Apollo 15

Apollo 15 has just gone behind the Moon near the end of their second orbit. While out of contact with Earth, the only planned item is the Descent Orbit Insertion (DOI) burn which is quite critical, so just in case they burn the SPS for slightly too long and lose too much velocity, the Flight Plan includes details of the adjustments they should make if the change in velocity is greater than planned.

Since LOI, their orbit has measured about 294 by 109 km (159 by 59 nautical miles) with the high point, or apocynthion, occurring near the landing site at Hadley Rille. The DOI burn will occur at this orbit's low point, or pericynthion, on the opposite side of the Moon from Hadley. The burn, which is intended to slow the spacecraft by 213.9 fps (65.2 m/s), will have the effect of dropping the high point until it becomes the orbit's low point of only about 16 kilometres. Since an accurate landing depends on the precision of this velocity change, the crew must adjust for any errors manually. Also, as there is some uncertainty about the shape of the Moon, and considering that some of the mountains adjacent to the landing site rise 3.4 km (1.8 nm) above the plain at Hadley, burning the SPS longer than planned is particularly undesirable. A small overburn of the 24.5-second SPS firing could risk the spacecraft impacting with the lunar surface.

If the velocity is 1 to 2.2 feet per second under what is desired, the minus-X thrusters would be used to increase the stack's velocity (remember, the docked CSM-LM is facing backwards). As these thrusters impinge upon the LM, they cannot be used for larger velocity corrections. The Flight Plan instructs that if the velocity correction is more than 2.2 feet per second, the entire stack should be turned 180 degrees so that the forward facing engines can be used. Correcting for a velocity between 2.2 and 10 feet per second low can be accomplished using the plus-X thrusters. Corrections larger than 10 feet per second are performed using the SPS.

In case there is a gross error in the burn which is not picked up by the crew, provision is made for a "bail-out burn". Soon after the DOI burn, the spacecraft is maneuvered into the correct attitude for the bail-out burn should it be needed. Immediately after AOS, Mission Control will use radio tracking information to independently check the geometry of their new orbit. If they are not happy with it they will instruct the crew to carry out the bail-out burn, based on details given at 83:26 in the Flight Plan.

After a nominal burn, the new pericynthion will actually be about 250 miles to the east, or uprange of Hadley, and this will be the point from which Falcon's descent will eventually commence in a maneuver called Powered Descent Initiation or PDI.

Although many terms are used for the high and low points in a lunar orbit, such as "apolune" and "perilune" or "aposelene" and "periselene" respectively, the correct terms in this context are "apocynthion" and "pericynthion" because they refer to an orbit around the Moon of a spacecraft that came from Earth as opposed to one that was launched from the lunar surface.

Rev 3 begins at about 080:41.

Which is another way of saying that they believe they are in the correct orbit for the LM, Falcon, to fly off and do its stuff tomorrow.

As it monitored the progress of the burn, the computer decided when there had been enough change in the spacecraft's velocity and shutdown the engine when appropriate which was half a second before the estimated time.

Scott, from the 1971 Technical debrief: "The DOI was again a nominal burn. We shut down on time manually, but the G&N beat us to it. I guess, Al, you could see I put my hands on the switches and timed it. When the time ran out, I put the switches down. Al could watch the P_C [combustion pressure], and I guess he saw it. [To Worden] Why don't you just say what you saw?"

Worden, from the 1971 Technical debrief: "Well, I heard Jim counting down. I knew Dave was ready to throw the switches, and just as his hand started to move, the P_C dropped off. So the automatic shutdown and Dave's shutdown were almost simultaneous; except the automatic shutdown was, I think, just before his. It was perfect."

Scott, from the 1971 Technical debrief: "I took a stopwatch along because we were timing it in tenths of seconds, [which] was necessary for DOI. I think that unless you really have a double failure, you can't get in trouble on DOI, I don't think that is the problem that some people might have thought it was some time ago. That is a good, solid method of getting into a low orbit. Very little chance of getting into a bail-out situation."

Comm break.

