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I'm a beginner to trajectory design, and have to use NASA's GMAT software. It's been a few weeks now, and I have done the first four tutorials and managed to create a (very unrealistic and sub-optimal) trajectory taking a spacecraft from the Earth to a Sun-Earth L2 halo orbit and then Mars with B-Plane targeting. Very crude and using too many burns, but at least I consider it a step forward.

However, in order to properly solve complex trajectory problems one has to use patch points and control points, involving multiple spacecraft representing the different desired positions of the spacecraft, backpropagation, various commands to impose velocity and position continuity between them, etc.

I did the first 4 tutorials with ease and successfully learned from them, but then the fifth tutorial ("Optimal Lunar Flyby using Multiple Shooting"), which is supposed to be an introduction to using control points and patch points, is a massive leap from the previous one, and doesn't really explain the logic behind the method or steps in detail. I am still trying to do it, but in the meanwhile, are there any resources online for a beginner which I can use to better understand this method of trajectory optimizaton? Can't find much online other than mentions of the terms in academic papers.

For that matter, what is the most active online community for GMAT discussions? The link on GMAT's startup screen is dead, and its online presence in general is kind of weird. What I assume is the official website is a bit of a mess, having a lot of dead links. Surprising considering it's software made by NASA and used in various real-life missions.

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    $\begingroup$ This answer might help $\endgroup$ Commented Jan 26, 2022 at 12:46

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I definitely agree with you that the Lunar Flyby example isn't very helpful, as they seeded the script to converge pretty quickly, which isn't very helpful for users trying to get started with multiple shooting problems and patch points.

For the more complex problem of an Earth --> SE L2 --> Mars B Plane, I think your best bet is to start slow at first, creating each segment individually and then creating a larger script that incorporates seeded starting values for optimization through collocation. Based on your goal trajectory, the SE L2 orbit will likely be the most difficult to achieve, so I would suggest using something like the JPL Periodic Orbit Tool to get an initial guess that can seed some guesses (will need to convert from non-dimensional to dimensional units).

While you're getting started, it may be easier to go a bit smaller, and try to recreate the Lunar Flyby script from scratch, or add you're own take, doing something like a ballistic transfer or manifold insertion around the moon that can get you more familiar with the targeters and optimizers.

As for your final question, there really isn't an active community other than random stack exchange posts, as far as I've found. There is a public Jira page to the project, that I have found to be fairly useful in identifying if my problems are one-offs or actual bugs, and giving feedback to the team. Hopefully they re-open a community page at some point. Here is a ticket I submitted to their Jira with GMAT Functions for seeding Richardson's 3rd order Halo Solutions (see also CDS140B: Computation of Halo Orbit) that may be helpful for your SE L2 orbit (a very very early draft): https://gmat.atlassian.net/browse/GMT-8080

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  • $\begingroup$ This is great! I'd never heard of [Richardson's 3rd order Halo Solutions. I've added a few links that may be helpful to furture readers like me. $\endgroup$ Commented Jun 19, 2024 at 0:50

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