(AFJ) Undocking and the Descent orbit

Post-landing Activities

 

The First Lunar Landing

Corrected Transcript and Commentary Copyright © 1995 by Eric M. Jones.
All rights reserved.
Scan credits in the Image Library.
Video credits in the Video Library.
Last revised 15 September 2017.

[On Apollo 10, 11, and 12, prior to LM separation, the crews put the Command Module/Lunar Module combination into a circular orbit 60 nautical miles above the lunar surface. When the LM crews were ready for the descent, they separated from the Command Module. The accompanying 16-mm film clip (time lapse 56 sec, 11 Mb) shows the Apollo 11 separation as seen from the Command Module and then a full rotation around the LM thrust axis that Neil and Buzz executed so that Mike could do a visual inspection. The LM crew then performed a Descent Orbit Insertion (DOI burn) over the middle of the Farside in order to put themselves into a 60 by 9 nautical mile orbit. They would start the Powered Descent from the low point of that orbit.]

[Beginning with Apollo 14, the sequence of orbital procedures was modified so that the DOI burn was performed with the CSM Service Propulsion engine prior to LM separation. This change preserved LM fuel. Readers should note that the crews thought of the low-point in this orbit as being 50,000 feet and not 9 nautical miles.]

[Armstrong - "There was a necessity to land heavier weights on later flights, so they did a number of things to allow a maximum landing weight."]

[Because of the DOI burn, the Apollo 11 LM, Eagle, is in an orbit with a shorter period than is the Command Module, Columbia. In detail, the LM orbit is about 4 minutes shorter and, because they have completed about a quarter of an orbit, relative to the lunar surface, the LM is about a minute ahead of the CSM. However, because the Command Module is in a higher orbit, it comes into view of Earth at an earlier stage in its orbit than does the Lunar Module. Specifically, the Command Module is visible about 6 minutes before reaching the east limb while the LM - which is currently at an altitude of about 15 miles - comes into view about 3 minutes prior to limb passage. In summary, Eagle is about a minute ahead of Columbia, but Columbia has a three minute line-of sight advantage. The net result is that, if all has gone well with the burn, Mike Collins in Columbia will regain radio contact with Earth about two minutes before LM AOS (Acquisition of Signal).]

[Charlie Duke, the CapCom for the landing, calls about 10 seconds after Houston acquires the Command Module. AOS of the Command Module carrier signal comes at 102:14:52, with voice contact coming nearly a minute later. The delay in making voice contact is due to the need to get antennas on the spacecraft properly aligned.]

MP3 Audio Clip (39 min 19 sec; 10 Mb mp3) from the Flight Director's loop, starting at about 102:13:55. Clip courtesy Colin Mackellar, editor of the Honeysuckle Creek website.

MP3 Audio Clip (1 hr 00 min 35 sec; 10 Mb mp3) from the (tracking station) Network Controller's loop starting at about 102:13:55. Clip courtesy Colin Mackellar.

MP3 Audio Clip (15 min 46 sec; 8 Mb mp3) combining the Network Controller and Flight Director loops starting at about 102:13:55. Clip courtesy Colin Mackellar.

102:15:02 Duke: Columbia, Houston! We're standing by. Over. (Long Pause) Columbia, Houston. Over.

102:15:41 Collins: Houston, Columbia. Reading you loud and clear. How me?

102:15:43 Duke: Rog. Five-by, Mike. How did it (that is, the DOI burn) go? Over.

102:15:49 Collins: Listen, babe. Everything's going just swimmingly. Beautiful.

102:15:52 Duke: Great. We're standing by for Eagle.

102:15:57 Collins: Okay. He's coming along.

102:16:00 Duke: We copy. Out. (Pause) And, Columbia, Houston. We expect to lose your high gain sometime during the powered descent. Over.

102:16:19 Collins: Columbia. Roger. You don't much care do you?

102:16:22 Duke: No, sir.

[Both spacecraft are equipped with high-gain antennas which must be pointed accurately at Earth to maintain top-quality communications. Because the Command Module attitude relative to Earth changes as Mike maneuvers so that he can track the LM, the antenna must be moved to maintain pointing. However, the system of gimbals used to point the antenna has physical limits and Houston is telling Mike that, at some point during the descent, one or more of those limits will be reached. In order to re-acquire the high-gain, Mike would have to change the spacecraft attitude and, because he can maintain voice communications with both Earth and Eagle on his omni-directional antennas and because Houston will be pre-occupied with the landing, no attitude adjustment will be made.]

[Comm Break. Buzz Aldrin calls about 45 seconds after Eagle AOS.]

102:17:29 Duke: Five-by, Eagle. We're standing by for your burn report. Over.

[Armstrong - "They are referring to the strength and clarity of the signal on a scale of five. 'Five by five' meant 'loud and clear."]

[David Woods writes, "In this case, there has obviously been a large burn. They would like the burn to achieve a particular result in terms of velocity change (delta-v) in the three axes. In most cases. a single engine burn will fail to precisely achieve the required change of velocity - there will usually be a small under or over performance of the engine. The differences between the desired and achieved delta-v across the three axes are called the 'residuals'. For some (not all) burns, the crew are to use the RCS thrusters to bring the three residual values of delta-v to zero or null, essentially making up the deficit or cancelling the overperformance of the main engine to achieve a perfect result. This is called 'nulling the residuals'. However, it is often desirable for the engineers to know what the residuals were before they were taken out - 'before nulling'. This probably gives insight into the performance of the system."]

[Aldrin - "If the burn had been perfect, the computer would have displayed residuals in the three body-axes of 0, 0, 0. If there was some deviation, it was displayed to a tenth of a foot (per second). Apparently, we were supposed to null X and Z (the vertical and fore/aft axes, respectively). They didn't care about Y (left to right). The way they were nulled was (by firing the RCS) with the handcontroller. Neil was looking at them, and I was looking at them and I recorded them."]

[The spacecraft's X axis coincides with the thrust axis, with the positive direction being up - away from the engine toward the rendezvous hatch. The Z axis runs fore and aft, with the positive direction being out the windows. The Y axis runs left to right, with the positive direction being on Aldrin's (right) side of the spacecraft. By nulling the X and Z residuals, they are trying to avoid landing long or short of the target. They are ignoring the small north/south error.]

102:18:37 Collins: Eagle, this is Columbia. Houston would like you to re-acquire on the high gain. They've lost data with you. Over. (Pause)

102:18:50 Collins: Eagle, did you copy Columbia?

102:18:54 Duke: Eagle, Houston. Did you call? (Pause; static clears)

102:19:05 Aldrin: Eagle, Houston...(correcting himself) or Houston, Eagle. How do you read now?

102:19:08 Duke: Rog. (Making a mis-identification) Five-by, Neil. We copied up to the AGS residuals. Would you please repeat the AGS residuals and the trim - correction - the Sun check? Over.

[The AGS is the Abort Guidance System, the backup navigation system used primarily for an emergency return to orbit. The Primary Guidance and Navigation System provides more information and will be used during the landing. The two systems are cross-checked prior to descent; and close agreement between them gives confidence in both.]

102:19:54 Duke: Rog. Copy. Looks great.

[Comm Break, with static during most of the last minute.]

[The LM computer takes actions designated as Verbs, while Nouns are data. The PGNS and AGS both integrate accelerations to estimate the spacecraft velocities. The PGNS uses data from the inertial platform and the AGS uses less accurate data from body-mounted accelerometers.]

[Aldrin - "Somewhere, we had a state vector (three-axis position and velocity) update because of the tracking data that Houston got once we came around. But, how that happened and whether we were aware of it, I don't remember. I know that a lot of people got credit for developing the tracking filter that allowed them to do that. That neat capability contributed to the accuracy of our touchdown, even though nobody knew (exactly) where we were."]

[Armstrong - "I was less concerned about the state vector than I was about inertial platform drift. It had been a fair amount of time since we had last aligned the platform and, in time, it was going to drift. This sun check, here, was a gross check on platform drift. During the half orbit before DOI we rotated (the LM) to make the sextant look directly at the Sun and looked to see if it was in the crosshairs. I think that, to within some predetermined limits, some fraction of a degree, it was alright. The platform was drifted slightly, I think (0.08 degrees); but not enough to worry about."]

[Journal Contributor Paul Fjeld writes: " The predetermined limit for this gross check on platform drift was 0.25 degrees. There was quite a bit of controversy about the usefulness of this and other 'confidence builders' and they were discontinued for the later Apollos. Floyd Bennett, of NASA's guidance section (for whom Bennett Hill at the Apollo 15 site is named), thought that, in planning the descent, they were far too pessimistic in assessing the performance of the whole guidance and navigation system. The drift check, checks of the altitude by tracking the Command Module with the rendezvous radar, etc., caused more problems, and cost more time and money than was warranted by a slight gain in confidence. Bennett's Apollo Experience Report (NASA TN D-6846) discusses they way in which the landings and ascents were planned, and is one of the best of the series."]

[Armstrong - "We did landmark checking to check our position over the ground, and that was another check on the state vector."]

[Neil's double-paned window has scribe marks on both panes in the form of graduated vertical and horizontal scales, marked in degrees. During the landing, these scales will give Neil the means of locating the point on the ground where the computer thinks they are going to land. However, during this period before Powered Descent Initiation, Neil uses the scribe marks to determine how quickly objects on the surface move along the scale and, consequently, the LM's current altitude. Crudely, the LM altitude (in thousands of feet) is 360 times the radius of the Moon (in thousands of feet) divided by the LM's orbital period (in seconds) and, finally, that result divided by the tracking rate (in degrees per second). As an example, if we use a lunar radius of 5700 thousand feet and a LM period of 7200 seconds, the LM altitude (in thousands of feet) is 285 divided by the tracking rate (in degrees per second). In detail, the tracking rate as a function of position along the orbit depends not only on the spacecraft altitude but, also, on the shape of the orbit. As Buzz indicates in the next paragraph, he and Neil have a chart in the cabin on which Neil can compare tracking rates with expected values at various position along the orbit; and differences between the observations and the expected values allow him to estimate the altitude of the low-point in his orbit - called perilune - and the time at which they will reach it. See his comment two paragraphs below.]

[Aldrin, from the July 31, 1969 Technical Debrief - "We had two methods of computing altitude: one based on relative motion from the CSM and the other based on angular rate track of objects observed on the ground. We superimposed the two of them on one graph and re-arranged the graph a little bit with some rather last minute (pre-flight) data shuffling to give us something that the two of us could work on at the same time and to give indication of what the altitude and its time history appeared to be. With the communications difficulties that we were experiencing in trying to verify that we had a good lock-on (with Earth) at this point, I had the opportunity to get only about two or three range-rate marks (on the CSM). They appeared to give us a perilune altitude of very close to 50,000 feet, as far as I could interpolate them on the chart."]

[Armstrong, from the 1969 Technical Debrief - "The measurements against the ground course were indicative of altitude directly above the ground. The ground measurements were very consistent. If they made a horizontal line, it would indicate that you were going to hit a particular perilune (say) 50,000 feet. They (the actual measurements) didn't say that. They were very consistent (that is, they didn't jump around), but they came down a slope, which finally said that our perilune was going to be 51,000 feet. It started out at about 54,000 feet...and our last point was 51,000 feet. This indicated that either the ground was sloping (which it wasn't)...or that the line of apsides (the line connecting the high and low points in the orbit) was shifted a little bit (from its planned position). So, actually, perilune was coming a little bit before PDI...This was all very encouraging - that we were, in fact, going to hit the guidance box (an imaginary window in the sky) so far as (perilune) altitude was concerned from both (the radar and ground tracking) measurements. But I was quite encouraged that these (ground) measurements, made with the stopwatch, were consistent (that is, followed a smooth trend), in fact."]

[Aldrin, from the 1969 Technical Debrief - "When you're able to smooth the numbers and plot a reasonable number of them, your accuracy increases considerably. I think the pre-flight estimates were something on the order of a 6000-foot capability; and I think we demonstrated a much better capability than that."]

102:22:46 Collins: Eagle, this is Columbia. Houston lost you again. They're requesting another try at the high gain.

[Comm Break, with the static clearing after a half minute or so.]

[The LM computer tracks the signal it receives via the high-gain antenna and corrects the pointing to maintain maximum signal strength. The computer program which maintains proper pointing also contains a 'map' of the LM so that it can use information about the spacecraft orientation and, thereby, avoid trying to 'see' Earth through the spacecraft. Unbeknownst to anyone at this point in the mission, the computer has an incorrect LM map.]

[Armstrong - "I think that, later, they put in a five degree yaw (a rotation around the thrust axis) so that the high gain wasn't working so close to the spacecraft, so near its limit."]

[Houston will recommend a 10 degree right (clockwise) yaw maneuver at 102:27:22.]

102:24:02 Aldrin: Loud and clear.