Tsiolkovsky is a large, 200-km crater on the far Moon's side which was first photographed by the Soviet probe Luna 3 on 7 October 1959. In the poor imagery of the time, its dark, mare-like interior made it stand out from the other craters that pepper the far-side. The triumphant Soviets, in the manner of all explorers, promptly and appropriately named it after the Polish-Russian pioneer of spaceflight theory, Konstantin Tsiolkovsky (1857-1935). A dominant, W-shaped central peak of light coloured highland material rising out of the dark material makes this crater particularly distinctive and striking and it will be one of Al's major subjects for visual observation during his solo scientific program. As the spacecraft approaches a pericynthion of 18.5 km (10 nautical miles), its altitude is falling and Dave is noticing how the spherical shape of the Moon causes the central peak of Tsiolkovsky to appear to rise above the crater's rim.

AS15-91-12397 was taken later in the mission using a wider-angle lens and looks south into Tsiolkovsky showing the irregular shape of the dark area.

Long comm break.

After the execution of the burn, the last item detailed in the checklist for the thrusting operations is Verb 16, Noun 44. This command displays the altitudes, in nautical miles, of the orbit's pericynthion and apocynthion, relative to the height of the landing site. It also shows the time taken to make one orbit, the orbital period.

A "high-angle pass" comes from a phrase used by tactical fighter pilots.

Scott, from 1999 correspondence: "This generally means beginning a steep-angle dive-bombing run, as it might apply to a feature on the surface."

As the father of one of the editors of the Apollo Flight Journal is a retired fighter pilot, we thought we would seek his explanation of the term.

Lt. Col. Frank J. O'Brien, USAF, Ret.: "A high angle pass is a method of dive bombing that minimizes your exposure to ground fire, and is used for what we called 'slick' bombs, or those with no retarding device. In other words, for all dumb bombs of the 500#, 2000# variety. (When we were dropping napalm or 500# high drag bombs, we used about a 15 degree dive angle and started the run from about 3,000 feet. A lot more exciting experience.) You started by circling the target at about 13-15 thousand feet, depending on terrain elevation and your planned drop altitude. When cleared by the FAC (the Forward Air Controller; a pilot in a small, slow and low-flying aircraft who identifies targets on the ground), you started a diving 90 degree turn to the attack heading, and tried to establish a 60 degree dive angle. Shallower dive angles were not used because of more exposure to ground fire. The first few times you try this steep dive angle it seems a bit hairy, since we trained at George [Air Force Base, in the Mojave desert of California] with 45 degree dive angles, but when they are shooting at you it is a great motivator."

Lt. Col O'Brien added that for a more detailed explanation of dive bombing he would refer us to an "excellent work by a little known local author." The title is The Hungry Tigers, the topic in question being on pages 106-122. He did not add that the book was, in fact, written by Lt. Col Frank J. O'Brien, USAF, Ret.

On reacquiring the spacecraft around the Moon's eastern limb, the tracking stations on Earth began measuring its velocity and distance from Earth from frequency and delay changes in the received radio signal. From this, they calculated the orbit achieved by the DOI burn. They are in good agreement with the values calculated by the spacecraft's computer. Had Mission Control been unhappy with the orbit, they would have instructed the crew to "bail out". The crew would then have used the emergency burn given at 83:26 in the Flight Plan to alter their orbit.

Woods, from 1999 correspondence with Scott: "Since the DOI burn has the same effect as the LM DOI burns of Apollos 11 and 12, do you know why they did not have a bail-out contingency when A15 did?"

Scott, from 1999 correspondence: "For one thing, with the CSM we could go back up to 60 nautical miles and try DOI again - but with a LM DOI, the LM is committed and the only alternative is an abort. With the CSM, by going to the bail-out attitude immediately, we would be ready for such a contingency and would lose very little orbital motion and timing - to set up a second LOI."

Long comm break.

Apollo 15's attitude is currently fixed with respect to the stars, and as it orbits, the Moon has gradually moved away from its windows. The "landmark observation attitude" will reorient the spacecraft in preparation for viewing landmark J-1 such that the optics are facing the Moon as they pass over Mare Serenitatis. For a while they can see the Moon's surface again as the spacecraft changes attitude.

