reply to post by orangetom1999
You are stating in your post that the automatic pilot or automatic landing controls were not working or suspect.... If this were so..how did
they get back home??
Well, 'willy' has signed off, so I'll help. A little bit of Apollo history is in order.
Firstly, as they descended, they found that they were going 'long', somewhat downrange of the intended touchdown point. This is likely because of
some additonal forward velocity imparted to the spacecraft as air vented out of the connecting tunnel, used to move between the CM and the LM.
That was no big worry. But, as they got nearer the surface, Armstrong saw where the computer, and thus the A/P, was taking them, and it wasn't
appropriate for the landing. They needed someplace smooth, and relatively level. SO, he flew manually, which was planned anyway, but he took over a
bit sooner than planned.
The computer onboard also had some probs, too much data input, and it's discussed here, on a forum:
The story of the the 1202 computer alarm going off at a very inconvenient time during Eagle's powered descent to Mare Tranquilitatis is
well-known and often recounted in documentaries, as it ads a lot of drama to the story (as it did, undoubtedly, to the descent).
None of the documentaries I watched, however, offered a reason for the alarm (meaning data overload of the onboard computer, if I understand it
correctly) - fair enough, I thought, computer glitches occured before the invention of Windows.
I was surprised, then, that in the beautiful documentary, "In the Shadow of the Moon", Buzz Aldrin offered a simple explanation: "Being Dr.
Renezvous", and contrary to the flight manual, he had left the rendezvous radar on in case he needed it quickly, and the data flow resulting from
both the landing and the rendezvous radar caused the overload (The film is on YouTube here, and the story begins at 6:45 into the segment).
My questions - Is this the accepted explanation for the alarm? Were there any consequences from it? I guess Aldrin's explanation for him leaving the
rendezvous radar on is very reasonable. Was there a modification on subsequent flights to allow for the added data flow, or was the dual radar just
avoided? I also got the impression Aldrin rolled his eyes a bit that nobody anticipated that he would do what he did. What came out of the debriefings
regarding this error?
(And, the answer):
The story is told in considerable detail in Murray and Cox's "Apollo - The Race to the Moon". It suggests Aldrin's answer is correct, but
incomplete. The key was that the rendezvous radar was taking up a large portion of the computer's time, all the while performing a useless task.
The task related to the radar's mode switch, which could be in one of four settings. For the descent to the Moon, the radar was originally to be in
one mode. Not long before launch, it was decided to use a different mode, which required the development of new software and new procedures. The new
software was loaded, but then the engineers decided the new procedures were too much to introduce at such a late stage. So they came up with a method
of disabling the software which they thought was failsafe. Unfortunately, all they did was make the LM computer undertake an impossible task
(described in the book as trying to calculate an angle with a sine and cosine of 0). Doing that apparently used nearly one-fifth of the computer's
time, and contributed to the computer indicating that it was overloaded.
It sounds like it wouldn't have been that hard to remedy for later missions. What puzzles me is why it was never picked up in any of the simulations
- Aldrin presumably didn't do anything in the landing that he hadn't already done in the sims.
Anyway, I'm sure I haven't given the story full justice, so if anyone can tell it better than me, go right ahead.
edit: Forgot about the other half of your question. They had an entirely separate computer and navigation system on the CM, so all the LM computer
had to do was guide them up to orbit and rendezvous. Could have done that manually, too.
Ya know, if you watch Ron Howard's Apollo 13
there's a scene where they had to fire a correction burn during their trip back to Earth ---
I also found this short read (found one that's 37 pages, stuff I find interesting, but not everyone does
Google DonEyles.com if you wanna see
Apollo Mission 1202 Alarm
[edit on 30 July 2009 by weedwhacker]