Apollo 15

Apollo 15 has reached a parking orbit 170 kilometres or 93 nautical miles above Earth. This is a very low altitude for an Earth orbit but since the stack will make less than two revolutions, the decay in their altitude is of no consequence. While they are in this orbit, the crew will check their spacecraft as much as possible, following a list of items in the Launch Checklist, beginning at page L2-11.

Long Comm Break.

The parking orbit around Earth does not permit continuous communication with Mission Control. Indeed, the orbit is so low that when the spacecraft does come within range of a station, it is only above the horizon for a short period of time, even if it passes directly overhead. If it passes to one side or the other, the period of contact is further constrained; for this pass over the Canary Islands tracking station, communication lasts only 4 minutes and 11 seconds.

The Insertion and Systems checklist is split into 30 main sections covering 8 pages. The crew's workload is very high as there is a limited amount of time available to ensure the spacecraft and booster are fit to continue on to the Moon. Section 1, now completed, deals with shutting down the S-IVB control systems until they are needed again for the boost to the Moon, and checking that the booster's fuel and oxidizer pressures are within limits. The checklist includes a note, that after section 1, the other sections are not sequential; i.e. one section does not depend on others having been completed. However, Dave's comm later will suggest they are following the checklist items in the order they appear.

Scott, from 1998 correspondence: "These are set up in an optimum sequence before flight, and very seldom deviated. The checklist is normally followed precisely, to ensure there are no surprises."

Very long Comm Break.

The TSBs are Temporary Stowage Bags.

16-mm magazine A will be used to film the approach and dock with the Lunar Module currently housed in a shroud between the S-IVB, the IU and the CSM.

PICAPAR is a facility within the computer whereby it picks a pair of stars to be used for the realignment of the platform. Since the computer knows the attitude of the spacecraft and therefore where the optics are pointed, it can select a couple of stars within the field of view of the optics.

Communication is about to be handled through the Carnarvon tracking station in Australia. Apollo 15 still has just over one orbit of Earth to complete before Translunar Injection (TLI), the 5 minute, 50 second burn of the Saturn's S-IVB stage which takes them out of Earth orbit and on to the Moon.

Throughout the mission, one of Al Worden's jobs is to occasionally realign the guidance platform. He has just completed the first of 45 such realignments, a task usually known by the name of the program used, program number 52, or simply "P52".

Guidance and navigation are crucial to any journey and even more so in the ballistic dance of getting to the Moon and back. The crew must be able point the spacecraft to precise, well known directions, so that their engines will send them where they want to go. The spacecraft carries a gyroscopically stabilised platform within the IMU (Inertial Measurement Unit) which remains fixed in attitude while the spacecraft rotates around it, connected to it by three orthogonal gimbals. The only way to ensure that the platform is properly aligned is to compare it to a fixed attitude reference - the stars are almost always used. Initially, the CSM (Command/Service Module)'s platform was aligned before launch. Realignment is achieved by using P52 on the CMC (Command Module Computer). With the spacecraft held in a steady attitude the sextant is pointed at a specified star, marked, then to a second star, where another mark is taken. The computer, knowing the attitude of the spacecraft (relative to its own idea of where the stars are), now has new values of where the stars are located. The amount of drift the platform has experienced since the previous realignment is calculated, and displayed as the amount of correction needed to move, or "torque" the gimbals to bring the platform back into accurate alignment. Known as "gyro torquing angles", they are displayed on the DSKY as "Noun 93". These angles are usually very small, and are expressed in thousandths of a degree.

Scott, from the 1971 Technical debrief: "The post-insertion and systems configuration and checks went very smoothly. I don't think we had any problems at all. We took our time. We spent about 10 minutes or so just looking at the scenery after we cleaned everything up with the gimbal motors and all."

Worden, from the 1971 Technical debrief: "The guidance in the CMC was just dead-on, like what we looked at in simulation. I could almost repeat the numbers verbatim, because we had seen them so many times in simulation. It was just absolutely perfect, dead-on. The Z-torquing angle that we got after we got insertion was about half, as I recall, and that's just about what the first P52 showed, right in that ball park. We have the numbers written there somewhere [in their Flight Plan], but the guidance was right-on, super. We had no problems at all with the alignment. In fact, that was generally true with all the alignments. The first alignment went very smoothly. Tracking it in Orb Rate [the rate at which the vehicle was rotating as it orbited the Earth] was no problem. The Z-torquing angle came up about the same as they had called up."

Before Al began the platform realignment, the optics dust cover, on the opposite side of the CM from the hatch, was jettisoned to enable the spacecraft's optical instruments to be used. The cover protects the external surfaces of the sextant and scanning telescope during spacecraft preparation and launch. Al is to observe the jettison through the unity power SCT (Scanning Telescope).

Scott [to Worden], from the 1971 Technical debrief: "Optics cover jettison. Did you see any debris?"

Worden, from the 1971 Technical debrief: "Didn't see any debris. Didn't see anything through the optics when they went. All I heard was a slight thumping noise when the covers came off. That was it, never saw anything in the optics [relating to the jettison]."

Jim Irwin is referring back to section 14 of the checklist which asks that the flow rate during the purge of the fuel cell by hydrogen gas be monitored.

The spacecraft's three fuel cells use the chemical reaction of hydrogen and oxygen, stored in cryogenic tanks, to produce the majority of electrical power for the CSM. The by-products from this chemistry are heat and water. The heat is discarded through eight radiators around the upper circumference of the Service Module. The water can be used for drinking or equipment cooling. Impurities in the reactants build up on the electrolyte and a purge is required at regular intervals to remove them and restore full capability. Purges with oxygen are carried out daily, but once every two days suffices for hydrogen purges. Controls on the right-hand side of the Main Display Console are provided for these functions.

