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| Feb. 24, 1970, Russell A. Larson (left, Massachusetts Institute of Technology project manager for the lunar module flight program) and David G. Hoag (director of the Apollo Group at MIT’s Draper Lab) in the LM mock-up or simulator. |
[Don Eyles: "The Apollo 11 mission succeeded in landing on the moon despite two computer-related problems that affected the Lunar Module during the powered descent. An uncorrected problem in the rendezvous radar (RR) interface stole approximately 13% of the computer's duty cycle, resulting in five (1201 and 1202) program alarms and software restarts during the descent. In a less well-known problem, caused by erroneous data, the thrust of the LM's descent engine fluctuated wildly because the throttle control algorithm was only marginally stable."]
Apollo 11 (LM-5) /5/
At landing, the quantity of propellants remaining in each of the four tanks was as follows.
1. Oxidizer tank 1 : 5.7 percent
2. Oxidizer tank 2: 2.7 percent
3. Fuel tank 1: 3.9 percent
4. Fuel tank 2: 3.9 percent
The minimum-usable quantities, based on the indication of oxidizer tank 2, allowed a remaining hover time of 63.5 seconds.
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| Apollo 11 landing throttle profile |
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.
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.
[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).]
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| An artist's view of the final phase of the lunar landing |
Apollo 11 Descent Events Timing
There are many written list, audio tapes and videos about the Apollo 11 landing events available from various sources. Maybe the following YouTube video is the most informative of them all.
YouTube video: "Apollo 11 landing from PDI to Touchdown"
Rotation to a windows-up attitude was delayed slightly because of the selection of a slow rotational rate by the crew. This delay resulted in the slight delay in LR acquisition, which took place prior to completion of the rotation.
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| LM Automaitc Guidance programs and landing phases. If not touched the computer would have run first P63 then P64 and finally P65 to put the LM on the Moon surface to a nominal point. Due to various errors in computer estimations the commander usually switched to the P66 mode and landed it semi automated. |
The approach phase was consistent with premission planning. The descent headed into the area near West Crater because of an initial navigation error, approximately 3 nautical miles down range.
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| Apollo 11 LM's final landing point compared to the computer's target point and Armstrong's estimat about it. |
[Armstrong - "My guess was that the outer northeast slope of West Crater was where the computer was taking us."]
[Fjeld - "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."]
During the approach phase, the LPD indicated to the commander that the automatic system was guiding to a landing up range of West Crater. Later on, the landing appeared to be heading into the rock field just beyond West Crater. This uncertainty was caused by several factors: the time rate of change in LPD angle, errors introduced by terrain variations (primarily slope), and the lack of time for visual assessment because of crew diversion to guidance-program alarms. (Refer to the section entitled "Real-Time Analysis"). Therefore, not until the beginning of the landing phase did the commander try to avoid the large area of rough terrain by assuming manual control (P66 guidance) (see previous parts af this article about the LM landing programs available) at an altitude of 410 feet (550 ft) when the forward velocity was only 50 fps. An LPD input was made, but in discussions with the crew, it was determined that this input was inadvertent.
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| Apollo 11 Lunar Module seen from behind at its final landing spot |
The landing site is shown to have been moved, through manual maneuvering, approximately 1100 feet down range and 400 feet (200 ft) cross range from where the automatic guided descent (under P64 and P65 control) would have landed. The somewhat erratic behavior of the attitude and altitude-rate profiles can be best explained by Commander Neil A. Armstrong's comments to the Society of Experimental Test Pilots meeting in Los Angeles on September 26, 1969.
"I [was] just absolutely adamant about my God-given right to be wishy-washy about where I was going to land."
Touchdown have occurred 40 to 50 seconds prior to propellant depletion, only 20 to 30 seconds from the land-or-abort decision point and approximately 52 to 62 seconds longer than predicted for an automatic landing. The flying time below 500 feet was approximately 2 minutes 28 seconds.
[There seems to be some dispute about the last moments of the landing and how much the computer was right or wrong and if the Armstrong's correction was really required or not and how much did it change the final course. The landing was successful and that is the main important.]
Other Apollo Landings
Here are the videos about the other Apollo landings. They are very informative if you are familiar with the special vocabulary and abbreviations used.
YouTube video: "Apollo 12 landing from PDI to Touchdown"
YouTube video: "Apollo 14 landing from PDI to Touchdown"
YouTube video: "Apollo 15 landing from PDI to Touchdown"
YouTube video: "Apollo 16 landing from PDI to Touchdown"
YouTube video: "Apollo 17 landing from PDI to Touchdown"
Which landing was the best .. maybe you can decide?
REFERENCES
/1/ Apollo News Reference, 1968
/4/ MECHANICAL DESIGN OF THE LUNAR MODULE DESCENT ENGINE
Jack M. Cherne, Manager,
Engineering Design Department,
Power Systems Division, TRW Systems,
Redondo Beach, California, U.S.A.
/5/ APOLLO EXPERIENCE REPORT - DESCENT PROPULSION SYSTEM,
William R. Hammock, Jr.,
Eldon C. Currie, and
Arlie E. Fisher,
Manned Spacecraft Center, Houston, Texas 77058
/6/ APOLLO EXPERIENCE REPORT - MISSION PLANNING FOR LUNAR MODULE DESCENT AND ASCENT, Floyd V. Bennett, Manned Spacecraft Center
/7/ LMA790-2 - Lunar Module Vehicle Familiarization Manual - Nov 1, 1969
/8/ Don Eyles - Tales from the Lunar Module Guidance Computer, 2004
/9/ YouTube videos:
"LM - Capcom" audio and
"LM - Flight Control" audio.
/10/ NASA TN D-8227 -
M. D. Holley, W. L. Swingle, S. L. Bachman,
C. J. LeBlanc, H. T. Howard, and H. M. Biggs -
APOLLO EXPERIENCE REPORT - GUIDANCE AND CONTROL SYSTEMS:
NAVIGATION, AND CONTROL SYSTEM DEVELOPMENT PRIMARY
GUIDANCE, May 1976
/11/ NASA TN D-8086 -
D. Harold Shelton -
APOLLO EXPERIENCE REPORT - GUIDANCE AND CONTROL SYSTEMS -
LUNAR MODULE STABILIZATION AND CONTROL SYSTEM, November 1975
/12/ NASA TN D-7990 -
Kurten, P. M.:
Apollo Experience Report - Guidance and Control Systems:
Lunar Module Abort Guidance System., 1975.
/13/ NASA TN D-7289 Willium H. Peters, Kenneth J. Cox
APOLLO EXPERIENCE REPORT - GUIDANCE AND CONTROL SYSTEMS -
DIGITAL AUTOPILOT DESIGN DEVELOPMENT, June 1973
/14/ Wikipedia
/15/ Internet & Flicker
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