A11 - MCC scenario - NASSP v8 rev.1756 - LOI-1 Burn FAILURE putting me on a crash course to lunar surface

thermocalc

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Hi again, this is the 2nd part of my previous post, in which I described the NAVIGATIONAL problems I faced so far to enter into lunar orbit.

1) After TLI my MCC-1 was scrubbed, I did only MCC-2 with SPS, MCC-3 and MCC-4 were also scrubbed. Is it normal?
2) When the message of MCC-4 being scrubbed came (at 069:28:20) I also got an uplink for CSM SV update and Landing Site LLS 2 refsmmat, which I changed with P52 option 1 at 070:11:00; the next MCC message was for the PC+2 maneuver pad fast return, followed by the “wait for LOI-1 update” at about 73h, which I got at 073:03:20, BUT I got only this one until the TIG more than 2 h later…did I lost another LOI-1 PAD ? in Apollo 8 there was a “preliminary” LOI-1 and a “final”, also here in A11 the MCC was supposed to generate automatically only 1 or 2 LOI-1 pads? If 2 why I got only 1?
3) And here is the real problem I got: when at time 74h we need to start P30, with the target loads as for the received LOI-1 PAD the prediction V06 N42 for Ha, Hp was far off compared with the PAD values (see “photo 6-P30 at 74h”) with a NEGATIVE periapsis…
4) When the next execution of P30 was called at around TIG -40min, no new LOI-1 PAD were uplinked and the old values were still there in P30 (TIG time, velocities,..) and the new Ha-Hp predictions were still far from the PAD values and way different from the ones received before (compare “photo 7” with “photo 7-P30 at TIG-30m”.
5) Anyway, I carried out, made the LOI-1 burn and with “horror” I saw that indeed I was wrong all times and the burn set me into a collision course with the moon surface with a negative periapsis, see “photo 8 – LOI failed”.
6) Where is the problem? I saved scenarios almost every 30 – 60 min, so if you need some scenario at some particular time in the mission just let me know, see “photo 9_MCC uplink” for the chronology of all MCC updates I received so far… have all these sceneries.

Thanks a lot for your time in reading me, and sorry for the so many questions…and split posts, if I must NOT split them please kindly let me now and it will not happen again.
PC
 

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  • photo 6 - P30 at 74h strange values.JPG
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    photo 7 - P30 at TIG -30m.JPG
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  • photo 8 - LOI failure.JPG
    photo 8 - LOI failure.JPG
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  • photo 9 - MCC update list so far.JPG
    photo 9 - MCC update list so far.JPG
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indy91

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Hi again, this is the 2nd part of my previous post, in which I described the NAVIGATIONAL problems I faced so far to enter into lunar orbit.

1) After TLI my MCC-1 was scrubbed, I did only MCC-2 with SPS, MCC-3 and MCC-4 were also scrubbed. Is it normal?

Sounds fairly normal. MCC-2 and 4 are the desirable maneuver points, MCC-1 and 3 would only be used for bigger trajectory deviations. In the targeting done by the MCC it is highly likely that MCC-1 gets scrubbed because of that. I can't imagine a case where MCC-2 would also be scrubbed, there is always a bit of deviation after TLI. MCC-3 gets usually scrubbed because its DV would be small. It doesn't happen that long after MCC-2. MCC-4 is usually not scrubbed, but small. However, I wouldn't say that it is abnormal that this also gets scrubbed, it can definitely happen.

2) When the message of MCC-4 being scrubbed came (at 069:28:20) I also got an uplink for CSM SV update and Landing Site LLS 2 refsmmat, which I changed with P52 option 1 at 070:11:00; the next MCC message was for the PC+2 maneuver pad fast return, followed by the “wait for LOI-1 update” at about 73h, which I got at 073:03:20, BUT I got only this one until the TIG more than 2 h later…did I lost another LOI-1 PAD ? in Apollo 8 there was a “preliminary” LOI-1 and a “final”, also here in A11 the MCC was supposed to generate automatically only 1 or 2 LOI-1 pads? If 2 why I got only 1?

That also sounds normal, the flight plan has only one LOI-1 Maneuver PAD.

3) And here is the real problem I got: when at time 74h we need to start P30, with the target loads as for the received LOI-1 PAD the prediction V06 N42 for Ha, Hp was far off compared with the PAD values (see “photo 6-P30 at 74h”) with a NEGATIVE periapsis…

I need to add a comment here for the Maneuver PAD from the MCC I think. P30 doesn't simulate the LOI-1 burn as a multiple minute long maneuver. Instead it just adds the DV vector to your velocity at TIG as if the maneuver was instant. The resulting velocity gets used to calculate the HA and HP. For such a long burn those values are pretty useless. To inform the crew of this, on Apollo 11 they got both the actual expected HA/HP after LOI-1 and also the numbers that P30 will show:

"072:51:24 McCandless: LOI-1 ... 0169.2, plus 00610. ... The values which you would see on Noun 42 prior to LOI burn are HA, plus 431.3; HP, minus 128.2."

So that doesn't look too far off from what you were getting.

4) When the next execution of P30 was called at around TIG -40min, no new LOI-1 PAD were uplinked and the old values were still there in P30 (TIG time, velocities,..) and the new Ha-Hp predictions were still far from the PAD values and way different from the ones received before (compare “photo 7” with “photo 7-P30 at TIG-30m”.

And I think here is the problem. The velocity in P30 is not the same as before. When the AGC enters P40, it rotates the DV vector in pitch to compensate for the burn duration. This is quite useful for shorter burns. If you use e.g. +200 +0 +0 in P30 then that rotation will lead to your spacecraft being slightly below the local horizon at ignition, exactly on the local horizon in the middle of the burn and slightly above the horizon at cutoff. That way, on average, the burn is done on the local horizon, which is what you would want when you only use a DV in the x-coordinate in P30 (the +200 above).

This approximation doesn't work so good for very long burns like LOI-1, but that's not really the problem here. I think what caused your issue is the P40 TVC test. Two hours before the burn you use P30, then you enter P40 (which does the rotation of your DV vector as described above). Then you exit P40 after the test and about half an hour before TIG you do P30/P40 again. But P30 now doesn't have the DV as uplinked anymore, but it instead has the rotated DV vector! That is why the flight plan has this line:

"EXT DV - P30 (Reload N81 with PAD values)"

You need to reload them from the Maneuver PAD, or else when you enter P40 again for the burn, it will now have rotated your DV vector twice and is now very wrong. I am sure if you compare the DV in P30 at that time with the Maneuver PAD you will see them being quite different from each other.
 

thermocalc

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Hi Indy91 and thank you for your enlightening explanations, it was exactly as you said...the P40 TVC test screwed up the next P30 at TIG -30....I restarted from time TIG-40m and I did as you said, calling up P30 and entering again manually the LOI-1 burn pads and everything went well, I entered into lunar orbit at 169.1 x 60.2 nm, than I did the LOI-2 and I got into a final orbit of 60.3 x 59.7 nmi ... I just got the P22 LMK A-1 PAD and I am going to enter into the LM....(tomorrow, now for me is time to sleep, I had a long day).
thanks.
 
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