LM Computer and Apollo 11 Landing

[Kiwi]

Don Eyles, one of the Apollo Guidance Computer programmers at MIT, has a copy of an interesting paper he wrote in 2004 on his web site:
www.doneyles.com/LM/Tales.html

Note: Save the link if might want it for future reference as it doesn't appear to be accessible from his home page.

ABSTRACT: 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 interface stole approximately 13% of the computer's duty cycle, resulting in five program alarms and software restarts. 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. The explanation of these problems provides an opportunity to describe the operating system of the Apollo flight computers and the lunar landing guidance software.

Eyles can be seen in the NASA movie Apollo 14: Mission to Fra Mauro between approximately 0:06:24 and 0:06:41. These times are for a 27 minute 47 second version of the film that doesn't start with the NASA seal, but starts with Freedom 7 on the launch pad on 5 May 1961.

[Czero 101]

Great find, Kiwi!...



Cz

[Kiwi]

czero101 said:

Great find, Kiwi!...

Credit where it's due though, and this time it goes to... Ta Dah! ... Altair4. After following his Googling instructions and observing that he was muddling the atmospheric re-entry with the lunar landing, I scanned down Google's list and spied the name Don Eyles.

Just like JayUtah, Eyles has the talent of being able to explain complex things in an interesting and easygoing way that laypeople can understand, and earlier today I read his paper right through. I thought it was excellent, and parts of it were just as exciting as reading elsewhere about the landing.

As he explains,

P63 was called the braking phase because its only purpose was to shed horizontal velocity. It would end in about eight minutes when the spacecraft reached target conditions known as "high gate" at about 7400 feet altitude.

I like the way he personifies things at times:

The discomfiture of the CDUs was at its worst when the phase angle between the two 800-Hz waveforms was near 90 or 270 degrees — and Apollo 11 evidently hit one of these sweet spots.

And to his team's credit:

With a 10% margin and a 13% drain, the LGC simply did not have enough CPU time to perform all the functions that were required. Thanks to the flexibility of the Executive design — and quite unlike what would have happened with a boxcar structure — there was no collapse… The interesting effect of this train of events, during P63, was that the problem fixed itself… We had devised a real-time control system that under certain conditions was "fault tolerant".

A nice description of the type of problems they had to tackle:

A fixed-thrust engine and a very simple guidance equation could put a spacecraft through a spot on the lunar surface. But to get there right side up, moving slowly, with visibility and the ability to hover while choosing a landing area, required an engne that could balance lunar gravity while varying its thrust as the vehicle's mass decreased, as the vertical component of the thrust vector changed during attitude maneuvers, and as the astronaut requested changes in the descent rate.

And pretty amazing to read of a fortunate error:

...if the software had compensated at 0.3 seconds on Apollo 11, the throttle would have been unstable. The throttle oscillations, instead of settling down, would have become greater... An abort would have been inevitable. With all modesty, it appears to be the case that if the author had coded the "correct" compensation number in the throttle-control routine, Apollo 11 would not have landed. I invite someone with no personal stake and a grasp of the mathematics to reexamine this theory.

Isn't the internet wonderful at times?

[Kiwi]

With all modesty, it appears to be the case that if the author had coded the "correct" compensation number in the throttle-control routine, Apollo 11 would not have landed. I invite someone with no personal stake and a grasp of the mathematics to reexamine this theory.

Is there anybody here who could do that?

Perhaps somebody who's using new tools <plug mode=shameless>running on state-of-the-art supercomputers</plug>?

In my experience, few people ever respond to such an invitation and this sounds like a particulalry worthy one to me, if it hasn't been accepted and the matter settled already.