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Apollo 15

Splashdown Day

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

[This section covers the thirteenth and final day of the Apollo 15 mission, 7 August 1971.]

[Although moving at only 1,950 meters per second, Apollo 15's velocity has been increasing slowly since it crossed the invisible boundary where the Earth's gravitational pull began to dominate over the Moon's. Slowly at first, Endeavour's velocity will increase rapidly as it is drawn closer to the Earth. Since the spacecraft reached that point 43¼ hours ago, it has gained only 1,100 metres per second in velocity. In the next 8½ hours, its velocity will increase fivefold, to nearly 11,000 metres per second (36,000 feet per second).]

[Flight Plan pages 3-394, 3-395, 3-396, 3-397 and the current page, 3-398.]

286:32:26 Allen: Su Sois, good Endeavour crew. Su Sois. Rise and shine. It's splashdown day.

[According to journal reader Ken Glover, "Su Sois" seems to mean "To be in front of everyone". We would interpret that as "Let all of us here know that you are up and about."]

286:32:43 Allen: Good morning, Dave.

286:32:46 Scott: Good morning, Joe. That got everybody up.

286:32:55 Worden: Morning, J.P.

["J.P." is the nickname of the CapCom, Joe P. Allen.]

286:32:57 Allen: Morning, Alfredo.

286:33:03 Irwin: Joe, sounds like you're really in harmony this morning.

286:33:08 Allen: That's one.

[Long comm break.]

286:39:03 Scott: Okay, Houston; Endeavour. We got a postsleep checklist for you.

286:39:13 Allen: Okay, Dave. Go ahead.

[Immediately after waking, the crew reports their status regarding how they slept and, usually, whether any medication has been taken. They then receive an update from Mission Control of the spacecraft's systems, which includes recording the onboard readings of the oxygen and hydrogen tanks, and the propellant quantity for each of the four Service Module RCS (Reaction Control System) thruster quads.]

286:39:17 Scott: Okay. Okay. About 8 hours apiece on the sleep, and ready for your consumables.

286:39:26 Allen: Roger. At 286 plus 30; RCS total, 36 percent; quad A, 41; [B], 36; [C], 30; [D] 37. H₂ tank, 27, 24 and 29 [hydrogen tanks 1, 2 and 3, respectively]; O₂ tank, 43, 44 and 37 [again, tanks 1, 2 and 3]. I've got the world's smallest list of updates for your Flight Plan, and I've got the news summary when you're ready.

286:40:07 Scott: Okay. Give us a couple of minutes on those. Thank you.

[Very long comm break.]

286:51:46 Scott: Houston, Endeavour.

286:51:52 Allen: Go ahead.

286:51:57 Scott: Well, we just got our first view of the Earth this morning, and, can you believe it's getting larger and it's getting smaller? We see just a very, very thin sliver of a very large round ball.

286:52:20 Allen: Roger, Dave. I believe that.

[Joe Allen isn't simply being kind and responding in agreement to Jim's comment about the crescent Earth. The Flight Plan includes a diagram of the expected view of both the Earth and the Moon at this time.

The Earth takes up about 10° of the astronauts' field of view, about 20 times that of the full Moon as seen from the Earth. During yesterday's lunar eclipse, the spacecraft was well to one side (and south of) Earth's shadow. Now, in the final hours of the flight home, it is passing beneath the shadow, moving towards the Pacific on the eastern side of Earth's disc. By the time they land, that area will have moved around into daylight. Because Endeavour's flight path has carried it over the night-time side of the Earth, only the slightest glimpse of the daylight side can be seen. Most of the "view" is that of the Southern Pacific hemisphere, where it is midnight in Australia and the mid-winter darkness persists over the south pole. The Moon, meanwhile, is nearly full.]

286:52:31 Scott: And, go ahead with the updates, Joe.

286:52:44 Allen: Roger, Dave. These are the Flight Plan updates for today. And - would you believe - I don't have any DAP load changes to give you, which is fortunate. The first addition is at 288 plus 18. And it reads "X-ray to Off, Alpha Particle to Off."

[The Digital AutoPilot (DAP) is currently configured for Passive Thermal Control (PTC). Verb 48 is (11111) (x1111). Translated: CSM only, Plus-X translations using all four jets, 5 degree deadband, maneuvers at 0.2°/second, either Quads A and C, or B and D used for roll, and all four sets of RCS quads are to be used.]

286:53:29 Allen: Roger. And all the rest are just reminders, really. The first one at - on - at 290, on the UV photo's page, you've already changed the two frames line to read one frame at 20 seconds and the second frame at 2 seconds. A reminder to use mag Papa instead of mag Metro. And, finally, we'd like to remind you to enable all jets before beginning the maneuvers today. And we're - we're thinking now we'll likely do a midcourse 7 correction of about - probably around 5 feet per second [1.5 m/s]. Over.

[As Joe Allen notes at 286:58:39, the upcoming midcourse correction became necessary after the previous day's uncoupled thrusting in support of the science experiments. The crew had used single jets to hold their attitude as this gave finer control over what is now a very light spacecraft. To prevent any further degradation of their trajectory, all maneuvers will use all four RCS jets, working in pairs, to reduce the undesired velocity changes that occur with uncoupled maneuvers.]

[Midcourse 7 is formally known as the Corridor Control Burn and is used to fine tune their approach into the narrow corridor of the atmosphere. While Endeavour is still well within the limits for a successful entry, tweaking the trajectory now will ensure that the Command Module will not have to perform extra maneuvers as the computer works to balance the optimal entry profile and targeting to the recovery force. The burn will be performed at 291:58:48.]

286:54:36 Allen: It's also not certain, but we'll keep you posted on that. And that's all the official updates I have for you. I have a news summary, if you'd like to listen.

286:54:46 Scott: Okay. Everybody's [has their headsets plugged] in...; go ahead.

286:54:51 Allen: Roger. And this will be a short one. The rest of it, you can read for yourselves today in the papers. Congress has started a month-long summer recess, setting the pattern for a government-wide exodus likely to make Washington a virtual ghost town for the rest of August. The Senate finally quit at 7:30 Friday night, more than six hours after the House had adjourned at about one in the afternoon. And after passing an 18 billion dollar higher education bill and three key appropriations measures. Besides the Labor-HEW appropriations, the Senate approved, Friday, a $1 billion measure to provide public service jobs mainly for Vietnam veterans and a continuing resolution to fund agencies still without regular appropriations until the 15th of October. In Chile, four [sic] government ministers presented their resignations to president Salvador Allende on Friday, causing the first cabinet crisis since the President took office last November. I have a long list of baseball scores which I think I'll skip over here. In exhibition football, the Buffalo Bills downed the New Orleans Saints, 14 to 10, and the Cowboys won over the L.A. Rams, 45 to 21. In the American Golf Classic at the Firestone Country Club in Akron, Ohio, Jerry Heard is still leading with 7 under par, 133, at the halfway mark. And Bob Lunn is next with a 4 under par, 136. The United States basketball team was eliminated yesterday in the Pan-American games in Columbia, and the U.S. baseball team was upset by the Dominican Republic, 5 to 4. In tennis, Stan Smith is the last seeded player still in competition at the Western Championship in Cincinnati. And today in Chestnut Hill, Massachusetts, Marty Riessen meets Australian Ken Rosewall, and South African Cliff Drysdale meets John Newcombe in the semi-finals of the U.S. professional tennis tournament. And that's all I have from here. Over.

286:58:20 Scott: Okay. Thank you, Joe. That's interesting.

286:58:24 Scott: Hey, Houston, 15. By the way, where did the 5 feet per second come from? Should we get our trusty navigator up there to navigate some more for you?

286:58:39 Allen: Dave, it probably came for - from the uncoupled - thrusting we were doing yesterday. Other than that, I'm not really sure. It's - it's by no means certain. Anyway we're just, I guess - we'll be watching it today and get back with you, with the final bit of information on it.

286:59:01 Scott: Okay. Very good. Just thought maybe we'd get our navigator to navigate again, and it would probably go away, as most of them have done so far.

[Since firing their SPS (Service Propulsion System) engine to get them out of lunar orbit, Endeavour's course has been true. Both the opportunities to refine their trajectory have not been needed. Indeed, midcourse 7 was predicted yesterday to be so small that it might not be worth burning. However, as noted by Allen, the error in their trajectory has built up. Al Worden has been making parallel determinations of their trajectory by making angular measurements between the stars and Earth. He has been so good at it, he jokingly suggested to the flight controllers that if they practice, their answers might eventually converge with his. Normally, the ground's determination of the spacecraft's trajectory is thought to be the more accurate.]

[The forecast of the recovery area is excellent: two layers of scattered clouds - an upper layer of cirrus clouds (usually at about 30,000 feet, 10,000 meters) and a few fair weather clouds with bases at 2,000 feet (700 meters). Note that in the world of flying and aircraft, height above the ground is still (as of the year 2000) stated in feet rather than metres.]

[The reference to "personal minimums" refer to a pilots' self-imposed limits for flying, and are often more conservative than the officially published "minimums". For example, a pilot who has just earned his instrument rating in the United States is legally qualified and permitted to land in 200 foot (60 meter) ceilings (the base of the cloud cover) and one-half mile (800 meter) visibility. Landings in these conditions are sufficiently difficult (try it sometime!) that most new instrument pilots will avoid those conditions, opting instead for an airport which might have 500 foot ceilings and one mile visibility.]

287:00:46 Scott: Sounds like the recovery troops have things in hand as usual.

287:00:51 Allen: Yes, indeed.

[All the while the crew has been checking their systems, recording updates, and listening to the news, breakfast is being served.]

[Very long comm break.]

287:32:13 Scott: Roger. Gamma-ray, Gain Step to center.

287:33:19 Allen: And, Endeavour, this is Houston. SIC over and out.

287:33:29 Scott: Hey, Mr. SIC! Congratulations on a super job all the way. Sure appreciate it.

[SIC: Scientific Instrumentation Controller. Allen was the primary CapCom during Dave and Jim's very intense exploration of the lunar surface. Having trained with them for the event, he had proved to be a very capable and integral part of the team. Dave's praise is deeply felt. In the Apollo 15 Lunar Surface Journal, Dave had much to say about Joe's role in the success of the mission. He and Eric Jones talked about the choice of Joe Allen as the Apollo 15 EVA CapCom.]

[Scott - "We knew he was good."]

[Jones - "And you specifically picked Joe Allen for the job?"]

[Scott - "Absolutely. Without question. Joe Allen was selected to be the lunar surface CapCom because of his capabilities."]

[Jones - "By Dave Scott and Jim Irwin."]

