P52 Tips and Help

[thammond]

I'm to about 2 hours into the Apollo 7 mission and I'm performing the P52 IMU Realign checklist.


Does anyone have any general tips for this procedure? I'm guessing that this is something that I'll get better at with some practice, but it's been difficult to get a good mark as the star moves across the sextant very quickly. Maybe slow the time down during this procedure until I get better at it.


How close to the center does the mark have to be to prevent problems later on? Can I use the Mark Reject button if I have a bad mark? If so do you just manually rerun through the P52 steps until you catch back up to the current spot on the checklist?


Finally (and my biggest issue) the 2nd mark specifies to use star #41. But by the time I get to the V06 N92 point in the checklist for the 2nd mark, the trunnion angle on R2 is around 48 degrees and increasing over time. If I'm understanding everything correctly, the trunnion angle has a maximum angle of 49.775 degrees. Which doesn't allow me enough time to get the star centered and a mark taken before it exceeds 49.775 degrees. Would that mean that I would have to rotate the entire S-IVB & CSM stack to get a mark? That doesn't seem like that is what would have happened in the real Apollo 7 mission for this particular IMU realign?

 

[Thespacer]

Re the star moves too fast, I have found the issue exacerbated in earth orbit, in the SIVB stack during EPO on Apollo 8 (have not tried A7). Are you in inertial mode? If so, obviously it helps to null any rates as far as possible. I found that the DAP does not null as much as I thought it would, so initially I resorted to using the killrot function in the Project Apollo MFD. Cheating yes, but until I got proficient it was necessary.

If you’re in orb rate, then I’ll leave it to others to provide tips.

Tony

 

[thammond]

Yes I'm in orbital rate so that is making it a bit more difficult I'm sure.


I'm rerunning this portion of Apollo 7 at a slower time speed to see if that will help. I noticed this time around that the audio clip just before the P52 IMU Realign checklist started stated that the stars to use would be #2 & #4.


Star #4 looks like it would be a much better star to use for the 2nd mark. Am I doing something wrong to get star 41 selected?


Also when I tried rerunning at .1 time acceleration, the P52 seems to have gotten stuck at the F 04 06 portion of the program. The checklist has moved on beyond, but the AGC display has not. Not sure if it's because I was moving ahead of the checklist (performing actions before they were highlighted on the checklist), or if it's because I'm running at .1 acceleration?

 

[indy91]

How close to the center does the mark have to be to prevent problems later on? Can I use the Mark Reject button if I have a bad mark? If so do you just manually rerun through the P52 steps until you catch back up to the current spot on the checklist?

What you want is a star angle difference (F 06 05, shown after the second mark) of less than 0.03° (+00003). If you get more than that with the sextant you should repeat the P52. In orb rate achieving this is not super easy. I usually try to anticipate where the star is going to be, move the sextant there, and if the star actually passes right through the center I mark. If not, I readjust. After separation from the S-IVB it gets easier, as you can just null your attitude rates. As Thespacer said, the DAP isn't actually that great at nulling the rates. So I usually switch from CMC to SCS control and fine tweak the rates there. Sometimes I use 0.1x for this to get even more fine control, but that's kind of cheating. Same goes for the kill rotation in the PAMFD. But if you are new to P52 then this is definitely helping.

Yes, mark reject allows you to redo a mark if you didn't like it. I think it just goes back one step of P52, so you just have to do the mark again.

Finally (and my biggest issue) the 2nd mark specifies to use star #41. But by the time I get to the V06 N92 point in the checklist for the 2nd mark, the trunnion angle on R2 is around 48 degrees and increasing over time. If I'm understanding everything correctly, the trunnion angle has a maximum angle of 49.775 degrees. Which doesn't allow me enough time to get the star centered and a mark taken before it exceeds 49.775 degrees. Would that mean that I would have to rotate the entire S-IVB & CSM stack to get a mark? That doesn't seem like that is what would have happened in the real Apollo 7 mission for this particular IMU realign?

That sounds like some bad luck with timing. You could either rerun P52 entirely, where it will likely give you different stars. But even if you let the AGC decide on the star pair, you aren't actually bound to using those. So what you can do is, when you are supposed to mark and it doesn't work out, is just look for a different star and mark on that one. That will give you the usual

F 50 25, 00016 (terminate marks)

display. You PRO on that and then you need to confirm on which star you marked:

F 01 71, 000XX (XX for star code)

There it will show you the star that the AGC wanted you to mark on. But you can change this to the star code you actually used. V21E 000YYE, YY being the actual star you marked on.

I'm rerunning this portion of Apollo 7 at a slower time speed to see if that will help. I noticed this time around that the audio clip just before the P52 IMU Realign checklist started stated that the stars to use would be #2 & #4.