The Flight Plan requires that after each platform realignment, details of the task's result should be reported to Houston, either by voice or by calling up the angles on the DSKY display from where Houston can read them by telemetry. In this alignment, Al used Option 3 of P52, or the REFSMMAT (Reference to a Stable Member Matrix) option. This extraordinary acronym refers to the simple idea of a reference orientation which can be well defined and used by the crew in their platform alignments. The initial 5½ hours of the mission uses the precise orientation of the launch site at Kennedy Space Center at the time of launch as the reference to which the platform is aligned. As the flight progresses, other REFSMMAT alignments will be brought into play, eight in total, which include one based on the plane of the ecliptic, another based on the alignment of the landing site at the time of lunar landing, and others based on the computed alignment for major engine burns.

The realignment of the platform was generally done by sighting on stars, and computer carried a catalogue of the positions of 37 prominent stars distributed across the sky.

Scott, from 1998 correspondence: "But the crew had to know how to locate all 37 stars within the celestial sphere - one of the more interesting aspects of training. [We trained in] planetariums and [studied] the night sky while flying cross-country. The sky and its constellations became very familiar. Even though we did have a small diagram in the checklist, it was very important to be able to locate and positively identify each of the 37 stars. If the platform was too far from its desired orientation, the computer would not be able to point to the proper star."

The star maps from the G&C checklist are pages 6-8, 6-9 and 6-10.

The star reference numbers were given by their octal (base 8) number. The two stars used for Al Worden's first realignment were Antares (number 33) in the constellation Scorpius, and Dabih (number 41) in Capricornus. After the star sightings, the computer found that Al's measured angle between the stars, as given by Noun 05, differed from the angle the computer knows is between them, by only 0.01°. The difference between the intended platform orientation and its actual state was 0.019° in x, 0.021° in y, and 0.061° in z. These values were displayed through the Noun 93 display on the DSKY and were used to bring the platform back into correct alignment at 50 minutes into the flight.

The full list of stars, along with their reference numbers is as follows:-

Note that four of these are not stars as such but allow the crew member to refer to other celestial objects to the computer.

Comm break.

The "gray talkback" referred to by Dave Scott is an example of the indicators which were mounted around some of the instrument panels. Each talkback consisted of a small window with a black and white stripe pattern behind it - the "barber pole" often referred to during the mission. A gray flag could move in front of the barber pole stripes to indicate the status of a particular system. The gray flag would be driven by a control signal from the system in question and it told the crew what was going on in that system, therefore the indicator was called a talkback. Gray was essentially a normal or "doing nothing" indication. A barber pole in the talkback usually meant an abnormal or transient status.

This photograph, kindly supplied by Bruce Yarbro, shows the RCS talkbacks below the propellant switches in the Apollo 13 Command Module, Odyssey. The two talkbacks to the left are for the Command Module's RCS system, which will not be activated until re-entry. The four talkbacks to the right show half-barber pole in this photo and are the same as those in question.

For those who wonder at the term "barber pole", it comes from the red-and-white-striped pole that barbers display outside their establishments to make them identifiable to their clients. It harks back to a time when barbers indulged in dentistry and surgery. The photograph here is from a barber's shop in Maryhill Road, Glasgow.

Irwin, from the 1971 Technical debrief: "We had one secondary propulsion barber pole."

Scott, from the 1971 Technical debrief: "Yes, that's right."

Irwin, from the 1971 Technical debrief: "Insertion [means RCS-] B, B secondary, right?"

Scott, from the 1971 Technical debrief: "Yes, we sent that and it went great. ...[probably reading from notes] RCS-B secondary isolation valve barber pole, cycle to gray. It didn't come on at insertion. It came on at some other point."

Irwin, from the 1971 Technical debrief: "Yes, we noticed it when we did the check."

Scott, from the 1971 Technical debrief: "I made an SM [Service Module] RCS minimum impulse check, just to make sure that the RCS was working okay. I did that at 01:00 GET, and it worked fine. It was night and we could see the flashes. So I was fairly well convinced it was okay, that there wasn't any problem with it then. I don't remember what event would have triggered those barber poles unless somebody hit a switch, and nobody remembered hitting a switch. We talked about it, how did that thing get barber poled? When we noticed it, Al and I had been down in the LEB getting the helmet bags or something."

A recurring problem in the Apollo program was that the RCS propellant isolation valves [essentially, shut-off valves] occasionally closed during periods of heavy vibration or shock, rendering that particular RCS thruster unusable. Because the Service Module and Lunar Module used similar components, the valves in the RCS systems of both spacecraft could be affected by this. Indeed, one of the first post-landing steps for the Lunar Module was to cycle these valves to ensure they were open. Fortunately, this problem usually was regarded as a nuisance, since it was easily remedied. The postflight mission report concluded that variations in the supply voltage combined with the launch vibration was most likely to blame for closing these magnetic latching valves.

Acquisition of Signal (AOS) will be picked up 31 minutes later by the Goldstone station in California.

Flight Plan page 3-2.

During the hour and a half since launch, tracking stations around the world have been refining the estimates of Apollo 15's position and velocity, collectively known as its state vector. The latest estimates are to be relayed to the guidance systems in the S-IVB's IU (Instrument Unit), and to the CMC (Command Module Computer). Ground controllers cannot update these computers at will, however. Two switches on the Main Display Console can enable either the IU or the CMC to accept data from the ground, or to block it. Section 29 of the Insertion and Systems Checklist calls for Mission Control to send a freshly calculated state vector to the IU once the crew has commanded the IU's computer to accept the data via uplinked telemetry (Uptel in short).

At the apex of the Command Module, a tripod shaped assembly, the docking probe, is extended out as a check of its operation and in preparation for docking operations to come. At the tip of the tripod, an articulated knob with three simple, sprung latches will make the initial contact with the LM's docking assembly, the concave, conical "drogue." The shape of the drogue guides the probe to a central hole where the latches engage to achieve "soft dock." If the probe had not extended, there could be no link-up with the LM and therefore no landing. In this circumstance it is perfectly feasible for the crew to continue to carry out a lunar orbit mission though whether Mission Control would have allowed it is speculation.

The crew's request for Mission Control to monitor their arming of the Sequential Events Control System (SECS) arm and logic check is at section 30 in the checklist.

This is a restatement of the RCS valve problem as discussed at 000:55:52.