Dave is literal in his use of "down" as they are approaching their pericynthion and are descending as they cross Mare Crisium.

Woods, from 1999 correspondence with Scott: "Was it the spacecraft's apparent speed that struck you, or a sense of low-flying rather than orbiting or some other impression that prompted your comm?"

Scott, from 1999 correspondence: "This comment is due to not so much the speed, as speed is not apparent when still that high over the surface - its the elevation, or angle, of the horizon (mountains) above the 'normal' horizon, whereby the horizon is now higher in the window due to the mountains."

Long comm break.

Dave's comment illustrates the low altitude of their flight path at this point, and their altitude is dropping yet as they approach their pericynthion just east of Hadley. However, they are not really as low as the rim of Proclus. Rather, the curve of the Moon between them and Proclus means that the crater, sited south of them, will be slightly tilted away from them.

Although Littrow Rille is beyond the shore of Mare Serenitatis, its arcuate form, roughly parallel to the basin rim, suggests it is related to Serenitatis as a result of extensional tectonics, probably as a result of sinking of the mare basalt.

During landmark tracking, the crew uses the 28-power sextant to view the lunar surface. Since the sextant has a limited field of view, the spacecraft must have the optics pointed towards the surface, and, unfortunately for the sightseeing crew, the placement of the optics, on the opposite side of the spacecraft from the windows, makes it more difficult to see out. This may explain why these two photos are a little foggy, if the camera was viewing obliquely through the window glass.

Jim is describing Dorsa Smirnov, a very prominent ridge running about 130 km in a roughly north-south direction across the mare as they view it. From further away, the ridge is seen to form an arc which is concentric with the rim of the Serenitatis basin. Henize is prompting him to look for features which will pin down their origin, now known to be the result of compression.

Mare Serenitatis exhibits some of the best examples of wrinkle ridges and arcuate rilles on the Moon. These features are related and both are caused by the sinking of the relatively dense mare lavas with respect to the surrounding terrain. The centre of the mare sinks further than the edges. In the case of Mare Serenitatis, there is a smooth drop of 200 metres from the shore to the centre. Wrinkle ridges form where compression of the mare surface takes place and causes uplift. The very centre of the mare may display a circular formation of wrinkle ridges which can be mistaken for a ghost crater. Radial ridges may also form. Sometimes wrinkle ridges can betray the existence of features in the basin, especially ancient crater rings, now overlain by the layers of solidified lava which filled the basin to form the mare.

Both inside and outside the mare shore, rimae or rilles are often seen running roughly parallel to the basin rim, forming arc-shaped grooves where expansion has occurred. Often the land between two parallel faults drops slightly to produce a feature known as a 'graben'. Rimae Littrow, mentioned by Jim before he reached Serenitatis is a good example of this type of rille.

Readers should note that many of the smaller craters on the Moon visible from Earth are given the name of a prominent adjacent crater followed by letters of the alphabet.

Comm break.

Al is attempting to track landmark J-1, a feature on the surface of Mare Serenitatis just east of Dorsum Owen, to check the accuracy of the guidance system's knowledge of the spacecraft's orbit.

The spacecraft is approaching its 18-km pericynthion. To make an earthly comparison, the supersonic aircraft, Concorde, cruises at about the same altitude above Earth, but Apollo 15 is travelling three times faster at 5,960 km/h.

This ridge is an exception to the predominant north-south run of ridges in this area of the mare. It may reflect subsurface structure connected with the hills to which it is joined.

Comm break.

The mountains of the Apennines reach almost a quarter of the spacecraft's altitude and the sensation of speed is enhanced by their proximity to the surface.

Bodies in elliptical orbits, like Apollo 15's descent orbit, move faster when they are nearest to the object around which they are orbiting. However, compared to the size of the Moon, the difference between their pericynthion and apocynthion heights is small with little difference in their absolute velocity between these points. However, their close proximity to the Moon does affect their apparent speed. At apocynthion, 110 km up, the terrain parades past at a gentle 0.1° per second. At pericynthion, only 17.6 km altitude, craters and rilles sweep by at over 5° per second, mountain peaks even faster and, being much lower, the sense of perspective is more dramatic.