[Scott - "I don't recall how that came about, but when we got around to segmenting out the portions of the mission for the support crew, we made a very clear, definite decision that Allen would be the guy. Not that [Bob] Parker [their pre- and post-EVA CapCom] wasn't good. Parker was excellent. But Joe just had this sort of flair for it, as you can see. I mean he was able to fit in, he picked up the geology very quick, he was a master at diplomacy - on both sides. So it was a very easy decision of the guys we had. Karl Henize was very good. Parker was very good. But, for what we wanted to do with this exercise, Allen was the guy."]

287:33:45 Scott: Okay, very good.

[At about 287:40, the crew is scheduled to exit the PTC barbecue mode when the roll angle equals 71°. Because the time that this roll angle is achieved is so dependent on the time the PTC is started and the actual roll rate, it is impossible to predict ahead of time when the spacecraft will reach the 71 degree mark. In fact, the PTC will not be terminated until 288:27.]

287:55:26 Irwin: Houston, this is 15. [No answer.]

287:55:54 Irwin: Houston, this is 15 with the PRDs - readings.

287:55:59 Parker: Roger, 15. Go ahead.

287:56:03 Irwin: Good morning, Bob. Okay, for Al, it's 25034 and mine is 08041.

287:56:12 Parker: Roger. You got one for Dave?

287:56:16 Irwin: Okay. His is not working any longer.

287:56:19 Parker: Okay.

[Very long comm break.]

[Flight Plan page 3-399.]

[The instruments in the SIM bay have now completed their mission and will be shut down for good. Researchers in the back rooms are ecstatic over the quantity and quality of the data, which will keep them occupied for years.]

[Shutting down the SIM bay is more involved than simply hitting the "off" button. Each experiment is individually powered down and safed. Next, the Gamma-ray boom is partially retracted, and both the Mass Spectrometer and Gamma-ray booms are jettisoned. Although there is no operational requirement to get rid of the booms, this may be a test of their jettison systems. Apollo 15 is essentially an engineering test flight for the SIM bay and every opportunity to test its systems is taken.]

[As is the usual procedure after waking, Al Worden performs an alignment of the IMU (Inertial Measurement Unit) to correct for the drifting which the platform has experienced since the last alignment about 17 hours ago. An Option 3 alignment is performed, which will use the star sightings to establish the spacecraft's attitude accurately in space. Once the attitude is known, the gyros in the platform are "torqued", or rotated, to the reference attitude used for Passive Thermal Control (the PTC REFSMMAT).]

["PTC" orientation is somewhat of a misnomer here, as Endeavour will no longer perform the maneuver for the rest of the mission. To be sure, there are many possible platform orientations that are usable, but as the PTC orientation is satisfactory, there is little need to change it. It is as well suited for the cislunar sightings that follow as any other and has been used for both the inbound and outbound legs of the mission.]

[Obtaining an accurate platform alignment is especially important here. Navigation in cislunar space demands precise measurements of the angles between the Earth (or Moon) and various stars. By starting with a known attitude that has been established accurately, the crew can make highly precise fixes on their position.]

288:33:08 Parker: Apollo 15, Houston.

288:33:12 Irwin: Go ahead, Bob.

288:33:13 Parker: Roger. Correction of the Flight Plan. We'd like to have all crewmembers on the biomed harness for entry. I suspect that means Dave won't be doffing his about now.

288:33:34 Irwin: Okay, I guess we could give you two. The - our trusty CMP has his off and stowed for entry.

288:33:44 Parker: Roger. Copy. [Long pause.]

288:34:12 Parker: Okay, 15. Looks like the surgeons can live with that without too much trouble.

288:34:18 Irwin: Roger.

[As originally scheduled, Dave Scott would be removing his biomedical harness at the same time Jim Irwin would be putting his on, and the monitoring of only Jim would have continued through to entry. The episodes of premature ventricular contractions that both Jim and Dave experienced is certainly the motivation for this request.]

[At this time, a fresh Lithium Hydroxide canister is brought out from storage location A, and the used cartridge is stored in location A5. This will be the final LiOH change of the mission.]

[Long comm break.]

288:37:54 Irwin: Go ahead, Houston.

288:37:56 Parker: Okay. If you guys have a moment or two, we have some Flight Plan updates concerning entry - entry cue cards and Entry Checklist. Over.

288:38:08 Irwin: Stand by.

[Long comm break.]

288:41:24 Parker: Okay, Jim. Give me a call when you're ready.

288:41:29 Irwin: Okay.

[Everyone on board the spacecraft is currently busy with several tasks. Al Worden is performing the IMU realignment and the subsequent cislunar navigation sightings, Dave Scott and Jim Irwin are changing the Lithium Hydroxide canisters and getting reading on the command module RCS thruster temperatures. While some of these tasks are not necessarily time critical, it is far better to get them completed before moving on to discussing a new set of updates.]

[In preparation for the platform alignment, the Digital Autopilot is changed from the PTC configuration ([11111] [X1111] translated: CSM only, plus-X translations using all four jets, 5 degree deadband, maneuvers at 0.2 degrees/second, either Quads A and C, or B and D used for roll, and all four sets of RCS quads are to be used) to the configuration used for platform alignments ([11102][01111] translated: CSM only, plus-X translations using all four jets, 0.5 degree deadband, maneuvers at 0.5 degrees/second, Quads B and D used for roll, and all four sets of RCS quads are to be used) ]

[Long comm break.]

288:46:38 Parker: Roger, 15. We're ready to copy.

288:46:42 Worden: Okay. 5-C is 4.5; 5-D is 4.4; 6-A, 4.4; 4.4; 4.6; and 4.5.

288:46:57 Parker: Roger; copy.

[The system test meter displays parameters such as temperatures and pressures of many spacecraft systems.

Located in the CM's lower equipment bay, it consists of two rotary switches labeled A through D, and 1 through 6, plus a setting for the transponder. A voltmeter with a scale of 0 to 5 volts provides the output display. By selecting a combination of the two switches, a wide variety of readings can be obtained. When a specific reading is desired, such as the pressure in the oxygen manifold for fuel cell 3, the crewmember selects 2-B on the selection switches. The pressure is then displayed on the meter as volts, which will be converted to PSI by technicians on the ground.]

[The use of a voltmeter is perhaps unfortunate, as it doesn't allow a simple interpretation of the values presented. Documents on board the Command Module do provide suitable conversions for the crew. The awkwardness of obtaining this information is lessened though, as these parameters are not those which require constant monitoring. Using the system test meter also frees up valuable real estate on an already overcrowded Main Display Console.]

[The various setting for the system test meter are:

• 1-A: N₂ pressure Fuel Cell 1
• 1-B: N₂ pressure Fuel Cell 2
• 1-C: N₂ pressure Fuel Cell 3
• 1-D: O₂ pressure Fuel Cell 1
• 2-A: O₂ pressure Fuel Cell 2
• 2-B: O₂ pressure Fuel Cell 3
• 2-C: H₂ pressure Fuel Cell 1
• 2-D: H₂ pressure Fuel Cell 2
• 3-A: H₂ pressure Fuel Cell 3
• 3-B: Radiator outlet temperature Fuel Cell 1
• 3-C: Radiator outlet temperature Fuel Cell 2
• 3-D: Radiator outlet temperature Fuel Cell 3
• 4-A: O₂ Tank 3 Quantity
• 4-B: O₂ Tank 3 Pressure
• 4-C: H₂ Tank 3 Quantity
• 4-D: H₂ Tank 3 Pressure
• 5-A: SPS Oxidizer line temperature
• 5-B: Battery relay bus voltage
• 5-C: Injector valve temperature negative pitch thrusters
• 5-D: Injector valve temperature positive yaw thrusters
• 6-A: Injector valve temperature counterclockwise roll thrusters
• 6-B: Injector valve temperature positive pitch thrusters
• 6-C: Injector valve temperature negative yaw thrusters
• 6-D: Injector valve temperature clockwise roll thrusters
• 7-A: Battery compartment manifold pressure
• 7-D: CSM to LM Current
• Transponder-A: Transmitter power output
• Transponder-B: AGC Signal
• Transponder-C: Frequency lockup
• Transponder-D: Not used

The six valves read by Al Worden are for the valve temperatures of the Command Module RCS jets; five sets of two thrusters are arranged around the base of the CM, and another set of two thrusters at the apex above the main hatch. In order, the thrusters reported are negative pitch, positive yaw, counterclockwise roll, positive pitch, negative yaw, and clockwise roll.]

[Comm break.]

288:48:34 Irwin: Okay. We copy.

[The retrograde nature of this burn implies that the spacecraft is coming in slightly too fast and requires a burn against their direction of travel. If left unchecked, their re-entry would be slightly shallower than nominal, still within an acceptable range but requiring more correction during the passage through the atmosphere. As highlighted by the PAO announcer, there is also the interplay between the time of the CM's arrival and the rotation of the Earth. Their current error will have a profound effect on where they land. They want to get as close to the recovery forces as possible and at this stage, it is easy to get it right - and so they will.]

[Long comm break.]

288:58:05 Parker: 15, Houston. We have your torquing angles.

288:58:10 Scott: Roger; thank you.

[It is normally standard procedure for the crew to radio the ground the results of the platform alignment, which includes the amount of drift that has been detected and corrected in the platform, and a measure of the accuracy of the star sightings themselves. At the end of the P52 IMU alignment program, the amount of drift in all three axis that the platform has experienced (and is corrected for by the realignment process) is displayed on the DSKY (Display and Keyboard).]

[When the High Gain Antenna is being used (as is now), a higher telemetry data rate can be supported, which allows for more parameters to be relayed to the ground. Among the items that are included in the additional data are the DSKY displays. Rather than having Al go through the process of formally reading the DSKY, the Guidance Officer has informed Bob Parker that they can see the DSKY, and Al does not need to read the display to the ground. Two displays are of particular interest to the guidance team: Noun 96, which indicates the amount the gimbals have drifted, and hence, the amount they need to be torqued to return to proper alignment, and Noun 05, a quantity which indicates the overall accuracy of the star sightings.]

[Al's latest set of marks were dead on - a measured error of 000.00 degrees as reported by Noun 05 on the DSKY. Usually called out as "five balls", it is the best possible accuracy that the sextant is capable of resolving.]

[Long comm break.]

[Flight Plan page 3-400.]

289:01:31 Parker: Apollo 15. Request Omni Charlie, please.

289:01:37 Worden: Roger. Omni Charlie.

[Very long comm break.]

[Now that the guidance platform is accurately aligned to a known orientation, the business of determining the spacecraft's position and velocity can begin.]