Star #4 looks like it would be a much better star to use for the 2nd mark. Am I doing something wrong to get star 41 selected?

Nah, not really. It will just depend on timing as you are rotating all the time in orb rate and the AGC also wouldn't give you stars behind the Earth or too close to the horizon. So it just depends when the AGC runs the PICAPAR routine (pick a pair of stars). That happens when you press PRO on the F 50 25, code 00015. Even 30 seconds earlier or later could already result in different auto-selected stars.

Also when I tried rerunning at .1 time acceleration, the P52 seems to have gotten stuck at the F 04 06 portion of the program. The checklist has moved on beyond, but the AGC display has not. Not sure if it's because I was moving ahead of the checklist (performing actions before they were highlighted on the checklist), or if it's because I'm running at .1 acceleration?

The AGC and Checklist MFD can become desynced sometimes, 0.1x probably makes it worse, but the most common occurance for me is when there are multiple PRO key presses after each other than the Checklist MFD doesn't notice them all. It's not too bad, you just need to know which step to do to get back in sync.

 

[Thespacer]

I might just add that, again if in inertial mode, switching off some RCS quads / individual thrusters combined with SCS and min imp makes it quite possible to provide very fine attitude adjustments, so as to null rates for the purposes of getting a star (or horizon, if using P23) fix.

Tony

 

[thammond]

Thanks Indy and Tony,


I got through the P52 alignment checklist using your tips and help. I did use a different star then the AGC selected and slowed down time to make the mark (hopefully I can do this in real time in the future with a bit of practice) and ended up with the required star angle precision.

 

[meik84]

As we talk about it: I know that we can't simulate the real 'full-view' mirror of the SXT. But: what about a 'half-view' mirror like most old sextants (incl. mine from the german Bundesmarine, YOM 1961) had? It might be not as good as the real thing, but still better as this flickering epileptic seizure trigger we have right now. Sure, it takes some practice first, but at least I'm as fast and precise as my friend with his new super-fancy full-view sextant.

 

[Thespacer]

Would love a different approach to the flickering. I understand how it is implemented as a creative compromise, but it sure is painful. Particularly given all of the P23s conducted during Apollo 8 :lol:

Tony

 

[thammond]

Running into additional P52 issues, this time on the Apollo 7 scenario at about the T+5 hr mark. I felt like the procedure was MUCH easier this time around as opposed to doing it in the orb rate mode.

I had both stars dead center in the sextant when taking the marks, but I still ended up with a 00110 (1.10 deg) in R1 for the F 06 05. So I hit the FAIL button on the MFD which reran me through the P52 procedure.

Again I felt like I had both stars dead center in the sextant when making the mark. This time I ended up with a 00060 (.60 deg). So I hit fail again.

The 3rd time through the P52 program, after completing the V32E, PRO, I received the F50 25 and 00015 in R1, I than hit PRO. After clicking on PRO, I received a program alarm I think.

A orange light is on the PROG button on the DSKY. I have a F 05 09 displayed with a 00405 in R1, R2 & R3 are all zeros. I also have a yellow light on the PGNS button on panel 122.

Any ideas/help.

 

[indy91]

Maybe the optics wasn't properly zeroed? That would lead to a desync of the actual sextant trunnion and shaft vs. where the computer thinks it is. So try zeroing the optics again and look at V16N91 at the same time, that is the display for where the computer thinks the optics is positioned. Next to the DSKY in the LEB there is also a physical readout which shows the actual angles.

And alarm 405 is "Acceptable star pair is not available". Probably caused by the current attitude pointing the sextant too close to Earth.

 

[thammond]

I figured out that the program alarm is due to not having 2 usable stars available at my current orbital position and attitude. So I can try changing my attitude a bit or wait a bit and rerun V32 to see if I have 2 useable stars.


But I'm a bit perplexed that I performed "good" marks quicker while in orb rate (maybe I was just lucky), than now. How hard is it to get mark precision better than .03?

 

[indy91]

But I'm a bit perplexed that I performed "good" marks quicker while in orb rate (maybe I was just lucky), than now. How hard is it to get mark precision better than .03?

Under ideal conditions (no attitude rates whatsoever) you should always get +00000 or +00001, without any issues. You have all the time you need to position the sextant exactly above the markers. So if you get errors of 0.6° or 1.1° then there is definitely something else going on. When I was first using NASSP I didn't know the difference between sextant and scanning telescope and I used the scanning telescope with its much higher field of view for alignments. But even then you can get better than 0.03° error, it's just much harder.

