I think that with the Ap, Pe and Radius® of Kerbin (or whatever body you're orbiting) you can find the semi-major and eccentricity. From a quick search: You need the radius from the body center to your Ap or Pe, so let: rA=Ap+R and rP=Pe+R sma = (rA + rP)/2 which can also be written as (Ap + Pe + 2*R)/2 e = (rA - rP) / (rA + rP) which can also be rewritten in terms of Ap, Pe and R as (Ap - Pe) / (Ap + Pe + 2*R)