Day 10: Orbital Science	Journal Home Page	Settling down for the Journey Home

Apollo 15

Orbital Science and Trans-Earth Injection

Corrected Transcript and Commentary Copyright © 2000 by W. David Woods and Frank O'Brien. All rights reserved.

[Continuing the tenth day in the Apollo 15 mission, Endeavour has just gone behind the Moon towards the end of its 72nd orbit. Orbit 73 begins at about 219:55 GET and they have only two further orbits before they light the engine of the SPS (Service Propulsion System) for about 2 minutes, 20 seconds and send the spacecraft on a trajectory for Earth. Before that, they are to launch a subsatellite out of the SIM bay, and to put it in the required orbit, they must change the shape of their current path around the Moon.]

[The lunar photography part of the mission is essentially complete, as the later activities preclude maintaining the constant attitude necessary for photography. As the principal targets have all been imaged, the crew is being asked to simply run the Panoramic Camera and photograph whatever passes below to use up the remaining camera film. Although the camera will probably capture areas that have low scientific interest, it would be a larger crime to let the film go unused. However, when Mission Control review the camera's telemetry, they do not see the characteristics they expect from film depletion and begin analysing what the problem might be, fearing that, in some way, depletion of the film has not gone smoothly.]

[Several activities will occupy the crew's attention in the last three hours of lunar orbit. First, a small burn will be performed to place the CSM into a somewhat higher orbit. Raising the orbit is necessary to extend the lifetime of a small "subsatellite", a 35.6-kg satellite released from the aft end of SIM bay and used to investigate the various particles and fields in the lunar environment. "Particles and fields" is an expression for the types of science that the subsatellite performs, work continued in the late 1990s by the Lunar Prospector probe, where, rather than imaging a planetary body, the force fields, molecules and radiations which surround and interact with it are measured. Releasing the subsatellite is the crew's final activity in lunar orbit, occurring only an hour before the burn to take them home.]

[Trans-Earth Injection, certainly the most critical maneuver asked of the spacecraft's big engine, is a burn of their large engine 2 minutes, 18 seconds on the far side of the Moon scheduled to occur at 223:46:06. The news media of the time, always on the look-out for a gripping story, revelled in the apparent dependence of the crew's lives on a single engine. In reality however, just about the only stand-alone component of the SPS engine is the rather passive combustion chamber and its nozzle extending out the rear of the Service Module. With the importance of the TEI burn in mind, the Service Propulsion System was designed with redundancy in its plumbing, control electronics, valves and propellant injectors. Its hypergolic, pressure-fed strategy virtually ensure operation and, as the problem with the shorted SPS switch showed earlier in the mission, sometimes great care had to be taken keep the engine from igniting at undesirable times.]

[Flight Plan page 3-332.]

220:22:06 Allen: Endeavour. This is Houston.

220:22:12 Worden: Go ahead, Houston. Endeavour.

220:22:14 Allen: Roger. Could you tell us, please, the status of the Pan Camera switch?

220:22:23 Worden: Roger. Right now, it's in Standby and Power and it's been barber pole.

[A change in the Flight Plan, read up during the last orbit, asked that the Panoramic Camera be run until the film in it is depleted. To show this condition, an indicator for the camera on the SIM bay control panel would show a "barber pole" (stripes), upon which, Al Worden was to switch the camera to Standby. This is the start of efforts to characterise the telemetry seen to come from the camera.]

220:22:41 Worden: Roger. [Long pause.]

220:23:03 Allen: And, 15. We've powered up the drugstore to receive the film when you get home.

220:23:12 Worden: Roger. Better get a couple. [Long pause.]

[With over 1,500 photographs from the Panoramic Camera alone, each over a metre in length, there are two kilometres of film to be processed, once Al has retrieved the magazine containing the film and brought it back to Earth in the Command Module.]

220:23:43 Worden: Roger. Power, On and Operate. [Pause.]

[Mission Control are continuing their analysis of film depletion in the Panoramic Camera.]

[The crew are due to switch off the discrimination shield on the

Gamma-ray Spectrometer for about 10 minutes to help calibrate the discrimination process.]

220:24:04 Worden: Roger. You've got it.

220:24:06 Allen: Thank you. [Long pause.]

[Al gives Mission Control access to the computer's erasable memory so they can upload essential navigation data into it. The state vector consists of seven values which define the spacecraft's position and velocity in three axes at a particular time. The Shaping target load is the velocity change (Delta-V) and time-of-ignition (T_ig) required for the upcoming orbit shaping burn.]

220:25:21 Irwin: Joe, you'll have to give us that transmission again. We were off [garble] temporarily.

220:25:28 Allen: Roger, Jim. Just said that we've reverified that the Pan Camera is out of film and you can power it down for the final time at your convenience.

220:25:43 Irwin: Understand.[Long pause.]

220:26:33 Allen: Endeavour, we have a preliminary TEI-74 PAD, if you're ready to copy.

220:26:45 Irwin: Stand by one, Joe.

220:26:47 Allen: Roger, Jim. No hurry. [Long pause.]

220:27:15 Irwin: Okay, Joe. Ready to copy.

[While all three crewmembers are in the spacecraft, one of Jim Irwin's tasks is to copy down information read up by the CapCom. In the case of this TEI-PAD, there are standard forms which have boxes for each digit to help minimise errors.]

[Interpretation of the preliminary TEI-74 PAD is as follows:

Purpose: This is the first of two TEI-74 PADs, the second of which will be the final details of their burn to bring them home. The PAD contains the best information available at this time, and will certainly be able to get them home in the event communications are lost as it includes data on their re-entry in three days time. If used, the burn would occur towards the end of rev 74.

System: The maneuver will use the SPS (Service Propulsion System), and the primary Guidance and Navigation system.

CSM Weight (Noun 47): 35,852 pounds (16,259 kilograms).

Pitch and yaw trim (Noun 48): 0.62° and 0.96°. These are the angles to which the SPS engine should be aimed to place the thrust vector through the spacecraft's centre of gravity:

Time of ignition, T_ig (Noun 33): 223 hours, 48 minutes, 43.63 seconds.

Change in velocity (Noun 81), fps (m/s): x, +2,945.0 (+897.6); y, -769.6 (-234.6); z, -152.8 (-46.6). These velocities are expressed with respect to the

local vertical/local horizontal. The large positive x component shows the prograde nature of the burn and there is a sizeable out-of-plane or y component to match the plane of the Moon's orbit around Earth.

Spacecraft attitude at T_ig: Spacecraft attitude will be zero for all axes because the alignment of the guidance platform matches the required spacecraft attitude for the burn.

The next two items in the list define the Earth-based orbit that the burn will place them into.

H_A, expected apo of resulting orbit: N/A

H_P, expected perigee of resulting orbit: 22.3 nautical miles (41.3 kilometres).

This figure for perigee is purely theoretical. Its low value is well within the outer regions of the Earth's atmosphere and shows that this trajectory will bring the spacecraft in for a normal re-entry.

Delta-V_T (Noun 81): 3047.7 fps (928.9 m/s).

Burn duration or burn time: 2:22

Delta-V_C: 3029.4 fps (923.4 m/s). This figure is entered into the EMS Delta-V counter to allow backup control of the SPS engine. It is lower to account for the tail-off characteristics of the engine.

Sextant star: 37 (Nunki, or Sigma Sagittarii) visible in sextant when shaft and trunnion angles are 224.3° and 30.5° respectively.

Boresight Star: N/A

COAS Pitch Angle: N/A

COAS X Position Angle: N/A

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. In this context, a more important milestone is when atmospheric drag on the spacecraft imparts a deceleration of 0.05 g.

Expected splashdown point (Noun 61): 26.12° North, 157.98° West.

Range to go at the 0.05 g event: 1,083.8 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 0.05 g event to landing. This figure will be decremented by the EMS based on signals from its own accelerometer.

Expected velocity at the 0.05 g event: 36,179 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.

Predicted GET of 0.05 g event: 294 hours, 58 minutes, 40 seconds GET.

GDC Align stars: Stars 36, Vega (in Lyra) and 43, Deneb (in Cygnus) 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: x, 102°; y, 178°; z, 28°.

