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Feb 12, 2017

LM Descent to the Moon - Part 7 - Crew Comments (1969)

(Apollo 11 LM - DOI to Touchdown Crew Debriefing, 1969)

[The following is a partial reprint of NASA's Apollo 11 crew interviews /1/ during their quarantine that took three weeks after their splash down. Apollo 11 CM splashed down on July 24, 1969.]

The Apollo 11 mission, launched on July 16, 1969 and returned to Earth on July 24, 1969. Aboard the space craft were astronauts Neil A. Armstrong, commander; Michael Collins, Command Module (CM) pilot; and Edwin E. Aldrin Jr., Lunar Module (LM) pilot

1. Preparation for DOI [Descent Orbit Insertion]

"ALDRIN -  It was 40 minutes before DOI that we were scheduled to begin the P52 and we were about 2 minutes behind when we completed looking at the radar and VHF ranging and designated the radar down so that we could do the P52.

Figure 2. IMU-5 for Apollo GetN Equipment opened. Apollo inner, middle and outer IMU gimbal assemblies visible.

Figure 3. Apollo IMU, Naviation base and optical subsystem.

[P52 - IMU alignment program 52

Aligns the IMU to one of three orientations selected by the astronaut. The present IMU orientation is known and is stored in REFSMMAT.

The three possible orientations may be:

  1. Preferred orientation: An optimum orientation for a previously calculated maneuver. this orientation must be calculated and stored by a previously selected program.
  2. Nominal orientation
  3. RERSMMAT orientation

After a IMU orientation has been selected routine S52.2 is operated to compute the gimbal angles using the new orientation and the present vehicle attitude. CAL52A then uses these angles, stored in THETAD, +1, +2, to coarse align the IMU. The stars selection routine, R56, is then operated. If 2 stars are not available an alarm is flashed to notify the astronaut.

At this point the astronaut will maneuver the vehicle and select 2 stars either manually or automatically. After 2 stars have been selected the IMU is fine aligned using routine R51. If the rendezvous navigation process is operating (indicated by RNDVZFLG) P20 [Rendezvous navigation program 20] is displayed. Otherwise P00 [IDLE] is requested.

The program is called by the astronaut by DSKY entry.

Output -- the following may be flashed on the DSKY

  1. IMU orientation code
  2. Alarm code 215 - preferred IMU orientation not specified
  3. Time of next ignition
  4. Gimbal angles
  5. Alarm code 405 - two stars not available
  6. please perform p00

The mode display may be changed to 20]

ARMSTRONG - I don't think we had any difficulties with the DOI prep.

Figure 4. 1969-07-24, Apollo 11 Crew Boards U.S.S. Hornet Aircraft Carrier. Shown here are the three astronauts (L-R) Aldrin, Armstrong, and Collins leaving the recovery helicopter aboard the U.S.S. Hornet after their splashdown in the Pacific Ocean. Wearing biological isolation garments donned before leaving the spacecraft, the three went directly into the Mobile Quarantine Facility (MQF) on the aircraft carrier. The MQF served as their home for 21 days following the mission.

2. DPS/DOI burn

ARMSTRONG - At DOI ignition, which was our first DPS [Descent Propulsion System] maneuver, I could not hear the engine ignite. I could not feel it ignite, and the only way that I was sure that it had ignited was by looking at chamber pressure and accelerometer. Very low acceleration - -

COLLINS - I would think under zero g, it would throw you against your straps, one way or the other.

Figure 5. Lunar Module straps to hold astronauts in desired position under zero gravity.

ARMSTRONG - We're pulled down into the floor with the restraint, and the difference between that and the 10-percent throttle acceleration was not detectable to me, However, at 15 seconds, when we went to 4O percent, it definitely was detectable.

ALDRIN - On the restraints, I found that instead of being pulled straight down, the general tendency was to be pulled forward and outboard. So much so that this might have been a suit problem, as my right foot around the instep was taking a good bit of this load, being pulled down to the floor.

It did feel as though the suit was a little tight. Prior to power descent [powered descent phase, when the DPS engine runs all the way to the surface], the problem was obscured from my mind, but it was aggravated somewhat by the restraint pulling down and forward.

Figure 6. Lunar Module astronaut flight stations.

ARMSTRONG - I guess I noticed that last - I had expected a good bit of lateral shifting due to reports of previous flights.

ALDRIN - I was able to lean over and make entries on the data card without pulling it down; but as you can see, when you do make entries on them, you make them sideways.

Figure 7. Atronaut flight station.

