Apollo 17 DOI-1 Planning with RTCC?

[Wedge313]

Hello, I'm running Apollo 17, I'm currently in a 170 x 51 orbit around the moon after LOI. It looks like the orbit has me passing pretty much over the intended landing site. I'm trying to use the RTCC to plan the DOI-1 burn. Another learning opportunity for me, both in how to use the RTCC and in orbital mechanics.

Following the RTCC MFD Input Reference Guide, it looks like Apollo 17 did things a little differently than the others. The DOI procedure refers me to the "common functions" section. I've set up the General Purpose Maneuver as instructed (TYP=Apogee and Perigee Change, PNT=Time, GET=93:13:08 (from the flight plan), REF=Pad/LS, ApA=60.0, PeA=13.17). It then says to tweak the GET until LONG P is 31.36.

I'm unable to get LONG P to 31.36. Playing around with the GET, right around 93:12:36 I get a strange result. If I use 93:12:36.3 my LONG P is 33.10 E. If I go to 93:12:36.4 (only 0.1 sec later) the LONG P jumps to 68.16 W.

I don't understand what's going on here. Is my current orbit so far off that it's a case of "you can't get there from here"? Why is the LONG jumping about 90 degrees?

Going forward, should I go with the DOI-1 burn with LONG P of 33.10 E and hope my LM DOI-2 can correct for it? Or is there a better way I can plan this first DOI burn?

Hoping to better understand what's going on here.

I've attached a .scn of my current state.

As always, thanks for the help.

 

[indy91]

In terms of orbital mechanics, if you want to go from a 51x170 orbit to a 13x60 orbit then of course the maneuver is only possible between 51 and 60 NM. If you perform the maneuver at 60NM then you will always null the radial velocity and that is the only way to achieve the 13x60 orbit. But if you perform the maneuver near perilune, so at 51NM, then there are two ways to do the maneuver. One will get you into an orbit that has apolune after the maneuver point, and the other will be before apolune. The difference between the two is the radial velocity. One will have a positive rate after the burn, the other a negative rate. The difference between the two possible resulting orbits is the argument of periapsis, which of course also means the longitude of periapsis is different between the two.

The GPM processor automatically chooses the maneuver with the least DV, I think. That is why you see a sudden jump in LONG P location. The processor selects the other solution and the periapsis location is suddenly very different.

It's been a while since I looked at the Apollo 17 DOI-1. I think the problem with the normal method, using the descent planning processor, is that it always puts the DOI maneuver in a specific location relative to the landing site. But the maneuver nominally has a radial DV to put the perilune location in the right place so this logic with the placement of the DOI maneuver doesn't work. I think you will be fine performing the maneuver that gets you a 33.10° E perilune. It's close enough. Eventually I will need to revisit this topic but I probably need to find more original documentation about the burn planning to add any improvements.

 

[Wedge313]

Thanks for the explanation. I won't pretend I fully understand it!

But I went back and read a few articles, of the "orbital mechanics for dummies" category, to try to get a grasp of what you're saying. And then because I deal better with pictures than words I went back and played with the RTCC GPM, adjusting not only the GET but also the ApA and PeA values, and watching what happens to the resultant orbit. It's not too hard to get the LONG P to 31.36 E that way, just by asking for a slightly different orbit. And I think it helps me get closer to understanding what you're saying.

The fudged numbers gave me roughly a 58.9 x 15 orbit, and required about 5.9 fps less dV. But The dV I saved here would probably have to be made up by the LM DOI-2 or PDI later, so pick your poison I guess.

But of course I have already performed the DOI-1 burn using the originally computed values, and I'm too lazy to go back and re-do it! But if I have problems getting down to Taurus-Littrow I can go back and play with it again.

Thank you.