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

220:29:44 Allen: Roger. Delta-V_t is 3047.7; B_t, 2:22; and Delta-V_c, 3029.4. Over.

220:30:06 Irwin: Roger. Read on down the rest of that PAD, from there on down.

220:30:10 Allen: Oh, okay; coming at you, Jim. The sextant is 37, 224.3, 30.5; NA, NA, NA; latitude, plus 26.12; longitude, minus 157.98; 1083.8, 36179; and GET 294:58:40. Vega and Deneb; 102; 178; 028; 4 jet, 12 seconds. Over.

220:31:21 Irwin: Okay, Joe. Readback for TEI-74 preliminary. SPS/G&N; 35852; plus 0.62, plus 0.96; 223:48:43.63; plus 2945.0, minus 0769.6, minus 0152.8; zero for roll, pitch and yaw; Delta-V_T 3047.7, 2:22, 3029.4, 37, 224.3, 30.5; latitude, plus 26.12, minus 157.98; 1083.8, 36179; 294:58:40. Vega and Deneb; 102; 178; 028; 4 jet for 12 seconds.

220:32:36 Allen: That's correct, Jim, and your Noun 44 is NA and plus 0022.3 and readback's correct.

220:32:40 Irwin: Roger; copied. H_p is plus 0022.3. [Long pause.]

220:33:06 Allen: And Jim; this is Houston. Could you adjust your S-band volume for us, please? You have a side tone squeal when you transmit and the - the volume is - is fairly weak for us. Give us a count before you do it and after you do it. And, would you reverify the Delta-V_y for us? Over.

220:33:33 Irwin: Okay, Delta-V_y was minus 0769.6.

220:33:44 Allen: Okay, Jim. Thank you. And the volume is better. Thank you.

220:33:54 Irwin: And, Houston; Endeavour. We're just finishing up final stowage right now, and we'll be with you in a couple of minutes on the procedures.

220:34:03 Allen: Okay, we're standing by. [Long pause.]

220:34:33 Allen: Endeavour, at your convenience, Gain Step switch to center, please. And I have the shape SPS/G&N PAD, when you're ready for that.

[By placing the Gain Step switch to its centre position, they switch on the discrimination shield on the Gamma-ray Spectrometer which was switched off ten minutes ago.]

220:34:55 Allen: Roger, Jim. Shape, SPS/G&N; weight, 36171; plus 0.63, plus 0.98; 221:20:47.23; plus 0017.0; minus all zips, minus 0064.2; 355, 198, 010; 0076.1, plus 0054.3; and, Jim, why don't you readback from there, and I'll pick up again. Over.

220:36:23 Irwin: Okay. It's shape, SPS/G&N; 36171; plus 0.63, plus 0.98; 221:20:47.23; plus 0017.0; minus all zips, minus 0064.2; 355, 198, 010; 0076.1, plus 0054.3.

220:36:51 Allen: Right on, Jim. Delta-V_t is 0066.4, 0:03, 0054.8; sextant, 13, 164.3, 12.6; 001, down 09.1, left 4.6. The rest is NA. GDC align, Vega and Deneb; roll align, 102; 178; 028. Four jet, 12 seconds. Other is subsat launch, GET 222 plus 39 plus 27; roll, 266; pitch, 141; yaw, 038. High Gain [Antenna angle]: pitch, minus 70; yaw, plus 113. Standing by for readback starting with Delta-V_t. Over.

220:38:54 Irwin: Okay, Joe. 0066.4, 0:03, 0054.8; 13, 164.3, 12.6; 001, down 09.1, left 4.6; Vega and Deneb; 102; 178; 028. Four jet, 12 seconds; subsatellite launch, GET 222:39:27; roll, 266; pitch, 141; yaw, 038. High Gain is pitch, minus 70; yaw, plus 113. Over.

220:39:40 Allen: Right on the money. Thank you, sir.

[Interpretation of the shape burn PAD is as follows:

Purpose: The shape burn is a short maneuver, intended to raise the orbit of the CSM from its current 65.4- by 52.2-nautical mile (121.1- by 96.7-km) orbit to a nominal orbit of 76.1 by 54.3 nautical miles (140.9 by 100.6 km). The perturbations of lunar orbits are such that if this maneuver was not performed, the subsatellite would have an orbital lifetime of only a few months. After a successful burn, scientists can expect a full year out of the subsatellite.

System: The maneuver will use the Service Propulsion System, and the primary Guidance and Navigation system.

CSM Weight (Noun 47): 36,171 pounds (16,404 kilograms).

Pitch and yaw trim (Noun 48): 0.63° and 0.98°.

Time of ignition, T_ig (Noun 33): 221 hours, 20 minutes, 47.23 seconds.

Change in velocity (Noun 81), fps (m/s): x, +17.0 (+5.2); y, 0; z, -64.2 (-19.6). These velocities are all with respect to the Moon and the

local vertical and horizontal that the spacecraft makes with that body.

Spacecraft attitude: Roll, 355°; Pitch, 198°; Yaw, 010°. This is with respect to the attitude of the guidance platform, itself aligned to the

landing site REFSMMAT.

H_A, expected apocynthion of resulting orbit (Noun 44): 76.1 nautical miles (140.9 kilometres).

H_P, expected pericynthion of resulting orbit (Noun 44): 54.3 nautical miles (100.6 kilometres).

Delta-V_t (total velocity change): 66.4 fps (20.2 m/s).

Burn duration or burn time: 0:03

Delta-V_c: 54.8 fps (16.7 m/s).

Sextant star: Star 13 (Capella, or Alpha Aurigae) visible in sextant when shaft and trunnion angles are 164.3° and 12.6° respectively.

Boresight star: Star 1 (Alpheratz, in Andromeda) This is a second attitude check which is made by sighting on another celestial object with the COAS (Crew Optical Alignment Sight).

COAS Pitch Angle: down, 9.1°.

COAS X Position Angle: left, 4.6°.

GDC align stars: Stars to be used for GDC align purposes are Vega and Deneb. The align angles are roll, 102°; pitch, 178°; yaw, 28°. This would be used in the event that the IMU could not be used for aligning the gyro assemblies. The spacecraft would be maneuvered so that the two stars could be viewed in the scanning telescope in a certain configuration. On achieving this, the spacecraft would be in a known attitude, the same one given above. SPS propellants are settled in their tanks by firing the plus-X thrusters on all four of the Service Module RCS quads for 12 seconds.]

[Also read up at this time are the details of the launch of the subsatellite.

Subsatellite launch time: 222 hours, 39 minutes, 27 seconds.

Spacecraft attitude at launch: Roll, 266°; Pitch, 141°; Yaw, 38°.

As Allen will point out, the angles given for the HGA (High Gain Antenna) are meant to be used during the shaping burn.]

220:40:02 Irwin: Okay. [Pause.]

220:40:09 Scott: And, Houston, we're ready to talk over the procedures any time you are now.

220:40:14 Allen: Okay, Endeavour, I guess we're ready to start. I want to put one final note on the subsat launch. We'd like for you to reverify for us the talkbacks on the RCS after the launch.

220:40:38 Scott: Okay. Understand. Check the RCS talkbacks after the launch. We'll do that.

220:40:46 Allen: Okay, Dave; and I guess you might cast an eye up at the - the few - fuel cell - talkbacks as well. Over.

[Joe Allen is requesting that the crew monitor the "talkback" indicators for the RCS and fuel cell systems during the subsatellite launch. The talkbacks Joe refers to indicate the position (open/closed) of various valves for fuel and oxidizer in the Reaction Control System, and the hydrogen and oxygen reactant valves for the fuel cells. On previous Apollo flights, sudden jolts to the spacecraft would inadvertently close the valves. Earlier events, such as extraction of the LM from the S-IVB launch vehicle stage and the jettison of the door covering the SIM bay, had caused the RCS valves to close and Houston feels it prudent to monitor any changes to valve position. While easy to cycle and reopen, it could cause a great deal of confusion if a valve closure went unnoticed.]

220:41:16 Allen: Endeavour, this is Houston. Ready to tag up with you on the TEI procedures. And, Dave, I guess we're standing by for your recommendation. Do you want to read through the additions you now have in your P40 SPS Thrusting Checklist, or do you want us to go through the changes we want you to have there? It looks pretty much exactly like your TLI maneuver. There are just two changes; there are two difference from that, I guess. Over.