ARMSTRONG - The cut-off was a guided cut-off. What about the residuals?

ALDRIN - We burned both X and Z, and I'm sure they weren't in excess of .4.

ARMSTRONG - It was less than 1 ft/sec, but I don't recall the tenths.

Figure 8. S69-40205 (27 July 1969) -- The crewmen of the Apollo 11 lunar landing mission go through their post flight debriefing session on Sunday, July 27, 1969. Left to right, are astronauts Edwin E. Aldrin Jr., lunar module pilot; Michael Collins, command module pilot; and Neil A. Armstrong, commander. They are seated in the debriefing room of the Crew Reception Area of the Lunar Receiving Laboratory at the Manned Spacecraft Center.

6. Trimming residuals

ARMSTRONG - It's probably worth noting that the flight plan at this point does not adequately reflect the time requirements of the flight. I think the DOI rule in the flight plan says, "Trim Vx residuals."

ALDRIN - So does your checklist.

Figure 9. Typical DSKY 3COMP display. R1, R2 and R3 shown. VERB 6 NOUN 36 (DISPLAY DECIMAL IN R1, R2 AND R3 / TIME OF AGC CLOCK 00xxx HOUR, 000xx MIN, 0xx.xx SEC), program or phase is P00.

ARMSTRONG - That isn't right. This was a result of that orbital change that was put in late, and paperwork and so on just couldn't keep up with those last-minute changes. But, again, it shows that last-minute changes are always dangerous. You could follow the flight plan here and possibly foul up the procedure. Do you recall the VERB 82 [REQUEST ORBIT PARAM DISPLAY (R30)] values? 9.5 was perilune, I think.

Figure 10. VERB and NOUN buttons have a long history in MIT guidance and navigation keyboards.

ALDRIN - Preburn for NOUN 42 was 57.2 and 8.5. We had 57.2 and 9.1 after the maneuver.

   APOGEE,  xxxx.x NAUT MI
   PERIGEE, xxxx.x NAUT MI

ARMSTRONG - I guess we can't account for that.

Figure 11. Lunar-descent coordinates. G stands guidance.

ALDRIN - No. The NOUN 86 [N86 - VG (LV) 3COMP xxxx.x FT/SEC FOR EACH, DEC ONLY] [LV - Local Vertical] that we got out of the thrust program also differed from what the ground gave us in the pad, primarily, in the Z-component that's loaded into the AGS [Abort Guidance System]; that pad value is 9.0, and the computer came up with 9.5.

The coordinate frame that you load them in is frozen inertially, and if there are any discrepancies in the freezing of this, you will get a slightly different burn direction required out of the two guidance systems. I think that explains the larger AGS residual in the Z-direction of minus 0.7. I think we would have to have the guidance people verify that the difference in NOUN 86 produced that error in that direction.

Figure 12. S69-40209 (27 July 1969) -- The crewmen of the Apollo 11 lunar landing mission go through their post flight debriefing session on Sunday, July 27, 1969. Left to right, are astronauts Edwin E. Aldrin Jr., lunar module pilot; Michael Collins, command module pilot; and Neil A. Armstrong, commander. They are seated in the debriefing room of the Crew Reception Area of the Lunar Receiving Laboratory at the Manned Spacecraft Center (MSC). In the foreground are Donald K. Slayton (right), MSC Director of Flight Crew Operations; and Lloyd Reeder, training coordinator.

9. Radar tracking

ARMSTRONG - We had a good manual radar acquisition, and data from the radar agreed well with the VHF ranging information.

ALDRIN - Again, we had P20 in the background, but we didn't use it. This was a manual lockon.

[P20 - Rendezvous navigation program 20,

The purpose of this program is to control the rendezvous radar from startup through acquisition and lockon to the CSM [Command and Service Module] and to update either the LM [Lunar Module] or CSM state vector (as specified by the astronaut by DSKY [Lunar Module computer display and keyboard] entry) on the basis of the RR tracking data.

Calling sequence
   - astronaut request through DSKY V37E20E
        P20 - Rendezvous navigation program 20

P20 may be terminated in two ways
   - astronaut selection of idling program (P00)
     by keying V37E00E or
   - by keying in V56E
        V56 - TERMINATE TRACKING (P20 + P25)

Alarm or abort exit modes - range greater than 400 NM

Display output
   - TRKMKCNT = no of rendezvous tracking marks taken (counter)]

ARMSTRONG - The radar was depowered to cool during the DOI to PDI [Powered Descent Initiation] phase.