Al is completing a P52 platform realignment, option 3, using the landing site's orientation as the reference point for the platform.

Camera PADs will become a regular occurrence during lunar orbit when Al will start and stop the two main cameras in the SIM bay, the Mapping Camera (which is two cameras in one; the Metric Camera and the Stellar Camera) and the Panoramic Camera. Most photography with these two cameras takes place just to the daylit side of the terminator to take advantage of the low Sun angle and the long shadows which help to make topographic features stand out. The above times refer to when the spacecraft passes from lunar night to day over the far-side, just as they begin their fourth revolution. The change in shutter speed refers to photography, using the onboard Hasselblad and very high speed black and white film, which will occur at the same time as the Mapping and Panoramic Camera operation.

Long comm break.

Very long comm break.

Flight Plan page 3-90.

The above report is as required in the Flight Plan. Apollo 15 is the first mission to carry the SIM bay and is essentially a test flight for it. Everyone involved is therefore very interested to learn how well it performs, especially its various mechanical devices which have never been tested in the freefall vacuum of space. Throughout, Al will keep a note of the time it takes for the three deployable instruments to extend and retract and Mission Control can watch changes in the current going to the motors to monitor the health of these mechanisms. Al will also keep a note of when the non-imaging experiments are switched on and off. Investigators can then relate their data to the timeline and the absolute position of the spacecraft, a data analysis process that will take months to complete.

This may be a miskeying by Karl Henize.

Comm break.

Very long comm break.

Apollo 15 has gone behind the Moon and will commence its fourth revolution while over the far side. As the spacecraft crosses the terminator from night to day, the Panoramic and Mapping Cameras are activated for 11 minutes and 51 seconds to take advantage of the low angle lighting. The onboard Hasselblad is configured with the 80-mm lens and magazine R, containing very high speed (6,000 ASA) black and white film. Compare this to the film used aboard the Lunar Orbiter probes of 5 years earlier which was rated at only 2 ASA to give it some tolerance to radiation in the lightweight construction of that pioneering series of spacecraft. 6 images are scheduled to be taken right on the terminator. These are AS15-98-13303 to 13308.

The compilers of the Apollo 15 Index of 70-mm Photographs guess that the first of these very contrasty images may be of the Montes Haemus, the range of mountains on the rim of Serenitatis, but it seems likely that all six images are of the same, unidentified far-side landscape. AS15-98-13308 is one of the clearest of these images which are otherwise difficult to distinguish. Meanwhile, the crew begin their meal period.

With the photography completed, the two booms, each more than 7 metres long, are extended from the SIM bay. One boom carries the Gamma-ray Spectrometer and the other, the Mass Spectrometer. The booms take the instruments away from possible sources of contamination near the spacecraft and this is their first deployment. Again, Al times how long it takes for these to extend by monitoring the talkback indicators adjacent to the deploy switches. He records the results in the Flight Plan. The booms will be retracted during burns as they are not designed to withstand the forces of engine firings.

The same ingenious mechanism is employed for both booms.

Each has two spools of tempered steel tape which, when unrolled, form themselves into a 'c' profile; much like the way a handyman's measuring tape unrolls to form a curved cross-section which stiffens it. The spools are arranged so that as the two tapes come off and take up their tubular profile, one does so inside the other. A cable to connect the instrument to the SIM bay is wound around the mechanism's housing and is pulled out around the boom during deployment.

Flight Plan page 3-91.

The final task on this far-side pass is to switch the Mass Spectrometer's ion source to Standby. This brings on heaters to drive out gases within the instrument for the next hour until it is switched on at 086:00:00.

Rev 4 begins at about 080:42:30.

Comm break.

Karl is asking the crew to rewind the DSE tape so the ground can play it back to check the spacecraft's performance and collect the SIM bay data gathered during LOS.

Events in the Moon's past bequeathed it with a gravity field which, to scientists of the late 1960s, seemed unexpectedly uneven. Regional concentrations of mass intensify the gravity over certain areas. Consequently, the orbits of small bodies like spacecraft are unstable in the long term. Known as "mascons," short for "mass concentrations," these mascons are mostly associated with some of the great circular impact basins (Imbrium, Serenitatis, Crisium and Orientale) and are believed to be primarily due to dense mantle material being brought nearer to the surface by the basin-forming impact events, and, to a lesser extent, by the bulk of dense lavas now lying within those basins.

