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