Figure 13. S69-40307 (30 July 1969) -- The crewmen of the historic Apollo 11 lunar landing mission stand in the serving line as they prepare to dine in the Crew Reception Area of the Lunar Receiving Laboratory, Building 37, Manned Spacecraft Center. Left to right, are astronauts Edwin E. Aldrin Jr., Michael Collins, and Neil A. Armstrong. They are continuing their postflight debriefings. The three astronauts will be released from quarantine on Aug. 11, 1969.

16. Adequacy of procedures necessary to accomplish DPS maneuver

ARMSTRONG - The [inertial] platform drift check, a P52, was done against the Sun. This procedure seemed to work as we had planned; however, the variation in the data was somewhat larger that I would've guessed. Do you have those numbers?

ALDRIN - Yes. The technique that we used was to compare what the computer thought the little gimbal or the inner angle was and to point the rear detent at the Sun. We'd compare that with what the actual middle gimbal was. Now we did this in PGNS [Primary Guidance and Navigation System] pulse.

The way that we found to work out best was for Neil to me when, in the background, we'd have the AUTO maneuver display 50 18


N18, 3COMP,
      xxx.xx DEG
      xxx.xx DEG
      xxx.xx DEG ]

in P52. We'd call up on top of that VERB 6 NOUN 20 or 22

N20, 3COMP,
      xxx.xx DEG
      xxx.xx DEG
      xxx.xx DEG

N22, 3COMP,
      xxx.xx DEG
      xxx.xx DEG
      xxx.xx DEG ]

[ICDU - Inertial system Coupling Data Unit].

And I'd have NOUN 20 up. As soon as Neil would say "MARK", I'd hit ENTER [DSKY button, enter the command], record NOUN 20. Now the desire is to find out exactly what the computed value is in a close time period. So what I would do is hit the ENTER on the NOUN 20, visually recall what those numbers were, not write them down, but hit KEY RELEASE [DSKY button, it was used to release the control of the DSKY to other routines], which put me back to the 50 18 display. A PROCEED [DSKY button, confirms AGC so the program can continue] would recompute the numbers or maneuver. As soon as I would do that, those numbers would be frozen and the desired gimbal angles would be loaded in NOUN 22.

Then it was just a question of my calling them up, and they should not change the time I hit ENTER to record the gimbal angle that we had until it was recomputed as a desired one that did not exceed 3 seconds. Of course, we had pretty low rates. So I think that the comparison didn't suffer any from a lack of proper procedure. We did find that the numbers were a little larger than we thought they would be. We had it worked out with the ground how we arranged the signs on the differences, so we'd subtract NOUN 22 [NEW ICDU ANGLES] from NOUN 20 [ICDU ANGLES]. The first one was 0.19; second one, 0.16; and the third one, 0.11. The GO/NO-GO value was 0.25. So we're a little closer to this than we had hoped to be.

Figure 14. Lunar Module panels.

ARMSTRONG - The simulator is able to reproduce correctly the control modes that are required to fly it. It's an unusual control mode wherein you fly to in pitch and fly from in yaw. While flying AOT [Alignment Optical Telescope], you depend on the other crewmember to assure you that the roll gimbal angle is staying at a reasonable value.

The simulator was never able to simulate accurately what you would see through the Sun. We especially set up the AOT on the GetC [Guidance and Control] roof (MSC [Manned Spacecraft Center, nowadays JSC, Johnson Space Center]) to look at the actual view. In addition, on the way to the Moon, we looked at the Sun with the telescope; looked through the CSM telescope with the Sun filter on to get used to what the filtered view of the Sun would look like in the optics. It's somewhat different in the telescope than in the AOT in color and general appearance. I can't account for that, but it is different.

I thought the numbers ought to be both closer to zero if we didn't have any platform drift, or closer together in either case. But we had quite a spread, so I'm not sure that the check in general is really as good yet as it should be. In other words, our variation was 0.08 degree between our various measurements . The limit on the GO/NO-GO is 0.25. So, we were essentially using up a third of our margin just in variation between our marks. That's not really a good enough procedure for this important check of the platform. This procedure, being a GO/NO-GO for the PDI needs additional work prior to the next flight.

There are some alternative methods of understanding platform drift, which we just did not have time to implement. Perhaps the next flights will be able to look at some of these alternatives and decide on an even better method than the Sun check.

Figure 15. Lunar Module propellant quantity indicators on panel 1.