Since mascons are largely on the near-side, they accelerate spacecraft slightly when compared to the far-side. Over time, orbits are modified by being lowered on the near-side and raised on the far-side until, if not corrected, they cause the spacecraft to impact the Moon.

No previous spacecraft have matched Apollo 15's ground track and ground controllers have not accurately characterised the effect of mascons on its orbit. Based on what they know, Mission Control estimates the pericynthion of the orbit will drop by 0.7 km to 16.1 km. However, these predictions will turn out to be conservative.

Dave is asking about another parameter of orbital mechanics. If their orbit's inclination is not correct, then when it comes to the time of the descent to the surface, the spacecraft will aim to the north or south of the landing site rather than directly towards it.

Henize does not say whether this error is to the north or south but in 23 hours time, just before PDI, there will be a cross track error of 6.1 km (3.3 nautical miles) to the south.

The Gamma-ray Spectrometer is due another ten-minute period when the Gain Step Shield is switched off so the operation of its discriminator can be calibrated.

Comm break.

The Flight Plan included approximate expected times for the two booms to deploy and both of Al's reported times are about 20 seconds quicker than this.

Comm break.

This is quite useful information as it will allow the crew to check by eye that the booms are extending properly throughout the orbital science period.

About now, the Mass Spectrometer is powered up, though it will continue outgassing for another half hour. The ion source, which is at the heart of the instrument's operation, has not yet been switched on.

Long comm break.

According to the Flight Plan, the Mass Spectrometer Experiment switch was due to go to On at 85:25. At 86:00, the Ion Source is due to be switched to On. It is unclear why the Experiment switch is being put in Standby instead.

Comm break.

Both of these requests, the uplink and the TEI-12 PAD, are as directed in the Flight Plan. The latter is another abort PAD which, if required, would be used towards the end of the 12th revolution.

Apollo 15 is nearing the Hadley landing site within the Apennine range. These mountains are experiencing lunar dawn.

The middle of the day on the Moon is searingly hot while the night is cold to great extremes. Landing at Hadley is timed so that the Sun will have risen just 12°, giving oblique lighting with lots of shadows and aiding landmark identification during the final phase of the landing itself. It also ensures that, as well as the Sun, the landscape isn't radiating huge amounts of heat to overload the EVA suit's cooling ability. Though the Sun is illuminating the mountain peaks around the landing site, it has yet to reach the plain at Hadley. The crew's ability to see detail in the shadows of the Moon is unmatched by the much narrower dynamic range of their cameras, and part of their brief is to describe that which cameras cannot record.

The use of magazine R and its load of very fast black and white film at the last terminator crossing is an attempt to record these details on film.

Long comm break.

Comm break.

Since 84:00 GET, the spacecraft has been flying "sharp end forward" or plus-X forward attitude with the CM's apex pointing in the direction of flight. Throughout the coming rest period it will be flying in the "blunt end forward" or minus-X forward attitude with the SPS engine facing the direction of flight. This latter attitude is required for the proper operation of the Mass Spectrometer.

Comm break.

An interpretation of the TEI-12 PAD follows:

• Purpose: This PAD specifies the information required for an early return to Earth, if necessary, towards the end of the twelfth lunar orbit.
• System: It would utilise the SPS engine under the control of the G&N system.
• CSM Weight (Noun 47): 38,110 pounds (17,287 kg).
• Pitch and yaw trim (Noun 48): +0.58° and +1.00°.
• Time of ignition, T_IG (Noun 33): 101 hours, 36 minutes, 8.38 seconds.
• Change in velocity (Noun 81), fps (m/s): x, +2,845.0 (+867.2); y, -380.2 (-115.9); z, -63.8 (-19.4). These velocity components are expressed with respect to the local vertical.
• Spacecraft attitude: Roll, 180°; Pitch, 107°; Yaw, 354°. These attitude would be with respect to the orientation of the guidance platform.

