Flight Plan.]

[

LM Flight Plan page 3-106.]

[

CSM Flight Plan page 3-107.]

097:01:48 Mitchell: Go ahead, 15.

097:01:53 Scott: Okay; looks like FAO [Flight Activities Officer] owes us one, I had no trouble picking up Index Crater at all. The surface looks generally smooth. I saw no big boulders. I saw more craters than we're normally used to seeing [in the simulator], but I think that's because of the [low] resolution of the [Lunar Orbiter] photography. I can see a - a fair amount of boulders in the bottom of the rille. Most of the surface seems to be pretty subdued and rounded. I did see some debris around some of the fresher craters. And, of course, I don't know exactly what size, but if we're looking at 3-feet resolution on this, I'd - I'd say I didn't see anything larger than - I guess, at the most, maybe 15 feet or so, but that's - that was a unique rock. The - the area, in general, looks pretty good. I don't think we'll have any problem picking it up, and I don't think Al will have any problem at all identifying Index Crater.

[Index is a reasonably distinctive crater, by virtue of having a lobe on its north rim. It lies adjacent to the landing site and will be used both for taking sightings from orbit, a procedure Al will be carrying out over the next few revolutions, and to act as a guide for Dave as he approaches the landing site later today. It is the last of four craters of similar size which form a line leading up to the point of touchdown. The others were named Matthew, Mark and Luke in the order leading towards Index. A portion of AS15-87-11718, taken one orbit before landing, has had these features labelled.]

[By the time of the landing, the rising Sun will render many of the craters Dave is familiar with almost invisible. Dave talks about this further in the Apollo Lunar Surface Journal by Eric Jones before and after

104:39:32. Eric believes Dave's reference to 15 foot boulders must be to those lying within Hadley Rille as there is nothing like that up on the mare surface. Dave pointed out to me that the North Complex, a cluster of low hills and craters north of the landing site, did harbour this size of boulder.]

[Scott, from the 1971 Technical Debrief - "One of the questions on the landing site was general terrain relative to boulders, debris, and craters. A couple of months before the flight, we had worked out a plan whereby we do the low altitude landmark tracking technique without the spacecraft rate drive in order to take a look at the landmark through the sextant. There had been some question as to whether or not we could see anything. I took a look on REV 10, and it was as we had expected based on previous flights and fidelity of the optics. I could see the landing site very well. I could see Index Crater and the rille very well. I determined that there was no problem relative to boulders and debris, and it looked pretty smooth and flat. It was a comforting feeling to know that we wouldn't have a rockpile to land in. If the advertised resolution of 3 feet was correct, we had no problem with boulders on the order of 3 feet and above. This was subsequently verified when we got there. It was a nice thing to have behind us in the way of validating the surface at the landing site because of the poor resolution of photography we had from Orbiter. The technique worked very well. It was easy to track in Inertial Attitude. I think you found that during your J-1 track, also, didn't you Al? I think you could have done your landmark tracking without a spacecraft rate drive."]

[Worden, from the 1971 Technical Debrief - "I almost feel that way, yes. The J-1 tracking was really easy. With a high rate in the optics, it was fairly easy to track if it was off track. You have some roll in there, so that you're not coming through zero on the track."]

[Scott, from the 1971 Technical Debrief - "But the optics are very easy to control, have a very positive response, and once you lock on the target, you can stay right on it."]

[Worden, from the 1971 Technical Debrief - "That's right."]

[Scott, from the 1971 Technical Debrief - "That gets us down to lunar landmark recognition. Of course, the landing site at Hadley was particularly unique, when relative to landmarks. When I looked at it through the optics, I could recognise the craters that lead into Index and Index crater quite well, even though there didn't seem to be as many shadows, crater shadow, as I had expected."]

097:03:09 Scott: Rog.

[Long comm break.]

[Dave ought to be donning his biomedical harness and his suit, without helmet and gloves, about now.]

[Scott, from the 1971 Technical Debrief - "We set up a plan on the LOI day to try out our sequence of suiting for PDI day. As a result, we changed our minds on PDI day to make it a little bit more efficient."]

[Worden, from the 1971 Technical Debrief - "You two put your suits on and then went into the LM to zip them up."]

[Scott, from the 1971 Technical Debrief - "Because it's a lot easier zipping up the 7L-B suits in the LM and it gave us a chance to do the tunnelwork shirtsleeve. We helped you [Al] get your suit on. It is worth while to run through suit donning because the first day we did it we had you put your suit on. Then we put our suits on in the Command Module. That was a sort of chore. Jim suggested we suit up and go to the LM before zipping them up. That made it a lot easier. We recommend cleaning the tunnel out or putting the suits on unzipped, cleaning the tunnel out, and then the CDR and LMP transferring to the LM to do their suit zip. It would be a good idea to have a little trial run one of the days on the way out."]

[Worden, from the 1971 Technical Debrief - "As to the time line, that works out much better, too, because while you were over there putting suits on and zipping them up, that gave me a chance to put my suit on which is done in parallel rather than sequentially"]

097:10:43 Scott: Okay; hold for about 5 [minutes], will you please, Ed?

097:10:47 Mitchell: Will do.

[Comm break.]

097:13:54 Mitchell: Roger, 15. TEI-19: SPS/G&N 38009; plus 0.61, plus 1.05; 115:27:55.59; plus 2845.1, minus 0831.7, minus 0219.7; 180, 102, 346. The rest NA. Ullage, 4 jet, 12 seconds; and some notes. One is, burn is undocked; two, assume Circ.; three, longitude at T_ig is 166.0 [degrees] west; and the fourth note is, the attitude's based on landing-site REFSMMAT.

[Interpretation of the PAD follows:

Purpose: The purpose of the TEI-19 PAD is as a contingency in case an abort situation arises. It would return the spacecraft on an Earthward trajectory during Apollo 15's 19th revolution around the Moon.

System: The burn would be made using the SPS engine under control of the Guidance and Navigation system.

CSM weight (Noun 47): 38,009 pounds (17,241 kg).

Pitch and yaw trim (Noun 48): +0.61° and +1.05°.

Time of ignition, T_ig (Noun 33): 115 hours, 27 minutes, 55.59 seconds.

Change in velocity (Noun 81), fps (m/s): x, +2,845.1 (+867.15); y, -831.7 (-253.5); z, -219.7 (-67). These velocities are with respect to the

local vertical/local horizontal.

Spacecraft attitude: Roll, 180°; Pitch, 102°; Yaw, 346°. As Mitchell notes, these attitude angles are with respect to the

Landing site REFSMMAT.

SPS propellants are settled in their tanks by firing the plus-X thrusters on all four quads around the Service Module for 12 seconds.

The burn would be made without the weight of the LM. The PAD assumes that Al has completed the orbit circularisation burn. The spacecraft will be over the lunar longitude of 166° west on the lunar surface. The spacecraft's attitude for the PAD assumes that they are using the landing site orientation for their attitude reference.]

[Although this abort is without the LM, it does not necessarily mean that Al would be returning alone. If the LM crew need to return soon after landing, they could reach orbit, dock, transfer to the CSM and jettison the LM in plenty of time for this TEI-19 abort.]

[Woods, from 1999 correspondence with Scott - "Some of the abort PADs read up during lunar orbit have T_igs which are during the crew's scheduled sleep periods. It may seem somewhat odd that an option to return home would be scheduled just when a tired crew are switching off for the day and perhaps one would expect that a time would be chosen when the crew are likely to be alert for what would be a very critical burn, should it be used. Do you have any comment on the timing of Abort PAD T_igs?"]

[Scott, from 1999 correspondence - "Problems are not selective in time - therefore, it does not matter what the crew might be doing when the problem occurs, and an abort will be needed."]

097:16:00 Mitchell: That's affirm; and we have a few centiseconds update for your CMC clock, whenever you're ready.

[The basic unit of time in the computer is the centisecond (1/100 of a second). Mission Control can remotely update the computer's knowledge of time, once a crewmember has given them access to it.]

097:16:13 Mitchell: That's affirm.

097:16:16 Scott: You got it.

[Comm break.]

[Long comm break.]

[Mission Control has corrected the CSM computer's clock.]

097:23:35 Worden: Houston, 15. Go ahead.

097:23:38 Mitchell: Omni Charlie, please, Al.

097:23:43 Worden: Okay, Omni Charlie.

097:23:51 Mitchell: I haven't had a chance to say good morning, Al, so good morning.

097:23:58 Worden: Good morning, Edgar.

[Long comm break.]

[Finally, Ed Mitchell drops his businesslike stance and gets around to exchanging greetings with the crew.]

[In situations where the HGA (High Gain Antenna) is not being used, the crew can select one of four omni-directional antennas on the CM's skin to communicate with Earth.]

[Al is working on another platform realignment.]

097:29:23 Mitchell: Apollo 15, Houston.

097:29:28 Worden: Houston, 15. Go ahead.

097:29:30 Mitchell: Roger. We need to redo the REFSMMAT update that we gave you a little while ago. Could we have P00 and Accept please?

097:29:41 Worden: Okay, Ed. You caught me just in time, doing a P52.

[There are two P52 realignments in the Flight Plan about this time. The first, at 97:19, would have been carried out had there been no DOI trim burn and would be a conventional Option 3 P52 using the landing site REFSMMAT. The second is for cases where a DOI trim burn has occurred and it is therefore the one Al is to complete.]

[As well as raising the pericynthion of their orbit, the DOI trim burn has also changed the time of the landing slightly as the overall orbit is slightly higher than before and therefore of a slightly longer period.]

[The uplink of the updated REFSMMAT was quite timely, as Al was in the process of performing a platform realignment, and will be able to use the new values immediately. Had the updated REFSMMAT come later, it would not be used until the next scheduled alignment.]

097:29:44 Worden: I'll go back [and do it.]

097:29:45 Mitchell: ...what - what we were hoping to do.

097:29:50 Worden: Okay; you got it now.

097:29:51 Mitchell: Roger. Here it comes.

097:30:05 Mitchell: The story here, Al, is that, apparently, the block update changed some registers in the REFS - REFSMMAT and screwed it up a bit.

097:30:16 Worden: Roger.

[Comm break.]

[Houston has sent up several computer updates on this near-side pass, and one of these has accidentally written over storage used for the platforms orientation, the REFSMMAT. A note on the computer term, "registers" used by Ed Mitchell at 097:30:05: The contemporary definition of register is a computer component that is able to perform arithmetic and logical operations on data. The Apollo Guidance Computer had only one accumulator, a term which today is often used interchangeably with the word "register". In the software design of the computer, memory locations were reserved for well defined values, such as mathematical constants or frequently used variables (such as a REFSMMAT). This use of the term probably dates back to earlier computers, such as the Univac series, where registers and other hardware functions were referenced by a memory address.]

097:32:12 Worden: Roger, Ed. Understand.

097:32:13 Mitchell: Okay; proceed with your P52.

097:32:18 Worden: Okay; thank you.

[Long comm break.]

[As Al proceeds with the P52 realignment, he inadvertently uses Option 3 instead of 1. However, the spacecraft is about to disappear around the far side and Mission Control will not have a chance to pick this up until AOS.]

097:35:38 Worden: Roger, Ed.

[Very long comm break.]

[On this far-side pass, Dave and Jim begin their transfer to the Lunar Module as Apollo 15 begins its eleventh orbit.]

[During this period behind the Moon, Al will don his biomedical harness and suit without the helmet and gloves. He was scheduled to do this five minutes before LOS. However, it is unlikely he would have done so having had to execute a P52 alignment twice due to the error in the initial REFSMMAT. Other tasks for Al include purging the fuel cells, a waste dump and maneuvering to the undocking attitude. Meanwhile, Dave and Jim clear the tunnel of the probe and drogue to allow passage to the LM. Jim enters first and begins activating the lander's O₂ and water supplies, the electrical, communications and cooling systems and checking out the caution and warning system. At Jim's request, Al will switch off the power being fed to the LM through the tunnel umbilical cable. Dave enters with their helmets and gloves and joins in with the activation of the LM.]

[In the CM, Al prepares two cameras, the Hasselblad and the CM's DAC (Data Acquisition Camera, a 16-mm movie camera) in the left, forward-looking rendezvous window to photograph the undocking and separation of the two spacecraft. He replaces one of the CM's lithium hydroxide canisters putting number 10 into port B, stowing the exhausted number 8 in compartment B6. He then begins to aid the activation of the LM by supplying necessary information from the CSM's systems. These will include the current Ground Elapsed Time and a read-out of the spacecraft attitude with respect to the IMU to allow a coarse alignment of the LM's guidance system.]

[After AOS, radio traffic will be three way and the use of the individual spacecraft's call signs will resume. Al's CSM is Endeavour while Dave and Jim will be using Falcon for the LM.]

[LM Flight Plan page 3-108.]

[CSM Flight Plan page 3-109.]

098:20:33 Worden (onboard): Oh, what the hell.

098:21:40 Scott (onboard): Hey, Al.

098:21:41 Worden (onboard): (Yelling) Yes.

098:21:46 Scott (onboard): ...

098:21:47 Worden (onboard): (Yelling) What?

098:21:55 Scott (onboard): ...

098:21:57 Worden (onboard): (Yelling) What?

098:21:58 Scott (onboard): Did you ...?

098:22:00 Worden (onboard): (Yelling) No. You guys are too far ahead.

098:22:58 Worden (onboard): Okay. Let's see.

098:23:02 Scott (onboard): Ready?

098:23:03 Worden (onboard): You want P - Okay.

098:23:04 Scott (onboard): ...

098:23:06 Worden (onboard): Okay.

098:23:21 Scott (onboard): Okay there, Endeavour. How do you read the Falcon on...

098:23:23 Worden (onboard): Hey, loud and clear.

098:23:24 Scott (onboard): Say again?

098:23:25 Worden (onboard): Beautiful!

098:23:26 Scott (onboard): Good.

098:23:27 Irwin (onboard): And, Al, how do you read me?

098:23:28 Worden (onboard): Read you loud and clear, Jim.

098:23:30 Irwin (onboard): Ah, you're beautiful.

098:23:31 Worden (onboard): Hey, did you say something about T_ephem? Did you want a T_ephem, Dave?.

098:23:38 Scott (onboard): stand by.

098:23:39 Worden (onboard): Okay. I'll give it to you if you want.

098:23:43 Scott (onboard): Stand by. Configure for Simplex A.

098:23:45 Worden (onboard): Okay.

098:23:57 Scott (onboard): Okay, Al. How do you read on A?

098:23:59 Worden (onboard): Loud and clear, Dave.

098:24:00 Scott (onboard): That's nice.

098:24:01 Irwin (onboard): And, Al, how do you read me?

098:24:02 Worden (onboard): Loud and clear, Jim.

098:24:03 Irwin (onboard): Very good.

098:24:05 Scott (onboard): Okay. Now you can give me your T_ephem.

098:24:07 Worden (onboard): Okay.

098:24:18 Worden (onboard): Ready to copy?

098:24:19 Scott (onboard): Go.

098:24:20 Worden (onboard): Just like SIM. All balls in the R₁. R₂ is 32251. R₃ is 26157.

098:24:31 Scott (onboard): Okay. All balls, 32251, 26157.

098:24:34 Worden (onboard): Great.

098:24:44 Worden (onboard): Want a clock sync?

098:24:46 Scott (onboard): Stand by 1.

098:25:35 Scott [in Falcon, talking to Worden in Endeavour]: Okay, Al. Verb 06 - 6 - Verb 06 Noun 65; I'll give you a mark.

098:25:37 Worden (onboard): Give me a mark. I'm waiting.

098:25:46 Worden (onboard): Okay. 98:25:45.60.

098:25:53 Scott (onboard): Okay. 25:45.60.

098:25:56 Worden (onboard): Roger.

[As Apollo 15 emerges around the Moon's limb, Dave and Al are updating the clock in the LGC (Lunar Module Guidance Computer) with a time mark from the CMC (Command Module Computer). Noun 65 will present the CMC's time and Dave can compare it with the clock in the LM.]