ALDRIN - We turned the propellant quantity ON before DOI and I believe the quantity light came on at that point, which was expected as a possibility. Just recycling the switch off and back on again would extinguish the light. The values that we saw in fuel were about 94 and 95, which is what we generally saw in the simulator. The oxidizer value was somewhat lower than that.

The simulator values were 95 and 95. I don't believe that there was sufficient time during DOI for these to settle down completely. They did approach the maximum numbers with a reading of approximately 94. Anyway, they weren't dancing around the way we might have been led to expect them to do.

Figure 16. Steerable S band antenna coordinates. Antenna pitch around antenna Y-axis (+255 degr/-75 degr), antenna yaw around antenna X-axis (+/- 87 degr). When both antenna pitch and yaw are zero the antenna is pointing LM forward (LM Z-axis). There is 45 degrees tilt between antenna and LM X- and Y-axis (around antenna z-axis). Both have the same Z-axis (LM forward).

ARMSTRONG - The pre-PDI attitude prevented good S-band [The S band is a radio wave band with frequencies that range from 2 to 4 GHz] high gain contact. We had continual communications difficulty in this area until we finally yawed the spacecraft right between 10 and 15 degrees to give the high gain antenna more margin. This seemed to enable a satisfactory high bit rate condition, but it did degrade our ability to observe the surface through the LPD [Landing Point Designator] and make downrange and cross range position checks. I don't think that our altitude checks were significantly degraded.

ALDRIN - I can't explain why we had some dropouts there. The angles, 220 in pitch and yaw 30, are not ones that would lead you to believe they would give you trouble as far as interferences from the LM structure. It seemed to me that the initial locken was not bad. There is a certain rain dance you had to go through each time you'd come around to acquire lockon. Each time you'd have LOS [Loss Of Signal, other meanings "Line Of Sight"], we'd usually be on the OMNI's [In radio communication, an omnidirectional antenna is a class of antenna which radiates radio wave power uniformly in all directions].

Figure 17. Lunar Module antennas.

Of course, there's a choice of forward or aft. Then you'd want to switch to SLEW and slew in the proper values for the steerable before LOS on the other side, the ground would like you to not break lock in the slew mode, because in some cases the antenna would then drive into the stops, So, approaching LOS, you'd switch to maybe the aft OMNI and then you'd slew in some new numbers.

We'd make use of pitch 90 and yaw zero, to keep the antenna away from the stops. Once you drive it to those values, then you'd have to set in new numbers.

Figure 18. Lunar Module LM pilot's side, panel 12, Communications antennas section.

Coming around on the other side, you'd maybe switch from aft to forward to pick up the ground. Once you picked them up, you'd switch over to SLEW and you might have the right values down there or you might have to tweak them up. In any event, the initial contact would be made on one antenna; and then, after you establish contact, you have to take the chance of breaking it to switch over to the high gain. Occasionally, we got the jump on them a little bit because the ground was talking to the command module.

We saw that we had signal strength so I'd go ahead and try to lock on the S-band. It is a rather involved process that you have to go through. I didn't find that, if you left the antenna without an auto lockon signal, it would have a tendency to drive to the stops. At least from the indications, it didn't seem to be moving so rapidly that you couldn't, within several seconds if you knew what you were doing, stop it from where it was going and prevent it from hitting the stops.

Figure 19. Lunar Module LM pilot's side, panel 12.

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 rearranged the graph a little bit with some rather last-minute data shuffling to give us something that the two of us could work on at the same time and to give indication of 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 lockon at this point, I had the opportunity to get only about two or three range-rate marks. 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.

Those measurements give you altitude below the command module, essentially. And, of course, there are some modifications of the command module orbit, from the nominal preflight orbit that you expect. The numbers either have to be updated or you have to accept the error.

ARMSTRONG - The measurements against the ground course were indicative of altitude directly above the ground.

ALDRIN - The main purpose of the radar here was to confirm that we were in the same ballpark, the same kind of an orbit. And I think once you accomplish this several times, then it's adequate to go on with the truer altitude measuring device, which is from the ground.

Figure 20. Honeysuckle Creek, Monday 21 July 1969. Hamish Lindsay writes: “This picture was taken of the HSK antenna tracking the Apollo 11 Lunar Module just before Armstrong took his first step onto the lunar surface. Tom Reid, the Station Director, sent me out to record the moment. It was a wet and cold mid-winter morning in Autralia – we were suffering sleet showers at the time, which you can see on the hills behind.”

