Hello all, I have a spacecraft I am trying to get from Earth to Mars. I use an Earth propagator until the SC is out of the Earth's sphere of influence, then a deep space propagator until Mars' sphere of influence, and then I switch to a Mars propagator. The only stopping conditions the propagators have are the distances (i.e. propagate until 924,000 km from Earth with Earth propagator, propagate until from 578,000 km from Mars with deep space propagator, and so on).
I want GMAT to decide how much time the spacecraft should spend in each propagator since I don't how many days it would take the SC to go from, e.g., LEO to the end of Earth's SOI, or the edge of Mars' SOI to my desired Mars altitude or periapsis, so the only conditions I give are the desired arrival date to Mars, and the distances in the trip in which it should switch propagators. However, whenever there are no time-dependent stopping conditions in the propagators, GMAT propagates the spacecraft with what I assume is the aim of reducing delta-V as much as possible, leading to the trip lasting hundreds of years, which causes the optimizer to reach the end of GMAT's ephemeris data and the "PlanetaryEphem (sub)class exception: Requested epoch 95008.513478505 is not on the DE file" error. I sometimes get this error when I put impossible stopping conditions in the propagate commands, but this shouldn't be the case here. I have tried adding a second stopping condition to the last propagator in the sequence, being the desired arrival date, or adding a non-linear constraint on the epoch of the SC upon arrival, but the optimizer still tries to consider unrealistic dates. I also tried minimizing total travel time in the cost function, same problem. The mission sequences from my attempts can be seen in this link: https://imgur.com/a/myLEmM6 Finally, I created three variables: earth_prop_time, deepspace_prop_time and mars_prop_time, being the amount of days spent by the spacecraft in each propagator. I added an "ElapsedDays" stopping condition in each propagator, equaling those variables, and then a constraint on their sum being my desired total travel time. I added some plausible initial values to the Vary commands for them, but it does not vary them properly, disregarding my initial values in one iteration and leading to a non-converge message: https://imgur.com/iPKM0M7 So, is there a way to constrain the dates that the optimizer analyzes without specifying time conditions in the propagators? I essentially want the same thing that happens when you specify a time-dependent stopping condition, like ElapsedDays, but with different propagators for each part of the trip. If that isn't possible, and I must always specify how long each part of the trip should be, how can I get those intervals? PS: the commands labelled as "propagate to Mars periapsis" in the pictures are actually "propagate to Mars altitude", where the altitude equals the number I have on the constraint (500 km). Best regards, Panagiotis Karakatsanis
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