Question Help with Apollo 8 P23

[Zuppermati]

Hello everyone

Recently, I've restarted to play NASSP and, mostly thanks to the new MCC feature, I finally made it to TLI in Apollo 8 after years of failures.

I'm currently stuck at around T+4h and some minutes, when you have to run P23 for the first time and perform the star sightings.

Now: I might be dumb as hell, and the fact that english is not my first language is not helping, but I've literally spent the last week trying every single way to "superimpose" the star to the horizon of the earth but I'm always getting crazy delta values and program alarms.

Pardon me again since I'm not english, but for what I understand by reading this post:

https://www.orbiter-forum.com/showthread.php?p=597615

And translating what superimpose means in my language, I'm supposed to move the star in a way such that it "touches" the horizon, which (at least that's what I understand) means "move it into the border of the earth"

A screenshot here might help you to understand (yeah you cannot see the star since I was using the flashing screen you get by pressing V, but you can stay sure that at that moment the star was perfectly at the center of the cross): this imposition is what I understand as being "correct" and where I press Mark

https://imgur.com/a/euvaeZW

Now: I probably got it wrong, but there is literally nowhere around Internet that shows the proper way to do it visually, so I'm stuck with forum discussions talking about "superimposing SLOS into LLOS" and, as I've already said, I'm probably getting the whole thing completely wrong.

So, where do I have to put the star into? Cause I tried literally everything. The only time i get values close to 0 is by imposing it into the center of the earth, which I doubt is the correct way to do it.

Also: is it normal that the wasd keys behaves strangely with the coupling mode set to resolve? Like, w and s moves left and right and a and d move up/down? P52 doesn't do that.

Anyway, sorry for the long post. Thanks in advance for any help.
Regards

 

[Thespacer]

I have mixed results too, for what it’s worth, and only rarely have been able to get a “low score”. That said,

- check out this other post for some more thoughts: http://www.ibiblio.org/mscorbit/mscforum/index.php?topic=2725.0

- perhaps you are doing this, but it’s not clear from your post: make sure you have set up P23 correctly: earth near horizon, or far horizon. That will clearly affect where the star should be.

- If I recall correctly: if everything is in order, an imaginary line can be extended from the horizon to the star (when not superimposed) and should pass through the center of the earth (obviously if it was a “near” horizon setting, the line would have to continue to extend beyond the horizon). It’s not clear from your pic if that would be the case. Granted, hard to confirm with the way the optics are handled in orbiter.

- Direct vs resolved method of controlling the optics, this seems to be working as intended. Technical details are in the Apollo Ops Handbook, G&N Chapter (2), paragraph 2.2.4.10: https://history.nasa.gov/afj/aohindex.html

Tony

 

[MrFickles]

Now, this is probably not the correct procedure, but what I've done recently regarding P23s is to ignore whatever delta is in the first P23, and just accept it no matter what. There's always a possibility that the state vectors were wrong in the first place.

During the 2nd P23, if the numbers agree with the first (low deltas), then you've probably done both P23 correctly. Otherwise, if both P23s disagree, then you've probably screwed up one of them, only until the third P23 can you confirm the numbers.

Remember, you can always update SV manually to practice performing P23s.

 

[Zuppermati]

I already took a look to the topic. No help. I even tried to turn the spacecraft around with the rcs into the horizon but nothin'.

Tried to put the star into the intersection point of the segment between the star and the center of the earth. Still got crazy values.

P23 is setted up correctly (i guess): 00014 in the first row (star code), 00100 in the second row and 00110 in the third for near earth sighting

 

[Zuppermati]

Now, this is probably not the correct procedure, but what I've done recently regarding P23s is to ignore whatever delta is in the first P23, and just accept it no matter what. There's always a possibility that the state vectors were wrong in the first place.

During the 2nd P23, if the numbers agree with the first (low deltas), then you've probably done both P23 correctly. Otherwise, if both P23s disagree, then you've probably screwed up one of them, only until the third P23 can you confirm the numbers.

Remember, you can always update SV manually to practice performing P23s.

I don't think it's a problem with the state vector: I'm 200% sure I didn't touch it unless the checklist required it and I inserted the values they told me to

 

[MrFickles]

P23 is setted up correctly (i guess): 00014 in the first row (star code), 00100 in the second row and 00110 in the third for near earth sighting

Line 2 should be all zeros if you're not using a landmark:
R1: 00014 (star 14)
R2: 00000 (no landmark)
R3: 00110 (near earth horizon)

I don't think it's a problem with the state vector: I'm 200% sure I didn't touch it unless the checklist required it and I inserted the values they told me to

Exactly. At the first P23 after separation from the S-IVB, I believe that the state vector would not have been updated at all since launch. There's a good chance that a large error had accumulated in the 4-5 hours. Which is why I tend to ignore the large delta V and R, and just accept the first P23.

If everything is done correctly, on the 2nd run through of P23, you should get low error values. That way you know that everything is working fine.
Otherwise, you can always do a SV update and run P23 immediately after. That way if you get low errors, you know you've done the P23 correctly.

 

[Zuppermati]

Line 2 should be all zeros if you're not using a landmark:
R1: 00014 (star 14)
R2: 00000 (no landmark)
R3: 00110 (near earth horizon)

Wait a second: quoting your own post in this discussion

https://www.orbiter-forum.com/showthread.php?p=597615

Ensure you have a proper IMU alignment first, otherwise use P51 and P52 to align the IMU.
When entering P23, enter the star, landmark, horizon that will be used for the sighting.
V05 N70
R1 000AB
R2 XXC00
R3 XXCE0

AB = Star code
C = Earth (1) or Moon (2)
E = Near horizon (1) or Far horizon (2)

By this description, shouldn't you set R2 to 00100?

