Apollo 17 LOPC Burn RTCC Planning Question

Wedge313

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Hello!

Again I'm asking for some RTCC help. I'm getting ready for the LM ascent. I'm trying to set up for the CSM plane change, but it looks like I'm trying to do two things at once here- a plane change and a circularization. And I can't figure out how to plan it.

Unless I missed something, before the LOPC I'm in a 69.9 x 54.2 orbit. After the burn in addition to the plane change I want to be in a 63 x 61.3 orbit. I can figure out how to use the RTCC to calculate each maneuver separately, but it looks like I should be able to accomplish both goals with one burn, and I can't figure it out.

Thanks
 

rcflyinghokie

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@indy91 can probably give a better answer to this, but when I flew 17, I believe I just did the plane change burn itself. Did you check the resultant orbit after computing the LOPC?
 

Wedge313

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I did the plane change by itself, ended up with a cross range of 0000.1, and my CSM orbit was 69.8 x 54.0.

(The flight plan shows after the LOPC an expected orbit of 63 x 61.3.)

Here's why I'm asking: I've gone through the LM ascent twice now, and the whole geometry seems off, after insertion I end up (the LM) in a 28.6 x 9.3 orbit. I pressed on through TPI and I "rendezvous" with the CSM, but I'm carrying WAY too much dV (I need to lose 87 fps!). The RCS can't come close to killing that and I zoom on by the CSM (a quick wave as I shoot by).

I'm trying to figure out why my ascent plan is so far off, and my first thought was that maybe having the CSM in a 70 x 54 orbit was causing the bad solution?

If that's not the case (if the LM should be able to rendezvous with the CSM in that 70 x 54 orbit), then my error is somewhere else.

I'm just trying to work it out, but if the CSM orbit isn't the issue I can move on and try to find where I'm going wrong.

Thanks.
 

indy91

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Did you target the circularization burn to get into the 69.9 x 54.2 orbit? This doesn't really work right in Orbiter. The actual lunar gravity perturbations will change this orbit to near circular by the time of LOPC-1. But that can't be simulated in Orbiter yet. So it's probably best to actually circularize the orbit with the circ burn.

One trick you might be able to do is, calculate the LOPC-1 burn as normal, remember the TIG and DVY component and then go to the general purpose maneuver processor. It does have an option to perform a maneuver that does a plane change and also a circularization maneuver at the same time. So maybe you could tweak the inputs there to get a maneuver that does both, circularization and the same amount of plane change. Probably takes a bit of trial and error though.

For your rendezvous, I don't understand how you get into a 28.6 x 9.3 orbit. Insertion should happen with an apolune 15 NM below the CSM. So even if the CSM is at 54 NM altitude at the rendezvous point, then the LM apolune should at least be 39 NM. The lunar launch targeting display uses an input vertical insertion velocity of 32 ft/s and it calculates the horizontal velocity required. Those two values of course need to be input to P12. So does that display really give you a VH value that results in a 28.6 NM apolune? Because that looks more like the default value that P12 is loading, which leads to a 30 NM apolune.
 

Wedge313

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For your rendezvous, I don't understand how you get into a 28.6 x 9.3 orbit.
Yeah, I didn't understand how I got there either. But problem solved. As is normally the case I screwed up, before lift off on P12 the F 06 76 VH value was 5509.5, my PAD value was 5526.4, and for some reason I didn't catch it.

With the correct VH entered the ascent ended up with a 38.6 x 9.7 orbit, after TPI my TPF dV showed 31.5 fps (vs. the 87.2 fps I got with the bad insertion).

I hit the braking gates right on, and am currently station keeping with the CSM.

So the answer to my original question is: no, the CSM doesn't need to be in a circular orbit, 70 x 54 is OK, as long as the "crew" doesn't make stupid mistakes.

Thanks
 

indy91

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Well you gave it a good test for a non-circular case haha. And it's probably ok if you stay in that elliptical orbit until later, even the TEI targeting shouldn't have a problem with that.

Have you been doing the tweak burn using the two impulse processor? I feel like that would have given you a larger tweak, because essentially what happened to you was the insertion velocity being about 17 ft/s too small.
 

Wedge313

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Weirdly, I ran this ascent/TPI without looking at the tweak because I wanted to see if the corrected P12 VH was indeed the error, sort of a rush-through proof-of-concept thing, and went straight into the TPI and MCCs, and it all worked out well. Now that I know where my original error was I'll go back and do it correctly with the tweak.

Before the correction my tweak dVs were large (I forget exactly what they were), that was my first clue something was wrong.

If we do actually learn from our mistakes, I'm getting real smart real fast.
 

Wedge313

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Addendum: I was looking at the Apollo 17 In Real Time website to find the LM De-orbit Burn PAD numbers (the AFJ stops at Day 5 for some reason?) They have a "Mission Status" screen that shows the CSM in a 53 x 75 orbit at the time of docking. Not sure how accurate their info is, but that's close to the 54 x 70 orbit I'm in. Don't know why the flight plan shows 61.3 x 63.

I don't feel so bad, maybe they couldn't figure out how to do a plane change and orbit circularization at the same time either?:unsure:
 
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