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60 Seconds, the last few moments before Apollo 11 touched down

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60 Seconds, the last few moments before Apollo 11 touched down

Old 1st Jul 2021, 00:59
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60 Seconds, the last few moments before Apollo 11 touched down

Not really an aviation topic but here goes

It’s well known that Apollo 11, under Neil Armstrong’s control had to be guided away from the pre planned automatic touchdown point, he took over and manually ‘flew’ the lunar lander away from this boulder strewn location to find a less hazardous, smoother landing point

This maneuvering used up a lot of fuel and they started to get quite low, the controllers were so concerned they started to call out their fuel quantity in seconds remaining

I believe they were down to less than 30 seconds when they landed.

Of course this fuel remaining was only for the descent and landing phase, the ascent fuel was separate in the upper, launch section of the lander

Question is, what if they had completely depleted their descent fuel, would the lander have gone into an automatic abort, assuming this would have required jettisoning the lower, landing stage, would there have been time to do this before an uncontrolled ‘crash’ into the lunar surface even though there’s not much gravity they would have continued to ‘fall’ ?

Any space experts ?
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Old 1st Jul 2021, 02:15
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They always anticipated the need for a 'landing abort' - and it was practiced endlessly in the simulator in Houston. Had the descent stage fuel gotten critically low, Houston would have called for a landing abort since they assumed the LM would be under active control when the abort occurred (and if they ran out of fuel the LM would instantly become uncontrolled). Not saying they couldn't have successfully aborted after fuel exhaustion, but it would have been even more difficult. That being said, it was later determined that they had more fuel left than they thought (not sure how they determined that, it's not like they could go check the tanks...).
There always was some concern that a landing abort - especially at very low altitude - might not be successful. No one knew just how accurate the LM simulator really was. Apparently - at least for the maneuvers they actually performed - it was pretty darned good since the astronauts reported later than the real thing was actually easier to control than the simulator (and much easier than Lunar Landing Research Vehicle (LLRV) - of which they crashed several). If they needed to abort, the lightoff of the ascent stage was pretty much instantaneous - if you've ever seen the footage of the Apollo 17 liftoff from the moon, it gives you an idea of just how sudden the ascent motor comes on.

But the whole Apollo 11 landing was a pretty nerve racking affair - and the extra time that Neil spent looking for a good landing spot just added to the suspense.
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Old 1st Jul 2021, 03:50
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There are several websites that have the audio from the LM and Flight Control.
The way the landing was controlled from PDI to Touchdown still gives me a lump in my throat after all these years.
It was a privilege to watch it happen.
Pure genius.
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Old 1st Jul 2021, 04:23
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Originally Posted by tdracer View Post
They always anticipated the need for a 'landing abort' - and it was practiced endlessly in the simulator in Houston. Had the descent stage fuel gotten critically low, Houston would have called for a landing abort since they assumed the LM would be under active control when the abort occurred (and if they ran out of fuel the LM would instantly become uncontrolled). Not saying they couldn't have successfully aborted after fuel exhaustion, but it would have been even more difficult. That being said, it was later determined that they had more fuel left than they thought (not sure how they determined that, it's not like they could go check the tanks...).
There always was some concern that a landing abort - especially at very low altitude - might not be successful. No one knew just how accurate the LM simulator really was. Apparently - at least for the maneuvers they actually performed - it was pretty darned good since the astronauts reported later than the real thing was actually easier to control than the simulator (and much easier than Lunar Landing Research Vehicle (LLRV) - of which they crashed several). If they needed to abort, the lightoff of the ascent stage was pretty much instantaneous - if you've ever seen the footage of the Apollo 17 liftoff from the moon, it gives you an idea of just how sudden the ascent motor comes on.

But the whole Apollo 11 landing was a pretty nerve racking affair - and the extra time that Neil spent looking for a good landing spot just added to the suspense.

Well

you mention mission control would have called for an abort if fuel became critically low, how much lower would that be ?


They were down to less than 30 seconds remaining I think, seems like they were already critical

Of course Neil and Buzz were on the spot and could tell they were going to make it before running out but it really wasn’t that clear to mission control


Hence the ‘we’ve got a lot of guys turning blue’ remark by the Capcom after they touched down


It sounds like the lander wouldn’t automatically go into an abort if they did completely deplete their fuel and they’d have to accomplish this manually, you say the ascent stage light off was instantaneous but they would have to jettison the lower, landing stage wouldn’t they ?and instantly reverse their descent rate that would take precious seconds


I’d imagine there was an altitude beneath which that was impossible
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Old 1st Jul 2021, 05:03
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Abort options and risks explained in this article from 1966.

https://www.wired.com/2012/05/the-ea...-crashed-1966/

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Old 1st Jul 2021, 12:23
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The following presentation is one of the best I’ve seen which goes through the entire descent of the LM and how the computer was working, what the procedures were, and how the LM was actually doing exactly what is was programmed to do. It’s quite long, but completely fascinating. I’ve got quite a bit of time on my hands these days.

