Apollo 7 P23

AliDarwesh

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Hi all.

After having passed all the P22's with flying colors, most of them with single digits results (0000x) now this...
Whatever I do, I have huge results from the P23 where you have to align star 21 with the near horizon of the moon.
from the star's perspective, the near horizon is actually dark? but when doing a p23 with the far horizon (R3 220), results aren't better.

doing a state vector update and a p52 just before the p23 also doesn't help.

I did notice that when finished the auto mnvr (50 18), the sextant is quite far from the moon. this was also the case when I did a SV update and P52 just before the P23.

So this is how i do things: after the auto mnvr I make sure the optics are zeroed and then I mnvr the CSM with the RCS until the moon is in sight of the sextant. then I mark. then trunnion bias will be all 00000 then I do PRO, which will point the optics to the star (21 in this case), then i press V and the star will already be very close to the moon. then I use WASD to align the star with the lunar horizon.

am I doing something wrong? I literally reran it for 50+ times with achieving semi-good results (2 digits, 000xx) only once. otherwise my results are in 3-4 digits.

I attached a save file. maybe something got corrupted again? It should be fine this time as I didn't have any restarts and I didn't use more than x10 time compression.
I hoped I could solve this one on my own, but it seems I'm stuck yet again...
 

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indy91

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P23 in low Earth orbit just doesn't work very well. The star/horizon technique needs the horizon to not move so much, which is obviously the case here. Let me quote the Apollo 7 mission report:

Midcourse navigation/star horizon/landmark.- A number of star/earth horizon measurements were scheduled, but all attempts to perform these sightings were unsuccessful. This failure resulted partially from the difficulty of the control task at the relatively high earth-orbital rates, but primarily from the crew's inability to define a horizon locator, which was the primary purpose of these tests. The dichroic filter in the sextant landmark line-of-sight did not aid in land/sea definition and actually smoothed out the horizon such that it was impossible at earth orbital ranges to define a locator for repeatable sightings. The crew stated that at longer ranges, the sightings should be accomplished with ease. The capability for performing star/lunar landmark sighting was demonstrated using the star Alphard and lunar landmark 5 (crater Diophantus).

We even have trouble getting good results from P23 in normal circumstances with the star/Earth horizon technique. The Earth in Orbiter is a sphere, while the AGC expects an ellipsoid. And I also believe there is some issue with the way our sextant view is rotated, but I could never quite figure out what exactly that issue is. So P23 works "ok" at the best of times, but for Apollo 7, just like in the real mission, it's fairly hopeless. They did try it so it's included in NASSP as well. Maybe it should come with a disclaimer that it won't work well.
 

AliDarwesh

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I did these star/earth horizon marks earlier in the mission with average but what looked like consistent results, but the star/lunar horizon results seem totally random.
 

indy91

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Ah sorry, didn't read that right, it's only star/lunar horizon from Earth orbit. When I try your scenario and do a P52 with the Moon as the target (star code 50) then it seems to point fairly well at it. That tells me that the alignment has to be good, the state vector has to be good and the lunar ephemeris (for calculating the direction towards the Moon) is also not broken. I also did a P21 as a sanity check. So your scenario should be ok.

I didn't even remember that the NASSP flight plan calls for lunar horizon marks. I'm not really getting any better results with it when I try your scenario, so I can't really help you. Seems like during the actual mission they only used the lunar landmark option. That should work better.
 

AliDarwesh

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147:05:47 Eisele: Roger. I don't know if you read this on the computer. I've got P23 up, and I've got Alphard placed on landmark on the Moon, and these are the shaft and trunnion angles. Do you read them down there? [Long pause]
147:05:59 Swigert: Roger. I'm copying them.
147:06:00 Eisele: Okay. I'll tell you that was one whale of a lot easier than that crazy Earth horizon business. [Long pause

Would it be possible to incorporate the landmark in a future update? It even seems more logical to do a star/lunar landmark than simply a star/lunar horizon because then at least there is no confusion as to where to align the star on the lunar horizon exactly.

Good to know my scenario isn't broken this time :) thanks for checking.
 
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