How does KSP calculate orbit lines?

[TheVortexian]

I'm trying to recreate a KSP-like game in Java, and I have no idea how KSP calculates Apoapsis, Periapsis, etc. when your ship is moving in space.

[bewing]

Welcome to the forums. Just so long as you assume that there is only one gravitational influence on a ship, there are closed-form solutions for converting from (x, y, z, dxdt, dydt, dzdt) to the standard Keplerian Orbital Parameters.

Have you looked at wikipedia? Are you a math whiz who knows everything there is to know about quaternions?

 

[tomf]

This website has a bunch of equations you are going to need http://www.braeunig.us/space/orbmech.htm

[Snark]

Also, this doesn't directly answer your question... but if you want to see lots and lots of actual code for calculating orbital parameters, it's worth looking at the source code for MechJeb, which (like every mod, and unlike the source code for KSP) is publicly posted.  Lots of good stuff in there.  It'll give you a feel for the kind of code you'll need, even if it doesn't have the actual literal stuff you're looking for.  Basically, look for any file there that has "orbit" or "orbital" in the file name.