102:24:04 Duke: Roger. We see your Verb 47.

[Aldrin - "When we were looking at something on the computer, they could see it, too. So the verbal report was just added confirmation."]

[Armstrong - "Of course, they couldn't get anything when we were out of the line-of-sight, and had to rely on our (post AOS) reports. I don't think they had any ability to store that (post-burn) information in the computer."]

[The LM computer has only a very limited memory, so data that is no longer of use is not kept.]

[Journal Contributor Frank O'Brien notes, "Verb 47 is the command used to initialize the Abort Guidance System (AGS), using PGNS data".]

102:24:23 Duke: Roger. We'll work on it. (Long Pause)

[The crew is in the Push-to Talk communications mode and we only hear them when they chose to broadcast to Houston.]

[Aldrin - "There were switches on the handcontroller and on the electrical connector that went to the suit. I was using that one because I would have gotten my hand slapped if I had touched the controller."]

102:24:41 Duke: Roger.

102:24:45 Aldrin: Did you copy the star...I mean the Sun check, Charlie?

102:24:48 Duke: That's affirmative. We did, Buzz. Out. (Long Pause; with intermittent static)

102:25:35 Duke: Eagle, Houston. The AGS initialization looked good to us. Over.

102:25:43 Aldrin: Roger. (Long Pause)

[Armstrong - "It was a fairly simple procedure to send a state vector from one computer (the PGNS in this case) to the other (the AGS)."]

[Aldrin - "The AGS initialization was done by a Verb, an instruction with maybe a two- or three-digit code (Verb 47, as noted above)."]

[Armstrong - "At the time, there were inertial guidance systems in aircraft, using inertial platforms. But the computations usually had an altimeter-based smoothing input into the Earth-radius measurement so that the calculation of position was stable. The error would oscillate but wasn't like the error in three dimensions which is unstable and will continue to build. So, although aircraft had similar devices, the calculations were different. Aircraft didn't have anything like the AGS, because I don't think the accuracy would last very long."]

102:26:37 Duke: Roger. Copy. (Long Pause)

[Armstrong - "The visual check was something that we devised ourselves, barnyard math of v = r w. 'r' would be your altitude (that you wanted to know), omega (w) was your angular rate that you determined by watching a point on the ground, and the velocity (v) was pretty well known. We measured omega by measuring the speed at which an object on the ground passed through a certain number of degrees on the (LPD) grid on the window. We just timed it on a stopwatch and had a little plot to compare it with. As the altitude decreased, we could see it was converging pretty well. It gave us an alternate check of our altitude. The importance of this is that, if we weren't in a reasonably close altitude band to our intended starting altitude, the landing guidance would not necessarily converge - the solution wouldn't converge - so it was important to us at that point that the altitude be about right when we started."]

[500 series codes were reserved for radar-related computer functions.]

102:27:09 Aldrin: Rog. I say again...(Listens) Okay. That wasn't an alarm; that was a code. Okay.

102:27:14 Duke: Rog. We saw that. (Pause)

[Armstrong - "We'd have to have a half day in that simulator, again, to remember some of these things."]

[Aldrin - "We spent a lot of time in the LM trainer - maybe 30 or 40 percent of our time - but there was a lot in the CSM (trainer) and on other things. Your sense of well-being is generated by the degree of familiarity you have with what might go wrong in a challenging way. To me, when you're doing what we were doing for the first time, there's a level of importance that's a good bit different from the second or third time, because it has been done already and because the world has seen it. When you're part of the pioneering effort, there's a focusing of an individual's concentration and level of attention that is at the exclusion of a lot of other things. It's a kind of gun-barrel vision."]

[The 15 July 1969 Apollo 11 Crew Training Summary indicates that out of 959 hours of training, Neil spent 285 hours - 30 percent - in the various LM simulators. Buzz did 1017 hours of training, of which 332 hours - 33 percent - was spent in the LM simulators. These figures do not include Neil's LLTV flights nor the 56 hours each of them spent in briefings about LM systems.]

[Armstrong - "I would add, along that same line, that the time requirements necessitated that we accept the conclusions and recommendations of the crews that went before us. So, we did not spend an extraordinary amount of time on things that had already been done and had worked as expected. We focused a great deal of our attention on those things which had not been done before and which we would be expected to pass on to the crews behind us."]

[Aldrin - "Whether they wanted it or not."]

[NASA photo S69-35504 shows Neil, Mike and Buzz debriefing the Apollo 10 crew on 3 June 1969, about a week after Tom Stafford, John Young, and Gene Cernan returned from the Moon. Clockwise from the near left, the people around the table are Collins, Aldrin, Cernan, Stafford, Armstrong, and Young.]

[Armstrong, from the 1969 Technical Debrief - "We did have one program alarm...prior to ignition, that (indicated) we had the radar out of position...a 500 series alarm...which I don't have any way of accounting for. Certainly the switches were in the right positions. They hadn't been changed since pre-launch. But we did, in fact, go to the Descent position on the antenna and leave it there for a half a minute or so, and then go back to Auto and that cleared the alarm."]

[O'Brien - "The landing radar could be moved between two positions: Before pitchover, the radar was in the Descent position, and as the LM rotated to a more upright attitude during the approach phase, the radar was moved to the Hover position. These positions could be commanded manually, in addition to the Auto setting, where the radar was under the control of the guidance computer. The purpose of moving the radar was to keep the antenna pointed as directly towards the surface as possible."]

[This yaw maneuver is a ten degree clockwise rotation around the axis that extends out through the forward hatch axis. Houston is trying to put the high-gain antenna in a position where interference from the spacecraft structure will be reduced.]

102:28:08 Duke: Eagle, Houston. If you read, you're Go for powered descent. Over.

102:28:18 Collins: (Relaying) Eagle, this is Columbia. They just gave you a Go for powered descent.

[They are five minutes from the planned Powered Descent Initiation. Starting at about 102:27:50, Flight Director Gene Kranz asked members of his White Team of Flight Controllers for a Go/No Go for Powered Descent: RETRO (Retrofire Officer, determined LM engine burn times and aborts); FIDO (Flight Dynamics Officer, planned, monitored, and adjusted the flight path as required); GUIDANCE (Guidance Officer, responsible for the on-board navigation Systems and guidance computers); CONTROL (responsible for LM control systems); TELMU (Telemetry, Electrical and EVA Mobility Officer, monitored LM electrical and environmental control systems); GNC (Guidance, Navigation, and Controls Systems Engineer, monitored all vehicle guidance, navigation, and control systems, and the RCS systems); EECOM (pronounced 'e-com', Electrical, Environmental and Consumables Manager); Surgeon. They all responded ‘Go'.]

102:28:34 Collins: Eagle, this is Columbia. (Static fades) You're Go for PDI and they recommend you yaw right 10 degrees and try the high gain again. (Pause; No answer) Eagle, you read Columbia?

102:28:48 Aldrin: Rog. We read you.

102:28:49 Collins: Okay.

102:28:51 Duke: Eagle, Houston. We read you now. You're Go for PDI. Over.

102:28:57 Aldrin: Roger. Understand. (Reading the checklist) Stabilization and Control circuit breakers. DECA Gimbal AC, Closed?

102:29:07 Armstrong: (Faintly) What?

[Neil is not on voice activated comm (VOX) but, rather is in Push-to-Talk (PTT), squeezing a comm switch when he wants to be heard in Houston. Here, we hear him faintly through Buzz's comm circuit.]

[Fjeld - "Neil is supposed to change his rate-scale switch from 5 degrees per second to 25 degrees per second but, for some reason, does not make the change. This procedural error will cause problems when Neil rolls the LM into a face-up orientation at 102:36:46."]

[David Woods and Frank O'Brien (A15FJ at 104:21:52) write, "Command Override is a shorthand for 'Descent Engine Command Override', which will allow either pilot to assume control of the descent engine's thrust using the Thrust/Translational Hand Controllers, if required.":]

102:29:29 Aldrin: Roger.

102:29:33 Duke: Mark. 3:30 'til ignition.

102:29:38 Aldrin: Roger. Copy. (Returning to the checklist) Thrust translation, four jets. Balance couple, On. TCA throttle, Minimum. Throttle, Auto CDR. Prop(ellant) button, Reset. Prop button. (Pause) Okay. Abort/Abort Stage, Reset. (Pause) Att(itude) Control, three of them to Mode Control. (Reviewing the present situation) Okay, Mode Control is set. AGS is reading 400 plus 1. Standing by for (slight pause?) arming. (Pause)

[David Woods and Frank O'Brien (A15FJ at 104:21:52) write, "Attitude control gives control over Roll, Pitch and Yaw rotations to the computer. So when he says 'three of them', he is referring to the three switches that control this mode."]

[Danny Ross Lunsford calls our attention to the likelihood that Buzz is referring here to 'Engine Arm, Descent', which occurs at 102:32:50.]

[Lunsford writes "I know the cadence of Aldrin's voice - I can hear it in my head - he was like a computer reading off his internal checklist - his cool in the final stages of descent is amazing. He feeds the data to Neil as if they were parking the family car. So, I am quite certain that Aldrin says "Standing by for engine arm descent".]

[I hear the final word as 'arming' but, given the frequency with which Buzz's transmissions are clipped, I recognize the distinct possibility of alternate transcriptions after 'Standing by for'. Indeed, the slight pause after 'for' could be a clipped syllable or word.]

[Gary Neff has produced two high-quality versions of the Apollo 11 landing film. The longer of two starts here and covers 15 min 59 seconds. The shorter version starts at 102:34:24.]

[Frank O'Brien pulled Buzz's transmission out the noise. "'Verb 77' may make some sense here, as it is setting attitude control modes for the Digital Autopilot."]

[This is the 16-mm movie camera mounted in Buzz's window. See Gary Neff's discussion of the Apollo 12 landing film.]

[Armstrong - "My guess is that we only had one film cartridge and that we intended to run that all we could during the descent, at as close to regular speed as we could. I don't remember what the time limitation on those cartridges were."]

[Journal Contributor Ulli Lotzmann notes that six DAC magazines were transferred to the LM prior to undocking.]

[Aldrin - "During present Shuttle missions, when things get kind of boring for the flight controllers, they delve back into history and put up on the screen things like five minutes of Apollo 11 EVA or three and a half minutes of the powered descent. Somebody spent an awful lot of time synchronizing film from the cameras that were in the mission control room with the views out our (LM) window that the sequence camera took. It's really neat! It's listed in the Public Affairs Office's Mission Minutes."]

[Mike Caplinger, systems engineer at Malin Space Science Systems for the Mars Descent Imager onboard the Mars Polar Lander writes " The Apollo 11 Lunar Module carried a 16-millimeter Maurer Data Acquisition Camera. The camera was used to record the descent, ascent, and some surface operations."]

[Lotzmann - "The LM camera was fitted with an 10-mm wide-angle lens. The 10-mm lens has a horizontal field of view of 54.9 degrees and a vertical field of view of 41.1 degrees. The lens was manufactured by Kern in Switzerland The camera was mounted above the right-hand window, looking forward and down."]

[The Apollo 11 LM also carried a second DAC body, apparently without a lens. The spare camera body was stowed in the ancillary bag in the CDR's checklist pocket stowage container. During the post-EVA jettison operation, the spare DAC body was off loaded along with the ancillary stowage bag.]

[Caplinger - "The Maurer camera weighed 2.8 pounds with a 140-foot, thin base film magazine attached. It had frame rates of 1, 6, and 12 fps in 'automatice" mode and 24 fps in semi-automatic mode, at all lens focal lengths, and shutter speeds of 1/60, 1/125, 1/500 and 1/1000 second, again, at all lens focal lengths. At one fps, a 140-foot 16mm magazine would have a maximum run duration of about 93 minutes. During the descent of Apollo 11, the camera was activated at 102:31:04 MET and the landing occurred at 102:45:47 MET, for a total duration of 14m 43s. From page 3-68 of the Apollo 11 Flight Plan, the camera was set to 6 fps (max. run duration 16 minutes), infinity focus. So the amount of film was just barely sufficient to record the descent. I assume that the magazine was changed prior to the EVA."]

[Ulli Lotzmann writes: "In Automatic mode, the sequence (or framing) rate can be changed from 1, 6 to 12 fps without interrupting the cameras operation. In the semiautomatic mode the frame rate (24fps) cannot be changed without switching the camera off."]

102:31:32 Duke: Eagle, Houston. If you'd like to try high gain, pitch 212, yaw 37. Over.

102:31:45 Aldrin: Roger. I think I've got you on high gain now.

102:31:49 Duke: Roger.

[During the remainder of the Descent, only Buzz is on Vox and Neil is heard only occasionally in Houston. However, his comments to Buzz on the LM intercom were recorded onboard and that record has been used in constructing the following.]