[Al will carry out two navigational exercises today using P23 in the computer, both of which will use the Moon as the local reference for measuring their current position. Yesterday, all four of Al's P23s were done with reference to the Earth but now they are approaching the home planet so fast, it does not give a consistent reference as Al tries to take multiple marks. In this exercise, he first calibrates the spacecraft's optical system by taking 5 marks on Alpheratz (Alpha Andromedae). Normally, he takes just a couple but yesterday, Mission Control specifically requested he take 5 marks. He then maneuvers to the correct attitude for the navigational sightings. The exercise consists of Al measuring the angles between selected stars and the Moon. Specifically, Al uses the limb of the Moon either nearest the star or opposite the star, depending on where the terminator is with respect to the star.]

[The measurements are between Altair (Alpha Aquilae) and the Moon's far horizon, Delta Capricorni and the Moon's far horizon, Beta Pegasi and the Moon's near horizon, Markab (Alpha Pegasi) and the Moon's near horizon, and finally Gamma Pegasi and the Moon's near horizon. The results are used by the computer to check that the state vector (the spacecraft's velocity and position at a specified time) would lead to the measured angles and, if not, what change needs to be made to the vector's position (Delta-R) and velocity (Delta-V). These values can be accessed by Al on the DSKY (Display and Keyboard).]

289:32:04 Scott: Houston, Apollo 15. We're ready for your entry updates now if you like.

289:32:11 Parker: Roger, 15. Understand you're ready for the update. Tell Al that was another super set of marks, certainly.

289:32:17 Scott: Yes, that one came out pretty fair, didn't it?

289:32:24 Parker: Okay, 15. The first change is in the Entry Checklist, second page; and, on page 1-2, we wish to delete step 21, which is the "DSKY Condition Light Test."

289:32:46 Scott: Okay. Step 21 deleted.

289:32:48 Parker: You may remember that one from the sim[mulator]; that's the one with the - gives us the P35 and turns the PIPAs off momentarily.

289:32:55 Scott: Yeah, I guess we don't want to do that today. [Garble]...

[The details of this reference are, as they say, "lost to history", but it appears that the DSKY light test has a bug in it that might put the computer into Program 35 (a rendezvous program) and shut off the IMU's accelerometers. Needless to say, this is not a good thing.]

[It is often said that the computers and other spacecraft components perform "flawlessly". While this is usually true, there are often many problems that the ground and crew are aware of that might limit a system's full range of capabilities. If a problem is discovered, and a workaround is found, an "operational limitation" is noted. So long as the system is not operated outside this limitation range, there should be no problems. Such limitations are not exclusive to spacecraft. In an aircraft owned by one of the Journal's editors (O'Brien), a problem was found in the engines' rocker arms that was known by to the engine manufacturer. The manufacturer specified that the ignition timing be changed, which reduced the power output of the engine somewhat. With this limitation in place, no other restrictions as to engine operation were required.]

[Operational limitations often inspire the retelling of the old joke:

Patient: Doctor, whenever I do this (gesturing), it really hurts!

Doctor: Well, don't do that, then!]

289:33:29 Scott: Okay. SPS Pilot Valves Main A and B, both Open, and - Configuration of the circuit breakers on panel 8. Go ahead.

289:33:36 Parker: Okay. Then if we go down to the entry cue card, down to the P67 section.

289:33:44 Scott: Okay. Go.

289:33:45 Parker: Okay. Down there near the middle where it says "Steering commands downrange error minus 6 to 0," that should be changed to "Downrange error minus 24 to 0." Over.

289:34:01 Scott: Rog. Just like it is in the checklist. Right? And I think we noticed that last night looking it over.

289:34:06 Parker: Okay. And next, on the second line under the Noun 68, there's a comment that says "negative [if plus: EMS]." And there's a certain amount of happiness with that statement down here. They say that you can have a positive H dot in P67 nominally and, therefore, the statement on the cue card that this is a fail indication is not a good idea. I understand [Apollo 15 backup CMP] Vance [Brand] discuss[ed] this with Al beforehand.

289:34:39 Scott: Okay. We'll scratch that. I don't think we'll get to a P66 - 67 turnover anyway, but we'll scratch that one out. Thank you, Bob.

[The "P66 to P67 turnover" refers to the two computer programs that might be executed at a point in the middle of in the entry phase. Program 66 is selected when the drag on the CM falls to a predetermined low level, effectively the point where it has left the sensible atmosphere, and is used to maintain attitude and provide targeting. P67 is then called when the CM slams back into the atmosphere again.]

[Normally P66 would not be executed, for it is used only when the CM has "skipped" out of the Earth's atmosphere, perhaps due to an entry angle that was too shallow, or if too much upwards lift is applied during the early part of the entry phase. One scenario is that this would be a planned maneuver, for example, to "stretch" the entry to avoid poor weather. On the other hand, if the entry angle were too shallow, the CM would swing around the Earth to a several thousand mile apogee and re-enter the atmosphere after a full earth orbit. Under worse case conditions, the spacecraft's power and oxygen supplies would likely expire long before the entry occurs.]

[Noun 68 (used in conjunction with Verb 16) is used during entry, and displays the bank angle (roll) of the Command Module, its velocity, and altitude rate. The procedure that Bob Parker refers to describes the kind of entry where the spacecraft is gaining altitude relative to the Earth's surface, such as the case where it has skipped off the top of the atmosphere.]

289:35:22 Scott: Okay. We'll make sure of that. And I guess the problem with R-12 was that, once we got all the LM data onboard, we didn't have any place to put it, or else we didn't have any place to put the LM data. And if you have any better suggestions on where to put them, we'll be glad to do it.

289:35:39 Parker: I don't think we have any, right now, Dave. I guess the quickest thing would have been to just put it in the PGA bag with the hooks pointing up.

289:35:48 Scott: Okay. We'll do that.

[The "LM data", or more formally, the LM Flight Data File, is a set of systems and operations manuals and checklists for the LM. Rather than leaving documents that have no further relevance to the mission in the LM when it was jettisoned, Dave and Jim have brought them back to the CSM with them. They carry written information that the crew collected while operating the equipment and flying the spacecraft which may be useful when debriefing the mission. Most certainly, they have also been saved as a memento of the voyage.]

[The concern here is that this large and heavy stack of manuals (easily a foot thick and weighing 10 - 15 kilograms) might be resting on hooks used to secure the space suits. This weight will increase severalfold - to as much as 100 kilos - during a nominal 6-G entry. Parker is expressing worries that the flight data file might be resting on the hooks, which might punch through the aft bulkhead due to the weight of the documents. Such a puncture is serious, as it would result in the depressurizing of the Command Module. The easiest solution appears to be rearranging the material so that the hooks are not pressing against the bulkhead.]

[Scott, from the 1971 Technical Debrief - "We had all the entry stowage done, almost all of it done, the day before entry. On entry day, we found ourselves with an awful lot of time on our hands, which I think was a good idea. Everything was cinched down tightly, and I thought the ground pretty much agreed with where we put everything. I thought they had a fairly good handle on our stowage locations, and we stowed just exactly like we had it on the stowage map. Our comment going into the entry was, 'Gee, we've never had this much time in a simulation before.' We sat there and coasted along."]

289:36:03 Scott: Okay. No back lighting for the scroll and no lighting on the roll bug. Thank you. [Long pause.]

[Two sets of lights within the EMS (Entry Monitor System) not operating due to the short that tripped a circuit breaker earlier in the mission, at 033:48:00. As there was no clear explanation for the circuit breaker tripping, the ground has advised the crew to leave the circuit open for the duration of the mission. Only a few items are affected, including the EMS's scroll backlighting and the roll indicator. This should not present a serious problem for the crew, as the lighting is only to enhance the display's visibility.]

289:36:30 Parker: No, Dave. That's all I have for the moment.

289:36:33 Scott: Okay. That's pretty easy.

[Comm break.]

289:38:23 Parker: Stand by, Jim. We'll see if they're ready. [Long pause.]

289:38:46 Parker: 15, Houston. We'll have to stand by for another few hours to get you close enough to do that VHF comm check.

289:38:54 Irwin: All right. Roger. We understand. [Long pause.]

[The VHF communications system is intended only for short range communications and is limited to only a few thousand kilometres. Right now, Endeavour is nearly 75,000 kilometres from Earth - too far for reception. The VHF comm check will be performed about 6 hours from now at 294:25:05. At that point the spacecraft will be only 10,000 kilometres from the Earth.]

[The crew have begun using the Entry Checklist and are midway down page 1-1.]

289:39:57 Irwin: Houston, 15. Go.

289:39:59 Parker: Roger. Could we have a reading of the - an onboard reading of the potable tank quantity, please?

289:40:09 Irwin: 82 percent.

289:40:11 Parker: Copy; 82 percent.

[Comm break.]

289:42:11 Scott: Houston, 15. Go.

289:42:13 Parker: Roger. Could you guys check the Potable Tank Inlet valve again for us, and find out whether it is in the closed or open position? Once again, what we're seeing is the waste tank increase and the potable tank has been staying constant all morning.

289:42:30 Scott: Okay; last night when you called, I even went down and recycled that valve and made sure it was in the detent in the Open position, which it was.

289:42:38 Parker: Okay. In that case, could you go down and cycle it from closed to open again for us, please?

289:42:45 Scott: Roger. We're doing that right now. [Long pause.]

289:43:08 Scott: Okay, it was still open, and Al cycled it from open to closed and back to open.

289:43:15 Parker: Okay. Thank you. It's no big deal, Dave, nothing to worry about.

289:43:20 Scott: Might as well get everything all trimmed up.

289:43:22 Parker: That's what we're trying to do.

[Water is created as a byproduct of the reaction used to create electricity in the fuel cells. Some of the water is piped to the Command Module, where it is stored in the Potable Water Tank and used for drinking, food preparation and washing. However, far more water is created by the fuel cells than could ever be consumed by the crew. Some is stored in the Service Module, where it is used to cool equipment, and the remainder is shunted to the waste water tank, which is regularly dumped overboard.]

[The problem at hand is that the waste water tank is filling up, which would be the case where the potable water tank is full or the inlet valve to the tank is closed. Since the crew has verified that the inlet valve to the potable water tank indicates it is open, and the tank itself is not quite full, engineers on the ground will have to come up with other solutions as to why the waste tank is filling. If the waste tank were to fill, and overboard dump will be required. Dumping waste water is not a difficult procedure, but it will perturb the trajectory slightly, and the water vapor cloud that results will disrupt the tracking of the spacecraft somewhat. As the spacecraft is so close to the Earth, and the parameters for the next midcourse correction have already been calculated, any alteration of the trajectory is best avoided.]

[From the 1971 Post-flight Mission Report - "The potable water tank quantity began to decrease during meal preparation at approximately 277 hours and failed to refill for the remainder of the flight. The waste water tank continued to fill normally and, apparently, accepted fuel cell water for this period. A similar occurrence had been noted earlier, at 13½ hours, when the potable tank quantity decreased as the crew used the water, and remained constant until a waste water dump was performed at 28½ hours. This decrease had been attributed to a closed potable tank inlet valve until the crew verified in their debriefing that the valve had been open during this time. The amount of water drained from the tank verified that the tank instrumentation was reading correctly.]