ARMSTRONG - The ground measurements were very consistent. If they made a horizontal line, it would indicate that you were going to hit a particular perilune, in this case, 50 000 feet (in the middle of the chart). They didn't say that. They were very consistent, but they came down a slope, which said finally that our perilune was going to be 51 000 feet.

It steadied out at about 54 000 feet here at the bottom and our last point was 51 000 feet. This indicated that either the ground was sloping; and, in fact, it was about 10 000 feet lower than the landing site where we started (which is not consistent with the A-1 measurement that we made), or that the line of apsides was shifted a little bit. So actually perilune was coming a little bit before PDI.

Figure 21. Apollo Unified S Band Transponder.

[Unified S Band System Ranging Measurements

Allocating uplink/downlink frequency pairs in a fixed ratio of 221/240 permitted the use of coherent transponders on the spacecraft. Coherent in this sense means there is a specific temporal relationship between the radio uplink and downlink signal phases. Then the phase or timing differences can be more easily analyzed to determine speed and distance between the spacecraft and tracking station.

The Apollo spacecraft receives the uplink carrier, and with a phase locked loop system, generates a downlink carrier related in frequency by the ratio 240/221. When no uplink was received, the transponder downlink carrier was generated from a local oscillator at the nominal frequency. Uplink signals were derived from extremely precise time and frequency standards, and received downlinks were analysed in phase and frequency based on these same standards. A precise "two way" doppler shift was measured, and the resulting speed between the tracking station and spacecraft could be determined to within a few centimeters per second.

The Apollo Unified S Band System also provided distance measurements accurate to within 30 meters. The tracking station generated a pseudo-random-noise sequence at 994 kilobit/s and phase modulated it on the uplink carrier. The spacecraft transponder echoed this pseudo-noise signal back to earth on the downlink. The downlinked pseudo-noise was sent through a correlation process, meaning it was time-shifted to match the transmitted code, revealing the precise round trip light time to the spacecraft and back.

This pseudo-random sequence repeated after about 5 seconds, enough to measure distance out to 540,000 miles. These ranging measurements consumed an appreciable fraction of the downlink capacity and were only needed for short periods, typically during handover from one ground station to the next. After the new uplink station achieved a 2-way coherent transponder lock with the spacecraft, the ranging signal was turned off and the range measurement was continually updated by doppler velocity measurements.]

So we were actually reaching perilune a little bit before PDI, which would tend to slope the curve that way. This was all very encouraging that we were, in fact, going to hit the guidance box so far as altitude was concerned from both measurements (the radar measurements and the ground measurements). But I was quite encouraged that these measurements, made with the stopwatch, were consistent, in fact.

ALDRIN - When you're able to smooth the numbers and plot a reasonable number of than, your accuracy increases considerably. I think the preflight estimates were something on the order of a 6000-foot capability, and I think we demonstrated a much better capability than that.

Figure 22. July 24, 1969. President Nixon Greets the Returning Apollo 11 Astronauts. The Apollo 11 astronauts, left to right, Commander Neil A. Armstrong, Command Module Pilot Michael Collins and Lunar Module Pilot Edwin E. “Buzz” Aldrin Jr., inside the Mobile Quarantine Facility aboard the USS Hornet, listen to President Richard M. Nixon on July 24, 1969 as he welcomes them back to Earth and congratulates them on the successful mission. The astronauts had splashed down in the Pacific Ocean at 12:50 p.m. EDT about 900 miles southwest of Hawaii. Apollo 11 launched from Cape Kennedy on July 16, 1969, carrying the astronauts into an initial Earth-orbit of 114 by 116 miles. An estimated 530 million people watched Armstrong’s televised image and heard his voice describe the event as he took “…one small step for a man, one giant leap for mankind” on July 20, 1969.

17. PDI burn

ARMSTRONG - Our downrange position appeared to be good at the minus 3 and minus 1 minute point. I did not accurately catch the ignition point because I was watching the engine performance. But it appeared to be reasonable, certainly in the right ballpark. Our cross range position was difficult to tell accurately because of the skewed yaw attitude that we were obliged to maintain for COMM [Communications to Earth].

Figure 23. Lunar landing powered descent final LGC program phases. Either all automatic or to some level manual. But never fully manual (P67 has never been used during a real lunar landing) so computer assisted every lunar landing to the touch down.

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. 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, based on engine performance, so that we would still hit the right place. Then, it would be late throttling down so that it would brake toward a higher throttle level prior to the pitch over.