MP3 Audio Clip (15 min 06 sec; 2 Mb) clipped prepared by Mike Smithwick and downloaded from Mike's website (no longer available) by Thomas Schwagmeier. Neil's transmission at 102:31:49 is not included in Schwagmeier's download.

102:32:03 Aldrin: (To Houston) Say again the angles, though.

102:32:05 Duke: Roger.

102:32:06 Aldrin: I'll set them in to use them before we yaw around.

102:32:08 Duke: Rog. Pitch 212, yaw plus 37. (Pause)

[They are flying with engine forward and windows down. After they finish landmark tracking to confirm their trajectory during the first few minutes of the powered descent, they will yaw the spacecraft (a rotation around the thrust axis) to put themselves in a windows-up orientation. This will be done so that, as the spacecraft pitches toward an upright orientation for the landing, they will be looking out the windows at the landing site. At 102:36:11, in the onboard comm, Neil says they've passed over the crater Maskelyne W about three seconds early and, therefore, will be landing long. In a 24 September 1969 memo, Howard Tindall discussed factors which make pinpoint landings difficult. He cited "thrust from the LM water boiler" resulting in a 6,000 foot miss distance, or about 1/4 of the 4 miles by which Apollo 11 overflew its target. Readers should note that Paul Fjeld is doubtful about this conclusion. He writes: "The vent for the sublimators was angled up to a deflector which should have sprayed the gas evenly with little net thrust to the spacecraft. As far as I can find out, the design for the vent was never changed for the later, accurate LMs."]

[For Apollo 12 and the later flights, trajectory corrections will be read up to the crew during descent.]

[Armstrong - "We measured the position of certain prominent landmarks at a couple of points prior to ignition and then after ignition. Once that was finished, we did a 180-degree yaw (to the face-up position) so that the radar could see the surface. My recollection is that the landing radar wasn't really reliable above something like 30,000 feet. They had tried it on Apollo 10 (at 50,000 feet) to see if they could get altitude readouts, and we did try to use it (at that altitude) to get some measurements. We wanted to do that while we were still up at a reasonable altitude to help the guidance equation converge. The timing (of the 180-degree yaw) was a balance between seeing the surface and getting the radar information in the guidance solution."]

102:32:22 Armstrong (onboard): (To Buzz) Okay! What else is left to do here?

102:32:25 Aldrin (onboard): Engine Arm, Descent. 40 seconds.

102:32:34 Armstrong (onboard): Is the (16mm) camera running?

102:32:35 Aldrin (onboard): Camera's running. (Garbled)

102:32:50 Armstrong (onboard): Okay, the override at 5 seconds. (Pause) Descent armed.

[David Woods has released an annotated version of the DAC (16-mm film) footage that includes the air-to-ground audio and transcript plus commentary. Synchronization is superb, of course. Available on YouTube 14:57.]

[In his excellent book, How Apollo Flew To The Moon, David Woods tells us that, when Neil arms the descent engine, two indicator lights on the DSKY come on indicating that the landing radar is not providing adequate data to calculate the LM altitude and rate of descent. Later in the descent, when they get close enough to the surface that the radar is getting good data, those lights will go out.]

102:33:08 Aldrin: Proceed. One, Zero.

16-mm MOCR film clip with synchronized audio (37 sec)
Deke Slayton is on Charlie's right and Backup LMP Fred Haise is on Lovell's left. Audio/video compilation and synchronization Copyright © 2015 by Stephen Slater. YouTube presentation by David Woods.

102:33:11 Armstrong (onboard): Ignition.

102:33:11 Aldrin: Ignition. (Thrust) 10 percent (Pause; static)

102:33:19 Armstrong (onboard): Just about on time.

102:33:25 Aldrin (onboard): (Garbled) light is on. (Pause) 24, 25, 26, Throttle up. Looks good!

[Ten percent thrust means that the Descent Engine is generating ten percent of its maximum design power level of 10,500 pounds.]

[Armstrong - "One reason for using ten percent thrust had to do with the guidance strategy. The engine would run at ten percent until the guidance perceived that you were at the proper geometric position to go to full thrust. It allowed the computer to make a distance correction. In other words, it would accelerate from ten percent to full thrust within some time window. If it did it earlier, why, of course, that would bring the target closer to you; and if it did it later..."]

[Fjeld -"The LM Guidance Computer would calculate the ignition time for the correct distance, start the engine after 7 seconds of ullage and keep to ten percent thrust for exactly 26 seconds, a quantity the programmers called 'Zoomtime'. Within this period, the very slow Roll and Pitch gimbal drives would be able to trim the thrust through the center-of-mass, after which - as the programmers would say - the 'Flatout' call to 'Burnbaby' would be made. The timing of throttle-down and the following thrust and pointing commands corrected for errors in distance or position."]

[Aldrin - (To Neil) "Do you recall what the thrust level was for DOI? Was it ten percent all the way?"]

[Armstrong - "My guess is that it was."]

[During DOI, they ran the LM engine at 10 percent thrust for 15 seconds and then increased to 40 percent for the remaining 15 seconds.]

[That is, Houston wants Eagle to use the aft omni-directional antenna.]

102:33:46 Aldrin (onboard): (Garbled)'s holding. (Pause)

102:33:51 Collins: (To Eagle) They'd like to use the Omni (garbled).

102:34:01 Aldrin (onboard): (To Mike) Okay, we're reading you relayed to us, Mike. (Static fades)

102:34:05 Collins: (Making a mis-identification) Say again, Neil?

102:34:07 Aldrin: (To Mike) I'll leave it in Slew.

102:34:09 Armstrong: (To Mike) Relay to us.

102:34:10 Aldrin (onboard): (To Mike) See if they have got me now. I've got good signal strength in Slew.

[Fjeld - "Buzz's S-band antenna pointing switch is in manual (Slew) so he can dial in the antenna pitch and yaw angles directly. To do so, he has to turn to his right, away from the flight controls and computers, to the side panel where all the comm stuff is. The Rendezvous Radar mode dial also has a Slew setting."]

102:34:16 Duke: Eagle, we('ve) got you now. It's looking good. Over. (Pause) Eagle...

102:34:22 Aldrin: (To Neil) Okay, rate of descent looks good.

[Gary Neff has produced two high-quality versions of the Apollo 11 landing film. The shorter of the two starts here.]

102:34:29 Aldrin: Roger. Copy. (Pause)

102:34:34 Duke: Eagle, Houston. (Our recommendation is that), after yaw-around, (use pointing) angles: S-band pitch, minus 9, yaw plus 18.

102:34:51 Aldrin: Copy. (Pause)

102:34:55 Armstrong (onboard): Okay. (Garbled) Two minutes (into the burn); going good.

102:35:01 Aldrin: AGS and PGNS agree very closely.

102:35:07 Armstrong (onboard): RCS is good. No flags. DPS pressure is good. (Garbled)

102:35:10 Duke: Roger. (Long Pause)

102:35:14 Aldrin: Data, On. Altitude's a little high. (Pause)

102:35:28 Armstrong (onboard): Okay, what do you want? (Garbled) Let's get...Want to get rid of this radar?

102:35:37 Aldrin (onboard): Yeah.

[Fjeld - "Neil is referring to the Rendezvous Radar mode dial, which is in Auto Track and has been since he and Buzz did a post-DOI ranging test - another 'confidence builder'."]

[Fjeld - "Here, Neil puts the Rendezvous Radar mode switch in Slew - an action that leads to the Program Alarms and accompanying excitement in the landing. Because the dial is not set at LGC (computer control), once they start getting radar data, the computer will be continuously interrupted with useless (and false) 'warnings' that the radar's CSM tracking angle has changed. The computer will get overloaded five times during the descent and, later, this leads Neil to decide not use the computer for re-targeting the landing site. There was post-flight controversy about the possibility that Neil had made a mistake here, but I think the Crew Procedures Division, misunderstanding or using bad information from MIT, trained the crew that way."]

16-mm MOCR film clip with synchronized audio (36 sec)
Audio/video compilation and synchronization Copyright © 2015 by Stephen Slater. YouTube presentation by David Woods.

102:35:46 Aldrin: Houston. I'm getting a little fluctuation in the AC voltage now.

102:35:52 Duke: Roger.

102:35:54 Aldrin: Could be our meter, maybe, huh?

102:35:56 Duke: Standby. Looking good to us. You're still looking good at 3...Coming up 3 minutes.

102:36:11 Armstrong (onboard): Okay, we went by the three-minute point early. We're (going to land) long.

102:36:13 Aldrin: Rate of descent looking real good. Altitude's right about on.

102:36:18 Armstrong: (To Houston) Our position checks down range show us to be a little long.

102:36:21 Duke: Roger. Copy. (Heavy Static)

[In a post-mission analysis, Apollo Descent and Ascent Trajectories, Floyd Bennett notes that, at PDI, Eagle was about 3 miles farther downrange than planned, due to "small delta-V inputs to the spacecraft state in coasting flight. These inputs were from uncoupled RCS attitude maneuvers and cooling system venting not accounted for by the propagation of the predicted navigates state at PDI."]

[Journal Contributor Ron Wells calls attention to discussions in Gene Kranz's Failure is Not an Option which attribute the positional error at PDI to residual pressure in the tunnel between the LM and CSM at undocking at 100:12:00. In a 2002 e-mail, Kranz elaborates, "Floyd's note was correct on the velocity-induced position error at the start of the descent. There were several interrelated navigation problems, i.e. known deficiencies in the R2 lunar (gravitational) potential model, down-track (along the flight path toward the landing site) and cross-track (left or right, perpendicular to the flight path) propagation errors (errors that start out small but become larger as the flight proceeds), and errors induced by maneuvering of the spacecraft. The principal error induced by maneuvering of the spacecraft was, however, the incomplete vent of the tunnel propagated over one orbit after separation. We made a change in all future missions to get a MCC go-nogo on tunnel delta P before giving the crew a Go to undock. Page 82 of the Apollo 11 post mission report says ....'because of uncoupled attitude maneuvers such as hot fire tests, undocking impulse, station keeping, sublimator operation and possible tunnel and cabin venting. The net effect of these perturbations was a sizeable down-range miss.' To my recollection, the trajectory reconstruction determined that with the exception of the tunnel venting, most of the other perturbations were essentially self canceling. Further the post mission review indicated that the delta P gauge was too gross, the markings misleading and the tunnel had to be vented earlier in the timeline and the valve left in the tunnel vent position rather than returned to off."]

[And, finally, Hamish Lindsay, author of Tracking Apollo to the Moon notes that imperfect knowledge of the effects of mascons (mass concentrations) may have also contributed. In 2006, Hamish consulted with Jerry Bostick, who served as Flight Dynamics Officer on Gene Kranz's White Team. Bostick tells us, "It's one of those things that is hard to definitely prove one way or the other, but my opinion is that it was a combination of the tunnel pressure and us not completely understanding - being able to accurately model - the mass concentrations."]

[Armstrong - "We ended up three miles long."]

[Aldrin - "I think it's pretty remarkable that, this early in the burn, we could estimate that. Good guess."]

[Armstrong - "We picked a number of landmarks (to look at) while we were still in the face-down mode."]

[Aldrin - "I appreciate the 'we'. But you did (the tracking), because I wasn't looking out the window. I could care less about the landmarks. If it wasn't in the AGS or the DSKY, (then Buzz didn't see it)."]

[The AGS is the Abort Guidance System. The DSKY is the Display and Keypad, used by the crew to communicate with the LM computer.]

[Armstrong, from the 1969 Technical Debrief - "Our downrange position appeared to be good at the minus 3 and minus 1 minutes points (prior to ignition). I did not accurately catch the (time at the) ignition point because I was watching the engine performance. But it appeared to be reasonable, certainly in the right ballpark."]

[David Woods writes, "I think he was hoping to catch the exact time of ignition by taking a glance at the mission clock at the ignition point but it appears he was looking at the engine gauges so intently that he missed his opportunity."]

[Armstrong, from the 1969 Technical Debrief - "Our crossrange (north/south) position was difficult to tell accurately because of the skewed yaw attitude that we were obliged to maintain for comm. However, the downrange position marks after ignition indicated that we were long. Each one that was made indicated that we were 2 or 3 seconds long in range. (That is, they were reaching landmarks 2 or 3 seconds early. One second corresponds to about a mile of miss distance.) The fact that throttle down essentially came on time, rather than being delayed, indicated that the computer was a little bit confused at what our downrange position was. Had it known where it was, it would have throttled down later (to kill a little velocity). Landmark visibility was very good. We had no difficulty determining our position throughout all the face-down phase of powered descent. Correlating with known positions, based on the Apollo 10 pictures, was very easy and very useful."]

