[ericbarrett]

There is a high-quality mod which adds n-body gravity: https://github.com/mockingbirdnest/Principia

It will also add orbital perturbations/frozen orbits if you use Real Solar System: https://github.com/mockingbirdnest/Principia/blob/master/ast...

That said, to my knowledge there isn't yet a mod that adds high-altitude drag.