Thursday, July 25, 2019


My Brother's Role in the Moon Landing-- (re: Allan Klumpp)

Apollo 11: I’ve known for years that my brother designed the path of flight from the mother ship (the Lunar Module) to the moon.  But only a week ago, a friend perusing the internet found this account, written by Allan 25 years after the event.   Maralys Wills  

Apollo 11 Ignores Descent Guidance Faults, Lands Anyway

During the eleven minutes of the Apollo 11 lunar landing, several alarms appeared on the display of the Lunar Module (LM). As each appeared, Buzz Aldrin, LM pilot, immediately read it aloud for Neil Armstrong, LM commander, for the team of Gene Kranz, flight director for the lunar descent in Houston, and for the designers of the descent guidance, including myself, at MIT Instrumentation Laboratory in Cambridge. None of us had any idea what caused these alarms, whether the fault was minor or a prelude to disaster. Nonetheless, Kranz directed Armstrong to press on rather than opt for safety by aborting the descent and returning to the orbiting Command Module (CM).

In ensuing days, months, and years we found out what happened. The crew’s checklist called for turning on rendezvous radar during descent; it could be needed if the descent were to be aborted and the LM returned to the CM. But connections to the radar were incomplete, failing to synchronize its power supply with others. The radar’s power supply drifted in and out of phase. When out of phase, the radar ate up about 15% of the guidance computer’s time; there was only an 8% margin. Instrumentation Lab colleague Russ Larson now says that a time-consuming command from the astronauts aggravated the problem. As a result, the guidance computer was failing to finish its tasks, and it was complaining. The explanation became complete only this year at the design team’s 25th reunion at the lab, now renamed for its founder Charles Stark Draper.

My part of the official investigation showed that throttle and steering commands, which the guidance computer was supposed to issue every two seconds, were often incompletely computed, and were queued for later completion. Any attempt to queue a command when the queue was already full (about five commands) would cause the computer to flush the queue and issue the alarm. But when the radar’s power supply was in phase, queued commands, valid only at some remote past time, could be completed and issued in reverse order, momentarily taking control to guide the LM off its normal landing trajectory. Although flushing commands would cause alarms, issuing faulty commands would not. Simulations showed that faulty commands could put the LM on a crash course, and guidance would attempt to take the LM to the landing site via a trajectory that passed beneath the lunar surface.

A day or so before the reunion, the Boston Globe described an exchange between Larson and other members of the support staff at Houston as the alarms began. Not knowing what was happening, Jack Garman asked Larson what to do. Larson signaled thumbs up, Garman relayed the recommendation to Kranz, and Kranz directed Armstrong via Capsule Communicator Charlie Duke to press on.

At the reunion, I talked to Russ, and he confirmed the story. I asked what made him think the landing trajectory was safe, and he said his displays looked normal. I told him my simulations showed a crash course would look normal until it was too late. I asked why he had merely signaled thumbs up rather than giving his recommendation verbally. He said he was too scared to speak.

(529 words) @ Allan R. Klumpp,  1994

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