prop 2 s/c, one with a finite burn

Spacecraft, Thruster, Tank, Propagator, CoordinateSystem, etc.

prop 2 s/c, one with a finite burn

Postby joandunham » Fri Aug 15, 2014 7:44 pm

The problem is one of propagating two spacecraft, one with a finite burn to achieve rendezvous with the other after 5 days of flight. What I want is to start one spacecraft in its designed halo orbit (s/c halo) and launch a vehicle from the far side of the Moon (s/c Finson) to meet halo after 5 days. I want to propagate them together so that it will be possible to change the halo orbit and then find an optimal trajectory for Finson. I’ve had some problems with the GMAT propagation which may be related to the known issues GMT-1736 or GMT-3487.

For these scripts, I selected a specific point and iterated to that instead of what I want to achieve, which is zero distance between the two spacecraft.
thrust and nonthrust – A: Integrates halo outside of the Finson integration and maneuver iteration. Finson and halo both integrate properly.
-B: Propagate halo with Finson during the Finson burn. Get the error “MassFlow is not a known propagation parameter on halo”. This implies that the propagator is attempting to maneuver halo as well. Only Finson has a burn, so that is incorrect.
-C: Integrate halo separately for BurnDuration seconds, then try to integrate them together. Various ways of doing this all failed with one error or another. This one has a failure of “Propagator FourBodyProp failed to take a good final step (size = 42528.353427999995)”
-D: Used separate propagations for the maneuver but propagation together after the maneuver was unable to get the epochs to match after the maneuver. The message was “Spacecraft epochs do not match in a Propagator used in the Propagate command”.
-E: Separate propagations in the iteration resulted in completely incorrect trajectories for both spacecraft.

This works as I think it should when I use impulsive maneuvers instead of finite burns.

I would like to hear any suggestions you have on how to work around this problem. Right now, I think I will need to generate an ephem for halo and then use that for halo propagation while I maneuver Finson.
thrust and nonthrust sc - E.script
(18.42 KiB) Downloaded 212 times
thrust and nonthrust sc - D.script
(18.38 KiB) Downloaded 241 times
thrust and nonthrust sc - C.script
(18.3 KiB) Downloaded 223 times
thrust and nonthrust sc - B.script
(18.29 KiB) Downloaded 213 times
thrust and nonthrust sc -A.script
(18.24 KiB) Downloaded 225 times
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Joined: Sat Apr 19, 2014 3:52 pm

Re: prop 2 s/c, one with a finite burn

Postby joandunham » Tue Aug 19, 2014 9:08 pm

The attached script gives me the answers we wanted, but not quite how we thought it would work. Note the comment on the very final propagation: I get very different results for
Propagate FourBodyProp(Finson) ForHalo(halo) {Finson.ElapsedDays = 2}; halo goes nuts in this prop
As opposed to using:
Propagate ForHalo(halo) {halo.ElapsedDays = 2}
Propagate FourBodyProp(Finson) {Finson.ElapsedDays = 2}
First burn finite second burn impulsive.script
(18.95 KiB) Downloaded 228 times
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Re: prop 2 s/c, one with a finite burn

Postby JasonLaing » Wed Aug 20, 2014 6:50 pm

Hi Joan! Am I correct in assuming that you obtained the result you were looking for? I just wanted to let you know that I've started looking into this issue. I think you're correct in thinking that this is a manifestation of GMT-3487 (not so much 1736, as far as I can tell). The propagation of multiple spacecraft together with finite burn modeling is a known area for future work. I'll keep digging around on this side for something more definitive, and I'll let you know if I find anything!
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Re: prop 2 s/c, one with a finite burn

Postby joandunham » Thu Sep 18, 2014 12:23 am

Hi Jason -
Yes, the result I got with the script I posted was sufficient for the project the team is doing. At the moment, work has been suspended while most of the team is busy with proposals, but we expect to continue work on this.
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