Throughout the mission, large lists of numbers, called PADs, will be read up to the crew which give them the information necessary to carry out a particular maneuver. PAD stands for Pre-Advisory Data. Some of these "block data" are for planned maneuvers such as the TLI (Translunar Injection) or LOI (Lunar Orbit Insertion) burns. Other PADs, such as the "TLI plus 90" and "Lift-off plus 8" mentioned here are the first of 27 abort options which will be read up to the crew at scheduled times throughout the early and middle portions of the mission. Note that the TLI+90 PAD has nothing to do with TLI itself but would occur 90 minutes after a successful TLI burn in the event of an abort. However, mission planners have decided that at no time beyond Earth orbit will the crew be without a get-us-home PAD. Then, any time they might lose communication with Earth, they will have the information to hand to get themselves back manually.

To simplify the voice transmission of these huge lists of numbers and reduce the likelihood of errors, each type of PAD was precisely formatted both in Mission Control's and the crew's paperwork - all the crew needed to do was fill in the blanks.

Scott, from 1998 correspondence: "Another example of precision planning (a major key to Apollo success)."

The abort PADs are the responsibility of RETRO, one of the flight controllers in the front row of the MOCR (Mission Operations Control Room). Chuck Deiterich was one of those who occupied the RETRO console throughout Apollo.

Chuck Deiterich, from 2003 correspondence: "There is quite a bit of protocol in the PAD process. Empty PADs were in tablets of no carbon required (NCR) paper. We would make about 6 copies and use a red ballpoint on the top (original) so the CapCom would be sure what was part of the printed form and what was data."

Chuck supplied a sample PAD form from the Apollo 8 mission.

Deiterich, from 2003 correspondence: "You can see where the CapCom checked each item during the crew readback. A printed X on the PAD said no data goes here, a printed 0 indicated the computer would always have a zero here (ignition time for example), a heavy box on a square indicated a + or - sign should go here (sometimes a letter like D.) Only the red data was read up, thus saving comm time."

Page 2-15 of the Flight Plan includes a schedule of all the "Return to Earth Block Data" PADs. Three basic types of abort PAD are shown; "Complete P30" which defines all the parameters for a return to Earth, "P37" short versions which give data for the computer's P37 Return to Earth program, and the "Abbreviated P30" which assumes some data already has already been given in a full P30 abort PAD. The intention of these PADs is to ensure that at all stages of the flight, the crew have the information to hand with which they can get themselves on a homeward trajectory, in the event that they permanently lose communication with Earth.

Even at this early stage of the mission, and despite the huge workload, the crew have begun to fulfil their science objectives by taking a sequence of photographs of Earth using ultra-violet (UV) sensitive film, on magazine N. From the point of view of the couches, looking towards the apex of the CM, the extreme right-hand window, no. 5, has quartz panes fitted for high UV transmission rather than the coated tempered glass used in the other windows. A Hasselblad camera, fitted with a 105-mm f/4.3 Zeiss Sonnar UV transmitting lens, is used with four filters, each with a different spectral response, one being in the visual range. The CSM launch checklist, at page 2-19, describes the procedure for this particular sequence of photographs. Two images are taken at 1/60th of a second exposure with filter 1. With the camera's shutter set to B (time exposure), two more are taken through filter 2 with 20 second exposures. Filter 3's pair of exposures are at 1/250th while filter 4's are at 1/500th of a second. The eight photographs are AS15-99-13402 to 13409. AS15-99-13408 is an example of one of the exposures through filter 4. Although the checklist called for an additional photograph to be taken through the same window/lens combination on conventional colour daylight film, the crew have decided that, because of the pressure of time, they would delete this item.

Irwin, from the 1971 Technical debrief: "I don't know that we had a color mag[azine] out at that time. I think we just had a UV mag out."

Worden, from the 1971 Technical debrief: "I recall now, we did discuss that in-flight. I think we decided that the color mag would be nice if we could get the same area that we had taken the UV pictures of. But we couldn't do that because of the time. It wasn't valuable taking a color [picture] of some spot other than where we had taken the UV."

By the time their orbit brings them back over the spot where they took the UV pictures, they will be occupied with preparations for TLI. After the UV sequence is finished, the camera is configured with the 80-mm lens and colour film in preparation for photography of the T, D & E (Transposition, Docking and Extraction) maneuver after TLI.

When all three crewmen are occupying the Command Module, it is usually Jim Irwin who takes the task of being the secretary and writing down the data read up from Earth.

As Fullerton read the PAD, a complete P30 type, the Lunar Module Pilot, Jim Irwin, copied the values to the appropriate form in the checklist. An example of the form is on page 5-11 of the CSM Launch Checklist.

Interpretation of this particular PAD is as follows:

