Using large moons to assist with orbit insertion?

[Gordon Fecyk]

So I was watching this car jump from Minmus and noticed that a pass in front of the Mun tightened the resulting orbit considerably.

Then I send my Iktomi explorer to Duna, and managed to pass in front of Ike, or rather, I crossed Ike's orbit just ahead of it. The insertion resulted in a wildly elliptical orbit, but it only cost me about 280 m/s. Then I take a look at what I can do out at the new apoapsis, and found that passing in front of Ike again would tighten the resulting orbit a lot, with the new apoapsis matching Ike's altitude, and only for a cost of 5 to 10 m/s. I imagine I could then tighten the orbit further for only a few m/s more at Duna periapsis, but didn't get a chance to investigate that yet.

I want to understand what I'm seeing. I've read about and watched a lot of gravity assist examples on YouTube, but that car jump video had a very clear and beneficial example that I was then able to replicate. I remember that the assists are really directional changes at the body you're doing the assist at, but the new direction would instead represent some delta-v difference relevant to the parent body. But I'm wondering if a comparably simple rule would help me, something like, "pass ahead of a moon to slow down, pass behind it to speed up." At least in the two dimensions the typical orbital plane is in.

--

[bewing]

1 hour ago, Gordon Fecyk said:

"pass ahead of a moon to slow down, pass behind it to speed up." At least in the two dimensions the typical orbital plane is in.

Yup, that's it in a nutshell. If you come in tangential to the moon, and do a tight curve in front of it, you can lose up to twice that moon's orbital velocity. Passing behind it perpendicularly and coming out tangential can gain you up to twice the moon's orbital velocity. (But in reality 2x is an impossible ideal to achieve.) If you are going for one of these gravity boosts in a hyperbolic flyby, then burning retrograde (yes, retrograde) when you are at the Pe of the moon can increase the slingshot effect, because it keeps you in the SOI longer, and then longer you stay the more energy you gain/lose.

[Gordon Fecyk]

21 minutes ago, bewing said:

If you are going for one of these gravity boosts in a hyperbolic flyby, then burning retrograde (yes, retrograde) when you are at the Pe of the moon can increase the slingshot effect, because it keeps you in the SOI longer, and then longer you stay the more energy you gain/lose.

If you were to try explaining that before I saw the car jump assist it wouldn't have made sense. But now seeing that, I can understand why remaining in the moon's SOI longer would amplify the effect. At least in KSP's patched conics model anyway, as your ship gets dragged along in the SOI of said moon.

Also good to know that there's an upper limit to the delta-v savings, approaching twice the orbital velocity of the moon.

--

Edited November 20, 2016 by Gordon Fecyk
grammar fix

[MaxL_1023]

You can also use moons for plane changes as well by making a direct pass over a pole. It usually won't be a huge effect but it can make a difference over multiple orbits. 

[Foxster]

It is what makes the Jool system a surprisingly low-cost place to get to. You can fairly easily get braking assist from one of the inner moons for a low dV orbit of Jool. Then help again with leaving the system. 

Similarly with anywhere with a moon, though some are more useful than others. 

Edited November 20, 2016 by Foxster