Apollo 12 LM AGC STBY

[riclourenco]

Hello,

I have a problem with my Apollo 12 mission. This is the 2nd time I've done this mission, and I've been stuck for a week trying to figure out what I did wrong, taking into account that the first time I did the mission I didn't I had this problem, and everything went well. After the second EVA, when I start the checklist to power up the LEM, in the GET where we have to take the AGC out of standby by continuously pressing PRO, instead of the "restart" light , the PROG light comes on along with the master alarm. Initially I ignored it, and continued with the checklist, but later other problems arose, including after taking off to carry out the rendevouz, the LEM did not point to the CSM to pick up the rendevouz radar signal, and of course I was unable to continue. Does anyone have any idea what I could have done wrong? I leave here the scenario where this happens. Thanks.

 

[indy91]

The LGC clock didn't update properly, so very likely, when you are doing the lunar ascent, the clock is still many hours off and that is responsible for the LGC not finding the CSM.

There are two possible things going on. The AGC has a time limit for how long it can stay in standby mode and still be able to update the clock. That is why between the two EVAs you come out of standby mode for a moment and go right back in. See surface checklist page SUR-65, "PRO, V37E 06E, PRO". If this was not done and you just stayed in standby mode then you exceeded the time limit and the clock wouldn't have updated.

The other thing is a NASSP and/or Virtual AGC bug related to standby mode. Sometimes when coming out of standby mode, seemingly randomly, the necessary restart to update the clock doesn't happen. If this is what happened in your scenario it happened during the short procedure between the two EVAs. You can trigger the restart yourself, if you know the bug has happened, by doing V69E while still in P06. You can recognize it from not getting any restart light or program alarm when coming out of standby. Then you should immediately do the V69E. That did not help in the scenario you posted, so the problem likely occured earlier, between the two EVAs. All you can do in this scenario is update the clock manually. Then everything should work out fine.

But yeah, we have known about the standby mode bug for years and we still don't have a clue why it happens. We really need to try and figure it out some time.

 

[riclourenco]

Thanks! I went back to GET 130H between the two EVAs where we do the "PRO, V37E 06E, PRO" procedure. The PROG light comes on after I do V37E06E. I think something is not right here either. Also regarding the clock being several hours out of date, I had already managed to correct it the first time, using the V25 N36E. Then I confirmed the update with the V16 N65. When I said that the LEM didn't point to the CSM, I had managed to update the GET previously. I never took off from the moon without an updated GET, because I already knew it was going to go wrong, but it ended up not going well anyway. Here are two scenarios if you want to see; one at GET 130h, and the other after the moon launch. Unfortunately, I no longer have the scenarios I created after that, at the moment the LEM has to point to the CSM. But you can see that the GET is updated in that one.

 

[indy91]

Your IMU alignment is off by 6°. But only in one axis, yaw. I think I have seen this before. Did you use the "REFSMMAT + gravity" option in P57 at all? What can happen is this. You do a P57 using any other technique and the LGC saves your yaw angle sitting on the surface. Then an Orbiter-only thing happens (e.g. RCS hotfire) and you get rotated in yaw. If you do a P57 technique without any star after that then P57 will use data from that previous P57, assuming you never moved relative to the lunar surface. But that's just a theory for what could have happened in your case.

 

[riclourenco]

From what I remember, I did the P57 with the technique that is on the checklist. But of course I may have made a mistake that I didn't realize. I've done Apollo 11 several times for example, nothing like that ever happened, just like the previous Apollo 12.
Regarding GET 130H between the two EVAs where we do the "PRO, V37E 06E, PRO" in the other scenario, since the restart light doesn't come on, I assume the bug occurred there, should I do a V69E right away? I already tried it, and then I did V37E06E and put it back in STBY. But if I remove the STBY again after that, the PROG light comes on with the master alarm. Does this mean that the V69E did not work previously?

 

[indy91]

From what I remember, I did the P57 with the technique that is on the checklist. But of course I may have made a mistake that I didn't realize. I've done Apollo 11 several times for example, nothing like that ever happened, just like the previous Apollo 12.

It might not have been something you did wrong, just, in some circumstances the LM can rotate in yaw while sitting on the surface in Orbiter, while in reality that of course didn't happen. That said, I only see the technique 3 (Gravity + Celestial Body) in the checklist and I don't think the issue would happen there. So not really sure how you got that bad alignment. It's still pretty clear that it's only off in yaw, so it kind of has to be a rotation of the LM on the surface at a bad time.

Regarding GET 130H between the two EVAs where we do the "PRO, V37E 06E, PRO" in the other scenario, since the restart light doesn't come on, I assume the bug occurred there, should I do a V69E right away? I already tried it, and then I did V37E06E and put it back in STBY.

The restart light doesn't necessarily have to come on when the AGC internally does a hardware restart. I thought that the indication for the bug happening is either a restart or a program alarm. So if I didn't get either of these I would do the V69. That has to be done while still in P06. V69 should reliably work, but I am not 100% sure anymore if the lack of lights is really the indication for the bug.

But if I remove the STBY again after that, the PROG light comes on with the master alarm. Does this mean that the V69E did not work previously?

Internally what goes on is, the AGC continues with clock counts in standby mode, but not with its normal clock. Then, when you leave standby mode, the AGC should update its normal clock to current time. But this only happens if a hardware restart happens. And the bug is, for some reason, our AGC doesn't properly "wake up". The V69 helps it doing so. At that point the AGC clock time (accessible with V16 N65) should be updated.

 

[riclourenco]

So the alignment was really bad luck... If it weren't for that, right at the beginning when I managed to update the GET in the AGC, the rendevouz should have gone well after the take-off from the moon. So the only way to fix this would be to go back to the beginning of the PDI and land again I think. But in this case I prefer to start a new mission. Assuming that the alignment problem is unlikely to happen again, the only issue that may occur will be the restart problem, which can be resolved with V69E, or by updating the GET with V25 N36E, and checking if it is updated with V16 N65. We'll see how the next mission goes. Thanks for everything!

 

[indy91]

We have finally found the cause of the standby mode bug and it is now fixed in the latest NASSP release!

 

[riclourenco]

That's great! Thanks