• Purpose: In the event of a serious problem soon after a good Translunar Injection, this PAD has the details of a burn which would return them quickly to Earth. It has an ignition time of 90 minutes after TLI. In the event of such an emergency, the SPS (Service Propulsion System) engine would be used to slow the spacecraft to below Earth escape velocity, resulting in a high altitude ballistic arc that would take them directly to splashdown without any further orbits. Full procedures for the TLI plus 90 abort are given on page 4-13 of the CSM Launch Checklist.
• System: The burn would be under the control of the Guidance and Navigation System. Burns can also be controlled by the SCS (Stabilization Control System).
• CSM weight (Noun 47): Calculated to be 66,938 pounds (30,363 kilograms). Note that here, and in all the NASA documentation of the time, the term "weight" is freely used in the context where, strictly speaking, "mass" ought to be used. Weight is defined as the force applied by a mass in a gravity field. In free fall, mass remains the same while weight becomes essentially zero.
• Pitch and yaw trim (Noun 48): -0.52° and +1.90°. The SPS engine bell is gimbal mounted to allow its thrust vector to be aligned with the spacecraft's centre of mass. Two thumb wheels on the left of the Main Display Console allow adjustment of these pitch and yaw trim angles. Once a burn begins, the bell's aim is slowly altered automatically to compensate for shifts in the centre of mass.
• Time of ignition, T_IG (Noun 33): 4 hours, 19 minutes and 56.99 seconds, Ground Elapsed Time.
• Change in velocity (Noun 81), fps (m/s): X, -425.4 (-129.7); Y, +0.1 (+0.03); Z, +4,921.7 (+1,500.1). This is the change in velocity as defined along three orthogonal axes with respect to the local vertical.
The three velocity components in a PAD are always expressed with respect to the local vertical. Imagine a coordinate system based on a line from the spacecraft to the centre of Earth or the Moon. (Which you use depends on the sphere of influence you are in, i.e. which gravitational field is stronger.) This line is the Z-axis and is vertical at the point where it intersects the surface. The X-axis is perpendicular to this in whatever direction the spacecraft's orbit is taking it. It is therefore parallel to the local horizontal. The Y-axis is perpendicular to the orbital plane. This arrangement holds even for a very extended elliptical orbit like the one Apollo 15 will take to the Moon, where the spacecraft is clearly not travelling parallel to the local horizontal. For this burn, we can see that the largest component of velocity change is in the plus-Z direction which is towards Earth, countering the spacecraft's velocity away from the planet.
• Spacecraft attitude at T_IG: Roll, 180°; Pitch, 166°; Yaw, 2°. Although the velocity change is expressed with respect to the local vertical, spacecraft attitude is expressed with respect to the current alignment of the guidance platform. That alignment is currently set to the launch pad REFSMMAT.
• H_A (Noun 44): This is the height of the resulting orbit's apogee. Since the computer cannot display an altitude higher than 9999.9 nautical miles, and the maximum height of the CSM during a TLI+90 abort will be much higher than that, H_A is not applicable.
• H_P (Noun 44): 21.0 nautical miles (38.9 km). This is the height of the resulting orbit's perigee. Targeting a perigee so low will cause the spacecraft to intercept Earth's atmosphere and cause it to re-enter.
• Delta-V_T: 4,940.1 fps (1,505.7 m/s). This is the total velocity change that would be experienced by the spacecraft. It is a vector sum of the three components given earlier.
• Burn duration or burn time: 6 minutes, 34 seconds.
• Delta-V_C: 4,920.8 fps. This figure would be entered into the EMS (Entry Monitor System) Delta-V display.

Delta-V_C requires a little more depth of explanation. SPS engine burns are normally controlled by the G&N system. If it is a long burn, that is, greater than six seconds, the control is closed loop. The system monitors the achieved Delta-V and shuts down the engine at the appropriate time to reach the required Delta-V. In doing so, it takes account of the engine's tail-off impulse, which is the amount of thrust that the engine continues to impart after the shutdown command. It knows the thrust that is expected from this tail-off and can calculate the resulting tail-off Delta-V based on this and the spacecraft mass.

If the G&N system were to fail during a burn, the EMS provides a backup means of shutting down the engine at the right time. This equipment carries a separate accelerometer which measures Delta-V along the longitudinal axis of the spacecraft. Prior to the burn, the crew enter the expected Delta-V into a display on the EMS. As the burn progresses, the figure showing the remaining Delta-V drops towards zero, at which time the EMS sends a shutdown command to the SPS in case the G&N system has not already done so. However, the EMS has no knowledge of the tail-off thrust. The flight controllers take this into account and give the crew a low Delta-V figure for entering into the EMS so that if it is called upon to shut down the engine, it will do so early enough for the tail-off thrust to effect the correct total Delta-V.

Continuing with the explanation of the PAD:

• Sextant star: Star number 40, Altair (in the constellation of Aquila) should be visible through the sextant when its shaft and trunnion are set to the values 79.5° and 35.9° respectively.

The next five parameters all relate to re-entry, during which an important milestone is "Entry Interface," defined as being 400,000 feet (121.92 km) altitude. However, in this context, a more important milestone is when atmospheric drag on the spacecraft imparts a deceleration of 0.05g.

• Expected splashdown point (Noun 61): 16.04° north, 30.00° west; in the mid-Atlantic.
• Range to go at the 0.05 g event: 1,099.0 nautical miles. To set up their EMS (Entry Monitor System) before re-entry, the crew need to know the expected distance the CM would travel from the 0.05g event to landing. This figure will be decremented by the EMS based on signals from its own accelerometer.
• Expected velocity at the 0.05g event: 34,492 fps. This is another entry for the EMS. It is entered into the unit's Delta-V counter and will be decremented based on signals from its own accelerometer.
• GET of 0.05g event: Expected at 17 hours, 43 minutes and 58 seconds GET. This is when it is expected that the EMS and P64 will be triggered.
• GDC Align stars: Stars 43, Deneb (in Cygnus) and 36, Vega (in Lyra) are to be used to align the gyro assemblies if it is not possible to use the guidance platform for this purpose.
• GDC Align angles: Roll, 112°; Pitch, 128°; Yaw, 356°.

GDC Align is another term that needs a little more explanation. The spacecraft has two independent systems for determining attitude and changes in attitude. The primary system is the IMU and its stable platform, held orientated to the stars by gyroscopes. A secondary system, usually tied to the SCS, comprises a set of gyros that are attached to the spacecraft structure. In other words, being mounted directly to the body of the spacecraft, their mounts are not stable. Unlike the IMU, which measures absolute attitude, these gyro assemblies only measure the rate of attitude change. However, absolute attitude can be derived from these measurements by integrating the change, a job carried out by the GDCs (Gyro Display Couplers). This technique is imprecise so at regular times, the crew presses the GDC Align button to make the GDCs knowledge of attitude match the IMU's. The question then arises; what happens if the IMU is not working? The crew then have a backup method of aligning the GDCs by sighting two stars through the scanning telescope in a particular way. They know what the spacecraft's attitude should be when this is achieved and can dial this into the GDCs, properly aligning them.

The final note in the PAD concerned the ullage burn. Since the SPS propellant tanks are full, there is no need to perform a small RCS burn, known as the ullage burn, to settle their contents.

As CapCom Gordon Fullerton was reading up the PAD, the communication route was transferred to a ship in the Atlantic, USNS Vanguard. The changeover has caused Jim to miss some of the information. Journal Contributor Brian Lawrence adds, "I'm guessing that USNS means US Navy Ship? I DO know that it means that it's a ship crewed by civilians - where USS is a Navy-crewed vessel."