["During a postflight fill operation, with the waste tank inlet valve closed, and water introduced at the hydrogen separator, both the potable and waste water tanks filled.]

["The check valve between the fuel cell and waste tank dump leg was tested and found to leak excessively. A tear-down analysis of the check valve was performed and a piece of 300-series stainless steel wire (approximately 0.0085 by 0.14 inch [0.2 by 3.5 mm]) was found between the umbrella and the seating surface. This contaminant could cause the umbrella to leak and yet move around sufficiently to allow adequate seating at other times. The wire most probably came from a welder's cleaning brush and was introduced into the system during buildup. Safety wires and tag wires are of a larger diameter than the one found. The check valve at the potable water tank inlet is of a different configuration and is spring loaded closed. The 1-psi pressure required to open this valve is a large pressure drop compared to the other components at the low flow of 1-1/2 lb/hour, and would, therefore, cause the water to flow to the waste tank.]

["The potable water tank inlet check valve was found to be contaminated with aluminum hydroxide, a corrosion product, of aluminum and the buffer. The potable water tank inlet nozzle was clean and free of corrosion. The check valve corrosion is not believed to have caused the problem, but could have contributed by increasing the crack pressure of the valve."]

[Very long comm break.]

290:00:24 Parker: Apollo 15, Houston. Requesting Omni Delta.

290:00:28 Worden: Omni Delta.

[Flight Plan page 3-403. UV photography procedures are on page 3-402 of the Flight Plan.]

[Omni-directional antenna D, one of four low-power S-band antennae around the periphery of the Command Module, is used for communications with Earth while Endeavour is oriented correctly for UV photography. This session follows the pattern of previous periods of UV photography where a Hasselblad body is mated to a specialised UV-transmitting lens and mounted in window 5, itself fabricated from quartz to pass UV. The spacecraft's motion rates are stabilised before photography begins.]

[Eight frames are taken, AS15-99-13491 to 13498, two each through four filters, though only seven appear in the Index of 70-mm Photography. Samples of the sequence include AS15-99-13491 taken through filter 1. When compared to earlier photographs, for example 13487 taken yesterday, the increasing angular size of the Earth is readily apparent. However, the characteristics of the filter contribute a lot of background flare to the image. Frame 13495 is a much clearer shot by being taken through filter 3, while the flare becomes apparent again by 13498 through filter 4. The large, circular artifact is probably a reflection of the camera lens in the window.]

[Once the UV photography of the Earth is complete, a colour image is taken on colour film through the same combination. This is frame AS15-96-13136.]

[Long comm break.]

290:08:24 Worden: Omni Charlie.

[Long comm break.]

[Their maneuvering is taking antenna D away from Earth so C is to be selected to continue clear communications.]

[The final task in their program of UV photography is to take images of the Moon using the same methodology as for the Earth which will help calibrate the results of the experiment. These eight images are AS15-99-13499 to 13506. Those taken through filter 2 are not available and this may be due to them being essentially black. Examples from the others are displayed in this compilation of 13499, 13503 and 13505.]

[This is mistaken. Endeavour is at the correct attitude for the lunar UV photography. Soon, it will be maneuvered to another attitude for realigning the platform and afterwards, to yet another attitude for carrying out the midcourse 7 burn.]

290:15:27 Parker: Roger.

[Very long comm break.]

290:25:39 Worden: Okay, and we are finished with the UV photos.

290:25:42 Parker: Okay; that takes care of that one.

[Comm break.]

290:27:41 Scott: Go ahead, Bob.

290:27:42 Parker: Okay; if you guys got a minute, we can do a few things here. Number 1, if you give us Accept, we'll send you some state vector and target loads; and, number 2, we have a couple comments to read to you.

290:27:58 Scott: Okay; you have Accept, and stand by.

290:28:01 Parker: Roger; you're getting the uplink, and we're standing by.

[Long comm break.]

[The latest state vector, midcourse correction targeting information, and the updated PTC REFSMMAT have been sent to the onboard computer. These will be used for midcourse correction 7, scheduled for about two hours from now. ]

290:32:01 Scott: Roger. Thank you.

290:32:13 Scott: And, Houston, we're all on. You can go with your comments.

290:32:17 Parker: Okay; first one, news concerning the water business. We've been having you check that valve. It looks to us now as though there's a blockage into the potable tank. You got obviously - a - far more water than you need to survive the rest of the mission, but it does mean that the waste tank will be filling up, and there is a possibility that it will start to vent about an hour or two before EI [Entry Interface]. It's been discussed down here and decided that the best way to do this is - far as not perturbing the vector too much - is to let it relieve overboard and not to do a dump ahead of time or turn the water boiler on ahead of time. In line with this, we might just verify that the Pressure Relief valves down on panel 352 - the water control panel, is in the Relief position. It certainly should be there; it's just a - a little verification to make sure we're - we're not going wrong there. Second, I...

290:33:11 Scott: Okay, that's verified.

290:33:12 Parker: Okay; thank you. Second item is, we suggest that it might be reasonable to put some tape over the SPS light on the EMS to keep you from confusing with the .05g light, in case you have a problem there, Al. That's your option, obviously. It just a suggestion.

290:33:32 Scott: Oh, I don't think he'll have any confusion with that.

290:33:35 Parker: Okay.

290:33:36 Scott: We'll make sure we watch it.

[The SPS light, located on the Entry Monitor System panel, is illuminated any time there is power applied to the Service Propulsion System engine. Adjacent to this light is the 0.05G light on the EMS panel, which comes on when the spacecraft first hits the outer fringes of the Earth's atmosphere. Entry into the atmosphere is defined as when the drag on the spacecraft increases to a point where the deceleration reaches 0.49 meters/second.]

[The suggestion being passed up is for Al to cover the SPS light with some tape so as not to confuse it with the 0.05G light. Certainly an overly cautious, and well-intentioned request from the "back room"; Bob Parker is probably obligated to pass it along to the crew. Far from interpreting the request as patronizing (given the importance of the light and the many of hours training with the EMS), Dave Scott simply laughs it off.]

290:33:38 Parker: And the third question is, from your comments when we were talking about R-12, some people believe you may be asking us a question that you have a problem with stowage of some of the extra LM data file, or have you found a place for that already?

290:33:53 Scott: Oh, no; that's all tucked away in R-3 very - very neatly. We have no - no problem at all in the stowage. We just wanted to locate R-12 in a nice soft spot and secure it down, which it is.

290:34:04 Parker: Okay. We - we were - [ready to] crank up building 45 to find you a location if you needed it. [Pause.]

[Building 45 houses the Command Module and Lunar Module simulators. Bob Parker's somewhat tongue-in-cheek reference was that the a team was ready to start a full simulation run just to find a location for the extra LM Data Files.]

290:34:20 Parker: And, 15; Houston. We've got an entry PAD and a midcourse 7 PAD if you're ready to copy.

290:34:28 Scott: Stand by one, Bob. Let our pad leader get out his pencil. [Long pause.]

[The joke is that there is a person at the launch pad called the Pad Leader, namely Guenter Wendt - a celebrated character - who coordinated a team of technicians during the final moments before the crew were left to themselves atop the launch vehicle. Dave is playing with the title as one of Jim Irwin's roles is to copy down information read up from Earth, namely the Pre-Advisory Data or PADs.]

290:35:02 Parker: Okay. Purpose, midcourse 7. RCS/G&N; 26363; Noun 48's are NA and NA; 291:56:47.90; minus 0005.6; minus all balls, plus 0000.2; roll, 180, 311, 000; H_A, NA; H_P, plus 0022.3; 0005.6, 0:24, 0005.6; 31, 347.9, 35.3. The rest of the PAD is NA. GDC align stars are Vega and Deneb; roll, pitch, and yaw for the alignment are 100, 137, 316. Burn recommendation is 2 jets, plus-X, quads Bravo and Delta. And the H_P in the PAD on Noun 44 there is based on a MSFN trajectory after midcourse 7. Over.

[All these details are copied onto a standard form with the format predefined so as to reduce errors. The numbers will be fed into Program 30 in the computer as part of the pre-thrusting procedures for the burn.]

[Interpretation of the Midcourse correction #7 PAD, also known as the Corridor Control burn PAD follows:

Purpose: The burn is a small one to fine tune the spacecraft's trajectory and ensure it re-enters in the centre of the permitted corridor.

System: The Guidance and Navigation system will control the burn which will be performed using the RCS jets, not the SPS engine.

CSM weight (Noun 47): 26,363 pounds (11,958 kilograms).

Pitch and yaw trim (Noun 48): Since the RCS thrusters are being used for the burn, the gimbals for the SPS engine are not needed. Therefore there is a "NA" (Not applicable) entry in Noun 48.

Time of ignition, T_ig (Noun 33): 291 hours, 56 minutes, 47.90 seconds.

Change in velocity (Noun 81), fps (m/s): x, -5.6 (-1.7); y, 0.0 (0.0); z, +0.2 (+0.06).

Spacecraft attitude: Roll, 180°; Pitch, 311°; Yaw, 000°.

H_A, expected apogee of resulting orbit: Since the spacecraft is moving faster than the escape velocity of the Earth and is not in a sensible orbit, it makes no sense to define what the apogee resulting from the burn would be. Hence, this entry is NA.

H_P, expected perigee of resulting orbit: If the Earth lacked an atmosphere, the midcourse correction would bring the spacecraft to within 22.3 nautical miles (about 41.3 km) of the surface (Of course, the atmosphere does get in the way!).

Delta-V_t (Total velocity change): 5.6 feet/second (1.7 meters/second).

Burn duration or burn time: 24 seconds.

Sextant star: The star used for sextant sighting is number 31 (Arcturus or Alpha Boötis) which will be visible through the sextant when its shaft angle is 347.9° and its trunnion angle is 35.5°.

GDC align stars: Star number 36 (Vega, in the constellation of Lyra) and 43 (Deneb, in Cygnus) to be used for GDC align if the IMU cannot be used. The align angles are 100° in roll, 137° in pitch and 316° in Yaw.

It is recommended that the burn be made using the rearward facing thrusters (Plus-X) on two RCS quads, B and D (Bravo and Delta).]

[The "phonetic alphabet", where words are substituted for letters to enhance understanding during radio communications, was updated during the middle of the 1960's. Previously, the sequence began Able, Baker, Charlie, Dog, Easy and so on. Many letters were changed, and the alphabet began Alpha, Bravo, Charlie, Delta, Echo... Despite the "official retirement" of many of the words, old habits die hard, and a mix of the two systems was frequently heard.]