[These differences arise from a short circuit within one of the switches of the primary (or A) SPS control circuit.]

220:41:59 Allen: Okay, Dave. That sounds like a good way. Turn to page G5-4. And right down at the bottom of the page...

220:42:12 Scott: 5-4. Go.

220:42:13 Allen: Roger. Right at the bottom of the page after the two lines, "SPS Fuel/Oxidize Pressure" and "PUGS balanced", add the step "Cut-off minus 10 seconds, circuit breaker SPS Pilot Valve, Main A to open."

220:42:44 Scott: Okay: bottom line on page 5-4. Cut-off minus 10 seconds, "circuit breaker SPS Pilot Valve, Main A, open."

220:42:54 Allen: That's correct. And turning over a page to 5-5, you have an addition underneath the normal step which reads, "Circuit breaker SPS Pitch 1 and yaw 1 to open". Your addition reads, "Circuit breaker SPS Pilot Valve, Main B, open." And we have still another step to add in there, which is, "Circuit breaker EMS, two of them, Main A and Main B, to Open." And, also, we would like to delete, one step below that, the three lines, "Map Camera, On, to Off," "Pan Camera Power to Off," and "SM/AC Power to Off." Over.

220:43:50 Scott: Okay; on page 5-5, beneath the addition of "CB/SP - S - SPS Pilot Valve, Main B, open," add "CB EMS, two, open," and them delete the three lines on Map Camera, Pan Camera, and SM/AC power below that.

220:44:07 Allen: That's correct, Dave, and we're standing by for the other additions you've made, now.

220:44:16 Scott: Okay, we'll go back to the beginning, here. Okay; our initial configuration has the addition of both Pilot Valve circuit breakers open, and to verify that both EPS Group 5 circuit breakers are closed, and also on page 5-1, beneath the caution/warning test, EMS Function, Off, to verify, and the CB EMS A&B both closed. That's what we have on 5-1.

220:44:53 Allen: Okay; that's good. Turn the page.

220:44:59 Scott: Okay, on 5-2, under "TVC Check and Prep", second line, "CB SPS (10), closed". That's all on 5-2.

220:45:07 Allen: Roger.

220:45:11 Scott: Page 5-3, at minus 2 minutes, "CB SPS Pilot Valve, Main B, closed", instead of Delta-3 - "Delta-V Thrust A and B, Normal".

220:45:22 Allen: That is correct.

220:45:22 Scott: Okay. And on page 5-4, at 5 minutes prior to ignition when we get - 5 seconds prior to ignition when we get the flashing 99, we Delta-V Thrust A and B to Normal after the Pro. And then, on down at ignition, if we have a flashing 97, the SPS thrust fail, "CB SPS Pilot Valve A, closed". And, if everything goes according to Hoyle, why, at 5 seconds, we'll close the C - CB SPS Pilot Valve Mi - Main A. And then, of course, the step you just answered - just added - at the bottom of the page, at cut-off minus 10 seconds, to close the Pilot Valve - or to open the Pilot Valve Main A.

220:46:21 Allen: Okay, Dave. That's right on the money.

220:46:26 Scott: Okay. And then on 5-5, we just discussed those two, which were a "CB SPS Pilot Valve, Main B, open", and the "EMS circuit breakers, open".

220:46:36 Allen: Roger, Dave. That sounds good to us.

220:46:44 Scott: Okay; and I think we've practised one of those once before. I think we'll be able to handle that.

[The procedure changes are a continuation of the troubleshooting necessary from the intermittent short in the Direct Thrust switch that was discovered early in the flight. The A bank cannot be reliably controlled by the computer as it is highly liable to ignite the engine just by arming the circuit Essentially, the TEI burn will be accomplished starting with the good bank of valves, Bank B, under computer control. Upon ignition, the suspect bank, Bank A's valves will be opened, and will stay open until 10 seconds prior to cut-off when they are closed. The concern is that if the switch should short again, the engine would continue firing past the time of engine shutdown. By removing power from the Bank A valves, thus closing them, and relying on automatic control of Bank B, they have eliminated the possibility of any undesired engine operation.]

220:47:08 Scott: Yes.

220:47:09 Allen: ...marked up the cue card for the plane change maneuver, and we wonder if you're going to use that marked-up card for the shape burn. Over.

220:47:20 Scott: No, no. We'll use the checklist straight through on the rest of the burns.

220:47:29 Allen: Okay, Dave. That's fine with us as we know you're aware it's a single bank B burn, which has been marked correctly onto that cue card.

220:47:43 Scott: Rog. Yeah, I guess all - all our burns in orbit are just a straight single bank B.

[The SPS engine not only has redundant control circuitry and plumbing, it even has independent injectors introducing the propellants into the combustion chamber. However, the rated thrust of the engine is not achieved using just one system. Due to the problems with the valves in SPS Bank A, all the in-orbit burns are performed using only a single bank, Bank B. Only a little performance or capability is lost in this configuration, and all these burns are so short, that there would be no time to open the Bank A valves and close them in the short amount of time the burn would be performed. The last major use of the SPS engine, for the Trans-Earth Injection, will be the only time where both sets of valves will be used.]

220:48:06 Scott: And, coming out, we'll have the ball valves open and all the eyeballs watching.

220:48:15 Allen: Sounds like a good idea.

[Long comm break.]

[In preparation for the activities to come, the particles experiments in the SIM bay are switched off. The Gamma-ray boom and the Mapping Camera are both retracted and covers are closed.]

220:52:15 Allen: Go ahead.

220:52:20 Scott: We were just getting the hoses set up here for the burn and noticed that there's a little bit of water coming out of the - the blue hose in the suit loop. Thought you might be interested.

220:52:40 Allen: Okay, Dave. We copy that. Thank you.

220:52:46 Scott: Roger. [Pause.]

220:52:54 Scott: Not much. It's just, if you put your hand around the nozzle there, why, you can get some water on it.

220:53:03 Allen: Okay; we copy that. We think it's probably normal, but we'll look into it real closely. Sounds to me like the Endeavour has a few plumbers aboard as well as experts in other things.

220:53:24 Scott: Well, by the time this is over, I guess we will be plumbers.

220:53:29 Allen: We'll all be plumbers, Davy.

[Very long comm break.]

[Joe Allen's reference, is of course, to the quick fix that was needed to stop a water leak during the translunar coast.]

[From the Apollo 15 Mission Report - "Droplets of water came from two of the three blue [supply] hoses when they were relocated for the Trans-Earth Injection firing. Since cabin humidity continued to be normal and no recurrence of the problem was observed, most likely the condensation was an effect of the large primary coolant loop temperature transient on the suit heat exchanger during lunar orbit."]

[

Flight Plan page 3-333.]

[At ten minutes to go before ignition, the Digital Event Timer is started from 50:00. It will therefore count up to the time of the burn and help the crew pace their checklist properly.]

221:15:52 Allen: Endeavour, you're looking good. You're Go for the shaping burn.

221:15:59 Scott: Roger. Go for the shape.

[Long comm break.]

221:21:52 Scott: Okay, Houston. Endeavour with a burn status report.

221:21:56 Allen: Go ahead, Endeavour.

221:22:07 Scott: Okay. Burn was on time, and it was a 3-second burn. We had about 1.0 or 1.1 residual, and that was trimmed to 0.1, minus 0.2, minus 0.2. Delta-V_c was minus 11.0; fuel, 26.50; oxidizer, 26.25.

221:22:32 Allen: Roger, Dave. We copy that.

221:22:40 Scott: And it has us in an orbit, 76.0 by 54.3 [nautical miles, 140.8 by 100.6 km].

221:22:48 Allen: Sounds just right.

