How is the numerical integration handled as far as units? Is the numerical integration being doing done directly on the in-game untis or is there a unit conversion done to allow for speedier calculations? Are you tracking energy and momentum conservation and are you finding that the SPRK is handling orbits well? I see you have reference the Chambers paper, have you looked at possibly using the Bulirsch–Stoer algorithm for handling close encounters with other bodies aka inside SoI? In my experience the Bulirsch–Stoer Symplectic Integrator introduces less error when close to other bodies than a RK integrator, but the down side is that it can be slow because of step-size prediction.