098:26:06 Worden (onboard): And I'm coming up to get the umbilical now.

098:26:08 Scott (onboard): Read you loud and clear.

098:26:13 Mitchell: Okay, Falcon; reading you. We have a lot of noise on the loop - maybe it'll go away in a moment.

098:26:22 Irwin: Okay. Let's hope so. [Long pause.]

098:26:51 Scott: Okay, Endeavour, Another Noun 65 if you're ready.

[Communication with Earth, using the omni-directional antennas on the LM and CSM, are poor. The radio link using the HGA will be reestablished soon, improving the comm with Al but the LM comm will still be poor.]

098:27:00 Scott: 3, 2, 1...

098:27:02 Scott: ...Mark.

098:27:03 Worden: Okay. 98:26:59.60.

098:27:09 Scott: Okay. We're within three hundredths.

098:27:12 Worden: Okay.

098:27:41 Mitchell: Endeavour, Houston. Do you read?

098:27:46 Worden: Hello, Endeavour, Houston, this is Endeavour. Loud and clear.

098:27:50 Mitchell: Okay, Al. We read you through the static. Let us have Narrow on the High Gain [Antenna], please.

[CapCom Ed Mitchell is asking for the HGA to be switched to a narrow beam setting. This concentrates the antenna's radio energy across a narrower angle, making it stronger for a receiving antenna sited within the centre of the beam, but also requiring a greater pointing accuracy.]

[In the Reacq(uire) mode, the HGA will continue to point at the Earth as long as it stays within its limits of travel. If it loses lock, it will orient itself to a pair of angles dialled into two rotary knobs on the bottom right of panel 2.]

098:28:21 Mitchell: Okay, Falcon. Press on with secondary S-band. [Long pause.]

[Like most systems on both spacecraft, there are two redundant sets of S-band communications antennae. These are independent of the VHF system, which is used only for communications between the LM and CSM, and for ranging by the CSM. The LM's S-band system is used for communications between the LM crew and the Earth. The secondary S-band antenna is a steerable, high gain antenna mounted on the upper, starboard side of the ascent stage. This system is supplemented by two omnidirectional antennae, mounted on the forward and aft faces of the ascent stage. The omni antenna were used when the LM's attitude did not allow the steerable antenna to be pointed towards the Earth, or if the antenna had difficulties acquiring the Earth.]

098:29:15 Mitchell: Endeavour, Houston. Verify [that] on your last P52, you used option 1.

098:29:25 Worden: Houston, Endeavour. Go ahead.

098:29:28 Mitchell: Roger, Al. Verify that on your last P52, you used option 1.

098:29:39 Worden: I think that's a negative, Ed. I used option 3. Stand by one and I'll check.

098:29:45 Mitchell: Roger. We're standing by.

098:29:46 Scott: Houston, how do you read Falcon?

098:29:49 Mitchell: Falcon, Houston. You're coming through; we have a lot of static on the line and they're trying to clear it up now. You're way down in the mud.

[The expression "way down in the mud" means it is difficult for Mitchell to separate the crews voices from the noise. The signal to noise ratio is very poor.]

098:30:19 Worden: Houston, Endeavour.

098:30:22 Mitchell: Go ahead, Endeavour.

098:30:26 Worden: Okay, Ed. I did do that on an option 3.

098:30:30 Mitchell: Okay. We'll have some words for you, and we're ready to uplink to you, Endeavour, if you'll give us - P00 and Accept.

[Within what is a very busy timeline, Mission Control have to decide whether to accept the current alignment of the platform or to have Al repeat the P52 procedure.]

098:30:41 Mitchell: Falcon, Houston. Your S-band check is okay. You can press on, and I'll have your updates for you in a little while.

098:30:51 Scott: Roger. [Long pause.]

098:31:04 Mitchell: And, Apollo 15; Houston. I'm going to hold off reading you any PADs at the moment until we can clear up our comm on the ground.

[Mitchell refers to "Apollo 15" because he is referring to both spacecraft. He has PADs for both.]

098:31:28 Mitchell: Falcon, Houston. Bring up your steerable [antenna], please. See if that helps our comm.

098:31:35 Falcon: In work.

[Comm break.]

098:33:10 Irwin: Houston, this is Falcon. We're locked up on the - the High Gain. How do you read?

098:33:14 Mitchell: Okay, Jim. Reading you loud and clear now; that has seemed to improved our comm. And I'm - I'm ready to start with the PADs. I'll start with the CSM first, Al - whenever you're ready.

098:33:30 Scott: Okay; he's working right now. Are you ready for an E-memory dump on the Falcon?

098:33:35 Mitchell: Stand by.

098:34:06 Mitchell: Falcon, Houston. We're ready for the E-Mod [Erasable Memory Module] dump.

098:34:13 Scott: Okay; Stand by.

098:34:22 Mitchell: En - Endeavour, Houston. The computer's yours.

098:34:29 Worden: Roger, Houston. [Long pause.]

Public Affairs Officer - "The Erasable Memory Dump, which is in process at the moment, allows the Flight Controllers here in Mission Control, to look at the Erasable Memory Module in the Lunar Module Guidance Computer and see that everything is in order."

098:34:55 Mitchell: And, Endeavour; Houston. We will stay with the REFSMMAT you have.

[In view of Al using the incorrect option for his last P52, Mission Control have elected to stay with the alignment they have. The small error should be taken out when the LM crew align their platform and when Al next realigns the CSM platform.]

098:35:59 Mitchell: Falcon, Houston. Give us P00 and Data. I have an uplink for you.

098:36:06 Scott: Roger, P00 and Data.

098:36:09 Mitchell: And I'm ready to give PADs, Falcon, whenever you want them.

098:36:17 Irwin: [To Worden] Okay, Al; I'm checking it.

[Jim's conversation with Al in the CSM has inadvertently been switched to the main comm circuit.]

098:36:21 Mitchell: All right, your LM DAP [Digital Auto Pilot] data first. CSM weight, 37679; LM weight, 36630. Your GDA [Gimbal Drive Actuator] drive angles onboard are good.

[Spacecraft weight, or more accurately, its mass is very critical in calculating what is known as the "Moment of Inertia". This changes significantly as a function of mass and is used to calculate the firing times of the thrusters needed to establish known rates of rotation. Of course, all of these functions are under the control of the DAP (Digital AutoPilot), a set of algorithms in the CMC which are calibrated to accept values for vehicle mass in pounds. In metric units, the values loaded in the DAP are 17,091 kg for the CSM and 16,615 kg for the LM.]

[Readers ought to be aware of NASA's free, though incorrect use of the term "weight" when they are really referring to "mass".]

098:37:02 Scott: Go ahead.

098:37:03 Mitchell: ...plus 60442; 225, plus 29365; 226, plus 60449; 305, minus 01659; 662, minus 55021; 673, minus 32306.

[The numbers read up to Dave Scott are intended for the AGS. The three digits are a memory location in the AGS computer, and the five digits represent data to be loaded there.]

098:37:59 Mitchell: Affirm. The first 3 are plus, the last three are negative.

098:38:05 Scott: That's affirm. Houston, Falcon.

098:38:11 Mitchell: Go ahead, Falcon.

098:38:15 Scott: Okay. Everything is in order up to this point as far as the checkout goes, with the exception of the - LGC [LM Guidance Computer]. When we pushed in the LGC circuit breaker, we got a program light - with a 400 and R1. A Verb 5 Noun 9 gave us an 1105, which seems to be of little consequence; just thought you might like to know.

098:38:41 Mitchell: We copy, Dave.

[Comm break.]

[Error code 1105 means the computer thinks that a downlink is happening too fast. However, as this occurred on powering the LGC, Dave correctly assumes it is just the result of a messy start-up of what is a very complex, logical device, and that it is nothing to worry about.]

098:40:01 Scott: Roger.

[Comm break.]

[The four landing gear at the corners of the Lunar Module's descent stage are due to be deployed at about this time. The gear were designed to be folded in towards the descent stage to allow the LM to fit into the SLA (Spacecraft/Launch vehicle Adaptor), the conical fairing atop the Saturn V's S-IVB stage.]

098:41:14 Worden: Roger, Ed. Stand by one. [Long pause.]

098:41:43 Worden: Okay, Houston. Endeavour's ready to copy.

098:41:45 Mitchell: Okay. Falcon, the first one's a SEP[aration] time, if you would like that one, too.

098:41:56 Scott: Houston, Falcon. Go ahead; we'll stand by.

098:41:59 Mitchell: Okay. SEP: GET 100:13:56.00 - the pitch angle of 108 [degrees].

[This is the time that the two spacecraft will undock and separate. There is a box in the Flight Plan to accept this data at 100:13.]

098:42:29 Mitchell: That's affirm, Al. Okay, the next one is CSM DAP data.

098:42:42 Worden: Okay; go.

098:42:43 Mitchell: Roger. CSM weight, 37679 [pounds, 17,091 kg]; pitch [trim], 0.49; yaw [trim], 1.04.

098:43:02 Worden: Understand. The DAP data is weight, 37679; pitch trim is 0.49; and yaw trim is 1.04. And do you have some signs on those?

098:43:14 Mitchell: It's a good readback, and say again your last?

098:43:21 Worden: Roger, Ed. I need a sign on the pitch and yaw trims.

098:43:24 Mitchell: All right - Let me check that.

098:43:29 Worden: Okay. [Pause.]

[Mitchell checks with the controller sitting at the GNC console. For Apollo 15, the GNC controllers are Gary Coen, R. Watson, J. Kamman, L. Canin, J. DeAtkine.]

098:43:47 Worden: Understand; they're both plus.

098:43:49 Mitchell: Okay; your P24 is next.

[P24 is the program for "Rate aided optics tracking." Al will use it to rotate the spacecraft at an appropriate rate to roughly point the optics, on the back of the CM, towards the landing site while he views it through the sextant. In particular, Al is to sight the appropriately named "Index" crater, which is right beside Falcon's intended target. Sighting and marking its position will help Mission Control refine the position of the landing site.]

098:43:57 Mitchell: T-1, 100:46:29; T-2, 47:07; TCA, 47:37; T-3, 47:59; roll, pitch, and yaw are 008, 296, 000; north, 3 nautical miles. Noun 89, longitude-over-2, plus 2.149. Latitude and altitude are nominal.

[Explanation of P24 PAD follows:

T-1 (100:46:29 GET) is the time that the landmark is on the horizon and is 2 minutes, 8 seconds before closest approach.

T-2 (100:47:07) is 1 minute, 38 seconds after T1 and is when Al begins pitching the spacecraft at 2° per second to keep it at an approximately constant attitude with respect to the landmark.

TCA (100:47:37) is the time of closest approach.

T-3 (100:47:59) is 2 minutes, 30 seconds after T1 and 22 seconds after TCA. It marks the end of the landmark tracking mode.

Spacecraft attitude at T-2: x, 8°; y, 296°; z, 0°.

At TCA, the spacecraft will be 3 nautical miles (5.6 km) north of the landmark.

The computer deals in values of longitude divided by two to give greater accuracy within the constraints of its limited range of permitted values. Therefore the true value for the longitude is (2.149 times 2) +4.298°]

098:45:32 Mitchell: Good readback, Al.

098:45:39 Scott: And, Houston, this is Falcon. ED [Explosive Devices] batteries both reading 37 volts.

[The ED batteries are also known as the pyro(technic) batteries.]

098:45:52 Scott: Okay, Endeavour; Falcon. We're ready for a Docked IMU Coarse Align, if you are.

098:45:59 Worden: Okay, Falcon. Stand by one.

098:46:01 Scott: Okay; give us a call when you are in Min[imum] Deadband, Attitude hold.

098:46:05 Worden: Roger.

[The IMU in the LM is about to be aligned by reading the angles already in the CSM's IMU and transferring them across to the LM with appropriate translation between the two spacecrafts' coordinate systems.]

[As with the P52 realignments that Al makes in the CSM, the LM must be held steady while sightings are taken on the stars. The LM's RCS is not ready to be used for this so the CSM's RCS will be used instead.]

[The "deadband" is the range of attitudes on either side of the ideal that the vehicle is allowed to drift to before active measures are taken by the spacecraft's control system to correct it. While the spacecraft's attitude is within the deadband, there is no thruster activity. When it drifts to the limits of the deadband, the DAP works out which thrusters to fire to restore the correct attitude.]

[The deadband is usually set to narrow (±0.5°) or wide (±5.0°). The narrow setting is more costly in RCS propellant and is only used when high pointing accuracy is required.]

098:46:23 Scott: Okay; if you [could] read us your gimbal angles, please.

098:46:33 Worden: Okay; R1 is plus 001.24, R2 is plus 101.66, R3 is plus 005.37.

[Al uses Verb 6 (display decimal values) to call up the three values residing in Noun 20 (current gimbal angles). These angle readings are in hundredths of a degree.]

098:46:54 Worden: Roger.

[Long comm break.]

[Al holds the docked spacecraft in a steady attitude while Dave begins the alignment of the LM's guidance platform.]

[Performing this initial alignment of the IMU is done in two steps. First, a coarse alignment of the LM's IMU is performed while the spacecraft are docked, using the known orientation of the CSM's IMU as a reference. Since there is no computer-to-computer connection between the CSM and LM, gimbal angles of the CSM's IMU are recorded manually from the DSKY. To make these values usable for the LM's IMU, simple calculations are made with the recorded gimbal angles to account for the different orientations of the coordinate systems of the two spacecraft. Additionally, the difference in roll orientation between the two spacecraft that existed during docking is obtained from scribed marks in the docking tunnel (this value is generally quite small, in this case a nearly perfect -0.1° as reported by the crew at 034:44:38).]

[This part of the procedure was rather melodramatically portrayed in the movie "Apollo 13", during the emergency LM activation immediately after the explosion. It should be noted that this process uses only simple addition and subtraction, and doesn't require a slide rule!]

[Calculations completed, the crew then instructs the computer to drive the gimbals of the LM's IMU to this orientation. While not sufficiently accurate for precise maneuvering, the platform now has a reasonably good idea of "which way is up."]

[The second step in preparing the IMU for use is the fine alignment. Using the Alignment Optical Telescope (AOT) located between the two crewmembers, star sightings are taken which are used to determine the orientation of the spacecraft, and subsequently, the intended orientation of the platform.]

[The AOT itself is a remarkably ingenious device, whose elegance is in the simplicity of its design. Unlike the CSM's sophisticated sextant and telescope systems, the AOT is a simple, unity power telescope with a 60 degree field of view that is manually rotated between six fixed positions: Forward, forward right, aft right, aft, aft left, and forward left. It incorporates two methods of using the stars to determine the orientation of the platform. One is for in-flight use, the other for when the LM is on the surface.]

[Sighting the stars is done against an illuminated reticle on which is inscribed a pair of cross-hairs, for use when the LM is flying; and a pair of radial lines and spirals which come into play for surface realignments.]

[In both cases, the computer is told which of the six detents the AOT is currently in and which star is to be marked.]

[To mark on a star during flight, the LM is maneuvered to make the star move across the X and Y cross-hairs, with marks being taken when it is coincident with each axis. The computer can define two intersecting planes from these, whose vertex points to the star. A similar pair of marks on another star gives the two vectors the computer requires to calculate the IMU's orientation. Program 52 in the LM's computer is used for this procedure.]

[This first method is not used for the initial IMU alignment as the LM is still attached to the CSM, whose mass makes it undesirable to try maneuvering the entire stack from the light end. Instead they use the second method which uses P57 in the LM computer. This is normally used on the lunar surface. It is also a simple two step process once the computer knows which star is being viewed at which detent. First, the reticle is rotated until it is between the two radial lines (the "cursor") and the "Mark X" switch is pressed, yielding the "shaft angle".]

[The reticle is rotated again until the star is between the two "spirals" and the "Mark Y" switch is depressed, giving the "reticle angle".]

[Calculations performed by the computer convert this information into a vector to the star. This process is repeated with a second star, and when completed, the computer is able to determine the IMU's orientation which can now be accurately aligned.]

