Which side to approach for optimal capture burn?

[ErikTheAngry]

So, I did search. I honestly did. I think part of the problem is that I am not entirely sure what I'm searching for lol. Perhaps I'm incorrectly applying what is in my mind n-body physics, perhaps I'm just thinking too much, perhaps I'm not thinking enough.

In a nutshell I would like to know if there is a more or less effective side of the body (say the Mun) to approach from for a capture burn. In terms of "sides" I mean the prograde or retrograde sides, ending in either a prograde or retrograde orbit.

Approaching from retrograde is how you do a gravity assist (right?) so it would impart greater velocity. But since you're moving "with" the target (mun), that velocity is moot (right?).

Approaching from prograde is how you do a gravity... er... deceleration so it would reduce your velocity - on that same token, you're moving opposite of the target's path and so that deceleration would also be moot (right?)

My monitoring of dV usage has not yielded conclusive results, which would suggest that there is no more efficient side to approach from.

Edited July 20, 2015 by ErikTheAngry

[diomedea]

You're right about gravity assists (both to accelerate, coming from "behind" the body orbital path, or to decelerate, arriving in front of the body).

But about "optimal capture", there is no preference in KSP. Unlike space simulation programs that implement multibody gravitation, KSP uses only a two body gravitation model, so once a vessel enters the SoI of a body, there is no further influence on it from other bodies, therefore no advantage making a capture burn in relation to the position of those other bodies..

You're right instead that, should multibody be used, the side where the gravitation total effect is lower (facing the mainbody that is orbited by the body you plan to make the capture with) makes for lower orbital speed and therefore lower Oberth effect than the opposite side.

[ErikTheAngry]

Thank you Diomedea. Simple is good (single body physics).

I remember thinking it would be nice to have lagrange points, and noticing a mod in development for n-body physics. Then I saw the youtube video of orbits being perturbed by the mun and realized that lagrange points are definitely not worth all the station keeping I'd need for remotetech lol.

[ajburges]

There are more capture scenarios to consider...

When you capture from an elliptic orbit, retrograde encounters alow for an abort to free return trajectory. Prograde captures offer cheaper landings.

When you capture from a hyperbolic trajectory from the parent (Kerbol orbit -> Laythe) destination position and incoming orbit can create significant dV differences. The optimal approach trends to be to enter the parent in a prograde (matching the target) hyperbolic trajectory and encounter the destination on a tangent trajectory. This subtracts almost all of your target's orbital velocity from yours on SoI change. For Laythe this can be the difference between a 3 km/s aerobrake and up to a 8 km/s aerobrake (otherwise known as a firework).