Re: Apollo 11: A Night to remember

chippy@xxxxxxxxxxxx wrote:
The LM followed its descent trajectory until a point defined as 'Low
Gate' - about 500ft above the programmed landing site. ('High Gate' was
the point in the descent trajectory where the LM was pitched into an
attitude allowing the crew to observe the landing approach - ie: It
moved to a 'feet down, tilted slightly forward' attitude). At Low Gate
the PGNS killed any forward movement, and the LM began a vertical
descent.

It's interesting looking at the transcripts that at the pitchover point not a lot of work gets done; it's the first time the astronauts get a proper view of their landing sight and there's a lot of gawping goes on. Probably just as well that they're not on manual then...

However, this assumes that the landing site immediately below Low Gate
is suitable - not too steeply inclined, and clear of rocks. None of
the lunar mission commanders took the option of a computer guided
descent from Low Gate to touchdown, and all assumed manual command.

Probably in quite a few cases the commanders were 110% sure that they could do a much better job than the computers - which was quite likely the case, and that's not saying anything bad about the PGNS, just good things about the quality of the pilots (or rather the commanders, because the LMPs didn't actually pilot...) and the quality of the site surveys, which didn't have anywhere near the resolution to be certain that a site was in any way suitable.

The Apollo 17 debrief was quite interesting because Cernan decided to fly descent rates of his own devising rather than following the specified descent rates. Probably gave Houston kittens, let alone Schmitt...

113:00:32 Schmitt: Okay, you're 31 feet per second, going down through 500. 25 feet per second through 400. That's a little high, Geno.

[Schmitt - "A lot high!"]

[The nominal descent rate at 500 feet is 17 fps.]

[Cernan - "I had it under control; I knew what the response was and how quickly we could slow down. It was comfortable and it saved fuel. In the Navy, we land aboard carriers at 15 feet per second and I knew what that was; and I'd flown the LM Simulator so many times that I knew what 15 feet per second at 100 feet was like and knew that I could slow it down to 5 feet per second awfully quickly."]

--
Angus G Rae Science & Engineering Support Team
Computing Services
University of Edinburgh
The above opinions are mine, and Edinburgh University can't have them
.

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