 

[MrFickles]

Next line. If using a horizon, R2 must be set to all zeroes.

 

[indy91]

Here the page from the actual checklist:

https://history.nasa.gov/afj/ap15fj/csmgc/3-09.gif

ENH = Earth Near Horizon
EFH = Earth Far Horizon
LNH = Lunar Near Horizon
LFH = Lunar Far Horizon
EL = Earth Landmark
LL = Lunar Landmark

So yes, either R2 or R3 should always be all 0s.

 

[Zuppermati]

Next line. If using a horizon, R2 must be set to all zeroes.

Gosh I'm such a functional illiterate. Sorry, totally misinterpreted that.

Also thanks to indy91.

Now I get normal values at first try.

By the way, can I use the occasion and go slightly OT? Right before p23, a GDC realign is required. For this job, I've always interpreted the values from the FDAI 1 and tried to set the thumbwheel accordingly. However, since the FDAI's grid in NASSP is not quite the real thing and could lead to imprecisions, this time I just looked around and from what I understand, you can get the values from V16N20: correct?

 

[indy91]

By the way, can I use the occasion and go slightly OT? Right before p23, a GDC realign is required. For this job, I've always interpreted the values from the FDAI 1 and tried to set the thumbwheel accordingly. However, since the FDAI's grid in NASSP is not quite the real thing and could lead to imprecisions, this time I just looked around and from what I understand, you can get the values from V16N20: correct?

That works as well, yes. In the full GDC alignment procedure the FDAI attitude is just used as the initial guess though, you fine tune it by zeroing the FDAI error needles. That is quite precise I would say. But you can do it easily by using the N20 values and adjust the ASCP thumbwheels to that.

 

[Zuppermati]

Ok, thanks again

 

[Zuppermati]

Sorry again for reviving this post: I'm now at T+9h, at the second round of p23s, which requires to use landmarks.
I wanted to ask: while using a landmark, I'm also required to insert its longitude and latitude. The checklist mfd does not istruct you to do so, but I guess that, once you get the flashing V06 N89, you have to insert noun 25 again and insert the parameters by yourself, right?

If that's so: I have the apollo 8 flightplan and it states that in this phase,the landmark coordinates were:

LAT: 28,876 N
LONG/2: 56.292 W
ALT: 000.01

Which, according to both google maps and a document I've found on ibiblio, it's supposed to be a small island in Mexico. The thing is: either I've setted up p23 wrong (again), but this time I'm pretty sure it's correct:

R1: 00015
R2: 00100
R3: 00000

cause the telescope points way off into the Pacific ocean, and I doubt NASA made them make markings into the open ocean since, well, you have absolutely 0 points of reference for accurate markings

So the question is: am I doing anything wrong again? The telescope has to point to the ocean or at the island marking?

 

[indy91]

Sounds like you did everything right. The main issue I can think of is your onboard state vector. If you did the first round of P23s and didn't have good results and never uplinked another state vector to the CMC then it might be quite a bit off. Are you using the MCC or the non-MCC scenario? In either case, something that's a bit special about Apollo 8 is that most state vector uplinks were done to the LM state vector slot, to let the CMP play around with the CSM state vector at all times. Copying the LM state vector to the CSM SV slot is done with V47E. So try getting a CSM fresh state vector. Then it will hopefully point at the right landmark.

 

[Zuppermati]

I'm 99% sure I'm using MCC (mission control calculates everything for me so I can get all the infos I need by pressing tab and then 5, and they perform all the uplinks, which is a feature of mcc, right?).
Also, once you guys told me what I was doing wrong, I got really solid values for my P23, so I don't think it was that.

Anyway doing V47E itself doesn't seem to solve the issue (I suppose I can use it right before i start p23, right?). I'll try P27 (or whatever was used to update the state vector) as soon as I have time.

Thanks again

 

[Zuppermati]

By the way, quick update: turns out that the reason it didn't work was because I assumed that NASSP would use West coordinates for the latitude, like the checklist. Turns out the cmc uses East coordinates, so I had to insert not +56292 but -56292. Now it works thanks

 

[MrFickles]

Nice! With that, you're all set to get to the moon and back! That's all the skill set required for Apollo 8

 

[rwitham]

I'm also having difficulties with P23 at T+4h on Apollo 8 MCC. I've read all the above posts and I'm certain I've entered the correct star and landmark information into the CMC.

On the first P23 procedure (14 ENH), the horizon is not aligned in the SCT and there appears to be no way to align it for the following 2 reasons:

1. The OPT ZERO sw is supposed to be in the ZERO position according to the checklist, therefore preventing the SCT from being adjusted.

2. Even when placing OPT ZERO to OFF, the SCT will not move in the direction needed to align it with the horizon without completely "rotating the earth" upside down in the SCT. It's like the shaft or trunnion is completely to one side of its range.

I've tried just marking the initial location, then proceeded to the next step, however when it's time to superimpose the star on the horizon, the horizon is not in the SXT. Just the star in black space. And I have verified the SLOS is enabled with the 'v' key.

I've also tried "accepting" the first P23 deltas and going to the second P23 procedure (15 EFH), but I have a similar problem with the horizon mark and when superimposing the star at the horizon, the star is in the middle of the earth and I can't see the horizon in the SXT.

I've also tried doing a State Vector Update (using PAMFD) before starting any of the above. No difference.

I've included a scenario saved at about 2 minutes before the first P23 procedure if anyone's interesting is helping a newbie.

Note: A state vector update has not been performed recently in this scenario.

Thank you