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Old 1st Jul 2021, 12:36
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https://www.ibiblio.org/apollo/Docum...iderations.pdf

LUNAR PILOT MODULE - CONTROL CONSIDERATIONS


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Old 1st Jul 2021, 17:00
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No it wouldn’t have been an automated abort.

As for the fuel calls… they weren’t a direct count of seconds to tanks dry.

The rationale behind the timing calls was that NASA and the astronauts wanted to avoid if at all possible going through a process known as “abort stage” at low altitude.

That was a series of steps involving the descent stage being jettisoned from the ascent stage, various systems in the ascent stage being quickly powered up and pressurized and then the ascent engine firing to return the vehicle to orbit.. That sequence could take anything up to four seconds and if the LM was near the surface the crew might run out of altitude before the the ascent engine generated a rate of climb.

The other big danger issue with a low level abort stage was that it risked “frag” damage to the ascent stage if the descent stage hit the surface and components within it exploded..

So….the fuel timing was run such that when the timing got to zero on the stopwatch (managed by a gentleman called Bob Carlton) there were still approximately 20 seconds of fuel left in the descent stage. At zero on the watch the capcom in Houston (Charlie Duke on 11) would call “ fuel depletion”, which was a signal to the crew that they had to either:

1. Commit to continue to land in the next 20 seconds…

or

2. Abort by applying full power on the descent engine to immediately start climbing away, gaining altitude and building up a rate of climb. Once the descent engine ran out of fuel after a few seconds the abort staging could be done with the vehicle well clear of the surface and with a positive rate of climb.

HTH

Last edited by wiggy; 1st Jul 2021 at 21:11.
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Old 1st Jul 2021, 17:13
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I should perhaps add…at low fuel levels the gauging was inaccurate ( in part due to fuel sloshing around in the tanks.)

On 11 (and 12) that caused a fuel “low level” light to come on earlier then perhaps expected and the timings you hear on the recordings were all based on the time that light illuminated, not from a quantity gauge. As a result of the low level light coming on early the fuel calls on both missions were a bit pessimistic, post flight it was generally reckoned Eagle landed with maybe 40-45 seconds of fuel in the tanks.

https://www.watchtime.com/featured/t...-moon-landing/

After Apollo 12 the tanks were modified with anti slosh baffles to reduce gauging/indication problems.

edit to add: It’s all very comprehensively covered here:

https://www.hq.nasa.gov/alsj/a11/a11.landing.html

and specifically the comments at the 102 hrs 42 min 35 sec point, which includes..

[Post-flight analysis indicated that Neil landed with about 770 pounds of fuel remaining. Of this total, about 100 pounds would have been unusable. As indicated in an unnumbered figure from page 9-24 in the Apollo 11 Mission Report, the remainder would have been enough for about 45 seconds, including about 20 seconds for an abort.]

Last edited by wiggy; 1st Jul 2021 at 21:14.
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Old 2nd Jul 2021, 12:40
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Originally Posted by TukwillaFlyboy View Post
There are several websites that have the audio from the LM and Flight Control.
The way the landing was controlled from PDI to Touchdown still gives me a lump in my throat after all these years.
It was a privilege to watch it happen.
Pure genius.
Entirely agree. It was nail-biting and exhilarating at the time. Like you I still get a lump in my throat and prickling of the eyes hearing those words..
The calm way all those involved impressed me very much - and like many others I tried in my own small way to behave like that; prepare well and stay calm when things go wrong. It seemed (and still seems) an honour to be a very small part of that same community of pilots & engineers.

Superb engineering and a wonderful human achievement by people who believed in the capability and in themselves. he lack of any feasible back up plans for many of the elements of the flights - would we dare to do this now? So many things were singled down.
.
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Old 3rd Jul 2021, 01:08
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Originally Posted by biscuit74 View Post
Entirely agree. It was nail-biting and exhilarating at the time. Like you I still get a lump in my throat and prickling of the eyes hearing those words..
The calm way all those involved impressed me very much - and like many others I tried in my own small way to behave like that; prepare well and stay calm when things go wrong. It seemed (and still seems) an honour to be a very small part of that same community of pilots & engineers.
Neil Armstrong apparently had nerves of steel - the guy was totally unflappable. My favorite story - I think it was told by Alan Bean (Apollo 12) - was regarding when Neil had to eject from the LLRV literally seconds before it crashed (and then the parachute floated frighteningly close to the crash fireball). That happened in the morning - that afternoon he was in office like nothing had happened - Alan Bean was going to see him in his office for some reason - someone told Alan that Neil had crashed the LLRV that morning. So when Alan went into Neil's office, he immediately said something like 'did you crash the LLRV this morning?' Neil nonchalantly said 'yea' without even looking up from his desk - sort of a 'yea, I almost got killed, no biggie'.
A rare breed indeed.
BTW, the one that always gets me choked up is when I listen to Apollo 8 reading from the Book of Genesis on Christmas Eve 1968. So perfect, even if you're not religious...

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