As a fun side project I'm writing a turn-based game that tries to portrait solar system travel little bit more realistically then most of games do. I want to calculate the cost of traveling between planets based on delta-v differences graph like this one:

    enter image description here

I read up on delta-v budget and read through some of the Kerbal Space Program tutorials (and played some too) but I'm still not sure what "planet intercept" is.

If I start from low Kerbin orbit and make delta-v steps of: 950 (Kerbin intercept), 80 (Eve intercept) am I at high Eve orbit? If I stop accelerating will I stay on that orbit?

And more importantly, does this model make any sense at all?

  • 3
    $\begingroup$ Come on... Man up and use the real solar system as your question's reference... ;) imgur.com/r/space/SqdzxzF $\endgroup$
    – Erik
    Commented Nov 3, 2013 at 1:59

1 Answer 1


There's two parts to this question (and answer), KSP and real life. I'll get KSP out of the way first.

KSP uses a simplified two-body physics model rather than full N-body physics, for performance reasons. So, each celestial body has a sphere of influence, a region in which any spacecraft are affected by its gravity and nothing else. An intercept, for the purposes of a delta-v chart, means getting close enough that you enter its sphere of influence. This is represented on the orbital map as a circular icon with the words "Mun Encounter" (or similar); leaving a sphere of influence shows up as "Mun Escape".

Note that just because you are in something's sphere of influence doesn't mean you are in a stable orbit around it. That 850dv to intercept the Mun will get you a flyby of the Mun, entering its SoI, passing it on a hyperbolic trajectory, and then exiting its SoI to return to Kerbin orbit. If you want a stable orbit, you'll need to expend additional dv circularizing. Whether this flyby is in "high" or "low" orbit depends on the precise details of your intercept burn; for bodies with atmosphere, such as Eve, you can even tune your intercept to fly through the planet's atmosphere, allowing you to aerobrake.

In real life, things are a bit fuzzier. The concept of a sphere of influence still exists, but even within something's SoI you are still affected by the gravity of other bodies. What "intercept" means will depend on the mission - if you're trying to dock with a space station, you need to get within touching distance, whereas for a flyby of another planet you just need to get close enough for the cameras.

For the purposes of your game, I would say:

  • Yes, it does make sense to model travel costs using delta-v.
  • If you want to distinguish between flybys and orbits, separating "delta-v to intercept" and "delta-v to enter a stable orbit after intercept" makes sense; if you assume all intercepts will be for orbits you can combine the two.
  • Don't forget to give craft that can aerobrake a discount, when intercepting things with atmosphere.
  • Also don't forget that there are a variety of transfer orbits you can use with different delta-v/time/return trajectory tradeoffs, including ones not mentioned on that page like the bi-elliptic transfer and cunning gravity slingshot manouvers off other planets

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.