Henize notes that other items on the standard form are not applicable (NA) to this PAD. Additional notes include the settlement of the SPS propellants be achieved by firing the plus-X thrusters on all four quads around the Service Module for 12 seconds. The burn, if required, would be made after jettisoning the Lunar Module and with the spacecraft still in the descent orbit; i.e. without a circularisation burn having been made. Finally, Earth landing would then occur at about 196 hours into the mission and in the mid-Pacific landing area. Note that this last item was not specified in the TEI-5 abort PAD, read up at 081:34:19 GET.

Comm break.

A fresh lithium hydroxide canister, number 9, is being placed in receptacle A while the exhausted one, number 7, is stowed in compartment B6.

Off is the central position of the Ion Source switch between Standby, its current position, and On.

Mission Control are requesting that they keep the Discriminator for the Mass Spectrometer in its Low setting because of scattered sunlight causing excessive background counts in the instrument's detection electronics.

Comm break.

Even the smallest anomaly in one of the spacecraft's systems is exhaustively analysed to try and ensure there will be no surprises. Both the SPAN (Spacecraft Analysis) room and the MER (Mission Evaluation Room) will be beavering away at diagrams and specifications of the Systems Test Meter switch, as well as working with the spacecraft contractors, trying to leave nothing to chance.

As the Mass Spectrometer is intended to sample the very tenuous lunar atmosphere, it is highly susceptible to contamination from the spacecraft. The two large radiators on the spacecraft are designed to lose only a certain amount of heat. As the spacecraft cycles between lunar night and day, a supplemental cooling system mounted on the SM uses the evaporation of water in a vacuum to carry away excess heat, essentially dealing with the temperature peaks. Although the Mass Spectrometer is carried on the end of a boom, it is feared the resulting water vapour cloud from the evaporator would saturate the experiment.

Comm break.

Flight Plan page 3-92.

86 hours into the mission is the official time for the crew to begin a 7.7 hour rest period. It will be ten minutes before conversation with Karl Henize finishes and they will have a call seven and a quarter hours later. The presleep checklist is being worked on just now which includes reading the status of batteries, RCS tanks and the PRDs.

Comm break.

Long comm break.

The E-Memory dump is the final item on the presleep checklist. It allows Mission Control to study the computer's memory and check its contents.

With the Ion Source, On, the Mass Spectrometer begins full operation which will continue for the next nine hours.

Designed to characterise the lunar atmosphere, the Mass Spectrometer measures the atomic weight of the atoms and molecules which enter an aperture on one side of the instrument.

When the spacecraft is flown in an attitude that has the minus-X axis facing forward (i.e. the SPS engine bell facing forward), particles from the lunar atmosphere are rammed into an inlet where they are electrically charged (ionised) by electrons from a filament source. A magnet diverts the path of the resultant ion stream towards two detectors. Simply stated, the heavier an atom or molecule, the more resistant its motion is to change by a magnetic field. By measuring the deflection of the particle stream, the masses of its constituent parts can be determined. The two detectors will detect particles in the mass range of 12 to 66 atomic mass units.

The inlet is mounted on the opposite side of the instrument from the CSM to shield it from gases emanating from the spacecraft. By changing the angle of attack of the inlet, it is possible to discriminate between CSM contaminants and genuine lunar atmosphere.

According to the 'Apollo 15 Preliminary Science Report', little difference was detected between the inlet pointing into the flight path and pointing into the spacecraft's wake. This implies that most of what the instrument detects is, essentially, pollution from the spacecraft.

The later reference work, the 'Lunar Sourcebook' by Heiken, Vaniman and French, elaborates on this by pointing out how longer term surface experiments placed by Apollo 12, 14, 15 and 17 were deluged with contaminants from Apollo operations which made it very difficult to extract natural data from their results. This is hardly surprising considering that estimates for the total mass of the natural lunar atmosphere are around 10 metric tonnes and a similar quantity of gases are released during each Apollo mission, mostly from operation of the descent and ascent engines.

Apollo 15 will begin Rev 5 at about 086:36 during the next far-side pass. The crew's rest period will continue until nearly the end of revolution 8.