098:50:46 Scott: Houston, Falcon. We got about 9 minutes until sunset here.

098:50:51 Mitchell: Okay, Dave. And we have a clock update for you of about 66 centiseconds if you'll give us P00 and Data, please.

098:51:01 Scott: Okay; P00 and Data. You got it.

098:51:09 Scott: And, Houston; Falcon. We'd like to do the RCS pressurization now since we're a tad ahead, if - you're ready to take that?

098:51:18 Mitchell: Okay; we're ready, Falcon; go ahead.

[Pressurisation of the LM's RCS is not due until about 99:10.]

098:51:48 Mitchell: And, Endeavour, Houston. When you get a moment, we'll take your read-outs from the last P52.

098:51:58 Worden: Okay, Ed.

[It will be a couple of minutes before Al recalls the required values from the computer and has them ready for Mission Control.]

098:52:08 Scott: Roger.

[Comm break.]

[The next time they pass over Hadley, in about 2 hours time, Al will be taking marks on Index crater using the sextant while the spacecraft is rotated to keep the optics pointed roughly at the landing site.]

098:53:31 Scott: Okay; a few interesting differences there. Index is much more subtle than we've seen on the simulator. And Earthlight is much sharper with a much deeper shadow.

098:53:43 Mitchell: Copy, Dave. [Long pause.]

[Earthlight Crater sits just north west of the South Cluster on the plain at Hadley. Its name commemorates a science fiction novel by Arthur C. Clarke of the same name. In the opening chapter, Clarke described a monorail ride across the Apennines near Hadley and then onward for about 900 kilometers to an astronomical observatory in the crater Plato on the northern edge of the Imbrium Basin.]

[Dave's comment about the subtlety of Index presages the difficulties he and Jim Irwin will later experience when they try to locate themselves on the surface.]

098:54:07 Mitchell: Say again, Endeavour.

098:54:11 Worden: Roger; I got the P52 numbers for you.

098:54:13 Mitchell: Ready to copy, Al.

098:54:18 Worden: Okay, Ed. Stars 41 and 42; Noun 05 was plus four balls 1. Gyro torquing angles were minus 00.010, minus 00.009, minus 00.025. And they were torqued out at 97:39 even.

[Al used stars 41 (Dabih in Capricornus) and 42 (Peacock, or in Norton's Star Atlas as Beta Pavo, a constellation of the southern skies) in the last P52 realignment when option 3 had been used instead of option 1. Al's measurement of the angle between these two stars was correct to 0.01° and the gimbals were moved by 0.01°, 0.009° and 0.025°.]

098:54:58 Worden: Roger. That was at 97:35.

[This is the time Al stopped the electricity supply from the Endeavour to Falcon. From now on, the LM will depend on its batteries as its only electrical supply.]

098:55:11 Worden: That was satisfactory both ways - far as I know.

098:55:14 Mitchell: Thank you.

098:55:18 Scott: Roger, Houston. It was good both ways.

098:55:28 Scott: Hey, Endeavour; Falcon. Did you copy that on the difference between Earthlight [Crater] and Index [Crater].

098:55:33 Worden: Roger.

098:55:34 Scott: Okay.

098:55:41 Scott: And, Houston. The RCS looks good up here on the Falcon.

[Dave is noting that the RCS pressurisation is complete and successful.]

098:55:50 Scott: Okay. We'll pick up the alignment now and get back to you later on with the RCS checkout.

098:55:55 Mitchell: Okay.

[Very long comm break.]

[

LM Flight Plan page 3-110.]

[

CSM Flight Plan page 3-111.]

[Dave is carrying out a P57 realignment of the Lunar Module's IMU. This is the Spiral/Cursor technique which is normally reserved for alignments on the lunar surface.]

[Irwin, from the 1971 Technical Debrief - [To Scott] "Your alignment went real well. You didn't have any trouble seeing the stars."]

[Scott, from the 1971 Technical Debrief - "Yes, I did too. I had a tough time seeing Dabih. Dabih was a good star as far as position goes, but it was a very difficult star to see as far as alignment goes. If you can pick out bright stars, it'll sure help you. I guess the message there is, even if you don't have a NAV[igation] star, I think I'd ensure I had a good bright star for those alignments through the AOT. But the alignment went very well. The P57 docked is a very practical technique. You get a good alignment out of it, and subsequent drift checks showed that we had a good platform."]

[Al has put his helmet and gloves on and is checking the integrity of the suit. He will remove them shortly after LOS in about half an hour having made a check of the integrity of the CM forward hatch.]

099:10:39 Scott: Houston, Falcon. That P57 up here docked works pretty well. [Long pause.]

099:11:22 Scott: Houston, Falcon, do you have the torquing angles?

[Rather than reading them out, Dave has brought the gimbal torquing angles up on the LM's DSKY from where Mission Control are able to read them by telemetry.]

099:11:29 Scott: Okay; we'll torque them at 11:30.

099:11:34 Mitchell: Roger. [We] observe the five balls there.

[Dave's sighting of the two stars chosen for the P52 alignment, and his measurement of the angle between them, was as accurate as the system is capable of; five balls means 000.00°. Crews like to have this number as low as possible and Mitchell is complimenting Dave on the accuracy of his work.]

099:12:03 Mitchell: Copy. [Long pause.]

["Deadbanding" is the CM's motion back and forth within the preset deadband range.]

[Dave is commenting that during his IMU alignment, the combined CM/LM was not completely steady, but was moving around slightly. At Dave's request at 098:46:01, Al set it to its minimum setting, ±0.5°. Although this amount sounds insignificant, it can complicate the alignment of the platform when crews are sighting on stars. He is also pointing out that stray reflections of light from the Earth are coming off the Command Module and interfering with the sighting process.]

[The Flight Plan asks the crew to take a note of the angle between the two spacecraft at 97:49. They had already done this during their first entry into the LM on the second day of the mission.]

099:12:37 Mitchell: We copy, Dave. [Long pause.]

099:13:11 Scott: Okay, Endeavour; Falcon. Mid [correcting himself] min[imum] deadband no longer required.

099:13:15 Worden: Okay.

099:13:22 Scott: Okay, Houston; Falcon. We're ready for the RCS checkout, if you are.

099:13:28 Mitchell: We're ready to go, Falcon.

099:13:32 Scott: Roger.

[Comm break.]

099:14:32 Mitchell: That's affirm, Dave.

099:14:37 Scott: Okay. And, Endeavour. We need you in a wide deadband Attitude Hold.

099:14:44 Worden: Okay; wide deadband Attitude Hold.

099:14:47 Scott: Roger.

[Dave is ensuring everyone is ready for the series of checks he is about to make to the RCS on the LM's ascent stage as he will be firing the thrusters.]

[Al must have the CSM's DAP set to use a wide deadband. During these RCS hot fire checks, Falcon will be moving the combined CSM/LM stack around by several degrees, and they do not want the CSM's RCS compensating for this by trying to tightly hold attitude and wasting its fuel.]

[Additionally, Dave is making sure that Mission Control are receiving telemetry from the LM at the high bit rate as they will wish to observe the many parameters in the RCS.]

099:17:38 Worden: I haven't detected any, Dave.

099:17:41 Scott: Okay.

099:17:45 Worden: You want me to go Free now?

099:17:47 Scott: Roger. Go Free, please.

[By switching the CMC to its 'Free' mode, the CSM control attitude control system stops trying to bring the spacecraft into the deadband. Dave seems to have noticed some attempt by the CSM to oppose his firings of Falcon's thrusters and wants the CSM RCS disabled in the meantime.]

099:18:16 Worden: Okay.

[Comm break.]

099:20:47 Scott: Okay, Endeavour. Hot fire check is complete. Everything looks good. You can go back to wide deadband Attitude Hold.

099:20:52 Worden: Okay. And I'm going to turn the roll jets off and put the hatch in, if you don't mind.

[The docking probe and drogue have been reinstalled, Al is closing the CM forward hatch while Jim closes the LM overhead hatch.]

099:21:07 Mitchell: And, Falcon; Houston.

099:21:08 Scott: And, Houston, Falcon. How did it look down there?

099:21:10 Mitchell: I was just going to tell you, Dave, hot fire check looked good here.

099:21:16 Scott: Okay. They're nice and positive, aren't they?

099:21:20 Mitchell: That's affirm. [Long pause.]

[Although Dave has spent many hours in the LM simulators, this is the first time he has had a chance to try the real thing and he seems to approve of the responsiveness of the RCS controls. Note that with Ed Mitchell, who flew on Apollo 14, he is talking to someone who has already been there.]

[Thruster B-3 aims up towards the LM. The reason for it being disabled may be to avoid impingement on part of the LM.]

099:21:56 Scott: Thank you. [Long pause.]

099:22:38 Worden: And, Falcon; Endeavour. I'm cocking the latches now.

099:22:41 Scott: Okay.

[Long comm break.]

[Al extends the probe to engage the three capture latches on the its tip with the LM's drogue. He then retracts the probe slightly so that it firmly grips the drogue. He then cocks the twelve docking latches around the rim of the tunnel ready for release. These have been holding the CSM and LM together since the early hours of the mission.]

099:29:18 Mitchell: Okay, Apollo 15; Houston. You're Go for undocking. You're 45 seconds from LOS, and we observed your rendezvous radar test. Falcon, also, we have not seen you reset the DAP [Digital Auto Pilot].

099:29:35 Scott: Okay, understand. I'll get the DAP reset, and the tapemeter looks like it works fine.

099:29:40 Mitchell: Very good, Dave. Glad to hear it.

[The tapemeter displays the distance and distance rate in two modes. During the descent it will be the altitude and altitude rate meter relative to the lunar surface using data from the landing radar. During ascent and rendezvous, it will display the range and range rate between the Command Module and the Lunar Module derived from rendezvous radar data. Proper operation of the tapemeter was a concern since the initial inspection of the LM at 34:30 when the crew found the glass cover of the tapemeter had broken.]

[As Al removes his helmet and gloves, Dave and Jim are putting theirs on in preparation for the landing. Aboard both spacecraft, this far-side pass is mostly spent preparing for undocking and separation. Al sets up a Hasselblad and a DAC (Data Acquisition Camera, the small, variable frame rate Maurer movie camera) for photography of the departing Lunar Module out of window 2 of the Command Module. He carries out a maneuver to trim the attitude of the combined stack with the CSM towards the Moon and the LM away from it. Note that this is only a trim to a maneuver carried out over two hours ago.]

[Dave and Jim do a suit pressure integrity check, but find it does not hold pressure well enough at the first try.]

[Irwin, from the 1971 Technical Debrief - "Probably our biggest surprise of the activation, was the pressure integrity check. When we obviously did not have integrity, we tried going to the secondary canister and still didn't have any integrity. We decided to press on through it and do the rate check, which we did. Then, later on, I guess it was about 10 minutes before undocking, we came back and redid the pressure integrity check. Of course, we cycled through it right from the start; and this time, it worked out great. I think we had a 1/10th drop in 1 minute."]

[Scott, from the 1971 Technical Debrief - "I guess on the first one, we had something like 1 psi drop in 1 minute, didn't we?"]

[Irwin, from the 1971 Technical Debrief - "Yes. It was obviously something open, and I don't know whether the valve was just not seating properly or just what it was."]

[Scott, from the 1971 Technical Debrief - "Okay, you cycled the valve back there several times. We both fiddled with the detent and had a good detent in it, but couldn't come up with an answer."]

[Irwin, from the 1971 Technical Debrief - "Well, I guess the ground never came back to us with anymore words on it either."]

[Scott, from the 1971 Technical Debrief - "There was a question in my mind as to what the mission rule was at that time. I guess the mission rule was to undock and press on, which we were going to do had we not gotten a good check. But it was a good thing we started that check a little early. It gave us a chance to come around and do it again. The message is to get ahead and stay ahead; that's why we stayed 10 to 15 minutes ahead. Every time we got to a point in the time line where we could do something, we went ahead and did it, even though it was a little early."]

[

LM Flight Plan page 3-112.]

[

CSM Flight Plan page 3-113.]

[They are to undock by Al extending the probe, then, once motions of the two craft relative to each other have settled, the three capture latches are released.]

[Immediately after undocking, he is to move the CSM away, or translate in a minus-X direction, at 0.3 metres per second (one foot per second) to separate the two spacecraft and avoid a collision. In the LM, Jim also films the undocking and separation and Dave pitches the LM up 90° and left 60° to allow Al to visually inspect the landing gear.]

[On coming around the Moon's eastern limb, Dave reports to Mission Control that things have not gone according to plan.]

100:18:21 Scott: Okay, Houston; this is the Falcon. We didn't get a Sep, and Al's been checking the umbilicals down on the probe.

100:18:33 Mitchell: Okay. We didn't read that, except no Sep. [Long pause.]

Public Affairs Officer - "Dave Scott reporting we have not gotten separation. They said they were going to be checking the probe umbilicals. We'll stand by for further reports."

[The two spacecraft have not separated because they have not undocked. Specifically, the term "Separation" refers to the maneuvers which puts distance between them after they have disengaged the capture latches and undocked.]

100:19:07 Mitchell: Okay, Falcon. We have copied. And we'll have some words in a minute.

100:19:13 Scott: Okay. There was not even any motion on the probe.

100:19:23 Mitchell: Roger. We copy. [Long pause.]

100:19:48 Mitchell: Falcon, Houston. We have no probe temp[erature data], which indicates the umbilical is probably not well connected.

100:19:58 Scott: Okay. Well, that's just what he's checking. Thank you.

100:20:47 Mitchell: And, 15; Houston. Be advised that we have plenty of time here on the Sep, up to 40 minutes or so. Procedures will be to get vertical or get - now get vertical on the orb rate ball and standard Sep procedures.

100:21:06 Scott: Okay, fine. We can handle that.

[The removable probe assembly is connected to the rest of the spacecraft via two umbilicals which supply power for the extension and latch release mechanisms, and which carry the signals from various sensors which indicate the health and state of the assembly. Since telemetry about the temperature of the probe is not reaching Mission Control, they are surmising that an umbilical is not connected properly and that the command to disengage the capture latches may not have got through.]

[Note that although the two spacecraft are being held together by the three capture latches, these are more than adequate to hold the seal between the CSM and LM once a hard dock and the docking latches had compressed the interface.]

100:21:41 Scott: Hey, Al, I hope you made sure the Extend/Release switch was Off when you went in there.

[Dave's worry is that if the switch to extend the probe is in the On position, and Al reconnects the umbilical, the probe would extend. With the docking latches unset, the craft would separate but with the CM forward hatch removed, the CM cabin would immediately evacuate.]

100:21:51 Scott: Okay.

[Comm break.]

100:23:09 Scott: Okay, very good.

100:23:18 Worden: Okay, David. I'm venting you down now.

[Al has replaced the forward hatch in the command module and is venting the trapped air out of the tunnel.]

100:23:27 Worden: And that probe was loose in the - the umbilical was loose in Victor [its socket].

100:23:31 Scott: Okay, I'm glad you found something.

Public Affairs Officer - "The indication here on the ground that we had..."

100:23:41 Falcon: Go ahead and take your time. And when you get all squared away, give us about 5 minutes or so, and we'll be all set.

100:23:45 Worden: Okay.

100:23:49 Mitchell: And there's plenty of time to get a good hatch integrity check, 15, and do the procedure leisurely.

100:23:59 Worden: Okay, Ed. That's in work now.

[Comm break.]

100:25:10 Irwin: Houston, Falcon. In the meantime, can I give you the gimbal angles under Verb 06 Noun 20,

100:25:24 Mitchell: This is Houston; go ahead.

100:25:30 Scott: Okay. The GET was 100:08:56; and the CSM, 35975, 10818, 35995. The LM, 30009, 28836, zip, zip, zip, zip four [00004].

100:26:01 Mitchell: We copy.

[Comm break.]