[Interpretation of the burn status report is as follows:

The burn was on time, at 221:20:42, and lasted for the expected 3 seconds. The resulting orbit was 76.0 by 54.3 nautical miles (140.8 by 100.6 km), almost exactly the desired 76.1 by 54.3 nautical miles (140.9 by 100.6 km). At the end of the burn, the total velocity error was about 1.0 to 1.1 fps (0.3 m/s). This needed to be trimmed to no more than 0.2 fps (0.06 m/s) in all three axis using the RCS, and they did so to 0.1, 0.2 and 0.2 feet/second in the X, Y and Z-axes, respectively. Delta-V_c is a measure of velocity change as determined by the accelerometer within the EMS, and was -11.0 fps (3.4 m/s). The imbalance between the two propellants consumed was ¼ pound more fuel than oxidizer.]

221:22:56 Allen: Okay, Dave. Great. And you just gained about 240 more days for our subsatellite, or something like that.

221:23:08 Scott: Very good.

[Comm break.]

221:25:54 Scott: Roger. Understand. Will do.

[Comm break.]

221:28:12 Scott: Roger. Thank you, Houston.

[Very long comm break.]

[Apparently, Al Worden has jumped immediately into the lower equipment bay after the burn to perform a platform alignment, using the TEI

REFSMMAT as an orientation, that was not due until after LOS. Because the ground can see the same computer entries and displays as the crew, it isn't necessary to read them off. This saves a few minutes of time, which is in short supply immediately before LOS.]

[

Flight Plan page 3-334.]

[Housekeeping duties during LOS includes purging the fuel cells with their hydrogen and oxygen reactants to remove contamination, and a dump of urine and waste water, the latter being excess water produced by the fuel cells. The Gamma-ray and Alpha Particle Spectrometers are started for an extra hour of data gathering though the boom is not extended.]

Public Affairs Officer - "This is Apollo Control at 222 hours, 15 minutes. We're about to acquire Endeavour on the 74th revolution. During this pass the subsatellite jettison is scheduled, and on this 74th revolution, after Endeavour goes behind the Moon, the Trans-Earth Injection burn will be performed. We will not be in contact at that time. The subsatellite will be jettisoned at the time the spacecraft is crossing the lunar equator on its way to apolune. [It] will be jettisoned to the north so that the spin axis will be perpendicular to the sun's line of sight and allow maximum sunlight on the solar cell. The spring ejection mechanism will give the subsatellite Delta-V of 4 feet per second [1.2 m/s] relative to the spacecraft. And the initial spin rate on the subsatellite will be 140 revolutions per minute. That's expected to stabilize out at 12 revolutions per minute when the booms are extended. We'll stand by for air/ground."

222:17:54 Irwin (onboard): Clear.

222:18:00 Worden (onboard): Oh, my. Look at this. That's a good picture.

222:18:05 Allen: Hello, Endeavour. This is Houston.

222:18:11 Scott: Houston, Endeavour. Go.

222:18:14 Allen: Roger, Endeavour. We're requesting you verify your systems - Data Systems, On, and the S-band Aux switch to Science, please.

[This is to carry science data from the SIM bay on the auxiliary S-band data channel.]

222:18:42 Scott: Okay; that's verified now, the data systems are on now.

222:18:46 Worden (onboard): Shoot!

222:18:48 Scott (onboard): They're easy to overlook, aren't they?

222:18:48 Allen: Okay, Dave; we copy that. I've got a map update, rev 75, when you're ready to copy. And I have a message for Al from the King [Farouk El-Baz], when he's ready to copy.

222:19:05 Worden: Go ahead, Joe.

222:19:07 Allen: Roger, Al. The message to you is to stand by to copy your final exam grade in orbital science and observation. It's an alpha-plus, with a subnote of "Well done." Over.

222:19:23 Worden: Tell the King, thank you very much, Joe.

222:19:25 Allen: Roger, Al. And I've got the map update...

222:19:27 Worden: And I expect to see him back in Houston soon.

222:19:29 Allen: Oh! Oh, no - no problem there.

222:19:31 Worden: Go ahead, Joe.

222:19:32 Allen: Roger. The map update, rev 75. Copy at 223 plus 20. And it is: LOS, 223 plus 29 plus 45; 180, 223 plus 52 plus 57; AOS with TEI, 224 plus 03 plus 03; AOS without TEI, 224 plus 15 plus 30. Over.

[The map update is yet another "defense in depth" strategy to handle problems if they arise during the preparations for the TEI burn. Interpretation of the map PAD is as follows:

Purpose: These are timings that define what the crew should expect whether or not the TEI burn is successful.

LOS on the final pass around the Moon: 223:29:45

Passing over the 180 degree longitude meridian of the Moon: 223:52:57

AOS, if the TEI was successful: 224:03:03

AOS, without the TEI and going into rev 75: 224:15:30

Although it could be considered far-fetched, if there was a problem with the clocks onboard the spacecraft, the TEI burn could be timed from the Loss Of Signal time, occurring almost exactly 19 minutes after the spacecraft passes behind the Moon.]

[It is also interesting to note just how nominal the Apollo 15 mission has been thus far, at least as far as flight dynamics are concerned. Despite all the maneuvers, including thrusting burns, major and minor, the time of LOS is only about 4 minutes different from the values calculated and marked in the Flight Plan long before the flight ever launched from Earth.]

222:20:46 Allen: Readback's correct, Jim; thank you.

[Very long comm break.]

[With less than 20 minutes to jettison of the subsatellite, preparations begin for the event and for recording it. A DAC (Data Acquisition Camera, actually a Maurer 16-mm movie camera) is mounted in the main hatch window. One of the crew will also elect to take photographs on one of the Hasselblad cameras.]

[The next task is to stabilise the spacecraft so the subsatellite's trajectory will be true. The RCS jets are configured to use only those thrusters that do not impinge upon the SIM bay (a common requirement for orbital activities) and to use only pairs of thrusters for fine control. Then the spacecraft's attitude is allowed to settle.]

[Having ensured that the switch to release the subsatellite is off, circuits to enable the firing of pyrotechnic bolts are powered. Mission Control verify it even as Dave Scott asks for verification.]

[

Flight Plan page 3-336.]

222:31:30 Scott: This is Endeavour. Can you verify the...

222:31:31 Allen: ...arm, and - your rates look good to us down here. Over.

222:31:41 Scott: Okay. You had us all figured out. We'll go Free.

[The reference to "going Free" is to the type of control the Command Module Computer will maintain over the spacecraft. Placing the switch in the "Free" mode, essentially disengages the CMC's autopilot, allowing the spacecraft to drift, and no correcting maneuvers from the thrusters will be performed. This is the desired mode for releasing the subsatellite. When the satellite is sprung out from its storage area at the aft end of the SIM bay, Newton's laws of "equal and opposite actions" will displace the CSM from it.]

222:31:59 Allen: And we know one of you will be watching out the window. We're particularly interested if the spin of the satellite is sweeping out a cone or if it seems to be a fairly flat spin as it comes out. Over.

222:32:15 Scott: Roger.

[Comm break.]

222:34:06 Worden: In work.

[Comm break.]

[As the CSM is simply drifting with no active control holding its attitude, even small motions by the crew inside the Command Module or the slosh of fuel in the propellant tanks are enough to cause the spacecraft to wander. Joe Allen is requesting that the crew invoke Verb 49 to have the computer adjust the spacecraft's attitude. Included in this request is the need to change the CMC mode to "Auto" from "Free". Once the orientation is corrected, the CSM will be allowed to drift again.]

222:35:22 Allen: Okay, Endeavour. We're recommending that you go back to Free at launch minus one minute.

222:35:32 Irwin: Okay; Free at launch minus one minute.

[Comm break.]

222:36:50 Scott: Roger; Free at launch.

[Comm break.]

[To ensure the spacecraft will not continue to drift from its desired attitude again in the minutes before the subsatellite launch, Joe Allen is relaying a request to have the computer actively maintain attitude up until the moment of launch. Part of the deployment sequence will now include disabling attitude control at the moment of launch. Once the subsatellite is well clear of the spacecraft, attitude control will be re-established.]

222:39:17 Scott: Three, two, one...

[One end of the subsatellite sits on a deployment mechanism with a spring preloaded against it to provide thrust for launch. When the "Launch" switch is turned on, a motor begins moving the deployment mechanism and the subsatellite along a track, opening a door of the housing in the process. When it reaches the end of the track, it engages a switch that fire the pyrotechnics to sever two bolts, freeing the subsatellite and allowing the spring to force it away from the spacecraft. A pin engages in a curving groove in a cylinder as the two spacecraft part, imparting a rotation to the subsatellite that will help stabilise it in its orbit. After launch, the deployment mechanism is retracted.]