The AGC actually does some calculation to account for aberration of light which will be on the order of 0.005°. As that is not reflected in the location of the star markers it probably screws up a perfect score (+00000) half of the time. But that just means you get +00001 instead.

 

[thammond]

Maybe the optics wasn't properly zeroed? That would lead to a desync of the actual sextant trunnion and shaft vs. where the computer thinks it is. So try zeroing the optics again and look at V16N91 at the same time, that is the display for where the computer thinks the optics is positioned. Next to the DSKY in the LEB there is also a physical readout which shows the actual angles.

And alarm 405 is "Acceptable star pair is not available". Probably caused by the current attitude pointing the sextant too close to Earth.

OK, I zeroed out the trunnion and shaft (the physical readouts in the LEB view were showing zero's) and called up the V16N91. I was getting something like 40 deg and 270 deg in R1 & R2. I clicked on the handle below the sextant to manually adjust the trunnion & shaft angles, as I was going to test if the R1 &R2 values changed if I manually changed the angles. When I clicked on it, I moved to the sextant view. I then went back up to the LEB view and now both R1 & R2 and all zeros?


Not sure if that is normal behavior or not, but I will try P52 again now.

 

[indy91]

OK, I zeroed out the trunnion and shaft (the physical readouts in the LEB view were showing zero's) and called up the V16N91. I was getting something like 40 deg and 270 deg in R1 & R2. I clicked on the handle below the sextant to manually adjust the trunnion & shaft angles, as I was going to test if the R1 &R2 values changed if I manually changed the angles. When I clicked on it, I moved to the sextant view. I then went back up to the LEB view and now both R1 & R2 and all zeros?


Not sure if that is normal behavior or not, but I will try P52 again now.

The AGC is waiting for 10 or so seconds to zero its counters. So you if switch to optics zero then the sextant will be driven to zero (under its own control, not by the AGC) and the AGC at the same time isn't really doing anything but wait those 10 seconds. Only then does it zero its counters and N91 will immediately switch to all zero. That's why the checklists often say to wait for a bit after switching to zero. The AGC will even give a program alarm, if you switch out of zero mode before that time.

So the normal behavior is that you switch to zero mode wait for at least 10 seconds and then both sextant and AGC counters (N91) should read zero. And definitely don't move the sextant manually during that time (although that shouldn't work anyway). If either the actual position or the one in the AGC still doesn't read zero by then, then there is another issue.

 

[thammond]

What do I need to do to get rotational control at this point in the mission. I turned the ROT CONT PWR DIRECT 1 & 2 to MNA/MNB. Anything else?

 

[meik84]

That will give you direct RCS control only (on/off when the RHC is hardover -sort of a "backup" mode). For "auto" RCS you'd need more, especially those AUTO RCS switches on panel 8. Anyway, how did you do the separation burn without auto RCS?:huh:
To the problem itself: maybe he faces the same issue I reported some time ago. I just tought that it is related to the dual use of the optics counters, but maybe there is more to the story...

 

[thammond]

That will give you direct RCS control only (on/off when the RHC is hardover -sort of a "backup" mode). For "auto" RCS you'd need more, especially those AUTO RCS switches on panel 8. Anyway, how did you do the separation burn without auto RCS?:huh:
To the problem itself: maybe he faces the same issue I reported some time ago. I just tought that it is related to the dual use of the optics counters, but maybe there is more to the story...

I did have rotation and translation during the separation maneuver and burn. But I believe some of the RCS switches were changed at the end of the separation burn checklist to "deactivate the RCS system". Since my SCT & SXT are aiming towards the earth right now, I need to "reactivate" the RCS so I can adjust my attitude in able to have useable stars for rerunning the P52. I'm trying to figure out what switches I need to change to get the RCS system reactivated.


As to Indy's explanation, it's possible that what I experienced was the 10 seconds or so delay from the mechanical zeroing of the trunnion and shaft angles to the clearing of the AGC counters. But as I think through the timing of it, it seemed as if it was a lot longer time. My best guess would be 30-60 seconds.

 

[meik84]

:huh: The P47 checklist doesn't do more then switching off the RHC DIRECT switches. That should have no effect on the whole auto logic.
Anyway, you should take a look at those:
SC CONT - SCS
MAN ATT - ACCEL CMD
RHC PWR NORM(!) (both) - AC/DC
RHC PWR DIRECT - OFF (to make sure we are on the auto system only)
AUTO RCS switches - MAIN A/B (there is quite a typical pattern for them, but any position except OFF (center) is good)
That should give you SCS acceleration control, i.e. the jets will fire as long as the RHC is out of detent or the key is pressed -btw: Num lock on? Orbiter control switched to "rotation" (the friendly women has to say it)?
If not, well that will be more interesting...