[Now that the problem with the probe's umbilical is fixed, the crew can work to regain the time line. While Al is rechecking the integrity of his hatch's seal, Jim is reading up the IMU angles for both the CSM and the LM. The Flight Plan requires that these are noted prior to undocking so Mission Control can verify their attitude. Al would have read the CSM angles across to the LM at their request. With the undocking still to occur, Jim's read-out is redundant.]

100:27:04 Worden: Roger; understand. Local vertical.

100:27:28 Scott: Houston, Falcon. You know, we've - You might run out of an attitude and a time, and it might save a little gas.

[The attitude of the stack would have aligned with the local vertical; CSM toward the Moon, LM away from it, at the planned time of undocking. As they have come further around in their orbit, the Moon's curve has altered the local vertical and they must remaneuver to match it in time for the undocking. The time for undocking is now uncertain and Dave is asking Mission Control to define undocking times and the appropriate angles for those times to which they should maneuver. This should prevent them trying to chase the Moon's curve.]

100:27:59 Mitchell: Go ahead.

100:28:03 Scott: Well, it's going to take fairly sizeable maneuver, and it takes a little while to maneuver [at] two-tenths of degree per second.

100:28:08 Mitchell: Roger; understand. We're getting another for you for 5 minutes from now.

100:28:23 Scott: Better make it 10.

100:28:27 Mitchell: Yes; we'll have one for every 5 minutes, Dave.

100:28:33 Scott: Okay. [Long pause.]

100:28:51 Mitchell: And 15; Houston. We're not going to be able to make the P24 this pass, we don't believe. So don't worry about it.

100:29:08 Worden: Roger.

Public Affairs Officer - "P24 is the landmark tracking of the landmark near the landing site which Al Worden was scheduled to perform from the Command Module. We'll be doing that again on the next revolution. And at the moment we're standing by for another attempt at undocking and separation."

[LM Flight Plan page 3-114.]

[

CSM Flight Plan page 3-115.]

100:30:24 Worden: Okay, Houston; Endeavour. Going towards 30 degree pitch.

100:30:30 Mitchell: That's affirm, Al.

100:30:51 Mitchell: And, Endeavour; Houston. That angle is good for 100 hours and 38 minutes [GET], and it's not very critical. We'll use it anytime around there.

100:31:08 Worden: Yes. Roger, Houston.

[Long comm break.]

[Note that off-scale-high does not mean that the probe was wildly hot. Depending on the sensor and the support electronics, a broken connection might be read as a value far above or below its expected range.]

[Worden, from the 1971 Technical Debrief - "I guess there was an anomaly that happened during maneuvering to an undocking attitude. I checked things off on the Flight Plan as we went. We went right down the line on the Flight Plan and the checklist. We released the docking latch, put the suit on, and did a suit circuit integrity check. We installed the hatch, got a LM/CM Delta-P, and went right on down the time line. We did a Verb 49 maneuver to the undocking attitude and the Sep attitude, went into P41 SCS [using the RCS to move away from the LM] and the whole thing. We went through the undocking, checklist and got the probe circuit breakers in. I guess the major thing is that everything was nominal, except when I went to Release on the probe Extend/Release switch; nothing happened. Nothing.]

["I rechecked the circuit breakers and hit the Extend switch again, but nothing happened. At that point, there wasn't anything I could check inside. The only two things that you've got are the circuit breakers and the switch. So, I figured that there had to be something back in the tunnel. I went back and pressurized the tunnel. I looked in the tunnel and there was nothing there that was out of order. So I thought I'd go ahead and check those connectors again. I pulled the connectors off and put them back on. I figured that if that wasn't it then we had a serious problem. I put the hatch back in, depressurized the tunnel, and went through the checklist again, depressurizing the tunnel. We got a new attitude from the ground, which was the local vertical attitude. That time it worked fine. That's really a mystery to me."]

[Scott, from the 1971 Technical Debrief - "We got a couple of good calls from the ground on that; one when we came around the corner. I called and told them we had not had a Sep and that you were in the tunnel checking the umbilicals. Right away they came back and said they had no TM [telemetry] on the probe, which gave them the indication that there was something loose on the umbilicals, and that was, of course, the last thing to check. Soon after you checked everything they reported getting their TM, so that was a pretty good confirmation that that was the problem. Then, I thought another good call was immediately or very soon after. They came up and said no problem on the time, that we had 40 minutes to get the Sep done and just go to the local vertical attitude, somewhere around there, which was a big help to us. Jim and I were trying to plan ahead to make sure we didn't get too far behind the time line and get hooked into having to delay PDI rev. We were trying to plan our next series of events for a late separation. It was nice to have that call, to know that we had 40 minutes to get things squared away and move on."]

[Worden, from the 1971 Technical Debrief - "The MCC-H came up with an attitude after you'd requested that they give us the time and an attitude. We went to that attitude and we were there 4 or 5 minutes before the time. It worked out fine."]

[Scott, from the 1971 Technical Debrief - "I thought that was a very good recovery for an off-nominal situation."]

[Slayton, from the 1971 Technical Debrief - "It sounds like you guys were ahead of it, though, by the time you came around the corner."]

[Worden, from the 1971 Technical Debrief - "Yes, that's right. There was only one way to go."]

[Scott, from the 1971 Technical Debrief - "You check the switches and the circuit breakers, and the next thing you have to do is go into the tunnel."]

[Worden, from the 1971 Technical Debrief - "Anyway, we got undocked and from there on it went pretty well, except that the undocking was too late to do that low altitude P24. So we skipped that."]

100:36:17 Scott: Okay, very good. Give me a minute.

100:36:19 Worden: Okay. And I'll stop the maneuver at local vertical.

100:36:26 Scott: Okay. Have you - Are you in Attitude Hold now?

100:36:29 Worden: Negative. Will be in just about another 10 degrees.

100:36:35 Scott: Okay.

100:36:44 Worden: Okay. As you get all set, I need about one minute to get P47 up.

100:36:47 Scott: Okay.

Public Affairs Officer - "That's Al Worden and Dave Scott talking back and forth between Endeavour and Falcon."

100:37:23 Worden: Okay, Dave. About a minute and a half.

100:37:28 Scott: Okay; a minute and a half until you get to your attitude?

100:37:31 Worden: Until we're ready to SEP.

100:37:32 Scott: Okay; good.

100:38:13 Worden: Okay; let's go on one minute.

100:38:16 Scott: Okay; you got P47 running. You can Go anytime you want to.

100:38:18 Worden: Okay; I've got P47 running also. [Long pause.]

[Program 47 is the Thrusting Monitor Program, which is used to display the results of all thrusting procedures that are not explicitly controlled by the spacecraft computer. Al is using it to monitor changes in the spacecraft's velocity imparted by the undocking procedure and by the subsequent separation maneuver. The display resulting from Verb 06, Noun 83 will show the accumulated velocity changes along each of the three axes.]

[Al set the event timer to count up to the moment of separation. The process is controlled by a switch on the upper-right of panel 2 on the Main Display Console. The centre position of this switch is Off. In the 'down' position, it retracts the probe. The 'up' position is momentary (i.e. it does not stay in that position when let go) and commands the probe to extend and the capture latched to disengage.]

[There are two procedures for undocking the LM when it is manned, one of which is called a "soft undock". The other simply has the probe being extended and the LM pushed away by the force of that extension. Since the Extend command also pulls in the capture latches, the LM continues to move away once the probe has reached the end of its 25-cm stroke.]

[The "soft undock" minimises unintended LM velocity with respect to the CSM. The Probe Extend switch is only momentarily held in Extend. While this extends the probe, it allows the three capture latches to reset so that at full extension the LM is not set adrift, but rather remains attached by the latches re-engaging in the drogue. Once the motions between the two vehicles have stabilised, the latches are released by another operation of the Probe Extend switch. The separation can then be completed by controlled firings of the RCS thrusters. This seems to be the method used by the Apollo 15 crew.]

[If the switch command to release the capture latches were to fail, the probe includes arrangements to allow a suited crewmember to manually release them from either side of the tunnel. The CMP can pull a handle from the CM side while a button in the centre of the probe tip can be accessed through the drogue by a LM crewmember. In either case, the cabin must be depressurised and the corresponding hatch must be removed to allow access.]

100:39:20 Scott: Okay; we're on the capture latches. Good.

100:39:22 Worden: Roger [Long pause.]

[The probe has been extended and the LM drogue has re-engaged on the probe's docking latches. Having waited for the relative motions between the two spacecraft to settle, the capture latches are disengaged and the CSM is translated in a minus-X direction by firing all four RCS thrusters for one second.]

100:39:43 Scott: Okay; good clean Sep. [Long pause.]

Public Affairs Officer - "You heard Al Worden and Dave Scott report separation and we confirm that on the ground, a good clean separation at 100 hours, 39 minutes, 39 seconds."

[Photographs AS15-87-11695 and AS15-87-11696 show the CSM pointing towards the LM while it is still about one fifth of the width of the picture frame in diameter. The Hasselblad camera is fitted with a 60-mm lens so by my calculations, the CSM is roughly 25 metres distant. Behind it is a massif on the eastern shore of Mare Serenitatis. AS15-87-11697, 98 and 99 are taken a few minutes later when the two craft are halfway across Serenitatis, north of the crater Bessel. The CSM has retreated to about 70 metres.]

100:40:31 Worden: Okay, Ed; if you think we can't get there for the P24s.

100:40:37 Mitchell: Negative. You've only got 6 minutes. They say "No way."

[Landmark tracking by the Command Module of the landing site is an essential part of obtaining accurate guidance information needed by the Lunar Module. The delay in the undocking and separation maneuver has made tracking of the landing site impossible on this pass. Other tasks, such as the critical LM inspection and a platform alignment are more pressing. The tracking procedure will be performed on the next orbit.]

[The LM crew, meanwhile can proceed with their preparations for the descent and landing. They will defer the check of the DPS (Descent Propulsion System, pronounced 'dips') until after they have observed the landing site.]

100:40:47 Irwin: Okay. Falcon's going to yaw left. [Long pause.]

[Dave is performing the pitch and yaw maneuver which will bring the four landing gear into Al's view and allow him to visually check that they have all extended properly.]

[Woods, from 1999 correspondence with Scott - "What would have happened if one or more of the gear hadn't deployed? Were there contingencies to try and deploy them by troubleshooting - probably - or by some external intervention - unlikely, I would have thought? Would it have been a case of do as much data gathering in lunar orbit as possible?"]

[Scott, from 1999 correspondence - "Yes, I am sure there were mission rules and procedures for a gear problem. At a minimum, one would probably attempt to release the gear with a CSM nudge; or if all else failed, land with the gear as is and hope that it locked down, being ready to abort if not."]

100:42:30 Worden: And, Falcon, Endeavour. Looks like you got one radar there that's - rotating away from me [DSE has 'rotating quite freely'].

[Comm break.]

[The landing radar is operated in two positions depending on where the LM is in its approach. The early part of the descent is carried out with the crew facing up. At about 3,000 metres, the LM pitches over for the final descent with the crew facing forward. The landing radar must be able to rotate to accomodate the changing attitude.]

100:43:39 Mitchell: Falcon, Houston. Low Bit Rate, Aft Omni, please. [Long pause.]

100:43:58 Mitchell: Endeavour...

100:43:39 Scott: Roger. We're Low Bit Rate and Aft Omni.

100:44:02 Mitchell: Roger. Thank you. [Long pause.]

100:45:02 Mitchell: Falcon, Houston. Give us, on the steerable, Pitch, 155; Yaw, minus 50.

100:45:17 Irwin (onboard): Roger. Pitch, 155; Yaw, minus 50.

[Comm break.]

[The steerable High Gain Antenna on the LM will allow telemetry to be transmitted at a higher bit rate while the DPS is tested.]

100:47:38 Mitchell: Okay, Falcon. We're reading you well now. Give us High Bit Rate, please.

100:47:48 Irwin: Roger. High Bit Rate.

100:47:53 Scott: And, Houston, we're ready to go with the DPS throttle check, whenever you are.

100:47:56 Mitchell: Okay; we're ready. Let's go.

100:48:01 Scott: Okay.

100:48:07 Mitchell: Endeavour, Houston. Give us P00 and Accept; we have an uplink for you.

[This is an uplink of the ground's latest calculation of Endeavour's state vector. They also send up a target load for the upcoming Circ maneuver.]

100:48:18 Worden: Got it?

100:48:21 Mitchell: Roger, Al.

100:48:40 Scott: Minimum is 11, soft stop, 52; max, 100.

[They are testing the Thrust/Translational Hand Controllers (TTHC) in the LM. Minimum thrust is registered as 11%, the "soft stop" is at the bottom end of what is called the "Critical Zone" for the engine. The DPS cannot be throttled between 65% and 92.5% thrust because of excessive erosion on the nozzle throat. They are reading the values off of one of the analog gauges on the Commander's panel.]

100:49:11 Scott: LMP is 11; soft stop, 51; max, 100.

100:49:22 Mitchell: Copy.

[Comm break.]

[DECA - Descent Engine Control Assembly.]

100:51:31 Scott: ...open.

100:51:37 Worden: Endeavour, Roger.

100:51:40 Mitchell: And say again, Dave.

100:51:41 Scott: Okay. You want to run it again real quickly, Ed?

100:51:43 Mitchell: Okay. Was the circuit breaker out?

100:51:44 Scott: ...take at it - take a look at it again with the - Yes. Roger. The circuit breaker is out.

100:51:54 Mitchell: Very good.

100:52:00 Scott: Okay. Do you want to look at the test again?

100:52:03 Mitchell: That's affirmative, Falcon. Let's have it again, please.

100:52:08 Scott: Roger-D.

100:52:30 Scott: Okay, Ed. CDR's at min, 11; soft stop, 51; max, 101.

100:52:41 Mitchell: We copy. And it looks good this time.

100:52:49 Scott: Okay. And the circuit breaker is still in.

100:52:52 Mitchell: Roger, Roger. And we're ready to give you an uplink, if you'll give us P00 and Data, please.

100:52:58 Scott: Stand by. Let's run the LMP check here.

100:53:14 Mitchell: And it looked good here, Falcon.

100:53:19 Scott: Okay. Thank you. [Long pause.]

[Al ought to be carrying out a P52 platform alignment just now but the delayed undocking has upset his time line. Soon Mission Control will tell him to skip it as they are happy with the current alignment anyway.]

100:53:41 Scott: No, I can't verify either. We checked them over before undocking and - I can't tell you whether it popped or it was open.

100:53:49 Mitchell: Okay; we understand. Thank you.

100:53:53 Scott: Roger. [Long pause.]

100:54:08 Scott: Okay, Houston. We'll take the uplink anytime you want to give it to us.

[The nearest LM uplink shown on the Flight Plan is one to send up gyroscope compensation numbers for the LM computer if they are required.]

[Comm break.]

100:55:36 Mitchell: And, Falcon, the computer's yours. [Long pause.]

100:55:50 Scott: Roger.

100:55:56 Scott: Falcon, Endeavour. I mean Endeavour, Falcon. [Long pause.]

100:56:11 Scott: Okay; Houston, Falcon here. Would you give the Endeavour a call. Tell them we're going to run the radar checkout now, please. We seem to have lost contact.

100:56:18 Mitchell: Roger. Endeavour, Houston. Falcon is calling, and he's ready for the rendezvous radar check. [Long pause.]

100:56:41 Mitchell: Endeavour, Houston. Do you copy? [Long pause.]

100:57:04 Mitchell: Endeavour, Houston. [Long pause.]

100:57:30 Mitchell: Endeavour, Houston. Do you read? [Long pause.]

100:58:04 Mitchell: Endeavour, Houston. Over.

100:58:14 Scott: Endeavour, Falcon. [Long pause.]

100:58:29 Scott: Endeavour, Falcon. Simplex A and B. How do you read?

100:58:35 Worden: I read 5 square, Falcon.

100:58:37 Scott: Okay. Well, we lost you there somewhere along the way. We need to check out the radar and Houston seems to not be able to get a hold of you either.