290:38:18 Irwin: Okay. Stand by. [Long pause.]

290:38:34 Irwin: Okay. I'm ready for entry, Bob.

290:38:36 Parker: Okay. Entry, area, mid-Pac; 000, 153, 000; 294:41:55, 267; plus 26.13, minus 158.13; 06.1; 36096, 6.49; 1082.4, 36178; 294:58:55; 00:28; Noun 69s are NA; 4.00, 02:13; 00:18, 03:37, 07:42; 04, 140.3, 37.5; 213, down 09.5, right 4.7; lift vector, up. Comments: 1, use non-exit EMS pattern; 2, RET for 90K, 6 plus 04; 3, RET for mains, 8 plus 30; 4, RET for landing, 13 plus 27; 5, constant g, roll right; 6, moonset, 294:56:37. Over.

[This is the preliminary entry PAD, based on the best estimate of tracking at the time. Its provision hedges against the remote possibility of a communication problem later. It will be revised slightly after the midcourse correction burn #7 is evaluated. While the PAD, as read to the crew, is essentially a string of numbers, as with the previous PAD, there is a very strict format associated with it. Both astronauts on the ground and in the spacecraft are using a form that contains all the necessary details for interpreting the numbers.]

[Before interpreting the PAD, it may be useful to explain some of the terminology used. The first important concept is that of Entry Interface, a completely arbitrary event which is defined as the time when the spacecraft (then consisting of only the Command Module) reaches an altitude of 400,000 feet (65.83 nautical miles, 121.92 km). It is chosen as it is easily calculated from the spacecraft's current state vector and does not depend on the vagaries of the atmosphere.]

[A second concept is the 0.05g event. This is the point at which the increasing drag of the atmosphere's outer fringes cause a deceleration measuring a twentieth of a g. For calculation sake, prior to entry actually occurring, it is taken to occur at an altitude of 297,432 feet (48.95 nautical miles, 90.66 km) but it will occur when the spacecraft's guidance system detects a change in velocity of 0.49 m/s² (0.05g). It triggers a change in the entry program run by the computer and also begins the monitoring of the trajectory by the EMS (Entry Monitoring System).]

[Note that some events in the PAD are tied to the Entry Interface event, others to the 0.05g event.]

[The data passed up for the entry PAD is interpreted as follows:

Purpose: Entry.

Landing target: The landing target is in the Mid-Pacific.

IMU gimbal angles required for trim at 0.05g: Roll, 000°; pitch, 153°; yaw, 000°.

Time of the horizon check: 294 hours, 41 minutes, 55 seconds GET.

Spacecraft pitch at horizon check: 267°. This is 17 minutes before time of entry.

Splashdown point: 26.13° north latitude, 158.13° west longitude.

Maximum number of g's during entry: 6.1.

Velocity at Entry Interface (400,000 feet altitude): 36,096 feet/second (11,002 meters/second).

Entry flight path angle at Entry Interface: 6.49°.

Range to go to splashdown point from 0.05g event: 1,082.4 nautical miles (2,004.6 km).

Predicted inertial velocity at 0.05g event: 36,178 feet/second (11,027 meters/second).

Time of Entry Interface: 294 hours, 58 minutes, 55 seconds GET.

Time from Entry Interface to 0.05g event: 0:28 (seconds).

Planned drag level (deceleration) during the constant g phase: 4.00g.

Time from Entry Interface until their velocity slows sufficiently to allow a circular orbit around the Earth: 2:13.

The practical implication of this is that this is the "capture point" where the CM will no longer be able to skip off the atmosphere. Since the spacecraft will already be within the Earth's sensible atmosphere at this point, drag will continue to slow the spacecraft and the return to Earth is assured.

Time from Entry Interface that the communications blackout begins: 0:18.

Time from Entry Interface that the communications blackout ends: 3:37.

Time from Entry Interface that the drogue parachutes will deploy: 7:42.

Sextant star: 04 (Achernar, Alpha Eridani.)

Sextant shaft angle at Entry Interface minus 2 minutes: 140.3°.

Sextant trunnion angle at Entry Interface minus 2 minutes: 37.5°.

The next three items refer to an attitude check made using the COAS sighted on a star two minutes before Entry Interface.

Boresight star: 213. Since this is not a recognised star number from the Apollo list, we have yet to learn what it refers to.

Boresight Star pitch angle on COAS: Down 9.5°.

Boresight Star X position on COAS: Right 4.7°.

Lift vector at Entry Interface: Up.

Comments in addition to the PAD:

The non-exit EMS pattern is to be used, that is, the entry is not expected to skip off the atmosphere and re-enter.

90,000 feet (27.4 km) will be reached at 6:04 after Entry Interface.

Time of main parachute deployment: 8:30 after Entry Interface.

Time of landing: 13:27 after Entry Interface.

When maneuvering to ensure a constant g force (done after the max-g portion of the entry to assure a constant deceleration), the crew is to roll right.

Moonset (used to provide one more check of the entry progress): 294:56:37.]

290:39:15 Irwin: Okay, Bob. Readback for the entry PAD. It's mid-Pac, 000, 153, 000; 294:41:55, 267; plus 26.13, minus 158.13; 06.1; 36096, 6.49; 1082.4, 36178; 294:58:55; 00:28; Noun 69 is NA; 4.00, 02:13; 00:18, 03:37, 07:42; 04, 140.3, 37.5; 213, down 09.5, right 4.7; up. Comments: Use non-exit EMS pattern; RET for 90K, 6 plus 04; for mains, 8 plus 30; landing, 13 plus 27; constant g, roll right; moonset, 294 plus 56 plus 37. Over.

290:42:37 Parker: Roger. Good readback, Jim. [Long pause.]

290:43:12 Parker: And, 15, we'd like Accept again. It looks like we found some errors in the load that we sent up.

290:43:20 Irwin: Okay; stand by. [Pause.]

290:43:25 Irwin: Okay; you have Accept.

290:43:27 Parker: Thank you.

290:43:44 Irwin: Houston, 15. Picked off the Verb keys zero - on top of you. Sorry about that. It's all yours.

290:43:50 Parker: Okay. [Long pause.]

290:44:11 Parker: 15, we'll clear it. Don't worry.

290:44:16 Irwin: Okay.

[Long comm break.]

290:48:57 Irwin: Roger, Bob. [Long pause.]

290:49:49 Parker: Apollo 15, Houston. We would like the VHF turned on even though it's too early to do the comm check. [We would] like it turned on in Simplex Alpha to warm it up, and so we can watch it.

290:50:12 Irwin: Okay, we're turning on Simplex Alpha.

290:50:39 Parker: Thank you, 15.

[Throughout the entire flight, Apollo 15 has communicated with the Earth through the S-band communications system, through a combination of the single high gain and the four omni-directional antennae. S-bands' high frequency (in the gigahertz range) is excellent for long-range communications, but its high power requirements and the highly directional nature of its signal make it impractical for use during entry. The VHF system, previously used for communication between the CSM and the LM, will handle the communication tasks between the spacecraft and the recovery force.]

[Unlike today's radios, which use frequency synthesis through a digital circuit, the 1960's vintage Apollo radios were purely analog devices, and required a period of warm-up to stabilize the circuitry.]

[Also in preparation for re-entry, the crew are donning Mae Wests in case they have to leave the spacecraft in a hurry after landing.]

[A P52 option 3 is being performed, using the PTC orientation. This realignment of the inertial platform is in preparation for the final time that its orientation is changed. Throughout the flight, the platform has been kept in one of a number of defined orientations, eight in all. Most platform realignments are simply to compensate for the drift that occurs in the platform's orientation over a period of time. Every so often, however, when he needs to swing the platform from one orientation to another, Al carries out a double P52. The first of these is simply to determine the platform's drift since the last P52, an important part of understanding the mechanical characteristics of the IMU in case this has a bearing on subsequent flights.]

[The second P52 is intended to swing the platform around to a new orientation. In this case, option 1 is being used which means that rather than being aligned to a REFSMMAT, it is aligned to an orientation which has been calculated to be relevant to an upcoming maneuver - in this case, re-entry. The practical upshot of this is that their FDAI (Flight Director Attitude Indicator, or "8-ball") will display angles which relate simply to their entry trajectory.]

[For both of these P52s, Al uses star 01 (Alpheratz or Alpha Andromedae) and star 10 (Mirfak or Alpha Pegasi) and achieves zero measuring errors each time. Soon, when he completes the first P52, he will bring up the torquing angles on the DSKY from where Mission Control can see them.]

[Long comm break.]

[Flight Plan page 3-405.]

291:00:39 Scott: Roger.

[Long comm break.]

[Al is now carrying out the second P52 to realign the platform to the entry orientation, known as the Entry REFSMMAT.]

[For this orientation, the IMU's X-axis is aligned along the direction of flight but parallel to the local horizontal as exists at Entry Interface. Its Z-axis is parallel to, but in the opposite direction of the vector that comes from the Earth's centre to the point of Entry Interface. The upshot of this arrangement is that if, at Entry Interface, the spacecraft were to have its heatshield pointing forward, its X-axis parallel to the local horizontal, its "wings" level and with the crew "heads down" the FDAI would display roll 0°, pitch 180° and yaw 0°.]

291:03:53 Parker: Roger, 12. We have the torquing angles.

291:03:56 Scott: Okay. That's through.

[The technical transcript does have "12" being spoken here rather than "15." A check of this section with the tapes is still required.]

[Again, ground controllers in Houston are able to monitor the DSKY inputs and displays. Of particular interest is the Verb 16, Noun 93 display ("Delta Gyro Angles"), which shows the amount the gyros have drifted since the last alignment.]

[Very long comm break.]

Public Affairs Officer - "This Apollo Control at 291 hours, 29 minutes. We're 27 minutes away from the midcourse burn. And Apollo 15 is 28,159 nautical miles [52,150 km] from Earth. Velocity: 11,347 feet per second [3,459 m/s]. In the next 3 and a half hours, at which time Apollo 15 will enter the atmosphere, that velocity will build to 36,000 feet per second [11,000 m/s]."

291:40:02 Parker: Apollo 15, request Omni Alpha, please.

[The crew are at the top of page 1-2 of the Entry Checklist.]

[Very long comm break.]

291:51:50 Parker: 15, at 5 minutes to go, you're looking good.

291:51:55 Scott: Roger. Thank you. We're all set.

[Comm break.]

291:55:04 Scott: Houston, the integration should all be done.

291:55:08 Parker: Rog. Somebody on the ground here hasn't seen it, apparently,

291:55:12 Scott: Okay, we've been sitting here for about 5, 6 minutes.

291:55:17 Parker: Rog. We couldn't tell. Sorry about that, Dave.