[Neil's landmark tracking indicated that they were going to land about 3 miles (4.8 km) beyond the center of the landing ellipse. As can be seen in the flown copy of LAM-2 that Mike had in the CM, the "Last Bst Pos prior L/O/", which is very close to the actual landing site, they landed about 6.6 km (4.0 miles) long. About 0.6 km of the difference is due to the fact that Neil has to take manual control and fly beyond the boulder field surrounding West Crater. The landmark tracking method available to Neil was far from precise, so the remaining difference is not signficant.]

[The PGNS, pronounced 'pings', is the Primary Guidance and Navigation System.]

102:36:43 Aldrin: Altitude rate looks right down the groove.

102:36:46 Armstrong (onboard): Roger, about 3 seconds long. Rolling over. (Pause) Okay, now watch that signal strength.

[Neil is about to yaw the vehicle to a face up, feet forward position. Yaw is a rotation around the LM thrust axis.]

[Kipp Teague, referring to the audio clips provided by Colin Mackellar and linked at 102:13:55: "This is a very unusual audio clip of the lunar landing featuring a thought-to-be-lost segment. It's in 'stereo' because the Flight Director loop is on the left channel and air-to-ground audio is on the right channel."]

["The segment that was thought to be lost is the final two minutes of the Flight Director loop. However, a staffer in the Audio Control Center at NASA Johnson Space Center found a backup copy of the dual-tracked tape, and provides this account of its history: 'The only tape that I could locate which contained your requested audio was an old 1/4-inch dub. The tape was in terrible condition. In fact, when I first discovered it, it was labeled 'Bad Tape'. The tape was poorly tensioned on its reel - causing our tape decks to not want to rewind, fast forward, or play the tape. It was losing so many oxide particles that the tape heads - and everything else along the tape path - would frequently get gummed up. It took a great deal of effort to get the tape to the point at which it would play back at all.'"

102:37:03 Armstrong (onboard): You know, I tell you. This is much harder to do than it was...(Pause)

[Armstrong, from the 1969 Technical Debrief - "The yaw around (to the right) was slow. We had inadvertently left the rate (scale) switch in 5 (degrees per second) rather than 25, and I was yawing at only a couple of degrees per second as opposed to the 5 to 7 that we had planned. The computer would not hold this rate of, say, 1 to 2 deg/sec. It was jumping up to 3 degrees and back, actually changing the sign and stopping the roll (means 'yaw') rate (as can be seen in the descent-and-landing film). It was then that I clearly realized that we weren't rolling as fast as was necessary and I noted that we were on the wrong scale switch (position). So I went to 25 and put in a 5-deg/sec command and it went right around. However, this delayed it somewhat and, consequently, we were in a slightly lower altitude at the completion of the yaw around than we had expected to be, so we were probably down to about 39,000 or 40,000 feet (altitude) at the time we had radar lockup - as opposed to about 41,500 that we expected to be."]

[A version of the DAC (16-mm) film starting at this point has been posted on YouTube by StormSpotterMike. It includes the audio from both Air-to-Ground and the Flight Director's Loop (10:14). Unfortunately, there are synchronization problems.]

102:37:18 Aldrin: (Under Charlie) Okay, Houston; (garbled) the ED Batts are Go...

102:37:19 Duke: ...You are Go to continue...

102:37:19 Aldrin: ...at 4 minutes.

102:37:22 Duke: Roger. You are Go. You are Go to continue powered descent. You are Go to continue powered descent.

102:37:30 Aldrin: Roger. (Static)

[The landing film shows a noticeable increase in the yaw rate at about this point.]

[Aldrin - "I can't ever recall having them exercise us in the simulator with the uncertainty of intermittent comm. It was distracting. In simulator training, either things were working normal, or there was something going wrong. It would have a degree of polarity to it. It either was or wasn't (working). But the uncertainty, particularly in communications, that was exhibited here, was frustrating. You didn't know where you were - whether you were on your own, or whether you were still under the close supervision of ground control. And that sort of reality is rarely simulated in training. And it was a forerunner of the computer alarms which further distracted me from my task, which was one of monitoring the computers and other instruments. You need some continuity when you're doing that. The whole reason for the whole litany (of readouts) that was coming from me was so that Neil didn't have to look at all the stuff. But, when something unusual starts happening so that we both have to appraise the situation, it's diverting."]

[Aldrin, from the 1969 Technical Debrief - "As I recall, there was a certain amount of manual tracking being done at this time with the S-band (high-gain antenna). During the initial parts of powered descent, the Auto track did not appear to maintain the highest signal strength. It dropped down to around 3.7 (versus a maximum of 5.0) and the ground wanted re-acquisition, so I tweaked it up manually. I got the impression that it was not completely impossible to conduct a manual track throughout powered descent. You'd not be able to do very much besides that. I think it would be possible to do, if you had sets of predetermined values that you could set in. We did have the S-band pitch and yaw angles immediately following the yaw maneuver...After the yaw, the S-band appeared to have a little bit better communications."]

102:37:49 Aldrin (onboard): Okay (Long Pause)

[They have yawed around far enough that the lunar surface in no longer visible in the 16-mm film shot out Buzz's window.]

[Table 5-I in the Apollo 11 Mission Report (13 Mb) gives the time of landing radar lock-on as 102:37:51. The 13-second difference between that time and the transcript time of Neil's call indicates the size of errors that can occur in transcript times.]

102:38:11 Aldrin: Yeah. Altitude light's out. (Pause) Delta-H is minus 2,900.

[The fact that the altitude light is out means they are now getting good radar returns from the lunar surface. "Delta-H is minus 2900" means that the radar altitude is 2900 feet lower than the PGNS altitude.]

Quick Time 16-mm Film Clip ( 56 sec; time lapse; 1.4Mb )

[This time-lapse clip by Gerald Megason starts with the 180-degree yaw and ends with touchdown. Use of VLC for this old format is recommended.]

[They have yawed to the right (a clockwise rotation around the thrust axis) to the windows-up position and, evidently, Buzz has glanced up from his computer. Note that the 16-mm capture is pointing down from above Buzz's window and the Earth is not captured in the film.]

[Aldrin - "There's no doubt, in listening to that, that that was my voice. But when I heard it, I figured that 'Well, Neil must have seen that, he was looking out the window."]

[Armstrong - "We were still pretty level when we rolled over. Earth was twenty-three degrees west of the zenith, so we would have been looking right up at it."]

[As indicated in Figure 5-5 in the Mission Report, the LM pitch is about 77 degrees. A pitch of 90 degrees would have them flying with their back parallel to the lunar surface and their feet forward. Once they are down on the surface, they will be at a pitch near zero. During the next three minutes, they will slowly come more upright, reaching a pitch of about 56 degrees just prior to entering P-64.]

[Fjeld - "At the end of the yaw maneuver, you can really see in the descent film how dramatic the propellant slosh problem is becoming. As the propellant level gets to near fifty percent, there is more room for the fluid to swirl and slosh yet still enough mass for that movement to really torque the LM out of its tight deadband. This is the reason that there is much more thruster activity than Neil has been expecting. In the absence of slosh, slow gimbaling of the engine would be keeping them level. The effects of the sloshing reaches its peak right about here, with the spacecraft wiggling 2 to 3 degrees back-and-forth and side-to-side every couple of seconds. Later, more seriously, the spacecraft motions caused by slosh will render the LPD essentially useless and then, at 102:44:45, will cause the propellant low-level sensor to latch nearly 30 seconds early."]

102:38:25 Duke: That's affirmative.

[They have switched to a data readout which shows Houston the difference between their altitude as determined by the radar and the inertial estimate provided by the PGNS.]

[Table 5-I in the Apollo 11 Mission Report (12 Mb) gives the time of this 1202 alarm as 102:39:02.]

102:38:30 Armstrong: (To Houston) It's a 1202.

102:38:32 Aldrin: 1202. (Pause)

[Altitude 33,500 feet. In the 16mm film record, the lunar surface can be seen in the very bottom of Buzz's window. As indicated in Figure 5-5 in the Mission Report, the LM is pitched back about 77 degrees at this time but will start coming noticeably more upright between now and P64.]

[The 1202 program alarm is being produced by data overflow in the computer. It is not an alarm that they had seen during simulations but, as Neil explained during a post-flight press conference "In simulations we have a large number of failures and we are usually spring-loaded to the abort position. And in this case in the real flight, we are spring-loaded to the land position."]

[David Woods has provided scans of pages 15 ( 1.0Mb ) and 16 ( 1.0Mb ) from the Apollo 15 Lunar Module Cue Cards, which cover various abort scenarios. On page 16, for a return to orbit using either the descent engine (lefthand column) or the ascent engine (righthand collumn), Then Neil would check the position of the Guidance Switch and then push either the Abort button or Abort Stage button, respectively.]

[Armstrong - "I clearly must have said this, and it strikes me as that it's probably right. At least, it was true for me and I think it was for Buzz. We had gone that far and we wanted to land. We didn't want to practice aborts. So I'm sure that we were focusing our attention on doing what was required in order to complete the landing."]

[Aldrin - "In simulations, someone's training you to give a certain response. So you want to do the right thing in the simulation. When it's not a simulation, you want to do the right thing to get the mission done."]

[Armstrong - "In the simulations we challenged the procedures and exercised the procedures and put all the participants - in the control center and in the simulator - in position of having to make fairly rapid and reasonable judgments in the situation that occurred."]

102:38:59 Armstrong: Roger. (To Buzz) 330.

[David Wood's suggests the possibility that '330' has to do with a comparison of, say, the radar's determination of height versus the Guidance and Navigation system's estimate. It is not a typo for 'P30', which is a program for setting up a burn.]

[In Houston, Steve Bales, the control room's expert in the LM guidance systems, has determined that the landing will not be jeopardized by the overflow. The overflow consists of an unexpected flow of data concerning rendezvous radar pointing. The computer has been programmed to recognize this data as being of secondary importance and will ignore it while it does more important computations. In my first draft of this paragraph, I described Bales as "a young expert".]

[Armstrong - "I don't doubt that Steve was a 'young expert' (he was 26), but Steve probably wasn't much different in age than most all of the other guys in the MOCR (Mission Operations Control Room)."]

[Journal Contributor Stephanie Hanus notes that Gene Kranz describes in his book Failure is Not an Option the final simulation done prior to the mission, with Dave Scott and Jim Irwin in the LM simulator. That landing simulation was aborted - unnecessarily - by Mission Control because of a 1201 program alarm.]

[Aldrin - "There had been some training - in a sim(ulation) that Fred Haise (the Apollo 13 LMP) was part of - where some things similar to this had come up, and Kranz instructed his people to go back and look at these sort of things. But, as best I can recall, whatever the flight director people had known about this alarm potential coming up and which were Go and which were not, I was the kind of the systems guy in the LM and I was not made aware of that. And it seems as though that was a flaw in communications. I was very much in the dark when this came up."]

[The Apollo guidance software was developed at the Charles Stark Draper Laboratory, Cambridge, Massachusetts under the direction of Margaret Hamilton. In September 2003, NASA recognized Hamilton's contribution with a NASA Exceptional Space Act Award.]

[Perspectives on the 1201/2 alarms and other guidance computer issues are provided by Fred Martin, Peter Adler, and Don Eyles. Eyles and Adler discuss the programming issues invovled in some detail.]

102:39:02 Aldrin: Okay. Looks like about 820...(Listens)

102:39:03 Duke: ...6 plus 25, throttle down.

102:39:06 Aldrin: Roger. Copy.

102:39:08 Armstrong: 6 plus 25.

[That is, they will reduce thrust 6 minutes 25 seconds into the burn.]

102:39:17 Duke: Roger. Copy. (Pause)

[O'Brien - " The code 16/68, to which Buzz refers, is the computer display Verb 16 Noun 68. This displays the range to the landing site, the time remaining in the braking phase - essentially, the time remaining before pitchover - and the LMs velocity. This display does not place a particularly heavy load on the computer, but when added to the existing load, was sufficient to generate the 1202 alarm."]

102:39:21 Armstrong (onboard) Were we...Was it (meaning the large Delta-H reading) coming down?

102:39:24 Aldrin: Yes, it's coming down beautifully.

102:39:28 Duke: Delta-H...

102:39:29 Armstrong: Roger, it looks good now.

102:39:30 Duke: Roger. Delta-H is looking good to us.

102:39:34 Aldrin: Wow! Throttle down...

102:39:35 Armstrong: Throttle down on time.

102:39:36 Duke: Roger. We copy throttle down...

102:39:38 Aldrin: You can feel it in here when it throttles down. Better than the (stationary) simulator.

102:39:42 Duke: Rog. (Pause)

[The onboard recording can be found at Mike Smithwick's Apollo 11 Audio site. Access the onboard clip by clicking on the "Show Playlist" under Day 5. the onboard is item 3 on the menu that appears. the clip starts at 102:31:49.]

102:39:50 Armstrong (onboard): Okay. (Pause) (Garbled) No flags. RCS is good. DPS (Descent Propulsion System, pronounced 'dips') is good. Pressure...Okay.