100:58:44 Worden: Okay. I'm in.

100:58:46 Mitchell: We're reading you now, Al.

100:58:48 Worden: Okay, Ed...

100:58:49 Scott: I was...

100:58:50 Worden: Okay, Ed; I was off for a couple of minutes reconfiguring inside here.

100:58:55 Mitchell: Roger.

100:58:56 Mitchell: Okay, Al. If you'll go back to Simplex A, then we'll give you the voice ranging, and we'll check out the radar.

100:59:06 Worden: Okay. I'm Simplex A.

100:59:10 Scott: Okay. Voice ranging coming up. We're going to check the radar.

[Comm break.]

[The prominent dish mounted high above the LM's forward hatch is the antenna for the Rendezvous Radar which locks onto the CSM and provides the crew and the LM computer with angle and distance information on its whereabouts. It does so with help from a transponder mounted on the CSM which allows it to operate as far as 750 km away.]

[

LM Flight Plan page 3-116.]

[

CSM Flight Plan page 3-117.]

101:00:50 Worden: Roger. Trans - Transponder is on.

101:01:07 Worden: Say again, Falcon. [Long pause.]

101:01:44 Scott: Okay, Endeavour; Falcon. Can you give us your range, please?

101:01:48 Worden: Okay, Falcon. Stand by one. [Long pause.]

101:02:01 Scott: Endeavour, Falcon.

101:02:06 Worden: Hello, Falcon. This is Endeavour. How do you read now?

101:02:08 Scott: Yes, five by. Can you give us your range, please?

101:02:10 Worden: Okay. Stand by one. [Long pause.]

101:02:33 Worden: Okay; .4 [nautical miles], Dave.

101:02:38 Scott: Okay; .4. We're looking at .78.

101:02:41 Worden: Roger. Let me reset.

101:02:44 Scott: Okay. Reset.

101:03:03 Worden: I have you at .41 now.

101:03:06 Scott: Okay. Maybe we're just in to close. We're looking at .79 mile. We'll press on.

101:03:11 Worden: Roger. [Long pause.]

[Worden, from the 1971 Technical Debrief - "We did check out the VHF against the rendezvous radar. I think I reset the VHF three times, and it came up each time with half the value that the rendezvous radar had in it. This made me wonder at the time how good the VHF was operating, and it subsequently turned out that it was operating just fine. I don't know what caused the difference in the range between the rendezvous radar and the VHF at that close range, because it was 0.79 mile, or something less than a mile, I think."]

[Scott, from the 1971 Technical Debrief - "Yes, we had on the Noun 78. In the LGC, we had 0.78; you had 0.4; and the tape meter had 0.78. Of course, the Noun 78 is just a tape meter read-out, but you did have, for some strange reason, just half value."]

[Worden, from the 1971 Technical Debrief - "I reset that thing three times, and I think it came up with the same value each time."]

[Scott, from the 1971 Technical Debrief - "I might add, in the LM, we could tell when you were resetting. It was audible, so we tried to observe a no-comm silence period while you were getting your reset. There's no question there that you reset."]

[The P52 realignment of the CSM's platform was due about 10 minutes ago but has been scrubbed to help Al catch up with his time line.]

101:04:16 Mitchell: Endeavour, Houston. Did you say standing by?

101:04:24 Worden: Yes, Houston, Endeavour's ready to copy.

101:04:27 Mitchell: Okay, Al. We'd like for you to go ahead and start going to the burn attitude. I'll give you the roll, pitch, and yaw, and you can get that in, and I - then I will give you the rest of the PAD.

101:04:45 Worden: Roger, Houston. Go ahead.

101:04:46 Mitchell: Roger. 000, 107, 358.

101:05:01 Worden: Roger; understand. 000, 107, 358. I'll put that in and be right back with you.

101:05:07 Mitchell: Roger. [Long pause.]

101:05:25 Worden: Okay, Houston; Endeavour. We're maneuvering. Go ahead with the rest of the PAD.

101:05:30 Mitchell: Okay. Circ.: SPS/G&N; 37679; plus 0.49, plus 1.04; GET is 101:38:58.19; plus 0068.3, minus four zero's 1, minus four zero's 7; 000, 107, 358; 0064.9, plus 0054.3; 0068.3, 0:04, 0057.2; sextant star 10, 171.3, 40.0. The rest NA. GDC align, Vega, Deneb; 288, 340, 346. Ullage four jet, 14 seconds.

101:07:15 Worden: Roger, Houston. Understand. P30 PAD, Circ. burn, SPS/G&N; 37679; plus 0.49, plus 1.04; 101:38:58.19; plus 0068.3, minus 0000.1, minus 0000.7; 000, 107, 358; 0064.9, plus 0054.3; 0068.3, 0:04, 0057.2; 10, 171.3, 40.0. Vega, Deneb, set stars; 288, 340, 346. Four jet, 14 seconds.

[The PAD is interpreted as follows:

Purpose: This PAD is for Al to circularise the orbit of the CSM.

System: It will use the SPS engine under the control of the Guidance and Navigation System.

CSM Weight (Noun 47): 37,679 pounds (17,091 kg).

Pitch and yaw trim (Noun 48): +0.49° and +1.04°.

Time of ignition, T_ig (Noun 33): 101 hours, 38 minutes, 58.19 seconds.

Change in velocity (Noun 81), fps (m/s): x, +68.3 (+20.8); y, -0.1 (-0.03); z, -0.7 (-0.2). The velocity components are expressed with respect to the

local vertical/local horizontal. The large positive value for x shows that this is a prograde maneuver that speeds the spacecraft up and has the effect of raising the pericynthion to about the same height as his apocynthion and thus circularising the orbit.

Spacecraft attitude: Roll: 000° Pitch: 107° Yaw: 358°. These attitude angles are expressed with respect to the

Landing site REFSMMAT.

H_A, expected apocynthion of resulting orbit: 64.9 nautical miles (120.1 km).

H_P, expected pericynthion of resulting orbit: 54.3 nautical miles (100.5 km).

Note that this orbit is not quite circular but is deliberately elliptical. FIDO in Mission Control is compensating for the perturbation of the CSM's trajectory by the mascons. Although the effect of these gravitational irregularities is not well understood by the flight dynamics team, they hope the orbit will tend to fully circularise itself under the influence of the mascons by the time Al next fires the SPS engine just prior to the return of the LM.

Delta-V_t: 68.3 fps (20.8 m/s). This is the total velocity change required from the burn.

Burn duration or burn time: 4 seconds.

Delta-V_c: 57.2 fps (17.4 m/s). This value is entered into the Delta-V display of the EMS so that instrument can provide backup control of the SPS engine. The value is smaller than Delta-V_t to account for the engine's tail-off thrust. The G&N system already takes this into account.

Sextant star: Star 10 (Mirfak [Alpha Persei]) visible in sextant when shaft and trunnion angles are 171.3° and 40.0° respectively.

GDC align stars: Stars Vega (number 36) and Deneb (number 43) to be used for backup GDC align in case the IMU is unavailable for this task. The align angles are 288°, 340° and 346°.

SPS propellants are settled in their tanks by firing the plus-X thrusters on all four quads around the Service Module for 14 seconds.]

[It was during preparations for this burn during Apollo 16 that an indicated fault in the SPS backup thrust vector control system delayed the Circ. burn and the landing by the LM by six hours, curtailing the lunar exploration somewhat.]

101:08:19 Worden: Okay, Go ahead.

101:08:21 Mitchell: Verb 21 Noun 01. Address 1765 and Enter 01605.

101:08:40 Worden: Understand. Verb 21 Noun 1 Enter, 1765 Enter, 01605 Enter.

101:08:46 Mitchell: That's affirmative, and that's - a short burn constant change, Al.

[The SPS engine doesn't come up to thrust instantly, and shut off instantly. Therefore, the build-up and tail off of engine thrust on short burns, becomes a non-trivial part of the total impulse. Since the guidance calculations occur only every two seconds or so, then during a four-second burn a guess must be made of what the performance of the engine will be (rather than being able to take advantage of averaging acceleration over a longer period of time).]

101:09:10 Mitchell: And, Al. Be advised that your sextant star will be occulted at 101:16 - 7 minutes from now. And a gentle reminder, this is a single bank, bank B burn.

101:09:23 Worden: Roger.

101:09:28 Mitchell: Okay. PDI zero PAD, when both vehicles are ready.

101:09:41 Irwin: Falcon's ready.

101:09:48 Mitchell: Roger, Falcon. And did you get the Circ. T_ig?

101:09:55 Irwin: Affirmative, Ed.

[Jim has also taken a copy of the PAD for Endeavour's Circularisation burn.]

101:10:03 Worden: Endeavour's ready.

101:10:06 Mitchell: Okay. PDI zero. Alpha: 102:39:35.35; Bravo: plus 0100.0, plus all zeros, plus 0001.8; 0138.3, plus 0009.0, 0100.1; 0:34; 000, 273; 0159.6; plus 0100.0, plus all zeros, plus 0002.4; Cocoa: 103:40:24.00; Delta: 105:22:30.00. Readback.

101:11:26 Irwin: Okay. Houston, this is Falcon with PDI zero readback. 102:39:35.35; plus 0100.0, plus all zips, plus 0001.8; 0138.3, plus 0009.0, 0100.1; 0:34; 000, 273; 0159.6; plus 0100.0, plus all zips, plus 0002.4; 103:40:24.00, 105:22:30.00. Over.

[The PDI-0 PAD is the first in a series of seven PADs read to the LM crew. This, and the next PAD (read up at 103:25:17), specify times and parameters needed for the LM to return to the CSM in the event that the powered descent is scrubbed. The first PAD, the PDI-0 Abort Pad, is executed at the point in the orbit where powered descent normally would have occurred but one revolution earlier, and is used when a determination that the landing should be scrubbed is made well before the time the descent is started. At the point of the abort, the LM is ahead of the CSM, and needs to slow down for the CSM to catch up. To do so, the descent engine is burned to put the LM into a orbit whose high point is greater than that of the CSM (the low point remains at about 9 miles or so).

There are 4 individual updates in the PDI-0 Abort PAD, A through D: Interpreting the PAD:

PDI-0 Abort PAD A (Alpha):

Ground elapsed time of the burn. A Noun 33 entry, the time will be 102:39:35.35

PDI-0 Abort PAD B (Bravo)

Change in velocity, with respect to the local vertical, fps (m/s): x, 100.0 (30.5); y, 0; z, 1.8 (0.55).

Height of apocynthion: 138.3 nautical miles (256 km).

Height of pericynthion: 9.0 nautical miles (16.7 km).

Delta-V (Overall change in velocity): 100.1 fps (30.5 m/s).

Burn Time: 0 minutes, 34 seconds.

FDAI (Flight Director Attitude Indicator - the "eight-ball") angles: Roll: 000°, Pitch: 273°

An entry made into the AGS (Abort Guidance System; see description below at

101:22:53), at address 373 (an octal address), which is the time of the burn, in minutes: 01596.

Velocity Targeting for the AGS, a Noun 86 entry:

Delta V_X: +100.0 fps (+30.5 m/s).

Delta V_Y: 0 fps.

Delta V_Z: +2.4 fps (+0.73 m/s).

PDI-0 Abort Pad C (Charlie (though Mitchell calls it Cocoa!))

Time of Ignition of the Coelliptic Sequence Initiation (CSI) maneuver, a Noun 11 entry. This maneuver is essentially a plane change, and brings the LM into the same orbital plane as the CSM.

Time of Ignition: 103:40:24.00.

PDI-0 Abort Pad D (Delta)

Time of Ignition of the Terminal Phase Initiation (TPI) maneuver, a Noun 37 entry. This maneuver targets the LM for its final approach to the CSM, ensuring proper distance and closing rates.

Time of Ignition: 105:22:30.00.]

[The next PAD, the PDI-1 +12, is intended to be used after a failure prevents the powered descent. As its name implies, it is initiated 12 minutes after the nominal start of the PDI. The format of the PDI-1 +12 Abort PAD is the same as the PDI-0 Abort PAD, with only the times being different.]

[The third PAD, the PDI-1 (or P63) PAD, provides the crew with the times and attitude information needed for powered descent. The fourth and fifth PADs are for aborts during the powered descent and landing, and the last two are for T-2 and T-3 aborts respectively. (See interpretation of the T-2 and T-3 PADs at 102:29:18.)]

101:12:21 Worden: Endeavour copied.

101:12:23 Mitchell: Roger, roger.

[Comm break.]

[The Gamma-ray and Alpha Particle experiments have been running by themselves since 95:12 and are about to be switched off until after the circularization burn. Note that the boom carrying the Gamma-ray Spectrometer is not extended even though the experiment is active.]

101:15:54 Irwin: Okay, Houston. We'll talk in a minute.

101:16:01 Mitchell: And, Falcon; Houston. We copy. Thank you.

[Long comm break.]

101:20:28 Worden: Okay, Ed. [Long pause.]

101:20:55 Mitchell: And we observed it here. Endeavour.

101:21:00 Worden: Okay.

101:21:06 Mitchell: And, Falcon, Houston. AGS K-vector.

101:21:14 Irwin: Go ahead, Houston. Ready to copy.

101:21:17 Mitchell: Roger. 100, 000, and 00006.

101:21:31 Irwin: Roger. 100, 00, 0006.

101:21:40 Mitchell: I think we needed 00006.

101:21:47 Irwin: Okay; understand. Four zeros 6.

101:21:50 Mitchell: That's affirmative. [Long pause.]

101:22:39 Mitchell: The Apollo 15, Houston. We're about 1 minute from LOS. We seem to be caught up [with the Flight Plan], and everything is looking good from here.

101:22:51 Irwin: Falcon, Roger. Thank you.

101:22:53 Worden: And Endeavour, Roger. Thank you.

[Very long comm break.]

[At LOS, towards the end of the twelfth orbit, Dave and Jim are activating, initializing and calibrating the Abort Guidance System (AGS, pronounced 'aggs'), a separate system from the Primary Guidance and Navigation System (PGNS, pronounced 'pings'). It consists of enough hardware to independently guide Falcon into a safe orbit from where Al can make a rendezvous. It is intended to be used only on those aborts where the PGNS has failed. The AGS is part of the AEA (Abort Electronics Assembly) which include "strap-down" gyros and accelerometers mounted directly on the structure of the spacecraft to save weight, rather than carry a separate IMU platform and the heavy gimbals which would have surrounded it. During normal flight, the crew constantly ensure the position and velocity solution in the AGS more or less matches that from the PGNS, so that in the event of an abort, it has enough knowledge to get the LM to a safe orbit.]

[Perhaps one of the most 'forgotten' computers ever built, the Apollo LM Abort Guidance Computer was an 18 bit, 4K computer with a 1 MHz system clock. Memory was 4K words (again, 18 bits/word), with 2K read-only, and 2K as read-write cores. Memory was fabricated in traditional core planes, rather than core "ropes" used in the PGNS. Data entry/readback was primitive compared to the DSKY; the interface was to simply read/write storage locations in the computer. Selecting the program or particular mode you wanted to run required that you change a branch table in the software. Read the

Apollo Lunar Surface Journal; each time you hear references to "413+10000" right after landing, or 400+00000 (much earlier), the LMP is entering (for example) at address 413, the value 10000. Voila! The program just changed. Addresses were strictly octal, and information on the data display was almost exclusively decimal. No scaling information was explicitly used, which demanded that the crew understood that a value meant hundereds of feet, or tenths of a foot per second. So, small and large number alike were entered as simple 5 digit strings, without decimal points or exponents. The software took care of scaling. All in all, it was a masterpiece of work. It's truly amazing to think that a reasonably sized computer offering guidance, attitude control and rendezvous was able to be done with so few resources.]

[Al's burn for this maneuver is due at 101:38:58 and will change Endeavour's velocity by nearly 21 metres per second, quite a sprightly acceleration due to the lack of the LM's mass and the less than half full SPS propellant tanks.]