222:39:31 Scott: And a gray.

222:39:35 Allen: We confirm that.

222:39:43 Scott: Tally Ho! [Long pause.]

222:40:05 Allen: Okay, Dave; copied that. Sounds great.

222:40:15 Allen: Can you see much? [Long pause.]

222:40:28 Scott: Oh, looks like it might be oscillating maybe 10 degrees at the most.

[As the subsatellite departs, not only is it filmed by the DAC, but also by a Hasselblad using magazine Q and a 250-mm lens. Eleven pictures are taken though it cannot be seen in two of them. Constant size portions of the other nine are brought together to show the departing craft.]

222:41:02 Allen: And, Endeavour, when you're ready, I've got a coming-home PAD to read to you.

222:41:10 Irwin: Stand by one, Joe.

222:41:11 Allen: Okay, Jim. And we would like Accept, please, on the computer.

222:41:21 Irwin: Stand by one.

[Comm break.]

[Mission Control want to upload a revised state vector and a target (T_ig and Delta-V) for the upcoming TEI maneuver.]

222:42:37 Allen: Very good, Dave. Thank you. [Pause.]

222:42:50 Irwin: And, Joe, I'm ready to copy that coming-home PAD.

222:42:54 Allen: Okay, Jim. I'm ready to read it to you. Its TEI-74, SPS/G&N; the weight, 35768; plus 0.57, plus 0.88; 223:48:45.05; plus 2945.2, minus 0761.3, minus 0171.4; all zips on roll, pitch and yaw; Noun 44 is NA, and plus 0022.1; 3046.8, 2:21, 3028.5; 37; 224.3, 30.5; 053, down 10.3, right 3.1; plus 26.11, minus 157.97; 1084.1, 36179; and the GET, 294:58:34. GDC align, Vega and Deneb; 102; 178; 028. Four jets, 12-second ullage. And, we'd like Accept, please. And I'm standing by for the readback. Over.

[Interpretation of the TEI-74 pad is as follows:

Purpose: This is the second and final of two TEI-74 PAD's. It incorporates the latest tracking data, the results from the shape burn, and the reduction of weight from the subsatellite deployment. Most of the values have changed, but only in very small amounts.

System: The maneuver will use the Service Propulsion System, and the primary Guidance and Navigation system.

CSM Weight (Noun 47): 35,768 pounds (16,221 kilograms).

Pitch and yaw trim (Noun 48): +0.57° and +0.88°.

Time of ignition, T_ig (Noun 33): 223 hours, 48 minutes, 45.05 seconds.

Change in velocity (Noun 81), fps (m/s): x, +2,945.2 (+897.7); y, -761.3 (-232.0); z, -171.4 (-52.2). The large positive x component shows that this burn would be prograde, i.e. with their orbital motion, increasing their velocity so that they escape the gravitational pull of the Moon. The y component indicates that the plane of their orbit would be changed, as would be expected to take them away from their highly inclined orbit to one which includes the Earth.

Spacecraft attitude: Spacecraft attitude will be zero for all axes because the alignment of the guidance platform matches the required spacecraft attitude for the burn.

H_A, expected apogee of resulting orbit: N/A.

H_P, expected perigee of resulting orbit: 22.1 nautical miles (40.9 km).

Delta-V_T: 3046.8 fps (928.7 m/s). This is the total velocity change imparted by the burn.

Burn duration or burn time: 2:21.

Delta-V_C: 3028.5 fps (923.1 m/s). This figure is entered into the EMS Delta-V counter to allow backup control of the SPS engine. It is lower to account for the tail-off characteristics of the engine.

Sextant star: 37 (Nunki, or Sigma Sagittarii) visible in sextant when shaft and trunnion angles are 224.3° and 30.5° respectively.

Boresight Star: 53

COAS Pitch Angle: Down, 10.3°.

COAS X Position Angle: Right, 3.1°.

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. In this context, a more important milestone is when atmospheric drag on the spacecraft imparts a deceleration of 0.05 g.

Expected splashdown point (Noun 61): 26.11° North, 157.97° West.

Range to go at the 0.05 g event: 1,084.1 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 0.05 g event to landing. This figure will be decremented by the EMS based on signals from its own accelerometer.

Expected velocity at the 0.05 g event: 36,171 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.

Predicted GET of 0.05 g event: 294 hours, 58 minutes, 34 seconds.

GDC Align stars: Stars 36, Vega (in Lyra) and 43, Deneb (in Cygnus) 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: x, 102°; y, 178°; z, 28°.

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

222:46:54 Allen: Okay, Jim. The readback is right on. I've got a TEI-75 preliminary PAD and a Flight Plan update PAD, when your ready.

222:47:11 Irwin: Okay, I'll take TEI-75.

222:47:13 Allen: Okay. SPS/G&N, TEI-75; 35768; plus 0.57, plus 0.88; 225:48:44.08; plus 2981.4, minus 0807.2, minus 0145.1; 000, 002, 001; all the rest, NA. Ullage, four jet, 12 seconds. Over.

[This TEI-75 PAD is interpreted as follows:

Purpose: In the event the crew does not perform the Trans-Earth Injection during this pass, another attempt can be made on the next orbit.

System: The maneuver will use the Service Propulsion System, and the primary Guidance and Navigation system.

CSM Weight (Noun 47): 35,768 pounds (16,221 kilograms).

Pitch and yaw trim (Noun 48): +0.57° and +0.88°.

Time of ignition, T_ig (Noun 33): 225 hours, 48 minutes, 44.08 seconds.

Change in velocity (Noun 81), fps (m/s): x, +2,981.4 (+908.7); y, -807.2 (-246.0); z, -143.1 (-43.6). The velocity components are expressed with respect to the

local vertical/local horizontal frame of reference.

Spacecraft attitude: Roll, 000°; Pitch, 002°; Yaw, 001°. The slight change in attitude reflects the fact that the Moon has gone further around in its orbit, taking the spacecraft with it.]

222:48:47 Allen: Sounds good, Jim. Thank you. And I'm standing by for your call for the Flight Plan update.

222:48:56 Irwin: Go ahead, Joe; I'll take that.

222:48:58 Allen: Okay, Jim. And it's your computer [i.e. the upload of a state vector and target have been completed and the crew can take control of the computer again]. The Flight Plan update begins at 223 plus 51. And change the "Verb 49 maneuver" from the numbers listed to the numbers "127, 270, 030." Over.

222:49:35 Irwin: Read you. For the Verb 49 that occurs at 223:51, change the numbers to "127, 270 and 030."

[The Verb 49 that Joe Allen refers to is to maneuver the spacecraft to an attitude appropriate for photographing the lunar surface. The reference to "it's your computer" is that the update to the computer for the TEI-74 burn had been completed. This update included both an updated state vector and the Tig and velocity vectors for the TEI burn.]

[Once the crew is heading home after the TEI burn, they are to turn the spacecraft around and photograph the receding Moon. Performed only minutes after the TEI burn, and only a few minutes before the spacecraft emerges from the Moon's shadow, this will give the crew an excellent vantage for their last closeup views of the surface.]

[When the spacecraft comes into view of Earth, communication will be via one of the omni-directional antennae mounted around the periphery of the Command Module. However, their revised attitude after TEI means that instead of using antenna D, they should use C as it will be more favourably sited.]

222:52:17 Irwin: Okay, Joe. Going back to 224:00, that'll be "Omni Charlie" instead of "Omni Dog." Then, on that Map Camera photo PAD, its 224:03:00 and stop on MSFN cue. Then down to 224:15, on the "Map Camera, Image Motion" will - it'll be "barber pole plus two" instead of "three". At 224:21, we'll delete the "Verb 49 maneuver." At 224:23, we'll delete the "Map Camera track." At 224:27, delete "Map Camera/Laser Experiment Covers, Closed." And at 224:40, do a "Verb 49 maneuver to 127, 295, 030; High Gain, pitch, 23; yaw, 229."

222:53:08 Allen: Right on, Jim. Thank you.