Figure 24. S69-41359 (10 Aug. 1969) -- Astronauts Michael Collins (left) and Edwin E. Aldrin Jr., are greeted by Dr. Robert R. Gilruth, director, Manned Spacecraft Center (MSC), and others upon their release from quarantine. The Apollo 11 crew left the Crew Reception Area (CRA) of the Lunar Receiving Laboratory (LRL) at 9 p.m., Aug. 10, 1969. While astronauts Neil A. Armstrong, commander, and Aldrin, lunar module pilot, descended in the Lunar Module (LM) "Eagle" to explore the Sea of Tranquility region of the moon, astronaut Collins, command module pilot, remained with the Command and Service Modules (CSM) "Columbia" in lunar orbit.

23. LPD [Landing Point Designator] altitude

ARMSTRONG - The LPD was not used until we were below 2500 feet, and it was followed for some number of computation cycles. The landing point moved downrange with time as evidenced by successive LPD readings.

Figure 25. Landing Point Designator. If the pilot keeps the two windows patterns superimposed the computer can pinpoint the predicted landing point by turning LM and showing an angle measurement on the DSKY display.

[LPD Description

At the beginning of the approach phase (P64) the LGC assumes an attitude such that the surface and the landing site are visible, and the commander visually scans the moon for the desired landing site. He will recognize it either by a marker placed by a previously landed spacecraft, as a visually identifiable landmark, or as an appropriate though unmarked site.

Meanwhile, the computer orients the LGC such that the thrust points in the direction required for reaching the current site (that resulting from previous steering, if any, where the LM will land if there is no further steering) and uses the remaining degree of freedom (about the thrust axis) to keep the LPD superimposed on the current site. (There are two LPD scales and a double window, one scale is on the inside window and one on the outside window, to allow the commander to register his eye.) The computer also repetitively calculates and displays the complement of the look angle of the current site.

The LM pilot repetitively reads the angle from the display and repeats it to the commander. The commander identifies the current site by sighting through the LPD and observes the angular error, if any, between the current site and the desired site. If the angular error is significant, he manipulates the controller to cause the computer to redefine the current site closer to the desired site. Through repetition of the total process, the commander literally steers the current landing site into coincidence with the desired site.]

FCOD REP. - Do you recall when you proceeded?

ARMSTRONG - It was very shortly after we were going into P64.

ALDRIN - We got P64 at 41 minutes 35 seconds [the automatic LGC switch from P63, BRAKING PHASE to P64, APPROACH PHASE, indicates throttle down]; then you went MANUAL, ATTITUDE CONTROL.

ARMSTRONG - I can't say whether that was before or after proceeding.

ALDRIN - It wasn't too long after that,

  • 41:35 - P64,
  • 42:05 - manual attitude control is good,
  • 42:17 - program alarm.

What I'm wondering is did the proceed have anything to do with maybe generating same more activity which would cause the program alarm? We weren't in 16 68 at that point.

[VERB 16,

        xxxx.x NAUT MI

        xxBxx MIN/SEC

        xxxxx. FEET ]

ARMSTRONG - I have no recollection of that area.

Figure 26. S69-41360, 08/10/69, Apollo 11 crewmen released from quarantine. Astronauts Michael Collins (left) and Edwin E. Aldrin Jr., are greeted by Dr. Robert R. Gilruth, Director, Manned Spacecraft Center, and others upon their release from quarantine. The Apollo 11 crew left the crew reception area of the Lunar Receiving Laboratory at 9 p.m., Aug. 10, 1969 (41359); Astronaut Neil A. Armstrong (center), is greeted by friends in the crew reception area of the Lunar Receiving Laboratory. Dr. Gilruth is pictured just to right of Armstrong. Donald K. Slayton, Director of Space Flight Crew Operations, is behind Armstrong (41360).

24. Final approach and landing

ARMSTRONG - Landmark visibility was very good. We had no difficulty determining our position throughout all the face-down phase of power descent. Correlating with known positions, based on the Apollo 10 pictures, was very easy and very useful.

ALDRIN - As I recall, there was a certain amount of manual tracking being done at this time with the S-band antenna. During the initial parts of power descent, the AUTO track did not appear to maintain the highest signal strength. It dropped down to around 3.7 and the ground wanted reacquisition 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 else besides that. I think it would be possible to do, if you had sets of predetermined values that you could set in.

Figure 27. DSKY ALT light (when out indicates that the LR is working).