[Worden, from the 1971 Technical Debrief - "The circularization burn was exciting, but it was perfectly nominal. They had updated the short-burn constants for the engine characteristics. And the circularization burn was done on Bank B only because of the [electrical short] problem we had with the SPS. I went to attitude. All the star checks worked fine. The burn was done on time, and the residuals were 00 and minus 0.5 [feet per second], which is a no-trim kind of maneuver."]

["It was a very nice burn, very smooth, and [there] is sure a difference [in acceleration] when you get the LM off. You can really feel that mother go. It's really quite impressive."]

[After the Circ. burn, he reads the details of the burn to the LM crew in case they need the information for redocking without landing, and he maneuvers the spacecraft's pitch and yaw angles to that required for the P24 landmark tracking procedure. He will maneuver to the correct roll angle later. Then, once he doffs his suit, he will reactivate the Gamma-ray and Alpha Particle experiments, now that the CSM is in the intended orbit for the main period of data collection and photography using the SIM bay. A final task is to prepare the DAC in the eyepiece of the sextant so that the tracking exercise can be filmed. Al will set it to run at one frame per second.]

[LM Flight Plan page 3-118.]

[

CSM Flight Plan page 3-119.]

[

LM Flight Plan page 3-120.]

[

CSM Flight Plan page 3-121.]

102:12:24 Mitchell: Endeavour, Houston. Standing by for your burn status. [Long pause.]

102:12:44 Mitchell: Endeavour, Houston. Standing by for burn status.

102:12:51 Worden: Hello, Houston. Endeavour. Roger. Stand by one.

102:12:55 Mitchell: Roger. Roger, Al.

102:13:00 Worden: Okay, Houston. The burn got off on time. Burn time, 4 seconds; V_GX, minus 0000.9 [fps]; and I trimmed that to 0 at - roll of 0 [degrees], pitch of 107, and yaw of 358. V_GX was plus all zero's, V_GY was plus all zero's, V_GZ was minus 0000.5. Delta-V_C was minus 11.2 [fps]; fuel was 29.25 [percent]; oxidizer, 29.15 [percent]; and unbalance meter was decreased 50. And I've got me in a 65.2 by 54.8 [nautical mile orbit, 120.7 by 101.4 km].

[V_G represents the velocity to be gained and Al is expressing it in each of the three axes. After the burn, these values are known as the residual velocities. Note that although Al said during the debriefing that the burn did not require trimming, he did, in fact, trim 0.9 fps out of the X-axis residual.]

102:13:54 Worden: Roger. The V_GX at shutdown was minus .9. [repeat] 0.9.

102:14:03 Mitchell: Okay. We copy. [Long pause.]

102:14:19 Mitchell: And, Al. I'm ready to give you a P24 PAD, when you're ready to copy.

102:14:45 Worden: Okay, Ed. Go ahead.

102:14:47 Mitchell: Roger. 15-1; T-1, 102:37:27; T-2, 42:17; TCA, 43:57; T-3, 44:45. The attitude is nominal, and you'll be off track 3 miles to the north.

102:15:21 Worden: Roger. I understand. [P]24 landmark tracking PAD, tracking 15-1; T-1, 102:37:27; 42:17; 43:57; 44:45. Nominal attitude is off track north 3 miles.

102:15:42 Mitchell: Good readback.

102:15:46 Worden: Roger.

[The numbers just read up are times of the various points in the P24 tracking profile. In this profile, the CSM will keep a 22° pitched down attitude, with respect to the local vertical, throughout the entire tracking pass. To achieve this, the spacecraft pitches down with respect to its stellar inertial frame of reference, throughout the entire period. This will keep Index crater, which will pass rather quickly below, within the range of the sextant.

T-1 is the time the landmark, in this case Index crater, will be on the horizon. It is not the time it will be visible through the sextant optics: 102:37:27 GET

T-2 is the time the target can be acquired in the sextant and is when the CSM begins to pitch down: 102:42:17 GET

TCA is when the CSM will be closest to the target: 102:43:57 GET

T-3 is the time the target will fall out of the sextant's field of view: 102:44:45 GET

Note that at the time of the closest approach, Endeavour, will not be directly overhead of Index crater, but will be 3 miles to the north. Also note that a value for the longitude-over-2 has not been given. Al will point this out at 102:34:32 but is told to use the value in the Flight Plan.]

102:15:57 Mitchell: Okay, Falcon. Ready to copy.

102:16:02 Irwin: Roger. The initial values: plus 02, minus 04, plus 03, plus 02, plus 90, minus 07. Cal values: plus 02, minus 04, plus 02, plus 21, plus 81, and minus 15.

102:16:26 Mitchell: Copied all of them. Those numbers look good, Falcon.

102:16:36 Irwin: And, Houston; Falcon. We're ready to go with the DPS.

102:16:41 Scott: Okay. And we're ready to go with the DPS pressure checkout any time you are.

[Two helium tanks, one storing gas at an ambient temperature, the other storing supercritical helium (SHe) at high pressure and cryogenic temperatures, provide a source of pressure to the four propellant tanks of the LM descent stage. This pressure forces their contents into the propellant lines leading to the engine. The supercritical helium will not be used for pressurisation until soon after the DPS engine is ignited. Initial pressurisation of the propellant tanks is provided by the contents of the ambient tank. Three valves are explosively opened by command from the crew which allow the helium, via a regulator, to the tanks.]

[The DPS pressure checkout consists of verifying the ambient helium has pressurized the tanks correctly and that the temperatures are within range. Much of this checkout is done in Mission Control, since lots of telemetered data can be seen there, but not in the LM cockpit. Therefore, the LM's steerable antenna must be in use to carry all the required high-bit-rate data.]

[Later, when the DPS engine is ignited and the descent to the Moon has begun, an explosively operated valve will be opened automatically, releasing the SHe to the propellant tanks. The SHe travels via redundant regulators and a heat exchanger which uses the warmth in the fuel to heat the SHe. The delay in using the SHe is to allow the fuel flow in the heat exchanger to be established before the cryogenic gas reaches it, preventing possible fuel freezing.]

102:16:49 Scott: Roger.

102:16:54 Mitchell: Endeavour, Houston. Give us Narrow please on your High Gain [Antenna].

102:17:02 Worden: Roger.

[Comm break.]

102:18:39 Mitchell: Stand by. Okay, Falcon. We believe you turned your PQM [Propellant Quantity Monitor] Off, and - and that's your PQGS [Propellant Quantity Gauging System] Off, and - probably your problem - your hed - Helium Monitor. I think you got it inadvertently. And, Endeavour, we'll take P00 and Accept.

102:19:23 Worden: Roger. You got it.

[Mission Control is uplinking a state vector to the CSM in preparation for Al doing the landmark tracking.]

102:19:28 Irwin: Oh, yes, Ed. Thank you.

102:19:30 Mitchell: Roger. DPS looks good from down here.

102:19:36 Scott: Okay. [Long pause.]

Public Affairs Officer - "The Descent Propulsion System on the Lunar Module is pressurized and looks good at this time."

102:20:05 Mitchell: And, Falcon; Houston. A couple of items, Dave do you have a warm feel for the LPD decal.

102:20:14 Scott: Roger, Ed. It was right on.

[Dave Scott was to check the alignment of the Landing Point Designator, a set of colored lines on the two panes of his forward window. The LPD is used during the approach phase of the landing. When Dave looks through his window from the correct position, the outer and inner grid will line up. Via Jim, the computer will supply angles which will indicate a position on the LPD and as Dave sees the LPD against the lunar surface, this will indicate where the computer thinks it is going to land the LM. If Dave wants to alter this point, left, right, downrange or uprange, he only has to "blip" the hand controller in the appropriate direction and the computer would maneuver the vehicle by fixed increments to achieve this change.]

102:20:28 Scott: Oxidizer is 100 [probably psi].

102:20:31 Mitchell: We copy. Thank you. And, Dave, be advised, there will be no gyro bias updates - or gyro drift updates [means PIPA bias updates].

102:20:44 Scott: Okay; very good.

102:21:07 Scott: Houston, Falcon going into landing radar checkout now.

102:21:13 Mitchell: Roger, Roger, Falcon. [Long pause.]

[The landing radar antenna is an assembly mounted on the underside of the descent stage. Four microwave beams at a frequency of around 10GHz provide information about the current altitude (using radar techniques) and the rate of change of altitude (using Doppler techniques) as the LM flies in for a landing. The computer has a simplified model of the terrain profile as part of its programming to compensate for the mountainous landscape which passes below Falcon during the approach to Hadley. The computer also compensates for whatever angle the antenna may be pointing away from the local vertical. The information coming from the landing radar is fed to the LM computer and also drives the tapemeter display.]

[The antenna has two positions depending on whether the LM is approaching the landing site on its side or hovering in a vertical attitude above it. Throughout most of the descent, it is angled 24° from the LM's vertical (X-axis). At "pitch-over", when the LM enters its hover mode, the antenna is rotated to aim in a direction parallel to the LM's X-axis and therefore pointing straight down.]

[Dave and Jim are testing the landing radar by having it apply simulated Doppler and radar signals to its processing electronics.]

102:22:16 Mitchell: Okay, Endeavour; Houston. Computer is yours, and you can start your maneuver.

102:22:35 Mitchell: Endeavour, Houston. The computer is yours. You can start your maneuver.

102:22:43 Worden: Roger, Houston; Endeavour. Thank you.

102:22:53 Mitchell: Endeavour, Houston. We're recommending a half of a degree per second for your maneuver. You've got quite aways to go.

102:23:02 Worden: Roger, Ed.

[The maneuver is to put the CSM in the correct attitude in roll for the upcoming P24 landmark tracking exercise over the landing site.]

102:23:14 Mitchell: Roger. It looks good here.

102:23:25 Scott: Roger.

[Comm break.]

102:24:55 Scott: Roger.

102:24:58 Mitchell: And I have PADs for Endeavour and Falcon, when you're both ready.

102:25:11 Scott: Falcon's ready.

102:25:14 Worden: Endeavour's ready.

102:25:17 Mitchell: Okay. Here we go. With Echo, 104:42:30.00; Foxtrot, plus 0108.2, plus all zeros, minus 0050.0; 0144.9, plus 0008.6, 0119.2; 0:36, 000, 270; 0282.5; plus 0108.5, plus all zeros, minus 0049.3; Golf, 107:37:30.00; Hotel, 109:18:45.00. Readback.

[There are 4 individual updates in the NO PDI+12 Abort PAD, E through H: Interpreting the PAD:

PDI-1 Abort PAD E (Echo):

Ground elapsed time of the burn. A Noun 33 entry, the time will be 104:42:30.00

PDI-1 Abort PAD F (Foxtrot)

Change in velocity, with respect to the

landing site REFSMMAT, fps (m/s): x, +108.2 (+33.0); y, 0; z, -50.0 (-15.2).

Height of Apocynthion: 144.9 nautical miles (268.4 km).

Height of pericynthion: 8.6 nautical miles (15.9 km).

Delta-V: 119.2 fps (36.3 m/s).

Burn Time: 0 minutes, 36 seconds

FDAI (Flight Director Attitude Indicator - the "eight ball") angles: Roll: 000°, Pitch: 270°.

An entry made into the AGS, at address 373, is the minutes until the next burn: 02825.

Velocity Targeting for the AGS, a Noun 86 entry:

Delta V_X: +108.5 fps (+33.1 m/s)

Delta V_Y: 0 fps

Delta V_Z: -49.3 fps (-15.0 m/s)

PDI-1 Abort Pad G (Golf)

This is the time of Ignition of the Coelliptic Sequence Initiation (CSI) maneuver, a Noun 11 entry. This maneuver is essentially a plane change, and brings the LM into the same orbital plane as the CSM.

Time of Ignition: 107:37:30.00

PDI-1 Abort Pad H (Hotel)

This maneuver targets the LM for its final approach to the CSM, ensuring proper distance and closing rates.

Time of Ignition of the Terminal Phase Initiation (TPI) maneuver, a Noun 37 entry: 109:18:45.00.]

102:27:20 Mitchell: Okay. You got cut out there. Your AGS Delta-V_Z, confirm [that the value has] a negative [sign] and [the GET for] Hotel [is] 109:18:45.00.

102:27:36 Irwin: That's confirmed, Ed.

102:27:38 Mitchell: Okay. Endeavour, give us Omni Charlie, please.

102:27:48 Worden: Endeavour on Omni Charlie.

102:27:50 Mitchell: Roger. And did you get the readbacks, Al?

102:27:55 Worden: Endeavour copied them - copied the PADs. Roger.

102:27:58 Mitchell: Okay. Here we go with India PDI PAD: 104:30:10.94; 11:03, plus 0002.9; 002, 110, 310; plus 56922; Juliet: 109:18:45.00; Kilo: 107:27:30.00; Lima: 104:50:49.67; Meco: 109:18:45.00; T-2 is at PDI plus 20:39; Nectar: 106:41:20.05. Readback.

[These are the third through seventh PDI PADs, and they provide the parameters for PDI, the Powered Descent Initiation, also known as PDI P63 Braking Phase, and for the various aborts through to T-3, the third launch opportunity after landing. PAD India is the only one that is not providing abort parameters. The name for PAD India originates from the first program of the Powered Descent, known as P63.]

[PDI Juliet is for aborts occurring during the first 6 minutes of the Powered Descent. Kilo is for any abort from 6 minutes from the start of PDI through touchdown, and prior to T-2, the second opportunity to launch after landing. T-2 aborts use PADs Lima and Mike, and the T-3 PAD uses November. Note that both the crew and CapCom are rather casual about using the official phonetic alphabet in their transmissions. In particular, "Meco" is the usual acronym for "Main Engine Cut-Off" but Ed is using it for the letter "M".

Interpreting the five PADs:

PDI-1 (P63) India.

Time of Ignition for PDI, a Noun 33 entry: 104:30:10.94.

Descent parameters, entered through Noun 61:

"Time to go", the duration of the powered descent: 11 minutes, 3 seconds.

Crossrange (out-of-plane) distance to the landing site: 2.9 nautical miles.

LM attitude at Powered Descent Initiation, a Noun 18 entry: Roll: 002°, Pitch: 110°, Yaw: 310°.

PDI-1 (up to 6 mins into PDI) Abort PAD Juliet.

Time of Ignition for the Transfer Phase Initiation (TPI): 109:18:45.00.

PDI-1 (from 6 mins to touchdown) Abort PAD Kilo (also a T-1 PAD).

Time of Ignition for the Transfer Phase Initiation (TPI): 107:27:30.00.

T-2 (PDI + 20:39) Abort PAD Lima.

Time of Ignition (from the surface): 104:50:49.67.

Time of Ignition for the Transfer Phase Initiation (TPI): 109:18:45.00.

T-3 (1 revolution) Abort PAD November

Time of ignition (from the surface): 106:41:20.05.]

[LM Flight Plan page 3-122.]

[

CSM Flight Plan page 3-123.]

102:30:16 Irwin: Roger. 20:39.

102:30:19 Mitchell: Let's try it again - 20:39.

102:30:25 Irwin: 20:39; thank you, Ed.

102:30:26 Mitchell: Good readback. Falcon, computer's yours.

[Long comm break.]

[While Jim has been copying numbers from Houston, Dave has been preparing to photograph the landing site with the Hasselblad and magazine KK (160 ASA colour transparency film). AS15-87-11701 to

AS15-87-11703 show the spectacular pair of craters Carmichael, and in the foreground, Hill. These craters were formerly called Macrobius A and B respectively. AS15-87-11706 looks straight down into the 11-km Brewster (formerly Römer L).]

[As Falcon approaches the eastern shore of Mare Serenitatis, Dave takes AS15-87-11707 to 11714. Looking towards the southwest lunar horizon,

AS15-87-11709 shows the bowl-shaped Clerke, and an unnamed ray crater within the system of rilles around the edge of Serenitatis. Within the mare, AS15-87-11710 shows one of these extensional rilles surrounded by a triplet of simple craters, each probably only a kilometre or two in diameter. The rightmost of these is included in AS15-87-11712 along with the wrinkle ridge, Dorsum Androvandi. It is easy to see this ridge as a sunken feature rather than the raised feature it really is, and it might need reminding that the Sun is shining from the left.]

