To do a gravity assist you do not need anything more than Newtonian two-body interaction. KSP already has that. You can think of it as a small ball (the vehicl

To do a gravity assist you do not need anything more than Newtonian two-body interaction. KSP already has that. You can think of it as a small ball (the vehicle) bouncing of a big heavy ball (the planet), but the forces are not those of elastic compression, rather those of gravity.