[Altitude 21,000 feet.]

102:40:13 Aldrin: Okay. I'm still on Slew so we may tend to lose (the high-gain) as we gradually pitch over. Let me try Auto again now and see what happens.

102:40:21 Duke: Roger.

[The current LM pitch is about 59 degrees.]

102:40:24 Duke: Roger. We got good data. (Pause)

[ O'Brien - "The high-gain S-Band antenna could be steered two different ways: in Slew mode, the antenna could be re-positioned (slewed) manually, or in the Auto mode the computer would keep the antenna pointing towards the Earth regardless of the LM attitude. Buzz is telling Houston that since the antenna is currently being pointed manually (due to the earlier communications problems), it will not be pointing correctly after pitchover. He wants to try the Auto setting again to see if the computer can now keep the antenna pointing correctly. This works successfully."]

[The S-band antenna is shown in close-up in a LM-9 photo by Randy Attwood.]

102:40:47 Armstrong (onboard): And I have the window. (Garbled) view out the window.

[Altitude 16,000 feet.]

[Neil probably has begun to see some of the lunar surface at the very bottom of his window.]

102:40:49 Duke: Eagle, Houston. It's Descent 2 fuel to Monitor. Over.

[Aldrin - "We had two different fuel monitoring systems, and these guys were going to tell us which was more accurate in their judgment."]

[Altitude 13,500 feet.]

[Buzz wants to know when the computer will switch from P63 to P64. Because pitchover occurs when P64 starts, I was probably 'spring-loaded' (Neil's phrase in the commentary following 102:45:44) to hear 'pitchover' rather than 'switchover' when I was preparing digital versions of the Air-to-Ground transcripts prior to the mission review done with Armstrong and Aldrin in 1991. So I changed the digital transcript from 'pitchover' to 'switchover'. That mistake persisted until 2014 when David Woods was preparing material for the Apollo 11 Flight Journal and told me that he thought Buzz had said 'switchover'. I listened to the audio, sent my thanks to David by e-mail, and eliminated the error.]

[O'Brien - "At 102:39:14, Buzz determined that many of the 1202 alarms could be avoided if he did not request the computer to display the time remaining before pitchover (Noun 68), and their first view of the landing site. Rather than risk further computer problems, he is asking Houston to provide that information."]

102:41:12 Duke: Eagle, you've got 30 seconds to P64.

102:41:19 Aldrin: Roger. (Pause)

102:41:27 Duke: Eagle, Houston. Coming up 8:30; you're looking great. (Pause)

102:41:35 Armstrong: P64.

102:41:37 Duke: We copy. (Long Pause)

[The computer has just initiated the Approach Phase program, P-64, and the LM pitch has decreased rapidly from about 55 degrees to 45 degrees. The spacecraft will continue to rotate upright and will be at a pitch of about 20 degrees when Neil takes manual control to overfly West Crater.]

[Table 5-I in the Apollo 11 Mission Report (13 Mb) gives P64 starting at 102:41:32. The error in the transcript time of Neil's call is small.]

102:41:51 Armstrong (onboard): Okay. 5000 (feet altitude). 100 feet per second (descent rate) is good. Going to check my attitude control. Attitude control is good

102:41:51 Duke: Eagle, you're looking great. Coming up 9 minutes. (Pause)

16-mm MOCR film clip with synchronized audio (56 sec)
Split video screen with Flight Director Gene Kranz on the left and Duke, Lovel, and Haise on the right. Audio/video compilation and synchronization Copyright © 2015 by Stephen Slater. YouTube presentation by David Woods.

102:42:05 Armstrong: Manual attitude control is good.

[Armstrong - "We wanted a check that we were getting response from the handcontroller before we were irretrievably committed to it. I think (that), like control-stick steering, you could (intentionally) overpower the autopilot and, if you let go, it would go back (to autopilot). We're in P64 and we just probably exercised the handcontroller to check its response like control-stick steering, and then just released the handcontroller and it went back to the (computer) commanded attitude."]

[Fjeld - "The PGNS Mode-Control toggle switch has to be moved from Auto to Att Hold (Attitude Hold) for Neil to manually command attitude rates to the Digital Autopilot (DAP). He does this after the P-64 pitchover to test his controls, then pushes the toggle back up to Auto so that the DAP will follow Guidance attitude commands. I think Neil was supposed to have done this before P64 so he could use all his time evaluating the landing site and redesignating a new one if necessary. The change may have been another consequence of the computer alarms."]

[This Final Approach film by René and Jonathan Cantin is a side-by-side composite of the landing film and Lunar Orbiter Frame 5076_h3, with a number of craters matched up in the two views.]

[GoneToPlaid has create a ground track (5 Mb) displayed on the LROC overlay in Google Moon. Points are labeled with time (mm:ss) before the landing and many are labeled to indicate key calls from the crew.]

102:42:13 Armstrong (onboard): Okay. 3000 at 70.

102:42:17 Aldrin: (Respoding to Charlie) Roger. Understand. Go for landing. (To Neil) 3000 feet.

102:42:19 Duke: Copy.

102:42:19 Aldrin: Program Alarm. (Pause) 1201

[Table 5-I in the Apollo 11 Mission Report (13 Mb) gives a time of the 1201 alarm as 102:41:32. The error in the transcript time of Buzz's call is small.]

102:42:25 Duke: Roger. 1201 alarm. (Pause) We're Go. Same type. We're Go.

102:42:31 Aldrin: 2000 feet. 2000 feet.

102:42:32 Armstrong: (onboard) (With some urgency in his voice, possibly as he sees West Crater) Give me an LPD (angle).

102:42:?? Aldrin: Into the AGS. (Then reading the LPD from the PNGS as displayed on the DSKY) 47 degrees.

102:42:?? Armstrong: (Repeating his request, perhaps not having heard Buzz's readout while he considered the boulder field) Give me an LPD.

102:42:?? Aldrin: (Repeating the LPD readout) 47 degrees.

102:42:35 Duke: Roger.

[The window in front of Neil is scribed with vertical and horizontal scales. The scales is marked in degrees. Buzz is reading a computer display which indicates where on the vertical scale Neil should look to find the place where the computer thinks the LM will land.]

[Randy Attwood has provide a photo of the LPD scale in LM-9, the H-mission-capable vehicle scheduled for Apollo 15 before that flight became a J-mission.]

[During the other Apollo landings, pitchover gave the Commanders their first looks at the landing site. Then, they could re-target the LM using the Landing Point Designator (LPD). If they twitched the handcontroller forward, the computer would move its target downrange by a small increment and similarly to the right, left, or uprange.]

[Fjeld - "One blip of the handcontroller moved the target 1/2 degree up/downrange or 2 degrees left/right per blip. The increments were all changed to 1 degree for Apollo 15 and the later flights."]

[Armstrong, from the 1969 Technical Debrief - "Normally, in this period of time - that is, from P64 onward, we'd be evaluating the landing site and checking our position and starting LPD activity. However, the concern here was not with the landing area we were going into but, rather, whether we could continue at all (because of the program alarms). Consequently, our attention was directed toward clearing the program alarms, keeping the machine flying, and assuring ourselves that control was adequate to continue without requiring an abort. Most of the attention was directed inside the cockpit during this time period and, in my view, this would account for our inability to study the landing site and final landing location during the final descent. It wasn't until we got below 2000 feet that we were actually able to look out and view the landing area."]

[Readers should note that only Neil is looking out the window. As mentioned previously, Buzz has his attention focused entirely on the computer. Neil's use of 'we' in this context is consistent with his general tendency to deflect attention from himself, even in situations like the Technical Debrief where the only people present were his crewmates, other astronauts, and various engineers.]

[Armstrong, from the 1969 Technical Debrief - "In the early phases of P64, I did find time to go out of Auto control and check the manual control in both pitch and yaw and found its response to be satisfactory. I zeroed the error needles and went back into Auto. I continued the descent in Auto...We proceeded on the flashing 64 and obtained LPD availability, but we did not use it because we really weren't looking outside the cockpit during this phase. As we approached the 1500-foot point, the program alarm seemed to be settling down and we committed ourselves to continue. We could see the landing area and the point at which the LPD was pointing, which was indicating we were landing just short (and slightly north) of a large rocky crater surrounded with the large boulder field with very large rocks covering a high percentage of the surface. I initially felt that that might be a good landing area if we could stop short of that crater, because it would have more scientific value to be close to a large crater. (However), continuing to monitor the LPD, it became obvious that I could not stop short enough to find a safe landing area."]

[Armstrong and Aldrin, from the Crew Observations chapter of the Apollo 11 Preliminary Science Report - "This crater was later identified as one we had informally called West Crater during our prelaunch training."]

[The 1970 book, First on the Moon, attributed to Armstrong, Collins, and Aldrin but assembled by Life magazine writers Gene Farmer and Dora Jane Hamblin, includes the following quotation from geologist Eugene Shoemaker (on page 296 in the 'Book Club' edition): "Tracking information indicated that Neil had flown past the middle of the landing ellipse and was several kiometers downrange. We knew that he had flown over a blocky-rim crater, that he had seen rays of ejecta as he passed over, and that the landing pattern (ground track?) had been rather like a fish hook. There were maybe six craters which could fit his description, but once we knew he was downrange we narrowed it to two. I believe that it was (geologist) Marita West, over in Building No. 2, who first suggested that the crater Neil had described was West Crater. All of us came to the same conclusion pretty rapidly."]

[As Neil will discuss at 102:55:16, the boulder field surrounding West Crater contains rocks up to 2 to 3 meters in size, big enough to cause the LM some real problems. On a detail from a 300 dpi scan of a Xerox copy of 1:5000 LM Lunar Surface Map LSE 2-48 ( 750 ), I have marked what is probably a boulder just east of West Crater. The grid squares on this 1:5000 map are 50 meters on a side and the north-south size of the boulder is about 1-2 meters and close to the resolution limit.]

[Armstrong - "My guess is that the outer northeast slope of West Crater is where it was taking us."]

[Fjeld - "On the main, vertical index line of the LPD, hash marks are painted at 2 degrees intervals. Lesser increments have to be eyeballed. Use of the LPD was tough enough with even a stable platform, but the LM has been in a constant slow pitch since P64 and the LPD numbers Neil is getting from Buzz are as much as 2 seconds old. In addition, one to two degree per second motions caused by propellant sloshing is making use of the LPD, I believe, futile! Post-mission analysis showed that the actual computer target is more than 500 feet west/northwest of where Neil thinks the LM is taking him."]

[Aldrin - "If you don't like what you see, there are maybe four classes of alternatives: left, right, down (short), or go over. And, overwhelmingly, the less traumatic one is to go over, even though there may be some question 'Well, if I go over, then I don't know where it is.' Whereas, if I land short of it, then I know where it is. I'm not on it, I'm in front of it. As I try to reconstruct it, going right is a hairy thing, going left is a hairy thing, and coming down and stopping (short)...since, to stop, really..."]

[Armstrong - "You got to go dead...You can't see very well..."]

[Aldrin - "And you might drive yourself into a...You know, it's just a bad deal."]

[Armstrong - "You might get down there and find out 'Jesus, I've got a terrible situation.'"]

[Aldrin "Right. Right. So the easier, the much more natural (thing to do) is to easily fly over."]

[Armstrong "Extend it."]

[Aldrin - "Where, in retrospect, that's the one that eats up more fuel. I would suspect that Pete and the rest of the guys coming along afterwards paid a lot more attention to thoughts of fuel consumption than we did."]

[Armstrong - "Probably."]

[Post-flight analysis indicated that Neil landed with about 770 pounds of fuel remaining. Of this total, about 100 pounds would have been unusable. As indicated in an unnumbered figure ( 63k ) from page 9-24 in the Apollo 11 Mission Report ( 13 Mb ), the remainder would have been enough for about 45 seconds, including about 20 seconds for an abort. Naturally, Neil had no intention of aborting.]

[The mission plan had PDI at 102:35:13 and the landing at 102:47:11, for a total burn of 11 minutes 58 seconds. In reality, PDI was at 102:33:07 with the landing at 102:45:41. The burn lasted 12 minutes 34 seconds, 36 seconds longer than planned, with virtually all the difference due to Neil's decision to fly beyond the boulder field. The longer burn was equivalent to using roughly 530 pounds of fuel. Had Neil not landed long, he would have had about 1190 of usable fuel remaining. The other five Commanders all landed with roughly 1100 to 1200 pounds of usable fuel remaining.]

[O'Brien - "The large difference between the post-flight analysis and the critical, low-fuel situation that appeared to exist - but didn't - was due to fuel sloshing in the tanks. As Neil pitched the LM over to fly past the crater, the propellant-quantity measuring devices could not accurately gauge the amount left in the tanks."]

