Apollo Guidance Computer code now available

[bknight]

...
The story behind Bales' notes is famous.  During the last MOCR simulation a 120x alarm had been thrown and Bales had mistakenly called an abort.  His backroom guy Jack Garman set him straight, and Bales dutifully recorded it.  And if you listen to the flight controllers' loop during Apollo 11's P63, you hear Bales call "go" on the 1202 alarm, and then say "That's the same thing we had!" referring back to the problematic sim.  He later reported that Garman had been on the spot with the recommendation on the guidance loop while Bales was still skimming his notes to see if 1202 was in the group of acceptable intermittent alarms.  But had they not simulated program alarms, Bales might well have called an abort on Apollo 11 without there needing to be one.  You never know what bit of your training is going to be the one that saves a mission.

Yes thanks to simulations they got through what would have been a definite downer!  I wonder if Neil would have ignored the alarm and just gone it an landed?  I remember a NOVA(?) documentary concerning Deak Slayton and the original Mercury astronauts chatting with each other.  During the show Alan Sheppard was musing if he might have ignored an abort when the radar was giving problems.  I believe that was fixed with a reset of the AGS.

[JayUtah]

I wonder if Neil would have ignored the alarm and just gone it an landed?

I rather doubt it because he was known as a fairly by-the-book pilot.  If Kranz had called an abort, I'm pretty sure Armstrong would have obeyed the order.

During the show Alan Sheppard was musing if he might have ignored an abort when the radar was giving problems.

Ed Mitchell was of the very firm opinion that Shepard would have ignored an abort order, which worried him slightly because it implies a "land or die trying" attitude.  Apollo 14 was Shepard's last flight anyway, so that would have encouraged him to push the envelope a little.

[bknight]

It was best for all concerned that the systems were working during the final moments of each flight.

[ka9q]

Kranz's opinion (IIRC) was that Shepard may well have attempted a landing even without the radar, but that the propellant margins were so thin that he would eventually have been forced to abort. I don't think even Shepard would commit deliberate suicide rather than fail to land on the moon.

I love the call that saved that landing. Just like the call that saved Apollo 12, it was nothing more than "try turning it back off and on again and see if that works". In the Apollo 14 case, the radar had incorrectly powered up in its low range mode so it was unable to sense the surface when first expected. Cycling power got it into the correct high range mode, and it switched normally to low range mode at the proper time.

[bknight]

When all else fails, re-boot.

[Kiwi]

...Bales might well have called an abort on Apollo 11 without there needing to be one.  You never know what bit of your training is going to be the one that saves a mission.

As happened on the very next mission with John Aaron's call of, "Flight, try S-C-E to Aux."

[Edited to add: Oops, already mentioned by Ka9q in post 18.]

[ka9q]

I think almost every Apollo lunar mission had some glitch that could very easily have aborted the mission.

Apollo 11: computer restarts during landing; low propellant margins
Apollo 12: lighting strike during launch
Apollo 13: (we know about that one)
Apollo 14: T&D docking problem; LM abort button intermittent short; landing radar startup glitch (more than their share)
Apollo 15:
Apollo 16: LM steerable antenna failure (not sure this ever really risked an abort); CSM gimbal motor problem
Apollo 17:

I can't think of a mission-threatening glitch in 15 or 17, but that doesn't mean there wasn't one.

[bknight]

I think almost every Apollo lunar mission had some glitch that could very easily have aborted the mission.

Apollo 11: computer restarts during landing; low propellant margins
Apollo 12: lighting strike during launch
Apollo 13: (we know about that one)
Apollo 14: T&D docking problem; LM abort button intermittent short; landing radar startup glitch (more than their share)
Apollo 15:
Apollo 16: LM steerable antenna failure (not sure this ever really risked an abort); CSM gimbal motor problem
Apollo 17:

I can't think of a mission-threatening glitch in 15 or 17, but that doesn't mean there wasn't one.

I remember that the explanation for the A11 low fuel was the rock strewn area, caused by the trajectory being long since they had delta v error nearly at mission abort rules.  I don't remember what reason was discovered for the higher delta v, though.

[ka9q]

Well, a contributing factor to the low propellant on Apollo 11 was crew distraction caused by the computer alarms. Armstrong thought he could have picked a better landing spot if he hadn't been distracted.

Also, the propellant wasn't quite as low as it seemed. The quantity light latched on when a low level sensor was uncovered, and this happened early because the propellant was sloshing around in the tanks.

[bknight]

I thought the high delta v error occurred when they were in P-63 or P-64, before taking over manual control.
I'm thinking it was one of the controllers and Kranz, talking about the high delta v in a NOVA series.  But you may be correct as to when it occurred.

[Zakalwe]

I thought the high delta v error occurred when they were in P-63 or P-64, before taking over manual control.
I'm thinking it was one of the controllers and Kranz, talking about the high delta v in a NOVA series.  But you may be correct as to when it occurred.

A contributory fact was that that they separated from the CSM before the docking tunnel had reached zero pressure. This imparted a small delta-V to the LM. That allied to an imperfect knowledge of the Lunar gravitational field caused them to land long.

[ka9q]

What's strange about that one is that even if there had been a slight blow-off from tunnel pressure, that should have been seen by the LM's IMU, which would have adjusted its state vector to reflect it. That is, unless they later initialized the LM's state vector to one produced before separation, which would seem like a strange thing to do. In fact, they were tracking the LM throughout its independent flight, although they might not have been confident enough in the results to actually upload it into the LM's computer. (Tracking only gives you range and range-rate along the moon-earth line, and just a fraction of an orbit of this would not constrain some of the numbers very well).

My bet is still on a bad lunar gravity model. You can have absolutely rock-stable, accurate and drift-free IMUs and accelerometers, but without a precise gravity model you're going to get bad results.

Not until the recent GRAIL mission did we finally get a really good model of the moon's gravity field, on both sides. (The far side was especially poorly known until then.)

[bknight]

Here is the mission report and the only aspect I can decipher is that the de-orbit burn occurred late, thereby allowing for the 20K feet up range error from the original landing point.  Page 54-55

https://www.hq.nasa.gov/alsj/a11/A11_MissionReport.pdf