[Two programs are used for the midcourse correction burn. First, the prethrusting program, Program 30 (P30 - External Delta-V) is started to enter the basic parameters such as the time of ignition and change in velocity. This information was provided to the crew in the midcourse PAD at 290:34:20. It is interesting to note that the burn duration is not entered. Determining the cutoff time will be done in "real time", with the Inertial Measurement Unit sensing the change in velocity during the burn. The burn time calculated by the back room is a very good estimate, but factors such as an incorrect assumption of the spacecraft weight, or substandard thruster performance can alter the exact time of the thruster firing.]

[Next, Program 41 (P41 - RCS Thrusting) is selected. P41 uses the data computed by P30 to calculate the steering commands, maneuver the spacecraft to the burn attitude and control the burn. P41 requires a bit of time to perform a numerical integration of the targeting and steering equations. As such, P41 must be started some time prior to the planned ignition to allow sufficient time for the integration to complete. In the exchange above, it appears that the ground has not seen that the integration has completed, and is worried that the burn might be need to postponed because the necessary calculations have not completed. In fact, the integration had completed, and it isn't quite clear why the ground isn't aware of this.]

[The omnidirectional antennae are only able to handle low bit-rate telemetry, and is limited to critical engineering data. This does not allow ground controllers luxury of the of viewing data that is displayed on the DSKY.]

[Comm break.]

[Flight Plan page 3-406.]

291:58:00 Scott: Okay, Houston; 15 with the burn status report.

291:58:02 Parker: Roger that.

291:58:04 Scott: Okay, T_ig was on time. Burn time was 21 and came right out on the money, and it clicked up a couple of seconds after the shutdown to minus .1, 0 and minus .1. Delta-V_c was plus .8.

291:58:21 Parker: Roger. Copy that, Dave. It looked good to us.

291:58:29 Scott: Good.

[Comm break.]

[With the last propulsive burn of the flight, tracking radars on Earth begin the task of providing the range and velocity data that will be used to generate a new state vector. This will be uplinked to the spacecraft in the next few minutes. Soon, Al will be matching this effort by making his own parallel determination of the state vector using sightings of three stars in conjunction with the Moon.]

292:00:12 Scott: Roger. Omni Bravo.

[Comm break.]

292:02:31 Parker: Apollo 15, Houston. If you'll give us Accept after you get maneuvered to attitude before you go into P23 to start the optics cal, we'll uplink you your new state vector, and then you can press on with the P23s as soon as that's up.

292:02:47 Scott: Roger. [Long pause.]

292:03:01 Scott: You've got it.

292:03:03 Parker: Roger.

[The spacecraft has reached the attitude for calibrating the optics. The computer can be placed in Accept and the new state vector received.]

[Comm break.]

292:04:23 Scott: Roger.

[Long comm break.]

[Now Al maneuvers to the sighting attitude. Once there, he uses the optics to measure three angles between the Moon and three stars. These are between Gamma Pegasi and the Moon's horizon nearest the star, Alpha Gruis and the Moon's horizon furthest from the star, and finally, Altair (Alpha Aquilae) and the Moon's horizon furthest from the star.]

292:08:31 Scott: Roger. Omni Charlie.

[Very long comm break.]

Public Affairs Officer - "This is Apollo Control at 292 hours, 42 minutes, Apollo 15 now 19,927 nautical miles [36,905 km] from Earth. Velocity: 13,434 feet per second [4,095 m/s]. We're 2 hours, 16 minutes away from Entry Interface and about 2½ hours away from landing."

292:56:48 Scott: Houston, 15.

292:56:51 Parker: 15, go ahead.

292:56:53 Scott: Roger. We're ready for logic sequence check.

292:57:00 Parker: And, 15, we're ready for a logic sequence check also.

[The crew are working through the lower half of page 1-2 of the Entry Checklist.]

[In less than two hours, the Command Module will separate from the Service Module. Then, after re-entry, when the CM reaches the denser region of the atmosphere, the various subsystems of the ELS (Earth Landing System) will operate. These complex events, which include the detonation of explosives, are controlled by the SECS (Sequential Events Control System) which orchestrate the Service Module Jettison Controllers and the ELS Sequence Controllers. A check is made of the system prior to operation. Circuit breakers feeding power to the logic, arming, separation and circuitry, are closed; their status checked by ground controllers and a Go given to arm the pyrotechnic devices that will be fired.]

292:57:19 Parker: And, 15, you're Go for Pyro Arm.

292:57:23 Scott: 15; Roger.

[Long comm break.]

[Flight Plan page 3-407.]

Public Affairs Officer - "The recovery helicopter will be piloted by Commander Stephen A. Coakley of Chula Vista, California. His co-pilot, Lieutenant Junior-Grade John M. Murphy, Jr., of La Jolla, California. Crewmen are Aviation Machinist Mate, First Class, Ernest L. Skeen of Oklahoma City, and Aviation Electronic Technician, Second Class, Thomas R. Hardenbergh of East Lansing, Michigan. A Manned Spacecraft Center Flight Surgeon, Dr. Clarence A. Jernigan of Dickinson, Texas will also be aboard Recovery helicopter. Swim 2, the prime flotation collar helicopter, is piloted by Lieutenant Commander, David D. Cameron, Jr., of Palo Alto, California; co-pilot, Lieutenant Junior Grade Stephen M. Lind of Olympia, Washington, crewman Aviation Machinist Mate, Second Class, John H. Driscoll of Whitehouse Station, New Jersey, and Aviation Electronics Technician, Third Class, Bryce E. Devenport, of Glenns Ferry, Idaho. The swim team leader on that helo is Lieutenant Junior Grade, Fred W. Schmidt of Northbrook, Illinois. Swimmer Number 2, Quartermaster, First Class, William C. "Jake" Jakubowski of Lachawanna, New York. Swimmer Number 3, Yeoman, Third Class, Rudy R. Davis of Ashland, Kentucky. Swim 1 is piloted by Lieutenant Donald M. Larsen of Wellman, Iowa; co-pilot. Lieutenant Junior Grade Eric J. Challain of Olympia, Washington. Crewmen are Aviation Machinist Mate, Second Class, Larry G. Parker of Ringold, Georgia and Aviation Anti-submarine Warfare Technician Airman Thomas F. Sharafik of Hayward, Wisconsin. The swim team leader on Swim 1 is Warrant Officer Jerry L. Todd, Sturgis, Michigan, swimmer number 2 is Ship Fitter, Third Class, Frank S. Schroeder, Malvern, Pennsylvania and swimmer number 3, Radioman Seaman Roy Alan Buehler, Carrollton, Missouri."

293:07:10 Parker: Apollo 15, Houston. Over.

293:07:14 Scott: Houston, 15. Go.

293:07:16 Parker: Roger. I thought we'd let you know, from our preliminary tracking, you're sitting right in the center of the [entry] corridor now.

293:07:23 Scott: Great. That's a nice place to be.

293:07:25 Parker: The best.

[Very long comm break.]

Public Affairs Officer - "This is Apollo Control. The flight crew aboard the helicopter which will provide photo coverage for recovery operations; the pilot is Lieutenant Commander John M. Quarterman of Brunswick, Georgia; the co-pilot Lieutenant Junior Grade Ronald D. Martin, Roanoke, Virginia. Crewmen are Aviation Structural Mechanic, Second Class, Douglas P. Walker of Selma, Alabama; and Aviation Machinist's Mate Airman Gregory G. Wahl of Santa Monica, California. The crew aboard the relay helicopter; pilot is Lieutenant Michael T. Boyce, Bainbridge Island, Washington; co-pilot..."

[With the corridor control burn completed, Al Worden heads to the lower equipment bay to make the 45th and final P52 IMU realignment of the mission. This corrects the tiny amount of drift that has occurred since he first aligned it to the entry orientation two hours ago. He sights on star 40 (Altair or Alpha Aquilae) and star 45 (Formalhaut or Alpha Piscis Austrini) and makes an error of only one hundredth of a degree in determining their position. The torquing angles are brought up on the DSKY where Mission Control can see them.]

293:20:53 Scott: Roger. Thank you.

[Very long comm break.]

[The crew are well into their checks for entry and landing. As a backup attitude reference for their FDAI displays, they align the GDC (Gyro Display Coupler) to match the IMU's reference, per the top of page 1-3 of the Entry Checklist. Should the IMU fail to give them good attitude information, they can switch to the GDC and have that drive the FDAI. The GDC receives information about the spacecraft's attitude from one of two BMAGs (Body Mounted Attitude Gyros). These are the attitude reference gyros normally used by the SCS (Spacecraft Control System) for overall attitude control but which can be used in case the G&C (Guidance and Control) has problems.]

[Seventeen minutes before they reach Entry Interface, a simple check will be made of the G&N system, whereby Al looks out of the centre window to see whether the Earth's horizon is aligned with a mark inscribed on the pane. In preparation for that, he now maneuvers the spacecraft to a defined attitude which will be held until after the Service Module is jettisoned and which should make that mark align with Earth's horizon. If it doesn't, the G&N system is not working properly and he will control the entry with the SCS instead.]

[The final check of the spacecraft's ability to align itself is carried out. Starcodes were included in the preliminary Entry PAD, along with appropriate angles, with which the COAS and the sextant could be checked while in the current attitude. With that, the spacecraft's optics have performed their last task - they are swung to a 90° shaft angle and powered down.]

Public Affairs Officer - "This is Apollo Control with a correction on the home state of Ronald C. Weaver, one of the crewmen on the Relay Helicopter. He is from Madcroft, Wyoming."

[On the EMS (Entry Monitor System), there is a long Mylar tape, 8.75 cm (3.5 inch) wide that displays a intricate pattern of lines which define the relation between velocity and deceleration G-forces during re-entry.

Two different patterns are printed on the tape; one for a nominal entry, and one for an entry that will skip off the atmosphere and re-enter later. Additionally, there are several test patterns, which are used to verify the EMS's circuits.

Final preparations for re-entry are beginning now, as Al Worden runs the EMS through a series of self-checks as detailed at the bottom of page 1-3 of the Entry Checklist. Most of the tests are of the EMS's internal circuitry, but it is vitally necessary to test the scroll which displays the acceleration vs. velocity trace. Several sets of patterns on the tape are set aside just for this self-test. As the tape can be moved forward only, it is not possible to back up the tape and "reuse" a test pattern. During the test, the scroll makes a pre-defined series of motions. Once Al is satisfied with its operation, he will scroll the tape past the remaining self-test patterns, and set it to the actual entry monitor pattern.]

293:34:46 Parker: Roger, 15. Go ahead.

293:34:48 Worden: Okay. EMS check worked fine.

293:34:51 Parker: That sounds good.