102:34:24 Worden: Houston, Endeavour.

102:34:27 Mitchell: Go ahead, Endeavour.

102:34:32 Worden: Roger, Ed. Just checking over the P24 PAD again. And you didn't update the longitude over 2 on this one. Did you want to do that?

102:34:41 Mitchell: Stand by. Negative, Al. Go with the one in the Flight Plan.

102:34:46 Worden: ...and low altitude.

[Comm break.]

102:36:19 Scott: Houston, Falcon. Go.

102:36:21 Mitchell: Roger. Check your CO₂ Sensor circuit breaker. We're showing off-scale low.

[Once again, the fact that the reading from the sensor is off-scale-low does not mean that the partial pressure of CO₂ is low. It just means that it cannot be measured - in this case because it is not being supplied with power.]

102:36:32 Irwin: Circuit breaker's closed, Ed.

102:36:34 Mitchell: Roger.

102:36:52 Mitchell: Endeavour, standby for T-1 minus 30 seconds.

102:36:57 Mitchell: Mark.

102:36:59 Worden: Roger.

[Long comm break.]

[Index Crater is at the spacecraft's theoretical horizon at 102:37:27 GET.]

[Al is preparing for the P24 landmark tracking task, the results from which will refine the flight control team's knowledge of where the landing site is and the targeting of the LM. Meanwhile, the LM crew take a sequence of six photographs of the landing site and the plain on the other side of Hadley Rille.

AS15-87-11718 is a fabulous image of the bay they will begin to explore in a couple of hours time. Note that this version of it, scanned by Kipp Teague is quite a large file. I have added a labelled version of this image to show the primary features of the site.]

102:41:24 Worden: Roger.

102:41:45 Mitchell: Standby for 30 seconds.

102:41:47 Mitchell: Mark.

102:42:04 Mitchell: 10 seconds.

102:42:07 Mitchell: Mark.

[Comm break.]

[CapCom Ed Mitchell is giving Al audio cues leading up to the pitch down portion of the landmark tracking sequence, beginning at 102:42:17, when the spacecraft is made to slowly pitch down so as to keep Index Crater in range of the sextant. At 102:43:57, Endeavour will be closest to Index and by 102:44:45, Al will lose sight of the crater.]

102:44:54 Worden: Okay, Houston. It's out of sight.

102:44:56 Mitchell: Roger, Endeavour. How did you feel about them, Al?

102:45:03 Worden: Oh, I felt good about them, Ed. Right on.

102:45:05 Mitchell: Very good. Thank you.

102:45:08 Worden: No question about the landmark. And every mark, I had the - the crater centered, Crater Index.

102:45:17 Mitchell: Very, very good. Thank you. And I have an update to the PDI abort PAD, when Endeavour and Falcon are ready.

102:45:37 Scott: The Falcon's ready.

102:45:46 Worden: Endeavour's ready.

102:45:47 Mitchell: Okay. It's item Kilo. Should be 107:20:30.00.

102:46:02 Irwin: Okay. Falcon copied Kilo as 107:20:30.00.

[This update is for the T1 Abort PAD, which is effective for the first six minutes of Powered Descent. The Time of Ignition for the Transfer Phase Initiation Maneuver is being updated to 107:20:30.00. (The previous time was 102:27:30.00.)]

102:46:08 Worden: Endeavour copies.

[Long comm break.]

[Long comm break.]

[In Falcon, Dave is carrying out his second realignment of the LM's guidance platform to the

landing site REFSMMAT. As the spacecraft is now in free flight rather than attached to the CSM, Dave will use P52 rather than P57. This technique requires Dave to maneuver the LM to make his chosen star move across the X and Y lines in the AOT's reticle, taking marks when it coincides with each line. A fuller description of the use of the AOT can be found in the commentary after 098:46:54. note that the Flight Plan does not define who should carry out these realignments.]

[Al is also carrying out a P52 platform realignment to the landing site REFSMMAT.]

102:56:50 Irwin: All right. Go ahead, Ed.

102:56:51 Mitchell: Let's see if you can reach Endeavour and ask him to bring the High Gain [Antenna] up. Flight Plan angles, please. Minus 69 and 114.

102:57:05 Irwin: Roger. Endeavour, this is Falcon. How do you read? Roger. Houston would like you to bring up the High Gain [Antenna] to a minus 69 and a 114.

[Comm break.]

102:59:26 Mitchell: That's affirmative. We have two of them, Falcon.

[Dave has the three angles by which the LM IMU must be moved to bring it back into realignment up on his DSKY, from where Mission Control can read then via telemetry.]

102:59:36 Mitchell: Copy.

102:59:41 Worden: Houston, Endeavour is up on High Gain.

102:59:44 Mitchell: Roger Endeavour.

103:00:12 Mitchell: And, Endeavour, we copy your Noun 93s.

[Mission Control has read Endeavour's torquing angles, via telemetry, off Al's DSKY. Noun 93 is the operand used to bring these values up on the DSKY.]

[

LM Flight Plan page 3-124.]

[

CSM Flight Plan page 3-125.]

[Long comm break.]

103:07:32 Worden: Roger.

[Comm break.]

[Al is calibrating the COAS (Crew Optical Alignment Sight) by using it to sight on the star Dabih in Capricorn. As part of that procedure, he is to monitor Noun 92 which gives the angles of Endeavour's optical system.]

[The documentation available to me at the time didn't cover this operation and I asked Dave Scott what he could tell me about it.]

[Scott, from 1999 correspondence - "COAS calibration - orient the spacecraft relative to one set of stars such that the plus-X axis points to some other known star; whereby the spacecraft orientation is based on a combination of both spacecraft attitude and optics angles. Then look through the COAS and the known star should be centered in the reticle. If not, the bias can be determined (e.g., one tic up and one tic right), and that will always be plus-X.]

["One brief comment related to the COAS calibration. It should be remembered that, fundamentally, the CSM (and LM) procedures were designed to enable the crew to operate the spacecraft without MCC in Houston - primarily for communications loss. The MCC capability was then incorporated as additional capability within most procedures. Therefore, when evaluating, describing or outlining a flight 'procedure' (CSM or LM), if the procedure works when you take MCC out of the loop (if MCC is in the loop), then the procedure is correct. There may be some exceptions to this basic philosophy, but I can't think of any offhand - even the lunar landing procedures did not require MCC involvement to be successful. However, 'Mission Techniques' (Data Priority/Tindall) and 'Mission' Rules are another matter. Therefore, the COAS calibration procedure had to be independent of Houston, although, as a check or to simplify or verify, Houston could have been involved. The COAS was important for many piloting and independent CSM operations, such as spacecraft orientation, rendezvous, docking, etc. - e.g., if the IMU and communications with MCC both failed while in lunar orbit, the COAS could be used to align the BMAGs/GDC/SCS and then align the CSM for any particular maneuver or operations; e.g., TEI (Trans-Earth Injection)."]

[Dave mentions Bill Tindall here who was famed within Apollo, not only for the important work he did in coordinating the development of mission techniques (i.e. How do we go to the Moon?), but also for the unique, chatty style of memo writing. Tindall's memos are an important resource to those studying the mechanics of the Apollo missions. The following memo, dated 11 November 1967, specifically dealt with the calibration of the COAS.]

[Tindall memo - "...the Command Module computer program - COLOSSUS - is being developed to permit use of the [COAS] as a backup reference source for aligning the IMU and for rendezvous navigation. This is to guard against some failure of the sextant and telescope as well as providing the capability to make line of sight measurements as might be helpful in a one man rendezvous situation. Since precise COAS alignment cannot be established prior to launch, a provision has been made in the spacecraft program to accept and utilize its alignment as determined inflight and specified with reference to the sextant. However, it is necessary that the ground supply so-called 'equivalent sextant' shaft and trunnion angles. ...]

["The inflight procedure is as follows. The IMU is aligned using the sextant. The spacecraft is then maneuvered in attitude such that the COAS is centered precisely on a known star and the Mark button is pushed to get the platform gimbal angles on telemetry to the ground. With this information, the ground is able to determine the 'equivalent sextant' shaft and trunnion angles. These are the angles which the sextant would have to assume to be centered on that star given that spacecraft orientation. Of course, it is impossible for the sextant to view the star with that spacecraft orientation since it would be located approximately on the spacecraft X-axis which is outside the field of view of the sextant. It is the equivalent sextant shaft and trunnion angles which must be relayed by voice to the crew.]

["When the crew later specifies to the computer that they are using the COAS, it displays these parameters which, if not correct, must be input. It then uses the optical observation obtained with the COAS just as if it were a sextant mark obtained with those shaft and trunnion angles..."]

[Tindall's memo predates Apollo 15 by nearly 4 years and the detail of the procedure is bound to have been refined in the meantime. During our review in 2004, Dave talked further about Bill Tindall and the pivotal role he played in making a horrendously complex system come together.]

[Woods - " I've heard a lot about the Tindallgrams but I'm interested to learn more about how the meetings operated."]

[O'Brien - " Were these particularly large meetings? Were they lively, rowdy?"]

[Scott - "They weren't rowdy but there were some strong debates. And Tindall would control the debates in terms of giving people the opportunity to talk, and then mix and match and make the trades. And then he would make a decision and say, 'I'm gonna recommend this to management. Anybody have any really strong objections?' And the guy who lost the debate may say, 'Yeah, it won't work!' And Tindall would say, 'OK, fine. We'll go this way and if it won't work, we'll come back and re-address it, but we'll make a decision today.' And the beauty of Tindall's meetings was he made decisions on the spot. And the people who attended had the authority to make a commitment on behalf of their subsystem or whatever. So if they made a commitment, and they went back to their management and their management didn't like it, then the guy shouldn't be given the authority to make the commitment. And people were also very well qualified in their disciplines. The electrical guys or comm guys or whatever were very, very good. Tindall forced them into making commitments. Forced them. He said, 'OK. What are you gonna do? You gonna put two batteries on or one battery?' Guy says, 'I don't know.' Tindall would say, 'OK. We put one. Right?' 'Well, OK.' 'Done. Now, next subject.' That kind of thing where he kept it moving.]

["They were good debates and anybody could stand up and debate the issue. But he kept it moving. He didn't get bogged down because he himself was a brilliant engineer. I think Tindall was a real key to the success of Apollo because of how he brought people together and had them communicate in very complex issues. I mean really big complicated trades."]

[O'Brien - " That must have been an incredibly difficult job for him because, okay, you have an EECOM guy, you have a GNC guy, you have a propulsion guy and their all experts in their fields and he has to know their job."]

[Scott - "Pretty much. Yeah. He was very good at it. If he didn't, he'd have them explain it. And in front of all their peers. If somebody found a hole in the explanation, they'd say, 'Hey, wait a minute, John. No, no, no. That's not the way it works. Remember it does this or that.' So this openness and discussion, but again, Tindall made decisions and he kept it moving. And his decisions were really recommendations to management."]

[Woods - " Tindall was mainly responsible for issues regarding the spacecraft. Was there a similar process to do with the launch vehicle at Kennedy?"]

[Scott - "Tindall was not just spacecraft. His data priority meant Mission Techniques. He was concerned with the whole operational part of the mission including the hardware and the software and how all that traded. So his domain, if you will, encompassed the whole mission. The launch vehicle had launch, 12 minutes or whatever. Tindall had the whole thing including launch. Launch aborts and things like that. So often a Tindall decision would affect the launch vehicle. Like the S-IVB. When do you vent the S-IVB and when do you do this and that with the S-IVB because it affected so much more of the mission."]

103:09:42 Mitchell: Okay, Dave. It looks very good. Time's okay. And be advised, both Endeavour and Falcon, that the P24 looked good. There will be an update, but we feel very confident about it.

103:10:00 Scott: Okay, Falcon here. Very good.

[Earlier, Dave Scott started Program 63, which calculates the time of ignition, T_ig, based on its own information and data uplinked up from Mission Control, and performs the first part of the powered descent burn, the braking phase. Houston is happy with the T_ig solution it has arrived at. Around now, he will maneuver Falcon to the correct attitude for PDI while Jim is using the alignment of the PGNS to align the AGS.]

103:10:19 Scott: Very good.

[Verb 47 initialized the AGS with the attitude and acceleration information from the PGNS. The AGS used gyros that are "strapped down", that is, fixed to the body of the LM, as opposed to the PGNS, whose gyros maintained a fixed attitude in space. Realignments of the AGS gyros using star sightings were not necessary, and being fixed, did not have to be "torqued" into proper alignment. As a result, a simple update from the PGNS was necessary to tell the AGS, essentially, "which way was up". This is very similar to the way that the BMAGs or Body Mounted Attitude Gyros in the CSM are aligned to the IMU with the GDC Align pushbutton.]

[Dave and Jim discussed this anomaly during the crew's technical debriefing which is reproduced after 099:29:40.]

103:11:44 Scott: Okay.

[Long comm break.]

[With Endeavour's orbit set for the next three days and the landmark tracking completed, Al puts the CSM into an attitude for gathering science data. The SIM bay is made to face the lunar surface while the plus-X axis points in the direction of travel, or "pointy-end-forward." This is an "orb rate" attitude in which the spacecraft keeps a constant attitude with respect to the ground below, but is slowly rotating with respect to the stars at a rate which matches the orbital period. P20 and option 5 sets and maintains this attitude. The deadband for holding the desired position is ±5°.]

103:13:01 Scott: Houston, Falcon. Go.

103:13:03 Mitchell: Roger, Dave. Talk about reviewing notes, we did so, also, and we found one we'd like to pass to you before LOS.

103:13:12 Scott: Okay.

103:13:14 Mitchell: Dave, we - we're working out a procedure down here that we simmed [that is, ran in the simulator], and it's in the event of a low thrusting DPS during PDI. We're prepared to call to you - an RCS thrust augmentation for one minute, at about one minute or a minute and a half into the burn. And recommend doing it on the LMP's TTCA [Thrust/Translational Control Assembly], if we have to do it at all. What do you think?

103:13:47 Scott: That's fine. We'll try that if we need it.

103:13:50 Mitchell: Okay, the procedures are very simple. We'll call it to you as we've measured your thrust.

103:13:57 Scott: Okay. And I guess you'll call "on" and call "off" with the TTCA, is that correct?

103:14:03 Mitchell: We can. That'll just be a one minute - a one minute burst.

103:14:11 Scott: Okay, fine. And by the way, when we went by PD - PDI-0, we took a couple of hacks at the altitude. It showed 10 [nautical] miles even.

103:14:20 Mitchell: Very good.

[Comm break.]

[The PDI-0 abort PAD was read to the crew during the previous near-side pass. It included a T_ig for the abort at 102:39:35 which was one orbit before PDI is due to occur. When Falcon reached that point, Dave interrogated the DSKY, probably requesting the values given by Noun 43, the third item of which displays current height.]

[Woods, from 1999 correspondence - "I'm interested to know why you felt it was useful to take this reading. It isn't mentioned in the Flight Plan and I wonder if it was prompted by some preflight discussion."]

[Scott, from 1999 correspondence - "As you note, T_ig occurs at the same physical point as PDI in the orbit - therefore, we knew that we would be starting PDI at 10 nm altitude - just where it is supposed to be. If we were low or high, we would have been able to start thinking about post-PDI guidance corrections, or that maybe we were not set up properly for PDI. It was basically a nice comfort factor that things were as they should have been. However, also note that the PDI-0 point was not at the same physical location over the surface as PDI, since the surface (and target point) would move under the orbit between PDI-0 and PDI."]

103:15:53 Mitchell: Falcon, Houston. You're about a minute and 40 seconds from LOS. We'd like to see your 400 plus 30000 before LOS.

[The AGS entry, 400 +30000 is used to align the AGS using the Primary Guidance System. Houston would like to have the alignment performed before loss of signal so that they have a chance to evaluate AGS performance.]

