To find the speed which you'll hit the ground at, use the formula Xf=Xi+v(t)+1/2a(t2) Assuming of course, that the height of your orbit at t is also your altitude, and that the Mun's gravity doesn't diminish with distance. Solve for t to find the time until impact, then place that t into this formula: Vf=Vi+a(t) Also, keep in mind that since your velocity is tangential to your orbit the initial vertical velocity should (theoretically) be 0. EDIT: Just saw the previous posts, I was always under the impression that in KSP the gravitation was modeled constantly(ish). Obviously not at extremely high altitudes, but this would situation would be close to the surface.