[Fjeld - "At the time of Apollo 11, each of the tanks had a small cruciform baffle near the anti-vortex can at the base, right next to the hole for the Propellant Quantity Gauge. Studies were done, early in LEM development, with a clear plastic tank half full that revealed that the slosh dynamics were not too severe and that the small vortex baffles were sufficient to keep propellant slosh at bay.]

["Well, we all know what happened on Apollo 11. Slosh uncovered the Quantity Gauge, latching the light early, losing the crew half a minute of flight time. It also made the LPD unreliable. Apollo 12 flew with the same configuration and its Quantity light came on early, exactly the same way! Worse, Pete actually used the LPD to redesignate his landing, but he maneuvered away from his target because the slosh was messing up his view through the grid. He popped out of P64 and tried to slam on the brakes but it was too late ("Gosh! I went by it!"), so he had to make his heroic flight around the crater, scaring Al a bit. He would have been better off never using the LPD, letting the flight path settle out, switching to manual just before the crater, and landing at his 'parking lot', uprange and north of the crater!]

["Engineers rigged high-rate telemetry on the propellant quantity gauge for Aquarius to get data on the slosh dynamics; but, of course, Apollo 13 never delivered this precious data. So Grumman engineers rigged an ingenious tank test and discovered that, not only did the tanks slosh in ring mode and lateral mode, but did so in a plunger fashion as well. It was a very complex situation, but they were able to come up with a baffle design that would reduce sloshing to an acceptable level. However, with a solution in hand, how were they going to put big baffles in propellant tanks that were never going to be rebuilt - because the contractor responsible had gotten out of the business by then? The answer was: Ship-in-a-Bottle style.]

["At White Sands, two Grumman techs demonstrated assembling and welding(!) a complex baffle system through the two inch Quantity System hole. NASA and Grumman management were seated around them for the demonstration and when the techs succeeded, they stood and cheered. So Antares had the baffles first and Ed Mitchell remarked how smooth and quiet the early part of the ride was (no RCS bangin')."]

[Aldrin - (Thinking about the ease of giving control back to the computer) "Did you ever train on one where if you had some rates and you wanted to get rid of 'em...I mean, you could almost, I would think, be flying down and decide that you want to go somewhere, do it manually and, instead of even trying to stop the rates yourself, just give it back and let the computer stop. Try that in training and see if it works."]

[Armstrong - "If you'd become accustomed to that in training, you might have used that mode if it felt comfortable. It's kind of like using cruise control on a car. You know, if you're used to using cruise control on a car, you're probably going to use it a lot more than the guy who doesn't normally use it. So I think that's probably a fair observation. But since we really hadn't had practice in doing it that way, I probably didn't gravitate to it."]

[Aldrin - "I suspect that ten (or) fifteen years from now, a bunch of guys are going to sit around and argue the same damn things over again. And they may even be doing it single stage to orbit and landing powered descents here on Earth and have a lot of these same concerns."]

[O'Brien - "It is interesting to note that switching back and forth between manual and computer control became significantly easier in later versions of the LM guidance software. In Eagle, Neil had to physically switch the guidance control out of automatic, manually adjust his trajectory, and if he wanted to - which he didn't - physically switch the computer back to automatically control of his descent. Even then, Eagle's computer was programmed only to adjust the vertical component of the descent, leaving the control of horizontal velocity to the crew. On later versions of the software (first available on Apollo 15, but see below), simple deflection of the handcontroller would allow manual control, and when the controller was released, the computer resumed control over the descent. A further enhancement was automatic control of the descent in both the vertical and horizontal planes, easing the workload on the Commander."]

[Fjeld - "After Pete Conrad's struggles during Apollo 12 to keep his horizontal rates nulled, MIT came up with an enhanced P66 which let the pilot bounce back and forth between automatic and manual horizontal nulling and between semi-automatic rate-of-descent and fully-manual throttle control. That meant that there were now four unique ways to fly the LM to touchdown. However, the result of all this great re-design work was never tried by any of the Commanders who had the capability. They all flew just like Neil, except that they learned to use the LM shadow. I think Frank O'Brien's Apollo 15 comment refers to a fabulous P66 design that Don Eyles of Draper Labs wanted to incorporate into the later landings but was never allowed to."]

102:42:41 Duke: Eagle, looking great. You're Go. (Long Pause) Roger. 1202. We copy it.

102:43:01 Aldrin: 35 degrees. 35 degrees. 750. Coming down at 23 (feet per second).

[GoneToPlaid has prepared a labeled comparison (0.1 Mb) between the view out Buzz's window captured by the DAC at about 102:42:57 (2 min 43 seconds before the landing) and a similar view made from the LROC overlay in Google Moon, using the known ground track and a detailed LM model.]

102:43:07 Aldrin: 700 feet, 21 (feet per second) down, 33 degrees.

102:43:10 Armstrong (onboard): Pretty rocky area.

102:43:11 Aldrin: 600 feet, down at 19.

102:43:15 Armstrong (onboard): I'm going to...

[As indicated in Table 5-I in the Apollo 11 Mission Report (12 Mb), at 102:43:22, Neil takes over manual control, going into P66.]

[Armstrong, from the 1969 Technical Debrief - "We then went into Manual and pitched the vehicle over to approximately zero pitch (thereby slowing the descent) and continued."]

[The sudden response of LM can be seen in Figure 5-5 from the Mission Report. When Neil takes manual control, he changes the vehicle pitch from about 18 degrees to about 5 degrees. Gary Neff notes that Figure 5-6, which presents the pitch and roll information for the final phase is mis-labeled. The titles of the two charts are reversed. The top chart actually shows pitch angle and the bottom chart shows roll angle.]

[As can be seen in Figure 5-6a (top), what Neil later described as "approximately zero pitch" is actually about 6 degrees, which he holds until about 102:43:40 when he starts increasing the pitch toward 16 degrees once he has overflown West Crater.]

102:43:26 Aldrin: Okay, 400 feet, down at 9 (feet per second). 58 (feet per second) forward.

[GoneToPlaid has prepared a labeled comparison (0.1 Mb) between the view out Buzz's window captured by the DAC at about 102:43:25 (2 min 18 seconds before the landing) and a similar view made from the LROC overlay in Google Moon, using the known ground track and a detailed LM model.]

[This is Buzz's first verbal report of forward velocity. Because Neil has the LM pitched at only 6 degrees, the forward velocity is higher than would normally have occurred at this point in the descent and Buzz may simply be calling attention to the situation. Buzz cannot see West Crater and can only judge what is going on from the computer readouts and the cross-pointers.]

102:43:35 Aldrin: 330, three and a half down. (Pause)

102:43:42 Aldrin: Okay, you're pegged on horizontal velocity.

[Armstrong - "I didn't (use the LPD to) redesignate. By this time I was flying the machine manually out to the west. That's indicated when Buzz says 'We're pegged on horizontal velocity.' I had tipped it over like a helicopter and was going out to the west."]

[Neil's statement that he 'tipped it over like a helicopter' can give the impression that he tipped the LM forward, past vertical. However, both Figure 5-5 and the mis-labeled upper half of Figure 5-6 indicate that, after reducing the LM pitch to about 5 degrees - still a backwards tilt, but just short of vertical - at 102:43:20 to slow his descent rate, he maintained that pitch until he was clear of the crater. By 102:43:40, he was getting clear and began tilting the LM back to slow his forward speed.]

[NASA photo S69-39265 shows Neil examining a helicopter rotor on 14 July 1969, possibly in preparation for a training flight.]

[Fjeld (from 2005 e-mail) - "When Aldrin says 'pegged on horizontal velocity', he is reading the cross-pointers, which would have been set on low multiply. If so, the indicator would have been pegged at the 20 fps edge of the display."]

[O'Brien (from 2005 e-mail) - "For readers not knowing the cross-pointer scale change, would be easy to assume that Neil has really cranked over the LM to zoom past West Crater. I now believe that his actions were far more gentle. Neil reduced the pitch angle to reduce the amount of horizontal braking, so the effect was that he wasn't trying to accelerate over West, he was simply not decelerating."]

[At 102:43:26, Buzz gave a computer readout of 58 fps forward velocity, which is a leisurely 64 km/hr. West Crater has a diameter of about 165 meters. Traveling that distance at 64 km/hr would have taken about 9 seconds.]

[Armstrong, from the 1969 Technical Debrief - "I was in the 20- to 30-ft/sec horizontal-velocity region (actually 50 to 60 ft/sec) when crossing the top (north rim) of the crater and the boulder field. I then proceeded to look for a satisfactory landing area and the one chosen was a relatively smooth area between some sizable craters and a ray-type boulder field."]

[By 102:43:50, Neil had the LM pitched back at 16 degrees to slow his forward speed. He maintained that attitude until about 102:44:05.]

[Until August 2005, I attributed '270' to Neil. However, Danny Ross Lunsford points out that both from the voice quality and the context, it is almost certainly Buzz who said it.]

102:44:00 Aldrin: Eight percent.

[Buzz's utterance at 102:44:00 has always sounded to me like "Take it down.", in line with his "Ease her down" at 102:43:52. However, Lunsford suggests that the actual words are "Eight percent", which makes more sense from the context. Specifically, Neil has just asked for a fuel reading and Buzz would have been ready to give him one. Indeed, it is extremely unlikely that Buzz wouldn't have done so.]

[Journal Contributor Paul Fjeld - "Up on the CDR's main panel beside the timers were two small digital EL (Electro-Luminescent) displays that showed fuel and oxidizer percent remaining. The crew could monitor either set of tanks by pushing the PRPLNT QTY MON switch to DES 1 or DES 2 depending on which was more conservative (Houston would make the call). The LMP would just look over and call out the lowest number."]

[With his forward velocity now down to about 20 feet-per-second and the LM approaching a good landing area, Neil quickly reduced the LM pitch to 6 degrees and began the final descent.]

[Aldrin, from the 1969 Technical Debrief - "I would estimate (from the transcript) that I called out that shadow business at around 260 feet, and it was certainly large at that point. I would have said that, at 260 feet, the shadow would have been way the hell and gone out there, but it wasn't. I could tell that we had our gear down and that we had an ascent and a descent stage. Had I looked out sooner, I'm sure I could have seen something identified as a shadow at 400 feet; maybe higher, I don't know. But anyway, at this altitude, it was usable. Since the ground is moving away (that is, there is dust blowing out from underneath the spacecraft), the shadow might be of some aid. But of course, you have to have it out your window."]

[As Neil described during the Technical Debriefing (extract below), during the final stages of the approach, Neil was flying with the LM rotated about 13 degrees to the left and, because of the spacecraft structure over the hatch, the shadow was not in his field-of-view.]

102:44:13 Aldrin: Altitude (and) velocity lights (on).

[David Woods notes that these indicator lights are immediately above the 7-4-1 key column of keys on the DSKY. They indicate that the computer is not getting good radar data. Paul Fjeld adds, "The radar lost track with the surface twice after Neil took over manual control: once nearly a minute into P66 for 20 seconds and then, for a few seconds, about 40 seconds before touchdown."]

[Table 5-I in the Apollo 11 Mission Report (13 Mb) shows this loss of good landing radar data occuring at 102:44:11.]

102:44:23 Aldrin: 11 forward. Coming down nicely.

102:44:25 Armstrong (onboard): Gonna be right over that crater.

[The crater Neil mentions is about 60 meters east of the spot where he will land and is known as Little West Crater]

102:44:26 Aldrin: 5 1/2 down.

102:44:29 Armstrong (onboard): I got a good spot (garbled).

102:44:31 Aldrin: 160 feet, 6 1/2 down.

102:44:33 Aldrin: 5 1/2 down, 9 forward. You're looking good.

102:44:40 Aldrin: 120 feet.

102:44:45 Aldrin: 100 feet, 3 1/2 down, 9 forward. Five percent (fuel remaining). Quantity light.

[Fjeld - "The quantity light latched at 102:44:31, and indicated that 5.6% of the original propellant load remained. This event started a 94-second countdown to a 'Bingo' fuel call which meant 'land in 20 seconds or abort.' So if the count gets down to zero, Neil will have 20 seconds to land, if he thinks he can get down in time. Otherwise, he will have to abort immediately. If you're 50 feet up at 'bingo fuel' with all of your horizontal rates nulled and are coming down to a good spot, you could certainly continue to land. With your horizontal rates nulled at 70 to 100 feet, it would be risky to land - perhaps giving you a landing at the limiting load of the landing gear. At anything over 100 feet, you'd punch the abort button, say goodbye to the moon, and stew for the rest of your life!"]

[The Descent Quantity Light - labeled DES QTY - is one of the Warning lights) at the top of Panel 1, which is to the right of the CDR's window. These warning lights are all 'Aviation Red' in color. A group of Caution lights, all in 'Aviation Yellow', are at the top of Panel 2 to the left of the LMP's window. Dave Scott remembers, "We spent considerable time with the contractors defining the color, title, and placement of C&W lights"]

102:45:02 Duke: 60 seconds (of fuel left before the 'Bingo' call).