[Long comm break.]

[Among the various requests for Mapping Camera configurations, a change is being made to maneuver the spacecraft to another attitude, perhaps to keep the quickly-receding Moon in view.]

[As TEI approaches, a series of checks are begun to ensure the burn occurs without a hitch. Guided by

page 1-16 of the CSM Systems Checklist, some contamination control is carried out by rigging up the vacuum cleaner and going around the cabin interior to remove residual dust and dirt left over from Dave and Jim's return from the lunar surface. Special attention is paid to those compartments that hold items returned from the Moon, and any dead pockets of air that would not be stirred by the normal motion of cabin air and where dirt might collect.]

[The Caution and Warning system is checked as per

page 1-17. Two positions of the Lamp Test switch on panel 2 of the Main Display Console allow lamps behind the Master Alarm and all the other C&W indicators above panel 2 to be tested.]

[Next, checks are made of the monitored status of both the Service Module and the Command Module RCS (Reaction Control System) packages, according to the steps on the bottom of

page 1-1. Talkback indicators are checked to be gray. For each SM RCS quad, temperatures are taken of the package itself and the helium tank that pressurises it. The pressures of that tank and of the manifold that feeds it to the propellant tanks are also noted, as are the indicated propellant quantities. Checks of the CM RSC are simpler, being concerned with only the helium temperature and pressures of its twin, redundant systems.]

[The last system check called for at this time in the Flight Plan is of the all-important SPS as per the top of

page 1-1 of the CSM Systems Checklist. The temperatures of the propellant tanks are monitored and if the reading is outside limits, appropriate heaters are switched on or off. The pressures of three tanks that are crucial to the operation of the SPS are noted. A helium tank pressurises the propellant tanks (source pressure: 27.9 MPa or 3,900 psia) and two tanks of nitrogen activate the valves that feed propellant to the engine (source pressure: 20 MPa or 2,900 psia). The pressures within the propellant tanks themselves are checked to read within 170 to 195 psia (1.17 to 1.34 MPa). Valve and switch positions are checked and the propellant quantities are noted along with the "unbalance" quantity, a measure of oxidiser consumption with respect to fuel.]

[As occurred with the Lunar Orbit Insertion (LOI) burn, the precise time of AOS gives a very good indication of the quality of the burn. If TEI does not occur, the spacecraft will remain in orbit 110 km above the surface and AOS will occur 45 minutes after LOS, as usual. The TEI burn will accelerate the spacecraft, but not only this, it will cause its altitude to quickly rise as it begins its coast to Earth. Therefore, AOS will occur much earlier (12 minutes, 27 seconds to be exact if the burn runs to its full extent). Intermediate burns will give intermediate times which would result in highly undesirable trajectories so Mission Control will be looking for AOS to occur exactly at the earlier time, and if not then, not until the later time.]

222:56:54 Scott: Oh, very good.

[Long comm break.]

[

Flight Plan page 3-337.]

223:01:07 Irwin: Go ahead.

223:01:09 Allen: Roger. Jim, this is a comment for you. When I was reading the Flight Plan update to you, we noticed that you might be coming up on something not too clear in the Flight Plan and it involves the long list of steps between 224 plus 00 and 224 plus about 15. And there are a number of steps in there that have to be accomplished before the T-start time on the Map Camera photo PAD. It may be, you - you'll want to start on a few of them, I guess, a little early. Over.

223:01:51 Irwin: Okay, thank you, Joe. [Long pause.]

[Since the T-start time is at 224:03, they need to get the mentioned steps between 224:00 and 224:15 completed earlier than planned.]

[The crew is now running the first of two computer programs necessary for their TEI burn. Program 30 (titled, "External Delta V") is used to verify or modify the "target" parameters for an upcoming thrusting maneuver. The "target" is simply the time of ignition, and the required velocity change in the X, Y and Z axis. Normally, the ground uplinks this information, and did so during the TEI-74 PAD at 222:44. This uplink saves the crew the task of manually updating the data, certainly very useful during the busy moments before a burn.]

[Parameters for many maneuvers, such as midcourse corrections, LOI and TEI are calculated on the ground, and not onboard the spacecraft. Using computer processing capabilities far exceeding the Command Module Computer, an optimal solution is calculated from many possible ones. From these calculations, only the time of ignition and velocity change along the X-axis of the CSM need to be relayed to the onboard computer.]

[Program 30 takes these two parameters and uses them to compute the necessary initial conditions for the burn, and the resulting orbital parameters from the burn (admittedly a nonsensical thing when leaving lunar orbit). Subsequently, having maneuvered to the required attitude and checked the alignment of the spacecraft by sighting on a star with the sextant, they will run P40 which will orchestrate the Guidance, Navigation and Control System throughout the execution of the TEI burn itself.]

223:02:42 Irwin: Roger; Omni Delta. ... to go.

[Very long comm break.]

223:16:53 Allen: Hello, Endeavour; this is Houston.

223:16:59 Irwin: Houston, Endeavour. Go.

223:17:05 Allen: Roger, Endeavour. I have three requests for you. The first, the Optics is in the CMC Mode, and we've noticed it drifting off. We'd like you to drive it manually back to a value less than 10 degrees in Trunnion and then zero it. We're also waiting for a DAP load and a P40. And I have a guaranteed last correction to your Flight Plan, when you're ready.

223:17:40 Irwin: Okay, number 1, we've still got to make the star check. We'll take care of that, and I hope - I hope you've already seen the DAP load and the P40, but we'll take a look at it again for you.

223:18:12 Irwin: Okay, Joe. We - I'm ready to take that last change to the Flight Plan.

223:18:16 Allen: Okay, Jim; stand by one. [Long pause.]

223:18:31 Allen: Okay, Jimmy, this is an easy one. At 224 plus 14, we want you to delete the line that reads "Map Camera, Image Motion to On, talkback barber pole in three to five seconds, then gray." Delete that line, please. Over.

223:18:55 Irwin: Okay, that's an easy one. I'll delete that line that says "Map Camera, Image Motion, On, talkback barber pole in three to five seconds, then gray."

223:19:03 Allen: Roger, Jim. Thank you.

[Long comm break.]

223:23:12 Scott: Okay, Houston, Endeavour. Go.

223:23:14 Allen: Roger. Dave, Al and Jim, be advised you're Go for Trans-Earth Injection. Set your sails for home. We're predicting good weather, a strong tail wind, and we'll be waiting on the dock. Over.

223:23:32 Scott: Okay. Thank you very much, Houston. We'll see you around the corner.

223:23:37 Allen: Roger. We'll be watching.

[Very long comm break.]

[Just before Loss of Signal, The crew set up the computer for the TEI burn. This program, used anytime the Service Propulsion System is fired, in known as Program 40 (titled, "SPS Thrusting"). Taking its target values from an earlier execution of Program 30 or one of the backup rendezvous programs, the necessary attitude is computed, using the latest IMU orientation as a reference. When started, P40 maneuvers the spacecraft to the attitude required for the burn. Next, the crew aligns the Stabilization and Control System (SCS) to the IMU as a backup reference, and performs checks on the engine gimbal systems.]

[Flight Plan page 3-338.]

[Certainly the most important burn of the mission is about to be performed, and no reminder is necessary that if the engine fails to fire, there is no way for the crew to return home. The criticality of the maneuver is evident when reviewing the mission rules for the burn. In other, less critical, firings of the SPS, any problem with the engine, attitude control or guidance system, the burn is terminated. As with the insertion into lunar orbit, if there are problems with the spacecraft, the goal is to attempt to complete the burn. Even if the trajectory home may be imperfect because of attitude problems, or if the engine prematurely shuts down and needs to be restarted, these errors are usually correctable during the scheduled midcourse corrections. If, however, the burn was allowed to be terminated early, for example, the spacecraft could be placed in a high lunar orbit, or on a path that will not intersect with the Earth.]

[As the time of ignition approaches, P40 counts down the time on the DSKY, and five seconds prior to ignition, flashes Verb 99, requesting the crew's approval to continue. By pressing "Pro" on the DSKY, approval is granted, and the engine is allowed to fire at the scheduled time using only the secondary (B) control system, with the primary (A) being brought online soon after to take the engine to its rated thrust.]