We did have S-band pitch and yaw angles immediately following the yaw maneuver, and those that were acquired at about 3000 feet . After the yaw, the S-band appeared to have a little bit better communications. It was Just about at the yaw-around maneuver (trajectory monitoring from the DSKY up to that point agreed very closely especially in H-dot [altitude rate] and VI [inertial speed] with the values we had on the charts). It was almost immediately after yaw around that the altitude light went out, indicating that we had our landing radar acquisition and lockon.

ARMSTRONG - The delta altitude [altitude correction from the radar, enabled to the LGC manually by astronauts] was -- 2600 or 2700, I believe, is the number that I remember. I think it was plus 2600 or 2700. The yaw around was slow. We had inadvertently left the rate switch in 5 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 rate.

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. 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 at the time when we had radar lockup, as opposed t o about 41,500 that we expected to be.

ALDRIN - There are no discrepancies noted in any of the systems that were checked throughout the first 4 minutes. The RCS [Reaction Control System (propellant levels)] was surprisingly high in its quantity indications. The supercritical [Lunar Module supercritical helium pressurization system "SHe"] did tend to rise a little bit after ignition and then it started back down again. I don't recall the maximum value that it reached. I guess the first indications that we had of anything going wrong was probably around 5 minutes, when we first started getting program alarm activities.

ARMSTRONG - We probably ought to say we did have one program alarm prior to this; sometime prior to ignition, that had the radar in the wrong spot. In any case, as I remember, we had a 500 series alarm [Radar alarms] that said that the radar was out of position, 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. After 5 minutes into descent, we started getting this series of program alarms; generally of the series-that indicated that the computer was being overloaded.

Normally, in this time period, that is, from P64 [THE LUNAR LANDING, APPROACH PHASE] 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. 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 final descent. It wasn't until we got below 2000 feet that we were actually able to look out and view the landing area.

ALDRIN - Let me say something here that answers the question that we had before about the AGS residuals on DOI. They were 0.1 before nulling and we nulled than to zero. X was minus 0.1, Y minus 0.4, Z minus 0.1, and we nulled X and Z to zero. Looking at the transcripts, we did have considerable loss of lock approaching PDI. And we did have to reacquire manually several times. It looked like we had some oscillations in the yaw angle on the antenna. The alarm that we had was 500 and we went to descent 1 and proceeded in the computer and then went back to AUTO again on the landing radar switch. This was prior to ignition and the ground recommended that we yaw right 10 degrees.

Figure 28. The RR was mainly meant to assist in finding the CSM during the ascent phase.
SPEAKER - You had the rendezvous radar on?

[Later it was found that the reason for program alarms was that the RR was running on background and using too much CPU time. Also according to another source: "Repeated LGC jobs to process rendezvous radar were scheduled because a misconfiguration of the radar switches. Thus, the core sets got filled up and a 1202 alarm was generated. The 1201 that came later in the landing was because the scheduling request that caused the actual overflow was one that had requested a VAC area."]

ALDRIN - The rendezvous radar was on, not through the computer, but through its own AUTO track.

Figure 29. LM Rendezvous Radar Antenna Assembly.

ARMSTRONG - We did not have the radar data feeding to the computer in the LGC position; but, apparently, if you have it in AUTO track, there's some requirement on the computer time. This is the way we've been doing it in all simulations. It was agreed on. We were in SLEW. Prior to this time, we'd been in AUTO track until such time as we started to lose lock in the pitch over. Then we went to SLEW, isn't that right?

Figure 30. Some MIT people behind the programs that landed the lunar modules. Front Row: Vince Megna, "Doc" Charles Stark Draper, Don Eyles, Dave Moore, Tony Cook. Back Row: Phil Felleman, Larry Berman, Allan Klumpp, Bob Werner, Robert Lones, Sam Drake.

ALDRIN - Are you talking about the program alarms during the descent? We've passed the point of having the rendezvous radar in AUTO. We'd switched it over to SLEW at that point.

ARMSTRONG - We were in SLEW with the circuit breakers in. Radar was turned on, but it was in SLEW. In the early phases of P64 [approach phase], I did find time to go out of AUTO-control and check the [semi] 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.

At that point, we proceeded on the flashing 64 and obtained the LPD availability, but we did not use it because we really weren't looking outside the cockpit during this phase. As we approached the l5OO-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 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. Continuing to monitor LPD, it became obvious that I could not stop short enough to find a safe landing area.

Figure 31. Crowds gathered to watch the Apollo 11 astronauts in their mobile quarantine facility on the move from Pearl Harbor to Hickam after being unloaded from the deck of the carrier Hornet. ADVERTISER LIBRARY PHOTO, July 24, 1969

25. Manual control/pitch over

ARMSTRONG - We then went into [semi] MANUAL and pitched the vehicle over to approximately zero pitch and continued. I was in the 20- to 30- ft/sec horizontal-velocity region when crossing the top of the crater and the boulder field.