The Lift-off + 8 PAD carries data for P37, a program in the computer that will calculate the details of a burn that will return the crew to Earth. An important condition for P37 is that the spacecraft must still be in Earth's sphere of influence, thus simplifying the calculations. The program takes the four values from the PAD; the specified time for ignition of the engine, a specified maximum change in velocity (or Delta-V), the longitude of the splashdown and the GET for the start of re-entry. These act as a set of constraints with which it calculates the desired trajectory and the details of the burn to achieve it. The CSM G&C Checklist provides spaces for the P37 data on pages 4-23 and 4-24.

The data read up by Fullerton is structurally different to other PADs as the maneuver is controlled by the IU on the launch vehicle, and not the computer in the CM. A form for filling in the numbers is available on page 2-21 of the launch checklist.

The timings for events relating to the launch vehicle are defined relative to a number of time bases, each of which start with a particular event. This allows complete sequences of events to occur relative to indeterminate points of the flight rather than to the overall mission time. The restart sequence for the S-IVB's single J-2 engine is tied to time base 6, itself determined by tracking of the stack's orbit. When TB-6 begins, all subsequent events to restart the engine such as tank repressurisation, engine chilldown, ullage, etc., follow on, leading to the engine start command 9 minutes, 30 seconds later, and ignition 8 seconds after that.

The crew also has tasks to perform in the minutes leading up to the TLI burn and they use their event timer to help them. Around 002:40:23, TB-6 begins and this is shown by both the 'Uplink Activity' and 'S-II Sep' lamps coming on. The former is illuminated for ten seconds, the latter for 38 seconds. At 9 minutes to ignition, the point at which the 'S-II Sep' lamp is extinguished, Dave will start the event timer counting up, having previously set it to 51:00. This will give a visual count-up to and beyond ignition to aid the crew in sequencing their final tasks before and during TLI. Items in the checklist are therefore shown with times from 51:00, through (1:)00:00 and upwards.

The PAD is interpreted as follows:

• TB-6 predict light: 002:40:23. This is predicted start of time base 6. It implies that T_IG (time of ignition) will be 9:38 later, at 002:50:01.
• Attitude for TLI: 180°, 45°, 001° in roll, pitch and yaw respectively. This attitude is stated with respect to the orientation that the guidance platform has held since launch.
• Duration of burn: 5 minutes, 55 seconds.
• Delta-V_C': 10,401.1 fps. This figure will be entered into the EMS to allow the crew to monitor the remaining velocity that is still to be added. This should be at zero when the burn comes to an end.
• V_I: 35,599 fps (10,850.6 m/s). Inertial velocity at engine cut-off.
• Separation attitude: The correct attitude for separation of the CSM from the launch vehicle is 359°, 77°, 320° in roll, pitch and yaw respectively, again with respect to the orientation of the guidance platform. Among the criteria for adopting this attitude is solar illumination of the LM to assist the docking procedure.
• Extraction attitude: The correct attitude for extraction of the LM from the S-IVB is 301°, 257°, 40° in roll, pitch and yaw respectively.
• R2 align: 045.0.
• R2 ignition: 038.0.
• ORDEAL Start: Relative to the count-up on the event timer, the ORDEAL should be started at 56:45. The ORDEAL (Orbital Rate Display - Earth And Lunar) drives the FDAI (Flight Director Attitude Indicator, or "8-ball") to make it display the spacecraft's attitude relative to the ground below.
• Yaw: 001
• LM extraction time: The LM will be extracted from the top of the S-IVB stage by the docked CSM at 4 hours, 16 minutes GET.

The term 'orbital rate' refers to a how a spacecraft is orientated in orbit. If a spacecraft is in orbit with a fixed attitude relative to the celestial sphere (i.e. the spacecraft keeps pointing to the same stars no matter where it is in its orbit), then its attitude relative to the body it is orbiting (Earth, Moon or whatever) is constantly changing. For example, at one point in its orbit, the front of a spacecraft can be pointing directly at the planet below. Half an orbit later, it will be pointing directly away. This is known as "stellar inertial."

Conversely, if the spacecraft is to be flown in 'orb-rate,' keeping the same face towards the surface (to point cameras for example), it must rotate around one of its axes, usually the pitch axis, at a rate which matches the orbital period. Normally, the FDAI displays the spacecraft's attitude relative to the celestial sphere (i.e. it normally shows the inertial attitude) but the function of the ORDEAL is to provide the correct drive signal to rotate the FDAI at a rate which also matches the orbital period. With the ORDEAL, the FDAI can be made to display attitudes relative to the surface below.

During Apollo 15's TLI, the crew are going to use the ORDEAL to drive the FDAI at a rate which matches the pitch rate of the S-IVB during its powered flight. This way, the crew can monitor the progress of the TLI as far as the vehicle's attitude is concerned, and they can take over manual attitude control if required during the burn. They must start the ORDEAL working at a precise time if the FDAI is to show zero attitude errors, otherwise the spacecraft's further motion around Earth will cause it to be offset one way or the other.

Within their high-tech environment, it may seem somewhat strange for the crew to be given large quantities of mostly numerical information in such a low-tech fashion. At first glance it may appear easier to simply have the data uplinked to them and stored in the computer. However, the computer was not designed as a repository of data in the sense that we have come to think of computers thirty years after Apollo. It functions more like a real-time controller, albeit a very sophisticated one, and not completely unlike the embedded controller chips found in a VCR or microwave oven. The abort PADs are, in essence, a 'checklist' of items that the crew have to sequence through (Program 30 can be quite long), and although there are minimum keystroke ("minkey") options, there was never a 'scripting capability' that would automatically execute a program using stored responses. Additionally, there are verbal comments included in the PADs which cannot be entered into the computer.

In light of later, post-Apollo computer systems, it was an incredible feat to get the programming into the CM computer's 32Kwords of storage; most of this being hardwired into rope core memory. There was only 2Kwords (4K bytes) of erasable storage in the machine, and this was used to the maximum. During the Apollo 11 landings, using the very similar LM computer, the resource that the 1201/1202 alarms were complaining about was the lack of erasable memory.