[During these preparations, virtually all of the Command Module systems are powered up. The heat generated by these systems is readily handled by the thermal control systems (part of the Environmental Control System) within the Service Module. Under moderate power loads, the heat from the Command and Service module systems is absorbed by a water-glycol solution (not unlike that found in the radiator of an automobile) and is rejected to space through two large radiators located around the lower region of the Service Module. A water evaporator system, similar in principle to that of the PLSS sublimators (the Portable Life Support System, worn by the crews on the lunar surface), is also available to remove larger amounts of heat.]

[Unfortunately, once the Command and Service Modules separate prior to entry, the elaborate heat dissipation systems used by the Command Module float away with the rest of Service Module. A special provision must be made, therefore, to manage the heat generated within the Command Module during the 30 minutes between separation and splashdown.]

[Although the active components of the heat rejection system are no longer available after separation, the water-glycol loop still is functional within the Command Module. Shortly before separation, a "chill-down" process is begun, where both the radiators, and the primary and secondary water evaporators are turned on, which cools the water-glycol to the 5°C (low 40°F) range. This doesn't cool the cabin (which remains at about 24°C) but exploits the cooling solution's ability to hold large amounts of heat with little change in temperature (a property known as specific heat capacity - water has, by far, the highest heat capacity of the common liquids). The total amount of heat that can be absorbed by the coolant is still quite limited, but is sufficient to last through entry and to splashdown.]

[The water-glycol solution is used only to cool the electronics within the spacecraft and the cabin. No attempt is made to try to actively cool the spacecraft exterior during its fiery plunge through the atmosphere. The heatshield of the Command Module is more than adequate (and in some cases, excessively adequate) to protect the structure.]

[Very long comm break.]

Public Affairs Officer - "This is Apollo Control. Congressman William Archer and his mother, are now in the viewing room. And at 293 hours, 45 minutes; Apollo 15 is 11,611 nautical miles [21,504 km] from Earth. Velocity: 17,024 feet per second [5,189 m/s]."

[To record the view out of the window during re-entry, the movie camera is mounted in a bracket in the right-hand rendezvous (forward-looking) window. A mirror in front of the camera alters the view.]

[Earlier in the day, at 288:46:42, Al read out the temperatures of the injector valves in some of the Command Module's RCS thrusters. These readings were all found to be within an acceptable range, so the crew can skip the step at the bottom of page 1-4 in the Entry Checklist that requires them to preheat these thrusters if needed.]

293:47:49 Scott: Roger, Houston. You've got it.

293:47:54 Parker: And, Roger; you're getting it.

[Comm break.]

293:49:53 Scott: Roger.

[Long comm break.]

293:55:16 Irwin: Okay. Go ahead, Bob.

293:55:20 Parker: Roger, Jim. It's still mid-Pacific; 000, 153, 000; 294:41:54; 267; plus 26.13, minus 158.13; 06.2; 36096, 6.51; 1084.9, 36178; 294:58:54; 00:29; Noun 59s are NA; 4.00, 02:12; 00:18, 03:37, 07:44. Boresight and sextant stars are NA, since you've done them; lift vector is up. Comments, 1, use non-exit EMS pattern; 2, RET of 90K, 6 plus 06; mains, 8 plus 32; landing, 13 plus 29; constant g is roll right; moonset, 294:56:37. Over.

[Now that the corridor correction burn has been successfully performed, tracking stations have generated more accurate times and detail for the upcoming entry. The updated PAD is little different from the one at 290:35:02, but is more authoritative.

The data passed up for the Entry PAD is interpreted as follows:

Purpose: Entry.

Landing target: The landing target is in the Mid-Pacific.

IMU gimbal angles required for trim at 0.05g: Roll, 000°; pitch, 153°; yaw, 000°.

Time of the horizon check: 294 hours, 41 minutes, 54 seconds.

Spacecraft pitch at horizon check: 267°. This is 17 minutes before time of Entry Interface.

Splashdown point: 26.13° north latitude, 158.13° west longitude.

Maximum number of g's during entry: 6.2.

Velocity at Entry Interface (400,000 feet altitude): 36,096 feet/second (11,002 meters/second).

Entry flight path angle at Entry Interface: 6.51°.

Range to go to splashdown point from 0.05g event: 1084.9 nautical miles (2,009.2 km).

Predicted inertial velocity at 0.05g event: 36,178 feet/second (11,027 meters/second).

Time of Entry Interface: 294 hours, 58 minutes, 54 seconds GET.

Time from Entry Interface to the 0.05g event: 0:29 (seconds)

Planned drag level (deceleration or g-force) during the constant g phase: 4.00g.

Time from Entry Interface that velocity will slow sufficiently to allow a circular orbit around the Earth: 2:12.

Time from Entry Interface that the communications blackout begins: 0:18.

Time from Entry Interface that the communications blackout ends: 3:37.

Time from Entry Interface that the drogue parachutes will deploy: 7:44.

Lift vector at Entry Interface: Up.

Comments in addition to the PAD:

The non-exit EMS pattern is to be used, that is, the entry is not expected to skip off the atmosphere and re-enter.

90,000 feet (27.4 km) will be reached at 6:06 after Entry Interface.

Time of main parachute deployment: 8:32 after Entry Interface.

Time of landing: 13:29 after Entry Interface.

When maneuvering to ensure a constant g force (done after the max-g portion of the entry to assure a constant deceleration), the crew is to roll right.

Moonset (used to provide one more check of the entry progress): 294:56:37.]

293:58:28 Parker: Roger, Jim. Good readback. And I have some information on landing area and weather and recovery forces, if you're ready to copy that.

293:58:40 Irwin: Roger. Go ahead.

293:58:41 Parker: Roger. Conditions in the recovery area continue to be good. Two thousand scattered, high scattered, visibility 10 miles. Winds are 10 knots, out of the east, and wave heights have come down to 3 feet. Altimeter at 3006. The recovery forces: the aircraft carrier is Okinawa. We have 3 helos: Swim 2, Swim 1, and Recovery. And Swim 2 is estimating to be on station, after splashdown, within 5 minutes. The two '130s in the area will be Hawaii Rescue 1 and Hawaii Rescue 2. Over.

[Bob Parker is reading a standard aviation weather forecast to the crew, and the conditions have changed little over the last four hours. By all standards, the weather is ideal; the winds have reduced to 10 knots from 15 and have changed direction only slightly (to east from east-northeast), and the waves have lowered by a foot. The altimeter setting he refers to is the barometric pressure at the surface, in this case 30.06 inches of mercury (1018 millibars in modern aircraft parlance, 101.8 kPa). Aircraft altimeters are wonderfully simple devices in principle, which operate by comparing the difference between the outside air pressure against that of a sealed, expanding bellows. Altitude is determined by the amount the sealed bellows expands as it reacts to outside air pressure changes, and is displayed on a clock-like instrument.

The "'130s" refer to Lockheed C-130 transports. As of this writing (2000), the C-130 is arguably one of the most enduring aircraft of all time. In the 40 years since it entered service, it has served in many of the world's air forces, and several civilian versions are in production.]

293:59:32 Irwin: Okay. Understand the weather is generally good. It's 2,000 scattered; 10; 10 knots from the east; 3 foot waves; altimeter, 3006. The Okinawa's there. The helos are Swim 2, 1 and Recovery; C-130's are Hawaii Rescue 1 and 2.

293:59:49 Parker: Roger, Jim. That's right. And noting on the altimeter, that means your Delta-H is minus 128.

293:59:57 Irwin: Roger.

[Long comm break.]

[Flight Plan page 3-408.]

[The crew are at the top of page 1-5 of the Entry Checklist, dealing with the final stowage of items in the cabin. One of the interesting lines in this list is for the crew to check for water in the tunnel area. This is one part of the CM's external skin which is not covered by the heatshield - the forward hatch being the only barrier between the cabin and space - the actual tunnel having been jettisoned with the Lunar Module in lunar orbit. This area may be cold from exposure to deep space (the Sun being off to the side during the PTC barbecue roll).]

[After a check of the batteries that will power the detonation of the various pyrotechnic devices, the crew are ready to power up and arm the SECS, then pressurise the Command Module's RCS system.]

294:06:30 Parker: Roger, 15. Go.

294:06:32 Worden: Rog. We're getting ready to activate Command Module RCS and turning Logic and Arms on.

294:06:41 Parker: And, 15, we're ready to watch that.

294:06:44 Worden: Okay. Logic 1 coming on now. Logic 2 on - now.

294:06:55 Parker: Roger. Your Go for Pyro Arm.

294:06:58 Worden: Roger. Go for Pyro Arm. Okay.

[Comm break.]

294:08:08 Scott: Roger, Houston.

[The Command Module RCS system has been completely inert during the mission, and now must be pressurized to become operational. Pyrotechnic squibs fire, opening up valves from helium tanks to the CM fuel and oxidizer tanks. Two independent sets of thrusters, fuel, oxidizer and helium tanks comprise the RCS system. After the valves on the helium tanks are opened, regulators maintain a pressure between 1,000 and 1,400 kPa (150 and 205 psi) on the fuel and oxidizer tanks.]

[Like the RCS systems on the Service Module and Lunar Module, the tanks are not pressurized by simply pumping high pressure gas into the tank. This might work on Earth, with the gas filling the upper half of the tank, while the liquid is expelled through a drain tube at the bottom. However, in a weightless environment, blobs of liquid simply float in the tank, often uncovering the outlet to the feed lines. Obviously, there is the need to assure that the fuel and oxidizer is expelled from the tank in a predictable manner.]

[By isolating the pressurizing helium outside a Teflon bladder within the tank, and containing the propellant within the bladder, there are no floating blobs of propellant and gas. The bladder exerts pressure on the tanks contents, which is then forced through the feed lines to the individual thrusters.]

[Comm break.]

[The crew moves on to page 2-1 of the Entry Checklist with the setting of the Digital Event Timer to count down to the time of Entry Interface (294:58:54 GET). The EMS is set up making it ready to be initialised by P61 and start operating once 0.05g is sensed by P63. At the bottom-left corner of the EMS panel is the RSI (Roll Stability Indicator), an uncalibrated dial which serves to graphically show the direction of the lift vector generated by the flying characteristics of the CM's shape. By rolling the spacecraft during entry, the direction of lift can be altered, giving a degree of steering to the control systems. The RSI is aligned at this stage. Note that at top and bottom of this dial are two lights which help Al verify that he is in the correct corridor. The upper or lower lights tell him whether there is a need for the lift vector to be up or down to regain the correct entry corridor.]

[The final items on page 2-1 in the checklist are to test the CM RCS thrusters.]

294:11:03 Parker: Roger. We're watching. [Long pause.]

294:11:46 Worden: Okay, Houston; 15. Ring 1, test now.

294:11:50 Parker: Roger. Ring 2 looks good. [Long pause.]

294:12:37 Parker: And, 15, ring 1 looks good to us also.

