Apollo 7 P23: Difficulty Performing Star/LNH Marks?

Wedge313

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Good morning. Here's my next point of confusion: performing the Apollo 7 P23 using a star and the lunar near horizon.

Initially I launched into the P23s attempting to do a star/earth horizon sighting, but I found it impossible to discern the earth horizon (I believe this actually was the case during the real flight). So, I then tried do a sighting using a star (I picked #21) and the lunar near horizon. For the P23 I entered R1 00021, R2 00000, R3 00210. But after the spacecraft maneuvers, the moon is not visible in the SCT. If I understand this step of P23, after this maneuver the zeroed optics should be aimed at a point on the lunar horizon below the selected star?

Out of curiosity I ran a P52 opt 3, That went well, so the CMC seems to know where the stars are. I tried a P52 and manually selected the moon (00050), and the optics pointed at the center of the moon. So, the CMC knows where the moon is. I tried updating my state vector, and that didn't help. I don't think the REFSMMAT matters here, does it? (Unless I'm headed for gimbal lock).

So, I'm guessing pilot error. Where am I going wrong? I'm attaching two saves, one as I begin P23 and one just after the maneuver. Thanks.
 

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indy91

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We don't have the correct software for Apollo 7, so there is an issue there. P23 in Colossus 237 does not calculate a vector to the Moon on its own. They fixed this for Apollo 10 I believe (Anomaly report COL-56, "Second body in P23", fixed by PCN-664, "P23 Calculation of Secondary Body to CSM Vector"). A vector to the Moon would only be calculated if you do P52 on the Moon or if you are high above the Earth; that's when the AGC would start using Moon gravity for trajectory calculations. Did you try P23 again after your P52 on the Moon? If not then that might be a valid workaround. P52 calculates the vector and stores it somewhere and P23 can then use it. I think.

Because of this issue and the fact that the star/Earth horizon technique doesn't really work in low Earth orbit, I am a bit on the fence to even include the P23 procedures for Apollo 7. You are just proving that it doesn't work or that you need workarounds for the software. All a bit annoying.
 

Wedge313

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Did you try P23 again after your P52 on the Moon?
OK, I tried a P52 using the moon as one of the marks, then tried the P23 again and ended up with the same (incorrect) orientation. So, it looks like your explanation is correct and there's a software issue. (But don't discount the extremely high possibility that I'm doing something wrong)

So I'll skip the P23s, which I'm not that good at anyway, and move on to more fun things like ECS Redundant Component Checks.

Thanks
 
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indy91

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Hmm I was sure I could trick the CMC on Apollo 7 to calculate a Moon vector that P23 could then use. I'll have to try it again, I'll come back to you.
 
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