Dorsai!

Actually it wouldnt even even be necessary to have n-body physics for realistic gravitation, lagrange points and such. You can just as well reduce the calculations to a series of two body problems, one for each large body in the kerbal system. All the gravitational vectors could then be added up to get the final vector on which the ship/station/probe is pulled. There is really no need to calculate a real n-body system if you put the planets themselfes "on rails" and neglect the gravitational pull your ship has on everything else because its so infinitismaly small