294:12:41 Scott: Roger, Houston.

294:12:45 Worden: Houston, 15.

294:12:46 Parker: Go.

294:12:47 Worden: Can you see the solenoids operating down there? We can't hear them up here.

294:12:57 Parker: Roger, 15. That's what we're watching. And we verified them all.

294:13:01 Scott: Okay; thank you, sir.

294:13:03 Parker: You're welcome.

[Journal reader, Brett Buck explains why they may not be able to hear the thrusters firing.]

[Journal reader Brett Buck - "I am almost certain that this phenomena is the result of the "priming" effect. The system had been evacuated to some degree before propellant loading, the propellant induced, and then the system had remained unpressurized and unfired until shortly before the test described above. The result is that there is residual trapped gas (very small amount) in the lines, and then no propellant between the two thruster valves in each side (fuel and oxidizer). A few minimum-impulse-bit firings are required to fill up the free space with propellant before the thrusters actually fire.]

["Typically, on the first few pulses sent to these thrusters, nothing happens in the way of thrust, and no reaction is seen in the gyros. After some amount of on-time (in the hundred milliseconds range), the valves and lines are "primed" and the thrusters begin providing thrust and associated rate change and vibration. I can confirm that the actuation of the valves without firing on the Apollo thrusters makes a bit of a sound all by itself (sounds like BBs being dropped into an empty coffee can, sort of a sharp metallic "tink" sound), but it's not really very loud, compared to the actual firing (which you can hear even in a vacuum chamber from vibration transmitted through the test stand)."] [Scott, from the 1971 Technical Debrief - "We went through the CM RCS check, and when Al did the minimum impulse on the hand controller, none of us heard anything. We heard the propellant run through the lines as expected. As I had remembered on Apollo 9, we could hear very positive minimum impulses in the CM RCS. I was very surprised when Al ran around the [attitude control] stick, we didn't hear anything. We quizzed the ground, and they confirmed they could see the solenoids. Then I realized that they had second thoughts about it, and decided that they couldn't really verify that we had RCS. We were thinking along the same lines, too. 'We ought to do something else here before we go 'Separate' to confirm we've got CM RCS.' So, we had transferred SM, transferred back to CM RCS, and the ground suggested that we try 'Acceleration Command' - we were about to reach the same conclusion - to get some good rates. [To Worden] You put in an 'Accel Command' and checked both rings and got some good solid rates. We could see the flashes then. Then after that, particularly after we separated, we could hear the minimum impulse, and there was no question. We had good solid bursts on minimum impulse."]

[Worden, from the 1971 Technical Debrief - "That's right. I think we were all surprised at the noise level when we first checked those. It was quite different from what we heard in the simulator."]

[Scott, from the 1971 Technical Debrief - "Yes. The simulator is much too loud. Except after we got separated and you were pulsing around the entry attitude, then you could hear them."]

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

[Scott, from the 1971 Technical Debrief - "They were very positive and very sharp."]

[Long comm break.]

294:18:01 Parker: Apollo 15, Houston. Over.

294:18:05 Worden: Houston, 15. Go ahead.

294:18:07 Parker: Roger. We were unable to monitor rates down here because we weren't set up for it. And we'd like to suggest that you might go back and repeat that check again. We suggest Accel[eration] Command in SCS; and you might try to monitor rates on board, and we'll try to monitor them down here. What we're looking at is only the solenoids down here and I guess, if you really push, that isn't a verification.

294:18:33 Worden: Okay. We'll do that. [Long pause.]

294:19:07 Worden: Okay, Houston; 15. We're ready to test them now.

294:19:13 Parker: All right. We're ready to watch. [Pause.]

294:19:26 Worden: Okay. They're loud and clear up here.

294:19:28 Parker: 15, we're monitoring good rates. [Long pause.]

294:19:54 Parker: 15, Houston. You copy? We monitor good rates down here also.

294:19:58 Worden: Okay, Houston. We copied, and we're testing ring 1 thrusters now.

294:20:46 Parker: And, 15, ring 1 looks okay to us again.

294:20:50 Scott: Okay, Houston. Thank you.

[Long comm break.]

[Journal reader Brett Buck - "I believe the 'priming' explanation is perfectly consistent with the observations. They fired a few min bits, heard prop run through the lines, but no apparent firing. After a few tries, the thruster started firing audibly and visually."]

[The crew is beginning to work through the checklist at the top of page 2-2 for the separation of the SM and CM. Next, they go to P61 on the computer to prepare the spacecraft's instrumentation for entry. Data from the entry PAD is entered in: the latitude and longitude of the landing and whether they fly heads up or down; their expected velocity, entry angle at EI and predicted maximum g force; and finally, their velocity and range to go at 0.05g, as well as the time between EI and 0.05g.]

[Once P61 has accepted the last of these, it moves the computer on to P62, which looks after the separation of the CM and SM as well as the maneuvering of the CM prior to re-entry.]

[Mission Control take the opportunity to have the crew check that their VHF communications are working.]

294:26:06 Parker: Apollo 15, Houston. Over. [No answer.]

294:26:44 Parker: Apollo 15, Houston. Over.

294:26:47 Scott: Go ahead, Houston.

294:26:49 Parker: Roger. We're ready to try a VHF check if you will. We'd like you to go to VHF Antenna, Left, and all of you turn off your S-bands and turn on your VHF T/Rs [Transmit/Receivers].

294:27:03 Irwin: Okay.

294:27:05 Parker: 15...

294:27:06 Irwin: Did that once... [Pause.]

294:27:18 Irwin: Did that once, Bob. And I read you loud and clear, but apparently you were not reading us.

294:27:23 Parker: That's apparently the case. And, Jim, I guess all we need is one of you to go to VHF T/R and S-band T/R, Off.

294:27:32 Irwin: Okay, I'll do that. [Pause.]

294:27:42 Irwin: Houston, this is Apollo 15 on Simplex A.

294:27:45 Parker: Roger. 15 on Simplex A; read you 5 by.

294:27:49 Irwin: Roger. I read you the same.

294:27:51 Parker: Okay. And I guess that finishes it.

[Very long comm break.]

[At 294:41:54, seventeen minutes prior to EI, Al is due to carry out the horizon check, a quick and simple test of the Guidance and Navigation System. If this system is doing its job right, then from his couch position on the left, Al should see the Earth's horizon through the windows aligned to an inscribed mark at 31.7°, ±5°.

Dave Scott's debriefing later indicates that this check occurred after CM/SM Sep.]

[Now, as part of the CM/SM Sep. checklist at the bottom of page 2-3 of the Entry Checklist, the spacecraft is yawed 45° to the left (to 315°) so it is not perpendicular with the entry flight path. This orientation assures that, after separation, the two modules will not collide during entry. As the Command Module is equipped with only rotational thrusters (there is no need for translational maneuvers during entry) the task of ensuring a sufficient separation distance is left to the SM. After the maneuver, the CM will be yawed back in line with their trajectory (to 0°).]

[Jettisoning the Service Module is an intricate task, intended to safely sever the CM-SM interface and also to ensure the Service Module is moved away from the Command Module. As the Service Module is primary source of power, oxygen and cooling, inadvertently separating the two modules would have disastrous consequences for the crew (to say the least!). As with most other events that occur once and demand the highest reliability, a carefully timed series of pyrotechnic are used. Orchestrated by the SECS (Sequential Event Control System), umbilical deadfacing, CM-SM separation, and SM evasive maneuvering is performed in just a few seconds.]

[Beginning the process, the CM pilot Al Worden earlier applied power to the logic circuits of the SECS and also armed the pyrotechnic system which connects the pyrotechnic batteries to their control circuits. After the arming sequence is completed, the actual separation is started by simply flipping the CM-SM SEP switch - it goes without saying that this switch is protected by a metal cover to ensure it doesn't get moved accidentally! The SECS takes over at this point, immediately starting the process of deadfacing the electrical, gaseous and fluid connections between the Command and Service Modules.]

[The first step is to send the command to the Service Module Jettison Controller located in the service module. Once the command is received, the controller starts a timer that will trigger the RCS jets on the SM to move the now derelict hardware away from the CM. Many of the systems onboard the SM are left active to perform the separation. The fuel cells in particular are left operating, for if there is a problem or delay in the separation, the crew would not become dependent on the limited capacity of the re-entry batteries. Electrical power is also needed onboard the Service Module after separation to power the jettison controller and the RCS solenoids (for moving the SM away), and to fire the pyros needed to sever the tie-down connections holding the CM and SM together.]

[Now, simply cutting the electrical connections at this point is ill advised, as the cables are still carrying power and signals between the two modules. Severing these connections with the pyro-powered guillotine surely will result in the shorting and arcing, and would damage the CM systems. To eliminate this possibility, all electrical connections from the SM terminate deep inside the CM as familiar cannon-style plug and socket connectors. As the separation sequence begins, a small pyrotechnic charge fires, moving a series of cams and levers to, quite literally, "unplug" the SM electrical connections from the CM. Reflecting the "one-shot" nature of this permanent disconnection, the plugs and sockets are then locked in place to prevent any possibility of recontacting each other.]

[Once the electrical connections are deadfaced within the CM, the hinged cover protecting the cabling between the CM and SM pivots away so as not to interfere with the CM as it separates. Pyrotechnic charges fire again, propelling guillotine blades through the cable bundles and plumbing that run between the two modules. Now isolated from the Command Module, the Service Module event controller fires three sets of two pyros, each set which is attached to one of the three tension ties which hold the Command Module securely to the Service Module. The Command Module rests on six support pads, which distribute the weight of the CM across the heat shield. Three of these supports contain the tension ties which pass through the base of the heat shield and connect to the CM internal structure. Once the tension ties are severed, springs underneath each of the support pads push the two modules apart.]

[The final event occurs moments after the separation sequence is initiated. Simultaneously, the minus-X thrusters fire, moving the Command and Service Modules apart, and the SM roll thrusters fire for 7.5 seconds to stabilize the SM as it moves away. The minus-X thrusters will continue firing until either the fuel depletes or the fuel cells give up.]

[Early Apollo missions (Apollo 7 through 12) revealed an unexpected problem with this technique. At first glance, firing the translational thrusters to back the SM away, and using the roll thrusters to stabilize it would seen to be a perfectly adequate solution for this maneuver. Imagine the surprise then, when the crews of the earlier missions looked out the window and saw that the SM had somehow "boomeranged" around and was tumbling alongside the Command Module! After extensive analysis engineers discovered that the residual fuel and oxidizer in the Service Module sump tanks acted as a sort of "spring", taking the energy imparted onto them from the RCS jets, and releasing it against the tank walls as it sloshed about.]

[In a paper (Prediction of Apollo Service Module Motion after Jettison [J. Spacecraft and