Trajectory planning before launch (or spacecraft design)

[eggrobin]

From #4050 and many earlier discussions; it would be nice to be able to do something like the GRAIL launch.

• We would want a flight-plan-like entity whose manœuvres are instantaneous changes in velocity, since we have no vessel. A lot of the logic remains, including anomalous manœuvres. However, since time can run either way, the anomalous manœuvres can be in the past or in the future.
• The initial or final state should be tunable using some widget ; osculating Keplerian elements are probably more useful coordinates here than anything cartesian.
• The initial state should be shown in some form that is suitable as a desired final state for MechJeb ascent guidance. Eventually we should talk to @lamont-granquist about supplying that via an API.

[lamont-granquist]

What MJ needs is probably just BCI Cartesian r and v coordinates and time of a possible initial point to hit on the desired orbit. Initially PVG would be quite sloppy since it doesn't understand J^2 so it would attach to the osculating orbit at some other point. But that could drive fixing PVG to know about J^2 (I've got a paper which works it all out, and it's not terribly difficult on the scale of PVG things that could get implemented). PVG could also learn about the Earth-Sun-Moon system and start to be a little bit of an n-body integrator if that ever gets required (a bit harder, but in LEO where the perturbations are small that should be feasible).

PVG doesn't currently know how to deal with a terminal coast to hit a point on an orbit, but that's a known problem and the solution to that would also drive solving the orbital finite burn problem. It would be able to handle positive or negative time terminal coasts to attach before or after the selected point.