The crucial importance of the data requires that the crew write it down and have 'hard-copy' available to them in case of the very systems failure that might invoke such an abort. Say, for instance, that the guidance computer fails. Having the abort PADs stored electronically would make them inaccessible. Or, say an oxygen tank blows on the way to the Moon, and you have to power down the entire Command Module, computer and all à la Apollo 13. It's tough to beat having a piece of paper with all the vital information for getting home written on it.

It's important to realize that although the computer is a critical part of the spacecraft, it isn't an absolute requirement for its operation. Early in the development of the computer, there were even serious doubts that it would remain functional for the entire mission! As a result, Apollo was designed to be flown without an operational computer. All the tasks that it normally manages could be done manually. (Making attitude adjustments, firing the engine, etc.) An essential design philosophy: Always try to have survivable options even when a critical piece of equipment fails.

Scott, from 1998 correspondence: "The design philosophy was even more precise than 'survivable' options - survivable' being exactly what? The back-up system was usually of a completely different design, never two - prime and backup - of the same 'kind.' This was one of the major factors in 'What Made Apollo a Success?' Operating, maintaining, and learning two completely different systems for one purpose was far more difficult and costly than having two identical systems for redundancy - but the concept proved its worth, time and again."

Although the RCS isolation valve problem was a previously known occurrence, Mission Control would like to understand it better, in case they are facing a new problem.

Comm break.

Like the computer in the Command Module, the computer in the IU can either accept data from the ground (via uptelemetry) or that data can be blocked, depending on a switch on Panel 2. The IU now has a more accurate knowledge of the vehicle's trajectory.

Comm break.

Now the Command Module's computer, the CMC, is set to receive the updated state vector. The CMC must be in an idle status (running P00, essentially a "do-nothing" program) with switches set to accept ground updates. P00 is commonly pronounced "pooh" by all the Apollo crews as in the character from A. A. Milne's book "Winnie the Pooh. As recounted in Murray & Cox's book, Apollo: The Race to the Moon, this program name even entered the daily lingo of the flight controllers. To 'go to poo' meant to go to sleep."

As an aside, the CMC has 39 programs available to the crew. Numbered in octal (base eight), they are arranged into categories depending on their function.

• 01-07 Prelaunch
• 10-17 Boost
• 20-27 Navigation
• 30-37 Targeting
• 40-47 Thrusting
• 50-57 Guidance system alignment
• 60-67 Re-entry
• 70-77 Abort

Note that not all available numbers within these groups are used.

Comm break.

Very long comm break.

The crew has moved on the "TLI Preparation" section of the launch checklist on page 2-29. They have settled into weightlessness easily and are not reporting any of the adaptation problems which other crews have experienced.

Scott, from the 1971 Technical debrief: "I had fullness of head as I expected to have. I had no other sensation whatsoever. On Apollo 9, I had felt some tendency not to want to move my head, but in this case I felt completely at ease. I noticed in looking around, that I felt quite well adapted immediately upon getting into orbit. I think that probably had to do with all the flying we did prior to the flight, the acrobatics and everything in the T-38. That's the one thing that I did different from Apollo 9. I really believe that was a help, because that was the only thing that was different. I felt much better this time than I had on Apollo 9."

Dave was CMP on Apollo 9 when Rusty Schweickart became sick. After this (and Frank Borman's similar illness during the Apollo 8 mission) crews began flying vigorous aerobatics in the T-38 jet aircraft they had available to them. Mike Collins discusses this attempt at inner-ear conditioning in Carrying the Fire. "The idea of the T-38 was not so much to fly weightless parabolas, but rather to perform a variety of violent aerobatic maneuvers, loops and rolls, to slosh that fluid (in the inner-ear canals) around, in poor but hopefully adequate imitation of the sloshing motion induced by moving around the Apollo cabin in weightlessness."

Journal contributor David Harland adds that the Russians report that having been up once makes the second flight a piece of cake, as if the body remembers how to cope with the weightless conditions. Also, although Dave flew with Neil Armstrong on Gemini VIII prior to the Apollo flight, they were strapped inside a very cramped spacecraft on a flight that was aborted early. Nausea induced by weightlessness only became a problem when crews experienced the freedom of movement permitted by the relatively roomy Apollo CM.

Worden, from the 1971 Technical debrief: "I had the same thing, a little fullness in the head. But I never at any time noticed any problems with equilibrium, sensation of spinning, or any problems with moving my head. The thought crossed my mind at the time that it was probably a result of zero-g flight. I was ready to move right away, get down in the LEB [Lower Equipment Bay] and get on with that part of it, Dave kept telling me to slow down a little bit. I think we both came to the conclusion that there wasn't really any reaction. We weren't getting any reaction out of it. We could proceed on normally after a few minutes."

Scott, from the 1971 Technical debrief: "Yes, that's right. How did you feel, Jim?"

Irwin, from the 1971 Technical debrief: "Well, I definitely had a fullness of head that persisted for 3 days. I had just a slight amount of vertigo. I didn't want to move my head very fast or move very fast in any direction. That was more pronounced, of course, once we got inserted [into Earth orbit]. That feeling gradually subsided, but I still had a slight amount of vertigo, even after 3 days."

Worden, from the 1971 Technical debrief: "I really felt like we were right at home when we got into orbit. I really felt very comfortable in the environment. Maybe that's part of it too. If you feel comfortable with that kind of environment, that may help you adapt more to it."

Irwin, from the 1971 Technical debrief: "I just didn't want to move very fast, but [I was] not nauseous."

Scott, from the 1971 Technical debrief: "That's the way I felt on Apollo 9. I just didn't want to go fast. It might just be the time of year, as far as anybody knows. But there were no problems. As far as any other anomalies, I can't think of anything else prior to TLI. We were well ahead of the checklist all the way. We had plenty of time to look out the window and watch the scenery. We took in a couple of looks at the sunrise and the Earth airglow and everything. I think the time line was well organized."

Worden, from the 1971 Technical debrief: "As a matter of fact, I thought we had a lot more time in flight to just look out the windows, see the Earth and see what was going on, etc., than we ever had in simulation. The time line seemed to work out so much better, for some reason, that we really had additional time, and it just flowed so smoothly that we didn't miss anything in the checklist."