[During the burn, the time remaining is displayed on the DSKY, as well as the velocity to go (counting down to zero), and the accumulated velocity (counting up to the initial value of velocity to be gained). Ten seconds before the planned shutdown, the A control system is taken offline in case the electrical short in it causes the engine to over-burn. When the computer has determined that the required Delta-V has been reached, the burn is complete and the engine is automatically shut down. The crew brings up a display of velocity errors remaining and manually trims them using the translational hand controller. Exact accuracy not always required in all axis, indeed, for the TEI burn, velocity in the X and Z axes need only be accurate to plus or minus 0.2 feet per second (0.06 m/s), and the Y axis need not be trimmed at all. In reality, these are surprisingly small tolerances; from an approximately 3,000 fps burn, an error of 0.2 fps is less than 0.001%!]

[

Flight Plan page 3-339.]

Public Affairs Officer - "This is Apollo Control at 224 hours, 1 minute. We are now about 2 minutes away from reacquiring radio contact with Endeavour, assuming the Trans-Earth Injection burn went as planned. That 2 minute and 21 second burn with the Service Propulsion System engine was targeting to increase the spacecraft's velocity by 3,046.8 feet per second [928.7 m/s] and would give us a splashdown in the Pacific Ocean about 285 [nautical] miles [528 km] north of Hawaii at 295 hours, 11 minutes, 35 seconds. We are about 1 minute now from a scheduled acquisition time. We are about 30 seconds from our expected acquisition time and the spacecraft velocity at this point should be about 7,500 feet per second [2,300 m/s]. We do expect that the communications will be noisy now, when we do regain radio contact, the spacecraft will be using the small Omni antenna and it'll be about 30 minutes before we have the High Gain Antenna up, which will give us the stronger signal strength. And we do have acquisition of signal."

224:04:05 Parker: Apollo 15, Houston. Over.

224:04:08 Scott: Hello, Houston. Endeavour's on the way home with a burn status report for you.

224:04:15 Parker: Roger. Sounds good. Standing by.

224:04:20 Scott: Roger. Ignition was on time. Burn time was 2 plus 21. No trim. Our residuals were minus 0.2, plus 0.6, and plus 0.2. Delta-V_c was minus 16.7; fuel, 2.35; oxidizer, 2.2; the unbalance was about minus 25. And what a smooth burn that one was.

224:04:49 Parker: Roger. Sounds very good to us, Dave.

224:05:01 Scott: Just can't beat these rocket engines for traveling.

224:05:05 Parker: I should hope not.

[Very long comm break.]

[Interpretation of the burn status report is as follows:

The burn was on time, at 223:48:45, and lasted for the expected 2 minutes, 21 seconds. At the end of the burn, the total velocity error was about -0.2, +0.6 and +0.2 fps (-0.06, +0.18 and +0.06 m/s) in the X, Y and Z axis, respectively. Rules for the burn require that the X and Z components be accurate to 0.2 fps (0.06 m/s), and that errors in the Y-axis need not be trimmed. Delta-V_c is a measure of the velocity to be changed as displayed by the EMS. Since that system does not take into account the tail-off thrust from the engine and had a lower Delta-V entered into it, a figure of -16.7 fps (5.1 m/s) is to be expected. Based on propellant readouts on panel 3 of the Main Display Console, the remaining fuel and oxidiser amounts to 2.35% and 2.2% respectively. The Propellant Utilization Gauging System, or

the PUGS, tells Jim that the engine burned 25 pounds of oxidiser less than would be required to maintain a mixture ration of 1.6.]

[Tom Stafford, the commander of Apollo 10, was very keen on the use of television cameras on Apollo and made special efforts to get a colour camera into his spacecraft as he relates on pages 121 to 122 of his biography We Have Capture. Soon after Charlie Brown emerged from behind the eastern limb, Stafford began beaming images of the receding Moon. The geometry of their trajectory even allowed them to send live images of the far-side crater Tsiolkovsky (128°E).]

[Scott, from the 1971 Technical Debrief - "We turned around to take a look at the Moon, and that was one of the nicest views we had the whole trip - knowing that we were on the way home, and getting to see all the terminators from the Moon. We made a number of comments, that we recorded, on what we saw. It was quite obvious that we were going straight up. You could see the results of the burn immediately. There was no question that we had a significant change in our velocity."]

224:20:11 Scott: Houston, 15. Go.

224:20:18 Parker: Roger. Two questions. One, we notice you have A/C roll jets selected. I guess we aren't sure whether you have B - A/C roll jets selected on DAP, we aren't sure whether you have A/C or B/D selected on the panel.

224:20:41 Worden: Okay. I'll check it... Okay, A/C, you're selected.

224:20:57 Parker: Okay, and could we pry out of you guys any comments on the Moon as you leave?

224:21:06 Scott: Well, we're almost speechless looking at the thing. It's amazing. Looks like we're going straight up; and we're leaving, there's no doubt about that. And we're right on the terminator. It shows very distinctly all the topography - all the topographic highs and lows. And we can see some major rilles. And we noticed one large lava filling within a depression, with domes very prominent within the lava fill. Oh, it's just really spectacular. We have one crater almost below us that has a flat floor with radial rilles and circumferential rilles extending from the central peaks. I think we saw that as we flew over.

224:22:05 Parker: Roger; copy. Sounds beautiful.

[Scott - "You get pretty familiar with it,comfortable with it, and when you leave it, it's sort of like, oh my, we're leaving. Going home. That's the good news. But it's the same thing as being on the surface. You sort of hate to leave. Sort of like going to a holiday, and the holiday's over and you gotta leave, and you've been in the warm sunshine in, you know, Morocco or Spain or somewhere or France or whatever. You're leaving a holiday. You gotta go home. You get to go home so, it was an old friend."]

[Woods - "And was it good for the soul, going there."]

[Scott - "Mmm, it's a esoteric kind of thing. That's a much longer, esoteric, if you will, discussion. Who knows?"]

[The Mapping Camera has been running since AOS taking images of the receding Moon. One image from this sequence was digitally scanned for the book

Full Moon by the artist Michael Light who has kindly donated a version, number 24 in his collection. This photograph encompasses an entire hemisphere and the major visible features include the crater Humboldt in the centre. Note the dark patches on either side of its floor. Mare Smythii (Smyth's Sea) is the dominant dark feature near the top of the image. This formation is barely visible from Earth as it straddles the equator and the 90°E meridian. It's appearance to the top of this image indicates the southerly extent of Endeavour's trajectory away from the Moon.]

224:22:29 Parker: Be looking for it in three or four days.

224:22:05 Scott: Roger. It's really spectacular though, and there's no question that we're leaving. As a matter of fact, the first glimpse we got, it was quite obvious that we're on the way.

[As they depart, they are snapping photographs on three magazines with three different lenses using the Hasselblads. Many shots are taken and so I have included a representative choice in the journal.]

[The 500-mm lens is used with a black and white film magazine, RR, to spot detail across the visible hemisphere. Though frame

AS15-95-12980 is partially obscured, it manages to view some of the landscape near the Moon's south pole. Major features are labelled in the version included here and of particular note is Vallis Schrödinger, an enormous gash, 310 kilometres long, that cuts clean across Sikorsky crater on its way to a giant crater off the edge of the frame, also named after the Austrian physicist, Erwin Schrödinger, 1887-1961. Many of the craters towards the bottom of this frame have floors filled with mare-type material and are considered part of Mare Australe (Southern Sea), especially Lyot, named after a French astronomer, Bernard Lyot, 1897-1952. The photographer, probably Al, takes a sequence of images across the width of the Moon just inside the terminator where the low-angle lighting shows the relief in the landscape. Seven of these frames - AS15-95-12982 to 12986, 12988, 12991 - have been montaged. Near the top of the image is Mare Smythii (Smyth's Sea), and Humboldt faces the camera near the middle, its dark patches easily visible. The northern terminator is photographed a little later in 12998 with the bottom edge of Mare Smythii visible at the bottom of the frame.]

