Apollo 15 DOI, RTCC MFD question

doug1956

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I am 81 hours into the Apollo 15 mission, and have a question about getting a good DOI from the RTCC MFD, but first some quick kudos and thanks. I've learned a lot about space flight, the Apollo program, NASSP, and especially about the value of browsing this forum for answers. Many have had similar problems and questions, and they've been answered by some vy smart people on this forum, including, but not exclusively, indy91 & rcrflyinghokie. Having watched the Apollo program as a youngster, (born in 1956), being able to simulate them to the degree that Orbiter and NASSP affords is the ultimate 'kid in the candy shop' experience. Thanks to all that have made this possible!
OK, after the LOI burn, I'm in an approximately 167.7 x 58.7 nm orbit. In RTCC MFD, I go TAR-LDP, INI I leave as is, optimum azimuth, 50,000 ft, do not simulate powered descent, leave PDI alone, N of 11, DFT 11 min, DFA 16 degrees. MOD is LM Maneuver Sequence, SEQ is DOI, HEI is 60 nm, TH1 is 82:00:00, nearest even hour preceding historical DOI burn. CLC gives me a DOI resulting in a 96.65 x 8.23 nm orbit.

RTCC-1.jpg

A far cry from the historical 58.4 x 9.6 nm. I'll attach the scn file. So am I missing something obvious? Granted the documentation for the RTCC MFD is a work-in-progress, but it is a great tool, and I've learned a lot about it from experience, and I prefer to figure things out myself, but I'm stumped.
 

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indy91

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Hmm, I think your orbit is shaped incorrectly. The perilune after LOI must be placed quite exact in altitude and location so that DOI can happen at that perilune. Otherwise there will be a large DVZ component of the burn (-257.6 ft/s in your case) and the apolune after DOI is incorrect.

This could have happened anywhere before the DOI maneuver, either LOI or the MCCs must have been targeted incorrectly.

Which LOI solution did you choose? Was it 1 or 2? Those are for this kind of profile with a DOI instead of LOI-2. On Apollo 12 and earlier you would choose 7 or 8. If you did choose solution 1 or 2 the problem could be even earlier with the MCC targeting.

One thing you could do is perform a circularization (so a LOI-2) maneuver instead of DOI and then one orbit later do the CSM DOI maneuver. You would have to choose the option with CIRC + DOI as the calculated maneuvers. And the number of revs between DOI and PDI would need to be reduced from 11 to 10. PDI would be a few minutes later than in the flight plan, but it's not too bad usually.
 

doug1956

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Thanks for the reply! Yeah, after I posted this, I tried to 'wrap my head' around what was happening, and not happening, after the LOI burn, and I realized that, while the apolune and perilune numbers were pretty close, they were occurring at the wrong places in the orbit. The descent processor was giving numbers that would lower the perilune at the proper longitude, but since the orbital altitude was in the 90's at the time of the burn, there was no way the apolune was going to be below that, obviously. And yes, I did use 7 or 8 originally in the DOI solution, don't remember which. Rather than try to fix the orbit, I went back to a few hours before the DOI (I've learned to save frequently), and used 1 or 2, again, not sure which, did the LOI burn, and ended up remarkably close to the numbers in the Apollo 15 mission profile trajectory doc. I haven't done the DOI burn yet, but got a much better solution out of RTCC after the proper DOI burn. Thanks again for the reply, and the RTCC MFD, it's a great tool!
 

doug1956

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Another speed bump... Am to the point in the LM Timeline Book where the PDI pad is uplinked, app. GET 102:20:00. Assuming I use the RTCC MFD for that also. indy91, found your post from back in 2019 where you went through the procedure to do a Separation, DOI, and PDI in Apollo 11. It appears the MFD has been changed some since then, as the TUP button is in a different place, and there are other differences. I'm setting up the CSM & LM on the ini page, doing the AUT, M55, and M50, but not sure how to 'update the initial trajectory'. Am unable to get a response out of the PDI button. Looks like the VPS button on the ini page gets you to a Vector Panel Summary, which is where the TUP button is. I'm sure I'm missing something obvious, any help appreciated! Thanks!
 

rcflyinghokie

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Another speed bump... Am to the point in the LM Timeline Book where the PDI pad is uplinked, app. GET 102:20:00. Assuming I use the RTCC MFD for that also. indy91, found your post from back in 2019 where you went through the procedure to do a Separation, DOI, and PDI in Apollo 11. It appears the MFD has been changed some since then, as the TUP button is in a different place, and there are other differences. I'm setting up the CSM & LM on the ini page, doing the AUT, M55, and M50, but not sure how to 'update the initial trajectory'. Am unable to get a response out of the PDI button. Looks like the VPS button on the ini page gets you to a Vector Panel Summary, which is where the TUP button is. I'm sure I'm missing something obvious, any help appreciated! Thanks!
You shouldn't need to use the MPT to get a PDI pad. Your TLAND was computed when you did your DOI calculation and stored and you just need to go to the maneuver pad page and change the option to PDI (while in the LM) and it will populate.

For the MPT, if you want to use it, you update the vectors after initializing the CM and LM as you said with the VPS table. Update the DC vectors for both (left side is CM table right side is LM table) and use TUP there to update those as your anchor vectors.

But as I stated before, you shouldn't need MPT for PDI as its just generating the pad, and your landing site vector and TLAND should be stored in RTCC. You will want to update the landing time of course since it probably changed slightly after DOI.

If you want some quicker help or a little more interaction, we also have a pseudo discord "support" channel if you will hosted in this discord: https://discord.gg/gZQw95qb
 
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doug1956

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Ok, should have been obvious to check the maneuver pad page for a PDI option. The PDI pad is giving me a burn, but the TIG is about 13 hours later than the historical burn, with a correspondingly later landing time. You say I'll need to update the landing time, I'm not seeing how to do that. I think I've checked every page in the RTCC MFD, so please put me out of my misery. I'm getting the hang of this MFD, but still finding a few blind alleys...
 

rcflyinghokie

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Ok, should have been obvious to check the maneuver pad page for a PDI option. The PDI pad is giving me a burn, but the TIG is about 13 hours later than the historical burn, with a correspondingly later landing time. You say I'll need to update the landing time, I'm not seeing how to do that. I think I've checked every page in the RTCC MFD, so please put me out of my misery. I'm getting the hang of this MFD, but still finding a few blind alleys...
That landing time directs the TIG. And the landing time is computed when you computed DOI, so it's possible you burned DOI incorrectly or subsequently recalculated it using incorrect values changing the landing time. Feel free to post a scenario file here or jump in discord, we will see about getting you back on track!
 

doug1956

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Comes the dawn... I recalculated the DOI, then tried the PDI again and came up with times within 10 seconds of historical. Real world intruding, so will be a few hours before I continue, but thanks again for the candle!
 

rcflyinghokie

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Comes the dawn... I recalculated the DOI, then tried the PDI again and came up with times within 10 seconds of historical. Real world intruding, so will be a few hours before I continue, but thanks again for the candle!
Ah good! Make sure you perform a descent update in the LM as well (this is also in the flight plan) to make sure your LGC knows this time is different than preplanned.
 
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