Figure 32. Using the ROD switch and turning the LGC to a semi manual mode the commander can fly the LM like it was a helicopter.

I then proceeded to look for a satisfactory landing area and the one chosen was a relatively smooth area between some sizeable craters and a ray-type boulder field. I first noticed that we were, in fact, disturbing the dust on the surface when we were at 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.

Figure 33. Lunar dust.

I don't think that the 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 velocities than I thought would be necessary.

Figure 34. Lunar landing just before touch down.

As we got below 30 feet or so, I had selected the final touchdown area. For same reason that I am not sure of, we started to pick up left translational velocity and a backward velocity. That's the thing that 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 the backward rate with same possibly spastic 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 because we were close to running out of fuel. We were hitting our abort limit.

Figure 35. New York welcomes the Apollo 11 crew with a ticker tape parade on August 13th 1969. Courtesy of Kipp Teague’s Apollo Image Gallery.

28. Touchdown

ARMSTRONG - We continued to touchdown with a slight left translation. I couldn't precisely determine touchdown. Buzz called lunar contact, but I never saw the lunar contact lights [lunar contact whiskers are connected to the lunar contact indicator lights in the cockpit].

ALDRIN - I called contact light.

Figure 36. One of the many lunar contact lights blue on the far right in an illuminated LM panel 3.

ARMSTRONG - I'm sure you did, but I didn't hear it, nor did I see it. I heard you say something about contact, and I was spring loaded to the stop engine position, but I really don't know whether we had actually touched prior to contact or whether the engine off signal was before contact.

In any case, the engine shutdown was not very high above the surface. 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.

ALDRIN - We had a little right drift, and then, I guess just before touchdown, we drifted left.

ARMSTRONG - 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 I was going to land; I would be looking at a place just 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 time period. I felt that I was a little bit erratic.

Figure 37. Apollo 11 LM telemetry shows throttle oscillations during the last 140 s before touch down.

[During the last 140 seconds of the PDI burn (hover phase), a series of large oscillating-throttle changes occurred (figure above). These changes were approximately 15 percent peak-to-peak about a nominal throttle setting of approximately 26 percent.

There are 2 explanations:

  1. Large and rapid changes in vehicle attitudein pitch during the final phases of landing (Eyles: "Armstrong switched the autopilot from AUTO to ATT HOLD to manually fly over the rocky area") caused centrifugal accelerations on the inertial- measurement-unit accelerometers located at the top of the ascent stage high above the vehicle c.g., thereby giving a false indication of vertical acceleration. This false indication caused the lunar guidance computer to command a throttle change to compensate for an unreal change in vertical acceleration. Later, this problem was investigated under the analysis of the guidance and control system.
  2. The oscillatory character of the P66 throttle command was apparently due to the actual value of the descent engine time constant being smaller than that assumed. And so it was: the performance of the descent engine had been improved, but the ICD was not modified accordingly. The actual time lag for the descent engine was only about 0.075 seconds when it was assumed to be 0.3 seconds. Despite of that both Apollo 11 and 12 flew with 0.2 seconds of compensation for a 0.3 second throttle delay. As a result the throttle was barely stable (until later missions 14, 15, 16 and 17). 
Both confirm that it was not much due to Armstrong's handling.]

ALDRIN - 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 - 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 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 - 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 or little protuberances caning 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 it was quite difficult to do.

ARMSTRONG - 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 - I would think that it would be natural looking out the left window and seeing this moving this way 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 - Since we were moving left, we were yawed slightly to the left so 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, but Buzz could see it.

Then I saw it in the final phases of descent. I saw the shadow came 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. This is clearly a useful tool, but I just didn't get to observe it very long.

ALDRIN - Here's a log entry:

  • +46 seconds, 300 feet,
  • next min + 4 seconds. Watch your shadow, and at
  • +16 seconds, 220 feet.

So I would estimate 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. It was a good-size vehicle. 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, it might be of some aid. But of course, you have to have it out your window."

[Continues to "Lunar Surface" debriefing, see /1/ for more..]

Figure 38. An illustration of the LM ascent stage where the 2nd pilot is chilling out. That might be after the landing?


    July 31, 1969, VOL I,
    Lunar Descent Sections,
    Downgraded at 3-year intervals,
    declassified after 12 years

/2/ Internet

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