[The accompanying 16-mm film clip (4.0Mb) by Gerald Megason covers the last forty seconds of the descent. The camera was mounted in Buzz's window and, when the clip starts, Little West Crater is visible at the bottom of the window.]

[Fjeld - "This is a result of a second, brief radar dropout."]

[Neil wants to be moving forward as he lands so that he can be sure of not backing into an unnoticed or forgotten hole behind him.]

[Armstrong, from the 1969 Technical Debrief - "I first noticed that we were, in fact, disturbing the dust on the surface when we were something less than 100 feet; we were beginning to get a transparent sheet of moving dust that obscured visibility a little bit. As we got lower, the visibility continued to decrease. I don't think that the (visual) altitude determination was severely hurt by this blowing dust; but the thing that was confusing to me was that it was hard to pick out what your lateral and downrange velocities were, because you were seeing a lot of moving dust that you had to look through to pick up the stationary rocks and base your translational velocity decisions on that. I found that to be quite difficult. I spent more time trying to arrest translational velocity than I thought would be necessary."]

[What Buzz says here is sometimes transcribed as "Faint shadow" but I recently listened, once again, to both the Public Affairs tape and to the onboard tape and feel uncomfortable making a decision, primarily because the transmission is distorted and partially clipped. Buzz first saw the LM shadow when he looked out at 102:44:04, a fact which adds to my discomfort with the usual transcription. In 2006 I listened to the HSK recording of the Net 1 feed from Goldstone and am still not able to make a decision.]

[Fjeld - "Perhaps Buzz did say 'Faint shadow' but was referring to the now fuzzy edge of the shadow on the streaking dust layer."]

[David Harland suggests a transcription of 'Great shadow', but I still do not believe it is possible to definitively pull this one out of the noise.]

102:45:31 Duke: 30 seconds (until the 'Bingo' call).

102:45:32 Aldrin: Drifting forward just a little bit; that's good. (Garbled) (Pause)

[Armstrong, from the 1969 Technical Debrief - "As we got below 30 feet or so, I had selected the final touchdown area. For some reason I'm not sure of, we started to pick up left translational velocity and a backward velocity. That's the thing I certainly didn't want to do, because you don't like to be going backwards, unable to see where you're going. So I arrested this backward rate with some possibly spasmodic control motions, but I was unable to stop the left translational rate. As we approached the ground, I still had a left translational rate which made me reluctant to shut the engine off while I still had that rate. I was also reluctant to slow down my descent rate anymore than it was, or stop (the descent), because we were close to running out of fuel. We were hitting our abort limit."]

[Armstrong - "I guess that, at that altitude, running out of fuel wasn't a consideration. Because we would have let it just quit on us, probably, and let it fall on in."]

[Fjeld - "An engine cut-out at any height above 10 feet would have produced a touchdown harder than the landing gear was designed to withstand."] [On 19 September 2001, Neil was interviewed at NASA Johnson by historians Stephen Ambrose and Douglas Brinkley as part of the JSC Oral History project. During that interview, Neil said, "I wanted to make it as easy for myself as I could on that first (landing)—there's a lot of concern about coming close to running out of fuel, and I was very cognizant of that. But I did know that if I could have my speed stabilized and attitude stabilized, I could fall from a fairly good height, perhaps maybe forty feet or more in the low lunar gravity, (and) the gear would absorb that much fall. So I was perhaps probably less concerned about it than a lot of people watching down here on Earth. That's not to say I wasn't thinking about it, though, because I certainly was, but I thought it was important to try to get it down smoothly on the first try. We didn't know how that landing was actually going to go until that point. So I wanted to make it as gentle as I could."]

[In an e-mail response to a question from Journal Contributors Matt Gleason and Larry Jordan, Neil elaborated: "When my eyes tell me I am about 40 feet (12 meters) above the ground, the landing footpads are about 24 ft above the surface. The impact velocity under lunar gravity would be the square root of 2gh (two times the lunar gravitational acceleration {g} multiplied by the height {h}) or about 16 feet/second (5 m/sec or 17.5 km/hour). That should be acceptable. Uncertainties include the residual thrust of the descent engine as it runs out of fuel, the softness of the lunar surface, and the additional energy absorption of the landing legs and engine nozzle if the descent rate exceeds the gear design limit, and how much safety factor there is in the honeycomb crush calculation. They all should be favoring a safe impact."]

["It's only a barnyard calculation, but I probably could not judge 40-foot eye height well with all the blowing dust. But I would certainly prefer that drop to trying to go through the abort sequence at that altitude."]

[More commentary on the translational velocities follows the landing.]

[At least one of the probes hanging from three of the footpads has touched the surface. Each of them is 67 inches (1.73 meters) long. The ladder strut doesn't have a probe. Buzz made the call at 20:17:40 GMT/UTC on 20 July 1969.]

[Aldrin - "We asked that they take it off."]

[Journal Contributor Harald Kucharek notes that Apollo 11 photo S69-32396, taken on 4 April 1969, shows Eagle with a probe attached to the plus-Z footpad. This indicates that the probe was removed after that date. The probe attachment is highlighted in a detail.]

[Apollo 11 photograph AS11-40-5921 shows the area under the Descent Stage. A gouge mark made by the probe hanging down from the minus-Y (south) footpad is directly under the engine bell, a graphic demonstration that the spacecraft was drifitng left during the final seconds.]

[Armstrong, from the 1969 Technical Debrief - "We continued to touchdown with a slight left translation. I couldn't precisely determine (the moment of) touchdown. Buzz called lunar contact, but I never saw the lunar contact lights."]

[Aldrin, from the 1969 Technical Debrief - "I called contact light."]

[Armstrong, from the 1969 Technical Debrief - "I'm sure you did, but I didn't hear it, nor did I see it."]

16-mm MOCR film clip with synchronized audio (45 sec)
Audio/video compilation and synchronization Copyright © 2015 by Stephen Slater. YouTube presentation by David Woods.

102:45:44 Aldrin: Okay. Engine Stop.

[Neil had planned to shut the engine down when the contact light came on, but didn't manage to do it.]

[Armstrong, from the 1969 Technical Debrief - "I heard Buzz say something about contact, and I was spring-loaded to the stop engine position, but I really don't know...whether the engine-off signal was before (footpad) contact. In any event, the engine shutdown was not very high above the surface."]

[Armstrong - "We actually had the engine running until touchdown. Not that that was intended, necessarily. It was a very gentle touchdown. It was hard to tell when we were on."]

[Aldrin - "You wouldn't describe it as 'rock' (as in, 'dropping like a rock'). It was a sensation of settling."]

[Some of the other crews shut down 'in the air' (meaning 'prior to touchdown') and had a noticeable bump when they hit.]

[Aldrin - (Joking) "Well, they didn't want to jump so far to the ladder."]

[Readers should note that, although the Moon has no atmosphere, many of the astronauts used expression like 'in the air' to mean 'off the ground' and, after some thought, I have decided to follow their usage.]

[Armstrong, from the 1969 Technical Debrief - "The touchdown itself was relatively smooth; there was no tendency toward tipping over that I could feel. It just settled down like a helicopter on the ground, and landed."]

[On a final note about engine shutdown, Ken Glover calls attention to the following from an interview done with Neil on 19 September 2001 by historians Stephen Ambrose and Douglas Brinkley at NASA Johnson.]

[Brinkley: "Was there anything about your Moon walk and collecting of rocks and the like that surprised you at that time when you were on the Moon, like, 'I did not expect to encounter this,' or, 'I did not expect it to look like this'? Or included in that, the view of the rest of space from the Moon must have been quite an awesome thing to experience."]

[Armstrong: "I was surprised by a number of things, and I'm not sure (I can) recall them all now. I was surprised by the apparent closeness of the horizon. I was surprised by the trajectory of dust that you kicked up with your boot, and I was surprised that even though logic would have told me that there shouldn't be any, there was no dust when you kicked. You never had a cloud of dust there. That's a product of having an atmosphere, and when you don't have an atmosphere, you don't have any clouds of dust."]

[To clarify Neil's observation, on Earth, small particles don't travel very far due to air drag. Collectively, the particles kicked off the surface will stir the air it travels through, forming a dust cloud. Particles in the cloud will fall only slowly, and the cloud will last long enough to move with any breeze that is present. On the airless moon, each particle follows a ballistic trajectory. They don't form a cloud but, rather a sheet of individual particle all moving outward at about the same speed.]

[Armstrong - "I was absolutely dumbfounded when I shut the rocket engine off and the particles that were going out radially from the bottom of the engine fell all the way out over the horizon, and when I shut the engine off, they just raced out over the horizon and instantaneously disappeared, you know, just like it had been shut off for a week. That was remarkable. I'd never seen that. I'd never seen anything like that. And logic says, yes, that's the way it ought to be there, but I hadn't thought about it and I was surprised."]

102:45:46 Armstrong: Out of Detent. Auto.

[Armstrong, from a 1996 letter - "The Attitude Control Assembly [ACA] was the control stick. It had potentiometers or transducers or something similar to provide an output proportional to stick position. Output went to the LGC (LM Guidance Computer) to command the RCS jets to fire. 'Out of Detent' simply means the stick was moved away from its centered position. It was spring/detent centered like the turn signal control on your car."]

[Fjeld - "Because the Digital Autopilot (DAP) was in Attitude Hold, it was firing the jets like mad at touchdown to maintain the pre-touchdown attitude. By joggling the ACA, a new reference attitude was sent to the DAP. Since they weren't moving anymore, the new attitude needed no jet firing to maintain. Soon after, the DAP was cycled with the P68 landing confirmation program."]

['413' is an AGS address and has been a topic of considerable interest to Journal Contributors Marv Hein and Frank O'Brien. Frank supplied the following description.]

[O'Brien - "(The AGS is) a wonderful machine. If you have done any work on a computer that just has a switch register and display (such as an IMSAI, ALTAIR or one of my favorites, the KIM-1), you'd be comfortable with the AGS."]

["The way the AGS operated is that you had only an address and data display, 0-9 keypad, a Clear button, plus +/-, Enter and Readout. That's it. The ultimate in simple interfaces! How the AGS was operated was to press Clear, then a memory address. On a 5-octal character display, you got what was stored in that location. To change it, you typed a +/-, followed by 5 characters. Pressing Enter stored the value directly into memory. What you hear is the checklist item noted as: 413+10000. The key sequence is Clear, 413, Enter, +10000, Enter."]

["Address 413 contains the variable that indicates that the LM has landed - so any abort will be from the surface - which further tells the AGS to save the attitude information from its gyros. These gyros were 'strap-down' types, which means that they had a fixed orientation with respect to the LM body. They also had a nasty habit of drifting quite a bit. So, as soon as they landed, the AGS was to 'lock in', if you will, the attitude the LM was in. If the PGNS died - and it was the PGNS that oriented and re-aligned the AGS - at least they would have some approximate attitude information to abort with."]

102:45:58 Armstrong (onboard): Engine arm is off. (Pause) (Now on voice-activated comm) Houston, Tranquility Base here. The Eagle has landed.

102:46:06 Duke: (Responding to Neil's transmission but momentarily tongue-tied) Roger, Twan...(correcting himself) Tranquility. We copy you on the ground. You got a bunch of guys about to turn blue. We're breathing again. Thanks a lot.

102:46:16 Aldrin: Thank you.

[It is easy to understand how Charlie got momentarily tongue-tied when Neil reported the landing. There was not only the excitement and thrill of what they had all just achieved; but, also, after the intense concentration of the last few minutes, the sudden change in call sign from "Eagle" to "Tranquility".]

[After listening repeatedly to the tapes, Journal Contributor Ian Graham Clapp asks if it was really Buzz, not Neil, who thanked Charlie. To me, the cadence and inflection make it sound more like Buzz. In addition, I think it would have been more characteristic of Buzz to answer that quickly. Nonetheless, short transmissions like this one are often tough calls.]

102:46:23 Armstrong: Okay. (To Buzz) Let's get on with it. (To Houston) Okay. We're going to be busy for a minute.

[They will now prepare for immediate lift-off, in case, as an example, they have damaged an ascent fuel tank in the landing.]

[Aldrin - "Yeah, there are things that should happen."]

[Armstrong - "Crisp(ly)."]

[Aldrin - "Yeah, 'Crisp' is a good word. Because there are discrete abort times that you try to adhere to if you see something leaking, something going wrong."]

[Armstrong "Like, perhaps, if one footpad starts to sink into the surface and you're losing stability. Or you have a propellant tank pressure problem, or something that would cause us to go quickly."]