Flight Plan page 3-5.

5 by 5 is a radio term where the quality of the voice is given marks out of five. The first figure represents the strength or loudness of the voice, the second its readability. 5 by 5 is 'loud and clear' whereas 5 by 2, for example, would represent rather loud though distorted voice communications. 2 by 5 would represent faint though clear speech. Its use is due to the aviation background of the crews and many others in the program.

Comm break.

Three switches, to the far left of the Main Display Console, alter the way the spacecraft attitude is controlled in roll, pitch and yaw. They have three positions; Accel(eration) Command, Rate Command, Minimum Impulse.

Long comm break.

The pyrotechnic devices which will separate the CSM from the launch vehicle have been armed in case of a serious problem with the S-IVB.

Long comm break.

The LOX pressure has not decayed far since the end of the first S-IVB burn. The CVS (Continuous Vent System) in the LH₂ tank is properly regulating LH₂ pressure at 19 psi. In about 10 minutes, at 002:40:25, Time Base 6 will commence. This launch vehicle time base provides the relative timing sequence for the restart of the J-2 engine on the S-IVB. 42 seconds into TB6, the sequence begins by igniting a burner which will heat helium to provide pressurisation of the propellant tanks.

Comm break.

Comm break.

On the left of the Main Display Console there are a cluster of launch vehicle annunciator lights. Five of these are arranged in a pattern similar to the layout of engines on the S-IC and S-II stages and provide the crew with cues of each engine's status. Above are three lights which indicate other events. The central light of these three is labelled "S-II Sep," a label only relevant to the light's first functions as it is reused to indicate other later events. By using the same lights for multiple functions in the spacecraft, designers could save on precious panel space and weight by not duplicating monitoring lights for systems that would only be used for a tiny fraction of the journey.

As per page 2-30 of the Launch Checklist, the "Uplink Activity" and "S-II Sep" lights comes on, indicating the start of Time Base 6, the S-IVB restart sequence. TB-6 begins at 002:40:23 GET leading to an eventual ignition at 002:50:01 GET; 9 minutes, 38 seconds later. The "Uplink Activity" lamp goes out after ten seconds while the "S-II Sep" lamp goes out after 38 seconds, exactly nine minutes to ignition. To help the crew orchestrate their actions before and during TLI, the Mission Timer is preloaded to 51:00. When the "S-II Sep" lamp indicates nine minutes to go, the timer is started and counts up so that reaching 00:00 should coincide with ignition. The checklist is marked with times that match the 'count-up', aiding the crew in following events as they occur.

Originally, we were not sure how long the "S-II Sep" light came on for. The checklist makes it clear that the "Uplink Activity" light goes out after ten seconds. Eventually, it was pointed out by Journal contributor, Lennie Waugh, that the Apollo 12 Launch checklist specifically states that the "S-II Sep" comes on for 38 seconds.

The crew is now in the middle of page 2-30 and they are monitoring the repressurisation of the S-IVBs fuel and LOX tanks.

The fuel tank pressure has to rise from 19 psi [131 kPa] to its operational value of 30 psi [207 kPa].

Scott, from the 1971 Technical debrief: "The one thing that I noticed, which was a fair surprise, was that the helium repress was very slow compared to the CMS [Command Module Simulator]. In the simulator, helium repress goes very rapidly, and pressure on the oxidizer tank comes right up. In this case, it came up very slowly. It was almost an imperceptible beginning. I called the ground and questioned them on it. They called back and said it was a normal repress. I think we ought to have the simulator people take a look at that. It was a little bit of concern even though we had the ambient bottle if we didn't have the repress. But in the back of my mind, I was wondering what was wrong, and nothing was wrong."

The ambient bottle Dave refers to is one of the nine helium storage spheres mounted on the outside of the S-IVB's thrust structure next to the J-2 engine. Unlike the helium tanks mounted inside the S-IVB's fuel tank which are therefore very cold, the ambient spheres are not cooled and rely on high pressures to store their contents.

Long comm break.

During the TLI burn, the guidance of the vehicle will be monitored using the FDAI in front of the Commander. The Saturn third stage will pitch up at a preprogrammed rate as the burn continues and the ORDEAL will drive the FDAI at a matching rate to compensate for the pitch motion. This will allow the FDAI to indicate a zero attitude all through the burn. Therefore, if necessary, the burn could be steered manually by Dave, just by keeping the FDAI and the associated rate needles zeroed. Checklist lines at the bottom of page 2-30 and the top of 2-31 deal with setting up the FDAI and ORDEAL in preparation for this.

Comm break.

Average g is the process of averaging out the acceleration experienced during a burn. It is used to reduce the error inherent in sampled (digital) data. The DSKY blanks at about 1 minute, 45 seconds before ignition. Five seconds later, it returns displaying the time relative to ignition, the velocity to be gained and the overall inertial velocity of the spacecraft. This data is based on the output from the spacecraft's accelerometers and indicates that "Average g" mode has begun. These displays give the crew a confirmation that the Guidance & Navigation system is operating properly.

The motors which drive the gimbals to point Service Module's engine are powered up in case the CSM has to depart from an errant booster. The DSE (Data Storage Equipment), essentially a tape recorder, is set to record the spacecraft's telemetry throughout the burn.

Comm break.

At 1 minute, 24 seconds to go, the "S-II Sep" light comes on again. Soon after, the APS (Auxiliary Propulsion System) modules at the base of the S-IVB fire to settle the contents of the tanks. This ullage burn also provides an initial head of pressure of the propellants towards the engine's turbopumps.

18 seconds before ignition, the "S-II Sep" light goes out to signal the final phase of starting the J-2. On this starting sequence, the supercold fuel will flow through the combustion chamber walls for 8 seconds to condition them prior to discharge of the Start Tank's GH₂ through the turbines to spin them up. Previously, this was for only one or three seconds but the J-2 has been exposed to the heat of the Sun and is expected to take longer to chill down. A full description of the J-2 engine and its ignition sequence is at 000:02:58 in the Journal.