[Long comm break.]

[Al is scheduled to begin his meal period now.]

103:17:19 Mitchell: Falcon, Houston. LOS in 30 seconds.

103:17:24 Scott: Roger, Houston; Falcon. All set.

103:17:40 Worden: Omni Delta...

103:17:47 Worden: Endeavour; Roger.

[Long comm break.]

103:21:30 Mitchell: Endeavour, Houston.

103:21:36 Worden: Houston, Endeavour. Go ahead.

103:21:37 Mitchell: Roger, Al. Your a minute from LOS. We recommend, on the next pass, check your S-Band Squelch Switch, Off.

103:21:50 Worden: Roger. Roger. [Laughing.]

103:21:52 Mitchell: Thank you.

103:21:53 Worden: ..., was I?

[While Al is settling into his meal, Dave and Jim are making final preparations for the powered descent to the Moon's surface. As they begin the 14th orbit, they are putting on their helmets and gloves, checking out the Environmental Control System (ECS) and making sure that the appropriate switches are set for PDI. Biomedical data is switched to Left meaning from Dave who is occupying the left side of the cockpit.]

[LM Flight Plan page 3-126.]

[

CSM Flight Plan page 3-127.]

[During their far-side pass, the LM crew manage some photography. Seen in

AS15-87-11724 is Mare Ingenii, caught just before the Sun sets on it. The flooded ring which dominates this south-facing photograph is the crater Thompson which forms part of the mare. Later in their orbit, and also looking south, is AS15-87-11729, an excellent colour image of Tsiolkovsky. One of the LM's RCS thrusters is visible to the left of the image, suggesting that Dave took this and the previous image.]

[MP3 audio file. 8.03 MB. Clip courtesy John Stoll, ACR Senior Technician at NASA Johnson.]

104:05:42 Mitchell: Falcon, Houston. [Long pause.]

104:06:00 Mitchell: Falcon, Houston.

104:06:05 Scott: Houston, Falcon. Go.

104:06:08 Mitchell: Roger, Falcon. We're ready for your Ascent Bat On time and your ED Bat report. [Pause.]

104:06:18 Scott: Roger, Ed. The Ascent Bats were On at 103:50:45, and I'll check the ED Bats now. [Long pause.]

104:06:53 Scott: And, Houston, this is Falcon. ED batteries both check at 37 volts.

[Batteries in the ascent stage of the LM were brought online while Falcon was behind the Moon. The ED batteries are for detonating the Explosive Devices which separate the ascent stage from the descent stage in the event of an abort during the descent, or when the time comes for lunar lift-off when these pyrotechnic devices must work if the crew is to leave the surface.]

104:07:06 Irwin: Roger. Go ahead.

104:07:08 Mitchell: And, Falcon, give us P00 and Data, and we'll give you an uplink. [Pause.]

[Mission Control are going to uplink their most recent calculation of the LM's state vector, probably based on Al's landmark tracking exercise.]

104:07:22 Mitchell: Roger. India: 104:30:08.54.

104:07:32 Irwin (onboard): Yes? Okay.

[The PDI-1 PAD was read up at 102:27:58. CapCom Ed Mitchell is giving them two updates to this PAD: The revised time of ignition for PDI is now 104:30:08.54, 2.4 seconds earlier than before. The LM will have to steer 3.3 nautical miles (6.1 km) north during the descent to compensate for the their orbit being slightly out of the ideal plane.]

[Mitchell also gives them a value to be entered at address 231 in the AGS computer which Jim punches in using the DEDA (Data Entry and Display Assembly), the equivalent to the DSKY. The value possibly represents the RLS (Radius of Landing Site or the distance of the landing site from the Moon's centre) in terms understood by the simple logic of the AGS.]

104:08:13 Scott (onboard): Locked up yet?

104:08:14 Irwin (onboard): No.

104:08:18 Scott (onboard): Here it comes; don't change it. It's coming in.

104:08:33 Irwin (onboard): Throttle control, Auto.

104:08:35 Scott (onboard): Auto.

104:08:42 Irwin: Read you loud and clear, Ed. I'm ready for that update now.

104:08:45 Mitchell: Roger. India: 104:30:08.54; Noun 61 crossrange, plus 0003.3; DEDA 231, plus 56943.

104:09:14 Irwin: Roger. 104:30:08.54; crossrange, plus 0003.3; DEDA 231, plus 56943.

104:09:26 Mitchell: Readback is correct, and be advised that crossrange number means you're going from south to north. You'll probably see some roll during the PDI.

104:09:39 Irwin: Roger.

[Comm break.]

[MP3 audio file. 4.88 MB. Clip courtesy John Stoll, ACR Senior Technician at NASA Johnson.]

[Endeavour's orbit is, on average, higher than Falcon's and it is therefore slower. Al has only now reached AOS and he has relatively little to do while Mission Control concentrate on working with Dave and Jim leading up to the descent and through to a safe landing.]

104:11:42 Mitchell: Endeavour, Houston. Standing by. [Long pause.]

Public Affairs Officer - "And our Instrumentation and Communications Engineer reports that we should have lock on now with Endeavour. Al Worden in the Command Module."

104:12:01 Irwin (onboard): ... computer, because...

[Mission Control have completed their uplink to Falcon's computer and control of it is restored to the crew.]

104:12:12 Scott (onboard): Thirty-nine point - How's that?

104:12:13 SC (Tone)

104:12:20 Scott (onboard): (Laughter) Enter.

104:12:48 Scott (onboard): Okay. Looks like I just screwed. Okay.

104:13:11 Mitchell: Endeavour, Houston. How do you read?

104:13:19 Mitchell: Endeavour, Houston. You're on the scan limit. Go to Reacq when you're at the angles.

[Comm break.]

[The Flight Plan includes appropriate angles for Al to set the HGA (High Gain Antenna) to.]

104:14:46 Worden: Houston, Endeavour. Loud and clear.

104:14:48 Mitchell: Roger, Endeavour. I have an update for the PDI PAD, India. [Pause.]

104:15:03 Worden: Okay, Houston. Go ahead.

104:15:05 Mitchell: It's 104:30:08.54, Al.

104:15:17 Worden: Understand. PDI is 104:30:08.54.

104:15:23 Mitchell: Good readback.

[Comm break.]

[MP3 audio file. 6.24 MB. Clip courtesy John Stoll, ACR Senior Technician at NASA Johnson.]

104:16:41 Worden: Roger. Auto.

[Comm break.]

104:18:47 Irwin: Houston, Falcon. Go.

104:18:49 Mitchell: Roger. We did not see the 231 load go in. Can you verify that, please? [Pause.]

104:19:02 Irwin: In work.

[231 is the AGS entry which was requested at 104:09:14.]

104:19:15 Mitchell: That is affirmative, Falcon. [Long pause.]

104:19:46 Mitchell: Okay, Falcon. Thank you. [Long pause.]

Public Affairs Officer - "We're coming up now on 10 minutes until the beginning of this 12-minute powered burn to the lunar surface. At the initiation of this maneuver the Command Module Endeavour will be about 350 nautical miles [650 km] behind the Lunar Module. The Command Module will pass overhead at just about the time the Lunar Module is touching down. Aboard the Lunar Module Falcon the crew has completed updating their backup guidance system with the same information that has been loaded into the Primary Guidance [and Navigation] System."

104:20:37 Scott: Hey, Houston, Falcon on Vox. How do you read?

[Dave and Jim switch their communications to Vox. Rather than having to press a button to talk to Earth, voice-operated switches will transmit every time they speak, allowing Mission Control to monitor the crew's exchanges during the descent.]

104:20:44 Scott: Okay.

104:20:47 Scott: [Garble] off. [Long pause.]

Public Affairs Officer - "And the crew has now switched on the guidance program - program 63 which will guide the Lunar Module during the initial portion of the power descent, the principal braking phase."

104:21:19 Irwin: Okay. Propellant Quantity Monitors, Descent 1.

[The descent stage contains four tanks, two each for fuel and oxidiser. All four have systems for measuring the quantity of propellant contained within but the crew can only monitor one set of tanks (one fuel and one oxidiser) at a time. By default, Jim selects set 1. During powered descent, Mission Control will discuss which set is giving a more conservative reading before deciding a couple of minutes before landing to stay with set 1.]

104:21:23 Irwin: Okay; you ready for the DPS configuration card?

104:21:26 Scott: Rog.

[Since the crew are using the Vox mode of communication, this is one of the few occasions where we get to hear how they operate. Dave and Jim both have an intensive aviation background and are used to the strict protocols of challenge and response used in checklists used in every good pilot's routine prior to flight.]

[Jim has cue card F16 in front of him and reads each line to Dave who confirms that the action required in each has been carried out.]

104:21:31 Scott: DECA Gimbal AC is closed.

[The DECA (Descent Engine Control Assembly) package is mounted in the descent stage and translates all the signals from the crew's controls or from the computer to the appropriate gimbal actuators on the descent engine. The circuit breaker call is to route power to those actuators. They adjust the engine's direction of thrust, ensuring the thrust axis passes through the LM's centre of gravity.]

104:21:39 Scott: Roger, verified.

[Command Override is a shorthand for "Descent Engine Command Override", which will allow either pilot to assume control of the descent engine's thrust using the Thrust/Translational Hand Controllers, if required.]

[Jim then refers to a number of circuit breakers for Stabilization and Control that are located on both his and Dave Scott's panels, which control vital systems such as the hand controllers, the engine controller and engine arming and starting.]

[Most of these are critical systems, and are powered by two independent electrical busses referred to as simply the Commander's and LMP's busses. As a result, there is a set of circuit breakers for these systems on both the Commander's and the LMP's panels. By closing the breakers for a particular system on both panels, that piece of equipment will have a source of power if one bus fails. The Abort Electronics Assembly (AEA), which includes the Abort Guidance Computer, consumes a nontrivial amount of power, and is left off the Commander's bus to better balance the load on the busses.]

104:21:43 Scott: 25.

[This switch sets the sensitivity of the attitude error needles and the rotational rate indicators (small arrows on a scale at the periphery of the LM's two FDAIs.)]

104:21:45 Scott: Auto, CDR.

[The engine thrust controls are being set to allow the computer to control automatically, as opposed to manually by the crew. If manual control were to be selected (as it was in Apollo 13), another switch directs the engine to take its commands from the Commander's Thrust/Translational Hand Controller.]

104:21:47 Scott: 4 Jets.

[As with the CSM, attitude control can be done using all four of the quad jet assemblies around the LM, or just two of them. In the LM, the sixteen jets are arranged in two redundant sets, each of which fully capable of maneuvering the vehicle. When four RCS jets are used, both RCS systems are used adding redundancy if one system should fail. Additionally, using four jets for maneuvering gives more authority to control commands when the full descent stage is still attached.]

104:21:49 Scott: On.

[Balance Coupling is a function specific to the AGS, in which balanced sets of thrusters are selected for attitude maneuvering. The RCS system is designed as two independent systems, and each system controls half of the thrusters and is capable of handling the attitude control functions of the LM. The AGS doesn't have a sophisticated Digital AutoPilot (DAP) as the PGNS does (remember, the DAP is a software entity, not hardware) and the balance coupling provides the RCS jet selection logic necessary to perform a maneuver.]

104:21:51 Scott: Enable.

[The actuators for the engine gimballing system not only have power (from a previous step) but can now accept control commands.]

104:21:53 Scott: Off.

[Descent Engine Command Override allows the crew to override the engine thrust commanded by the computer. We believe it really only works "in one direction", that is, to add thrust by manipulating the Thrust/Translational Hand Controller. This is contrasted to the manual engine control, where the computer is no longer controlling the engine, and all thrust changes are made through the hand controller.]

104:21:55 Scott: Reset.

[This resets the Abort and Abort Stage 'discretes', essentially the representative bits in the computer. This is done, of course, to make sure that there aren't any stray bits that might cause an unexpected abort when the computer starts the powered descent program.]

104:21:57 Scott: Min.

[As with the Command Module, there are two commonly used settings for the attitude control deadband. The or Max(imum) deadband is 5° while Min is ½°.]

104:21:59 Scott: Mode Control.

[This gives control over Roll, Pitch and Yaw rotations to the computer.]

104:22:01 Scott: Auto, Auto.

[The mode control switch has two settings (other than 'off'); Auto and Attitude Hold. In the Auto mode, the computer has full control of the vehicles attitude, and can command it to whatever attitude the currently running program requires. Of course, the crew can stop any maneuver through a suitably assertive motion on the attitude hand controller.]

[Attitude Hold is a quasi-manual mode, where the computer attempts to hold the spacecraft at the current attitude. If a crewmember wishes to maneuver the vehicle, the computer will let him. When the desired attitude is reached, the crewmember will null the rotational rates, and once below a certain deadband value, the computer will again hold that attitude.]

[Attitude hold is particularly useful when there is lots of movement inside the spacecraft, such as crewmembers moving about or the motions of fuel sloshing in the tanks.]

[Additionally, Attitude Hold imparts an element of stability and ease of control to the LM. Without the computers intervention, it would be almost impossible to maneuver the LM and then hold it steady at the desired attitude.]

104:22:03 Scott: Both reset.

[As with the Abort Stage discrete, this is simply resetting the 'Stop Engine' discrete, so that it doesn't accidentally shut down.]

104:22:09 Scott: Soft stop. And you're - you're clipping a little bit on the first part, Jim.

104:22:17 Irwin: Okay. [Long Pause.]

[When the powered descent begins, the descent engine will start at 10% of full thrust while the control system senses the direction of the thrust and adjusts the gimbal actuators to align it with the LM's centre of gravity. In case Dave has to assume manual thrust control early in the descent, his TTHC is preset to its Minimum (10%) position, and in case his control lever fails during the descent, they can switch over to Jim's which has been preset to the "soft stop" position.]

[The sensitivity of the VOX switch for Jim's mike is set too low and doesn't operate until he is well into his utterance.]

104:22:32 Scott: Okay. [Garbled] you. [Long pause.]

[Verb 40 zeros the CDUs (Coupling Data Units). Noun 20 displays the ICDU angles for the three gimbals.]

104:23:07 Scott: Say again.

104:23:08 Irwin: AGS steering is in.

104:23:10 Scott: Okay.

[Guidance Mode Select has been switched to AGS from PGNS as a final check of the Abort system. If there are any problems with the AGS in maintaining attitude control, it will become apparent now. Guidance Mode Control will be switched back to PGNS at about PDI minus 2 minutes.]

[Comm break.]

[MP3 audio file. 2.31 MB. Clip courtesy John Stoll, ACR Senior Technician at NASA Johnson.]

104:25:11 Irwin: Five minutes.

104:25:12 Scott: Okay. [Garble].

104:25:13 Irwin: [Garbled, may be landing radar] breaker is in. Altitude transmitter.

104:25:18 Scott: Altitude transmitter is 3.7; velocity's 3.8.

[The altitude transmitter call is a request to Dave to verify the signal strength of the radar. There are two transmitters in the landing radar, one for altitude and one for velocity.]

104:25:36 Irwin: Reading me any better, now?

104:25:37 Scott: Yes. [Long pause.]

Public Affairs Officer - "Flight Director Glynn Lunney, at this moment getting a final status for powered descent."

104:26:09 Scott: Okay. Go for the final trim.

[This is the final attitude trim before PDI. If the LM has drifted from it's PDI attitude, this is the opportunity to correct it.]

104:26:19 Scott: Roger. Go for PDI.

[LM Flight Plan page 3-128.]

[

CSM Flight Plan page 3-129.]

[Readers can follow the progress of Falcon through landing, the surface activities and return to orbit by following the link to the

Apollo Lunar Surface Journal by Eric Jones. All the pages from the Flight Plan, including those for the surface activities are available by going to the Flight Plan index page.]

Day 5: Waking in the Descent Orbit	Journal Home Page	Apollo Lunar Surface Journal.	Solo Orbital Operations - 1