[Frame

12996 looks directly down to an area of battered highlands southeast of Mare Nectaris. At upper right is Petavius, a large, flat-floored crater 177 km in diameter which is somewhat similar in form to Humboldt with its dark patches and a cleft, Rimae Petavius, which cuts across its floor. These features get their name from a French historian and student of chronology, Denis Petau, 1583-1682. Just right of centre is Stevinus, a 75-km crater named after Simon Stevin, 1548-1620, a Belgian mathematician. The small bright feature to its left is Stevinus A. Below the centre of the image is the 70-km Rheita, named after Anton Maria Schyrleus of Rheita, a Czech astronomer who was a contemporary of Johannes Kepler.]

[Their new vantage now permits them to see Tycho for the first time in the mission. At about 100 million years old, Tycho is not only quite big, with a diameter of 86 kilometres and a depth of 4.5 km, it is also very young in lunar terms and this is borne out by the striking ray system which dominates the Moon's near side, is easily visible from Earth during a full Moon, even with the naked eye, and which is seen in

12997. The crater deservedly takes its name from one of the giants of pre-telescopic astronomy, Tycho Brahe, 1546-1601, whose careful visual measurements of the planetary positions laid a foundation for Johannes Kepler's analysis and conclusion that the planets moved in elliptical orbits, work which forms the basis for the trajectory mathematics that Mission Control now use to guide Endeavour on its journey.]

[Two of the eastern maria are caught with this 500-mm lens.

AS15-95-12995 is of Mare Fecunditatis (Sea of Fertility) showing the Messier pair of craters that the crew recently flew over, and the comet-tail rays that emanate westwards from them. The large crater that dominates the eastern side of the mare is Langrenus. Mare Crisium (Sea of Crisis) is well shown in 12999. From Earth, we are used to seeing this mare heavily foreshortened along its E-W axis. In this picture, the viewpoint foreshortens its N-S axis instead, but it also suggests the truth that Mare Crisium is indeed elliptical in shape along its E-W axis. The ray crater, Proclus, is just to the left and its rays dust the western half of the mare surface.]

[Also in use concurrently is magazine Q containing colour film. Initially the photographer uses the 80-mm (normal angle) lens but appears to be having difficulty getting a good view of the scenery, for the first seven images from this period are heavily masked by the CM hull, as in AS15-96-13081. In this view looking south, Vallis Schrödinger is visible right on the terminator as it cuts across Sikorsky. The mare-filled craters that make up Mare Australe are seen to the upper left. The view does improve for four frames, of which 13085 is a good example, before the lens is exchanged for the 250-mm. Mare Australe is better shown and Humboldt sits upper centre. Having fitted the telephoto lens, the photographer returns to the southern region and a composite of AS15-96-13089, 13090 and 13093 shows Vallis Schrödinger, Lyot and the other constituents of Mare Australe, and at the top of the image the distinct crater with a central peak is the 71-km Jenner. AS15-96-13093 is presented here in a higher resolution courtesy of journal contributor Kipp Teague. Petavius (upper centre) and Stevinus (centre) are the subjects of 13091; the ubiquitous Humboldt is centred in 13092. Four frames, three of which, AS15-96-13095, 13096 and 13097, have been composited, show the north pole of the Moon. Three mare are visible. Mare Crisium is partly visible to the left, at the bottom is the roughly circular outline of Mare Smythii, and above it is the irregular outline of Mare Marginis (The Border Sea) sandwiched between Neper and Jansky to the south. Goddard punctuates its northern edge.]

[Dick Gordon, the backup Commander and someone who knows about orbiting the Moon on Apollo 12 has come on the loop.]

224:23:02 Scott: Looks like we're going straight out, Dick.

224:23:04 Gordon: Yep. Reminiscing for me. Thank you.

224:23:10 Scott: Although, I'll tell you, we never got to see half of what we passed over, I'm sure. There's just so much up there.

224:23:21 Gordon: Well, I'll tell you, Dave, I'm not so sure you guys didn't get at least your share, or maybe a little more. Spectacular.

224:23:28 Scott: Rog. [Long pause.]

Public Affairs Officer - "That was Dick Gordon reminiscing with Dave Scott there. Gordon was backup commander for Apollo 15 and also was..."

224:23:57 Worden: Well, I guess our orbital geologist up here just figured out which way we were. I guess we were sort of momentarily disoriented there, because south is up, and we're looking right up and down the terminator. So I guess we're upside down looking at new territory that we haven't seen during the flight.

224:24:49 Worden: Jim just said "Gee, maybe I ought to look out my window." and, by golly, on his window he's looking up - he's looking up to the north. As a matter of fact, out window number 5 now, you can get a full half - half Moon view. And you can see it all in one big gulp, and boy, what a gulp.

Public Affairs Officer - "Endeavour's 1,300 [nautical] miles [2,400 km] from the Moon now."

[The boom carrying the Mass Spectrometer is deployed about now to sample the particles in the vicinity of the spacecraft. The instrument will be left to outgas for four hours, to be switched on when the crew settle for their rest period.]

224:25:44 Worden: That's a good point; we can't see it yet. [Long pause.]

224:26:27 Worden: Houston, 15.

224:26:29 Parker: Go ahead, 15.

224:26:33 Worden: Okay, Bob, I'm looking from Humboldt straight south now, and, in fact, you can draw a line between Humboldt and the - and a great ditch or scarp to the south [Vallis Schrödinger]. And there's quite a change in the light level or the intensity of the albedo in - between those two areas. And it looks like there's some - you know - very smooth fresh lava flows throughout that area. And, in fact, we noticed that while we were in orbit also, that in several of those areas around Humboldt there, and to the north of Humboldt, and to the east of Humboldt, that the flows in some of the craters there - (And they're quite distinct flows, you can see where they've lapped up against the sides, and you can also see where they've come - where they've spilled down over the sides, sort of reminiscent of the Coso Hills flow) - You can - we looked at these flows and realized that they looked fresh. And yet they had a lot more - the crater count was a lot higher on them than it was on the surrounding terrain. And this flow we're - kind of looking at now looks - it's a very large area, and it looks like it's just filled - seeped into some craters down there, some of the large craters. It looks much fresher in color. It hasn't - it doesn't look like it has been worked up as much. The craters that pit the lava flow are much sharper than they are, say further to the east. But the count seems to be much higher; the crater count seems to be much higher, even here.

224:28:17 Parker: Copy Al. Very interesting.

[Comm break.]

224:31:28 Parker: Well, is that a unanimous vote in the spacecraft, Dave?

224:31:35 Scott: Roger. I got three ayes on that one.

224:31:44 Parker: Roger. We'll add that, and continue to keep the score.

224:31:51 Scott: Good.

224:31:53 Parker: Someday I'll get up there.

224:31:54 Scott: Hey, Bob, we don't report to you the other kind.

224:31:58 Parker: Some day I'll get up there and make my report.

224:32:03 Scott: I hope so. [Long pause.]

224:32:20 Scott: We can see a point on the terminator now where we mentioned we saw a lava filling and some domes in a depression which is not the circular crater-like depression; it's a big cavity. And now, I guess, as the Sun angle has changed some, we can see where the lava has apparently spilled over a scarp into a deeper cavity, which is in - in shadow. And it's very clearly a filling of the cavity with two levels.

224:32:50 Parker: Roger, Dave. We call that a polye, don't we?

224:32:59 Scott: Well, I guess if we were in Hawaii.

224:33:05 Parker: You guys don't see any motion of that stuff, do you?

224:33:12 Scott: Stand by. We'll watch it.

[Comm break.]

224:34:41 Parker: Roger. I think that's unanimous on everyone, isn't it?

224:34:46 Scott: Roger.

[Long comm break.]

224:39:22 Scott: Roger. You've got it.

[Comm break.]

[During the coast between the Earth and Moon, the spacecraft is kept slowly rotating to distribute the Sun's heat across the Command Module's heatshield. This

PTC, or Passive Thermal Control rotation is started once the spacecraft's guidance platform has been realigned to an orientation which is appropriate to the maneuver, so-called PTC REFSMMAT which was also used during the coast to the Moon. First, Mission Control must send the details of the orientation for this REFSMMAT up to the spacecraft.]

224:41:17 Scott: Roger, Houston.

[Long comm break.]

Day 10: Orbital Science	Journal Home Page	Settling down for the Journey Home