[Following our review of the landing, I asked Neil about the value of the Lunar Landing Training Vehicle and it similar predecessor, the Lunar Landing Research Vehicle. These free-flying craft (shown here with test pilot Joe Walker at the controls) were built for NASA by Bell Aerosystems, Buffalo, NY and had a General Electric CF-700-2V jet engine mounted pointing downward so that, in effect, it removed 5/6th of the vehicle weight. According to Journal Contributor Ed Hengeveld, "lift for the remaining one-sixth of the LLTV's weight was provided by two 2250 Newton hydrogen peroxide lift rockets." Like the LM engine, these lift engines could be gimbaled. The vehicle also had sets of thrusters. A landing in the LLTV was not an exact duplicate of a LM landing on the Moon, but the two vehicles did share many performance similarities. Two LLRVs and three LLTVs were built and three crashed during test flights and training. All of the pilots - including Neil in the first of the three accidents - ejected safely. See, also, Gene Cernan's discussion of the LLTV at 113:43:49 in the Apollo 17 Journal.]

[As of August 2004, the surviving LLTV was on display in the lobby of Building 2 at NASA Johnson. John Osborn has provided an extensive set of photos.]

[Armstrong - "I would say that the Lunar Module flew better than the LLTV - fortunately. However, it was a reasonable simulation of the Lunar Module, and it was the best we had. The full-mission (but stationary) Lunar Module simulator worked very well, but the real in-flight situation provided by the LLTVs provided a reality that was hard to duplicate in a fixed-base simulator."]

[Also used in training and in the development of the landing systems was the Lunar Landing Research Facility - shown here with Neil standing in front of the LM mock-up. The facility consisted of a large, overhead gantry which allowed the cable-suspended LM mock-up to be moved forward and down - or up and back - in response to pilot input. The figure consists of a set of multiple exposures showing the mock-up as it comes in for a landing. Two frames captured by Ken Glover from film of a training exercise in late June 1969 show rear and side views of the vehicle approaching touchdown. Note that the 'craters' were painted on the flat tarmac. Buzz 'flew' the LLRF on June 27 and 28; Neil on June 30.]

[In response to a 1997 question about his experience with the LLRF, Apollo 15 Commander Dave Scott offered the following: "Ah yes, the LLRF -- what an experience! The LLRF was indeed a large Erector-Set structure at Langley (Research Center) that operated much as you indicated. It was intended to provide LLTV-type training without the risk and cost - although it must have cost a bunch! However, it was actually used very little compared to the LLTV for several reasons. I tried it several times and found it to be negative training, albeit well-intended. The major problem for me was the lag in response to control inputs. Even though they had some feed-forward loops and, at the time, some sophisticated control modes, the system could just not compensate for a vehicle swinging from a cable. Thus, the very short systems lag in a very dynamic and tight loop was just enough to confuse the human response -- which for LLTV-type flight was probably on the order of 0.2-to-0.5 seconds from input to expected output. It was an impressive-looking rig though! Should be a reasonable amout of documentation on the LLRF in the Langley files; and some of the old-timers should have some good stories. It was very important to Langley (as were a number of other Apollo-support activities, such as docking simulators), and the structure may even still be there!"]

[Neil has provided some comments on the LLRF. See, also, his comments about the LM engines during a panel discussion at the 1997 Naval Aviation Symposium.]

[We now move to discussion of the indoor, stationary simulator which was used as the primary training tool by the crews.]

[Aldrin - "The (stationary) LM simulator was excellent in its fidelity of reproduction. As an example, we had the actual computer codes. Obviously, it was a simulation and the data the onboard computer used had to be generated somewhere; but, instead of that being generated in the simulator, it was done outside. It was a real test of the systems. Whoever made the decisions to do that made very wise ones. They're probably doing that more and more with simulators now. The level of detail in the LM and Command Module simulators for Apollo was unquestionably a pioneering effort that contributed significantly to how effective aircraft simulators and other situational simulators were later on."]

[Armstrong - "It was extraordinary for its time and commercial aircraft simulators have undoubtedly benefited by duplicating the best parts."]

[Aldrin - "Certainly the visual portrayal and, of course, the instrument activation."]

[Armstrong - "What the LLTV gave you was not so much the seat-of-the-pants dynamics as the real-world visual. (Chuckling) That and the fact that, if you make a mistake, you can't hit the reset button."]

[Aldrin - "I think it's important to stress the evolution of capabilities to accomplish the mission. The big difference I would point to between our mission and the later ones is that they had a much easier time landing. They could let the computer do a lot of stabilizing whereas we had to do a lot of that manually. Once we took over manually at 500 feet - which we had planned to do anyway - there was no way we could give it back to the computer. Whatever we had in the way of an attitude, it would hold. If it's accumulating a drift, it will get progressively worse unless you do something against it. On later systems, they closed the loop on it so that, if you let go of the controller, the computer would null translation. That made the descent on the later missions a lot easier to do - plus the fact that they weren't doing it for the first time."]

[Fjeld - "The later crews never used this Auto mode. They all had to struggle with the controls as Neil did, except that they had his experience to draw on: they made sure they could see the shadow and that they were fat on fuel."]

[Aldrin - "I think it's important, historically, to keep some of these differences (between missions) in mind. Sometimes, when the TV networks are doing an anniversary program, they jazz up the tension. They always take about 5 seconds off the amount of fuel you have left, and they never put it in the perspective of how much fuel the other flights had. They're not interested in that and I think that ought to be included."]

[Readers should note, however, that Neil's heart rate at touch down was 150 beats per minute, the highest of any of the Commanders. The need to avoid West Crater was probably a contributing factor, although the drama of the moment was undoubtedly played a role as well. See the discussion at 103:25:37. In response to a question in 2002, Neil remembers that his resting heart rate "was usually about 60".]

102:46:xx Armstrong (onboard): Master Arm coming on.

102:46:xx Aldrin (onboard): I'll get the pressure check.

102:46:xx Armstrong (onboard): Okay. (Pause)

102:46:38 Aldrin: Very smooth touchdown. (Pause)

102:46:xx Armstrong (onboard): I didn't hear that (garbled)

102:46:xx Aldrin (onboard): (Garbled) and oxidizer.

102:46:xx Armstrong (onboard): Both (garbled) vent.

[Aldrin, from the 1969 Technical Debrief - "We had a little right drift and, then, I guess just before touchdown, we drifted left."]

[Armstrong, from the 1969 Technical Debrief - "I think I was probably over controlling a little bit in lateral. I was confused somewhat in that I couldn't really determine what my lateral velocities were due to the dust obscuration of the surface. I could see rocks and craters through this blowing dust. It was my intention to try and pick up a landing spot prior to the 100-foot mark and then pick out an area just beyond it such that I could keep my eyes on that all the way down through the descent and final touchdown. I wouldn't, in fact, be looking at the place (where) I was going to land; I would be looking at a place just (out) in front of it. That worked pretty well, but I was surprised that I had as much trouble as I did in determining translational velocities. I don't think I did a very good job of flying the vehicle smoothly in that period of time. I felt I was a little bit erratic."]

[Fjeld - "Armstrong made about four quick pitch-down maneuvers, three of them totaling about 12 degrees in 2-3 seconds. They were well within the design control regime of the LM despite his apology that they were 'spasmodic'."]

[Neil came down through 100 feet at about 102:44:45 and landed at about 102:45:40. According to the Mission Report "Landing on the surface occurred at 102:45:39.9 with negligible forward velocity, approximately 2.1 ft/sec to the crew's left, and 1.7 ft/sec vertically. Body rate transients (motions after first footpad touchdown) occurred and indicate that the right and forward landing gear touched almost simultaneously, giving a roll-left and a pitch-up motion to the vehicle. The left-directed lateral velocity resulted in a slight yaw right transient at the point of touchdown. These touchdown conditions, obtained from attitude rates and integration of acceleration data, were verified qualitatively, by the at-rest position of the lunar surface sensing probes and by surface build up around the rims of the footpads ( as photographed by the astronauts during the EVA)."]

[The fact that the right and forward footpads hit first indicates that Neil was still trying to correct his left translation and to impart a little forward motion.]

[Aldrin, from the 1969 Technical Debrief - "I was feeding data to him all the time. I don't know what he was doing with it, but that was raw computer data."]

[Armstrong, from the 1969 Technical Debrief - "The computer data seemed to be pretty good information, and I would say that my visual perception of both altitude and altitude rate was not as good as I thought it was going to be. In other words, I was a little more dependent on the (computer) information. I think I probably could have made a satisfactory determination of altitude and altitude rate by eye alone, but it wasn't as good as I thought it was going to be; and I think that it's not nearly so good as it is here on Earth."]

[Aldrin, from the 1969 Technical Debrief - "I got the impression by just glimpsing out that we were at the altitude of seeing the shadow. Shortly after that, the horizon tended to be obscured by a tan haze. This may have been just an impression of looking down at a 45-degree angle. The depth of the material being kicked up seemed to be fairly shallow. In other words, it was scooting along the surface but, since particles were being picked up and moved along the surface, you could see little rocks of little protuberances coming up through this, so you knew that it was solid there. It wasn't obscured to that point, but it did tend to mask out your ability to detect motion because there was so much motion of things moving out. There were these few little islands that were stationary. If you could sort that out and fix on those, then you could tend to get the impression of being stationary. But that was quite difficult to do."]

[Armstrong, from the 1969 Technical Debrief - "It was a little bit like landing an airplane when there's a real thin layer of ground fog, and you can see things through the fog. However, all this fog was moving at a great rate, which was a little bit confusing."]

[Aldrin, from the 1969 Technical Debrief - "I would think that it would be natural - looking out the left window and seeing this (dust) moving this way (left) - that you would get the impression of moving to the right, and you counteract by going to the left, which is how we touched down."]

[Armstrong, from the 1969 Technical Debrief - "Since we were moving left (toward the landing spot he had picked) , we were yawed slightly to the left so that I could get a good view of where we were going. I think we were yawed 13 degrees left; and, consequently, the shadow was not visible to me as it was behind the panel (Neil's window only gives a view from slightly right of forward around to 90 degrees left), but Buzz could see it. Then I saw it in the final phases of descent. I saw the shadow come into view, and it was a very good silhouette of the LM at the time I saw it. It was probably a couple of hundred feet out in front of the LM on the surface. (The shadow) is clearly a useful tool, but I just didn't get to observe it very long."]

[Figure 5-13 from the Apollo 11 Mission Report shows LM attitude (pitch, roll, and yaw) during the six seconds before the landing and the four seconds after. At the begining of that interval, Neil had the LM yawed left 15 degrees, which increased to 16 degree during the final two seconds before the landing, and then decreased to 13 degress as the LM settled on the surface.]

16-mm MOCR film clip with synchronized audio (36 sec)
Audio/video compilation and synchronization Copyright © 2015 by Stephen Slater. YouTube presentation by David Woods.

102:47:03 Aldrin: Okay. It looks like we're venting the oxidizer now.

[Armstrong - "I recall that there was some concern that surfaced in the last few days before launch. A systems concern about tank overpressurization due to the environment; (that is), to heating. So there were some procedural changes that were introduced - maybe not procedural changes but procedural concerns - and it was something that we discussed in those last few days before launch."]

[The page Sur-1 of the Surface Checklist contains three handwritten lines concerning the fuel and oxidizer vents.]

[Armstrong - "My recollection was that there was a new thought with regard to the heat sources that might be affecting those propellant tanks. And it was new procedures that were talked about, but I just don't remember those details."]

[Fjeld - "Engineering wanted to have the pressure in the tanks vented so that it didn't rise and cause the tank burst disks to rupture and perhaps spill propellant on the crew during EVA. Engineering also had the crew vent the very cold Supercritical Helium tank. As it turned out, the helium vented past the heat exchanger and fuel trapped there froze and blocked fuel down the line towards the still hot engine. Pressure built up in the fuel line for nearly an hour while the ground fretted about what would happen. Eventually, something let go. After the flight, the helium vent was re-designed."]

102:47:08 Aldrin: (Garbled)

102:47:09 Duke: ...T1. Over. Eagle, you are Stay for T1.

102:47:12 Armstrong: Roger. Understand, Stay for T1.

102:47:15 Duke: Roger. And we see you venting the Ox(idizer).

102:47:20 Armstrong: Roger. (Long Pause)

102:47:36 Aldrin: Radar circuit breaker. (Long Pause)

[Houston has decided, from a quick look at LM systems, that an immediate take-off will not be necessary. For the next several minutes, the crew will continue preparations to leave, if necessary, while Houston takes a more thorough look at the spacecraft systems. Along with other activities, the crew is now depressurizing the descent stage oxidizer tank.]

[Aldrin - "Do you think the Surface (science) guys knew what the systems guys were going to do in terms of venting?"]

[Armstrong - "There's a difference. This was a safety issue in the minds of the people who were worrying about it. And it took priority over science."]