Communications are now through the tracking station in Hawaii.

One second before ignition, the light for the number one engine in the cluster of five indicator lights comes on. Actual S-IVB ignition is at 002:50:02.6 GET and two seconds later the light goes out. When the light next comes on, it will be to announce engine cut-off and the end of the burn.

Comm break.

Fifty four seconds after ignition, the mixture ratio control valve operates, altering the ratio of fuel to oxidiser to ensure maximum propellant utilisation or PU. At this point, the thrust rises to its nominal level of about 890 kilonewton (200,000 pounds).

Scott, from the 1971 Technical debrief: "One minute after ignition on the S-IVB we had PU shift, which we hadn't been aware of and which we weren't expecting. We did feel a very noticeable change in thrust, and that hadn't been discussed preflight. It was something, I guess, we just missed along the way. It seemed strange to me that we didn't have it in our checklist or time line. We had the PU shift for launch, and I think it, might be a nice thing to stick in the TLI time line also, just so you'll know it's going to happen. It's no big deal."

Mission planners have taken account of the possibility that the TLI burn could have been delayed by one orbit. Knowing that the LH₂ fuel is constantly boiling off, they have allowed a small excess to compensate. Then, with a successful boost to the Moon at the first planned opportunity, the engine is set to run fuel-rich at a ratio of 4.5:1 for the first 56.5 seconds of the burn. After the PU shift, the mixture ratio changes to 5.0:1. Had TLI been delayed one orbit, almost the entire burn would have been made at the leaner ratio with the shift coming soon after ignition.

Comm break.

Scott, from the 1971 Technical debrief: "The new procedures that Mike Wash worked out for the TLI, putting that automatic and manual together, were really good. The ORDEAL setup was just right. The numbers came out just right, and ORDEAL track was right on zero until the last minute when the guidance starts trimming things out. Had we been required to fly a manual TLI, the ORDEAL would have been excellent because it really worked well."

Worden, from the 1971 Technical debrief: "That procedure is nice, too, because it is easy to keep up with. It is sequenced in the checklist so that there are plenty of check points in there so you can get everything squared away. It really works best."

Scott, from the 1971 Technical debrief: "Yes, if you ever had to step into a manual TLI, you could do it about any place and wouldn't be behind. I think you did a good job on that."

Scott, from 1998 correspondence: "The objectives and settings for ORDEAL during TLI were different from its use in Earth or lunar orbit. For TLI, ORDEAL was set to drive the FDAI [8-ball] at a pitch rate equal to the S-IVB IU-programmed attitude change such that the center of the FDAI remained at zero; that is, the artificial horizon was always level, or at zero displacement. Also, as I recall, the attitude error needle was nulled [remained at zero if the actual S-IVB pitch rate was correct]. This provided a display for the CDR to both monitor the S-IVB performance as well as to takeover and fly [control] the S-IVB in its proper pitch rotation in the event the IU failed. ORDEAL was not, in this case, set to an 'orbit rate,' it was set at a programmed pitch rate."

The PAD, read up to the crew at 001:38:07, gave an estimated burn time of 5 minutes and 55 seconds based on predicted engine performance. The actual time for engine cut-off is controlled by the guidance systems in the IU determining that the spacecraft has reached the required velocity. If the engine over- or under-performs, actual cut-off will change accordingly. A slightly low thrust means a longer burn. In this case, the J-2 is producing a slightly better than expected performance and this has shortened the burn by 4 seconds.

Scott, from the 1971 Technical debrief: "I guess we all felt that same low amplitude 10 or 12 cps [cycles per second] vibration all the way through S-IVB burn, just like we did during the launch. And we got a call from the ground on, it seems to me, a 3-second-early shutdown."

The Number 1 light on the launch vehicle indicator comes back on to indicate engine cut-off. It also signals the beginning of Time-Base 7, concerned with controlling the pressure in the S-IVB fuel tank, and with control of the Saturn's attitude in preparation for the separation and docking manoeuvre.

While the S-IVB continues firing during TLI, Mission Control are calculating the resulting apogee which would be reached if the booster's engine were to cut-off immediately. At the planned engine cut-off, the apogee was given as 538,342 km, based on a speed of 10,827 m/s. This apogee figure does not take into account the intervention of the Moon's gravity on the spacecraft's trajectory. Bringing this into consideration, the flight dynamics team calculate that the S-IVB burn will result in a 257.4 km (139 nautical miles) pericynthion (the trajectory's closest point to the Moon), instead of the intended 146.3 km (79 nautical miles). The upcoming manoeuvres to extract the LM from the top of the S-IVB will reduce the pericynthion by about 18.5 km (10 nautical miles). Two midcourse manoeuvres will further bring the achieved pericynthion to 125.9 km (68 nautical miles).

Soon after losing contact through Hawaii, the spacecraft is picked up by the MSFN (Manned Space Flight Network, pronounced 'misfin' by the crews). Coverage will be continuous for the coast to the Moon with this first portion being via Goldstone in California and subsequently through Honeysuckle Creek in Australia and Madrid in Spain. Communications will rotate through these three as Earth turns.

The orb rate maneuver puts the spacecraft heads down, rotating at 0.3° per second to keep it in a constant attitude with respect to Earth's surface below.

Worden, from the 1971 Technical debrief: "One further thing on TLI. I guess we wrote them down in the Flight Plan, but the [velocity] residuals on the CMC at the end of TLI were very close to zero. We wrote them down. The CMC kept very good track of the TLI burn."

At cut-off, the DSKY displayed their current speed as 35,599 fps (10.850.6 m/s). V_G is a measure of velocity to be gained, and was 145 fps (44.2 m/s). H-dot is the rate of change of height (or the time-derivative of height) and was 4,353 fps (1,326.8 m/s) and the height was 167.4 nautical miles (310 km). The value for H-dot is much less than the final velocity because a large component of that velocity was still parallel to Earth's surface. As Earth curves away from the departing spacecraft, the altitude will mount steadily, and H-dot will become a greater proportion of the overall velocity. Dave's call of V_C refers to the velocity display on the EMS after the burn finished.