Gravity Assists for Dummies

[Reinhart Mk.1]

I'm just now getting the hang of gravity assists but I often end up waiting entirely too long for encounters and lately it seems when i do make the encounter I'm more or less wasting time and delta v and my apoapsis doesn't seem to really JUMP like I see it do in other videos... I'm starting from Kerbin, then get a Eve encounter, then another Kerbin encounter, then another Kerbin encounter. If I calculated it right my AP should be crossing Jool's orbit but these last couple of tries haven't really been raising it much. I feel like it would make trips so much easier but maybe I'm missing something with fine tuning...

[Guest]

I haven’t used them much in KSP but I think I understand the theory behind it. 

Basically what I think is that the effectiveness increases in relation to how perpendicular your entry to the encounter is with the vector of the body. 

(Making sure that you are passing the planet on the rear side of its passage and not passing in front of it. In front bleeds your speed off, behind pulls you like a slingshot.)

*anyone pls correct me if I have the wrong idea I’m self taught lol

Edited July 30, 2019 by Guest

[Snark]

1 hour ago, Dale Christopher said:

I haven’t used them much in KSP but I think I understand the theory behind it. 

Basically what I think is that the effectiveness increases in relation to how perpendicular your entry to the encounter is with the vector of the body. 

(Making sure that you are passing the planet on the rear side of its passage and not passing in front of it. In front bleeds your speed off, behind pulls you like a slingshot.)

*anyone pls correct me if I have the wrong idea I’m self taught lol

Nope, you've pretty much nailed it. Well done.

1 hour ago, Reinhart Mk.1 said:

I'm just now getting the hang of gravity assists but I often end up waiting entirely too long for encounters and lately it seems when i do make the encounter I'm more or less wasting time and delta v and my apoapsis doesn't seem to really JUMP like I see it do in other videos... I'm starting from Kerbin, then get a Eve encounter, then another Kerbin encounter, then another Kerbin encounter. If I calculated it right my AP should be crossing Jool's orbit but these last couple of tries haven't really been raising it much. I feel like it would make trips so much easier but maybe I'm missing something with fine tuning...

 First of all, make sure you're doing it for the right reasons. If you're doing it for the challenge of doing so, or to learn about orbital mechanics, great! But if you're trying to use it for practical benefit... honestly I think it's more trouble than it's worth. Hard and finicky to set up, limited in how much oomph it can give, and hard enough to aim that there's a good chance that you lose more dV to lack of precision than you gain from the gravity boost.

But if you're just in it for the challenge, of course that's a great reason.

Some practical advice:

Make your flyby as close as you possibly can. You want your Pe to be the absolute lowest you can manage without hitting surface or atmosphere.

Make sure you've got the angles right. You get the maximum benefit when your Pe is directly behind the body.

Don't go whizzing past too fast. If you go streaking past the body like a bullet, so that your path doesn't bend much, then you don't get much benefit.

 

[bewing]

And if you are zooming past too fast to get much benefit, even with your lowest possible Pe that you can arrange -- then you can get more of a gravity assist by burning a few dV retrograde at your Pe. It gives the CB more time to fling you gravitationally.

[Zhetaan]

@Reinhart Mk.1:

12 hours ago, bewing said:

And if you are zooming past too fast to get much benefit, even with your lowest possible Pe that you can arrange -- then you can get more of a gravity assist by burning a few dV retrograde at your Pe. It gives the CB more time to fling you gravitationally.

Alternatively, since you're zooming past at ludicrous speed anyway, you can try a powered assist, which is a prograde burn at the periapsis.  The Oberth Effect increases with your velocity, so if your gravity assist is sending you in the correct direction, then a small burn at high speed may give you the correct altitude, as well.  Beware that burning will change your orbital characteristics, so if you're doing this after a precalculated four-pass assist, then you will need to correct it so that you actually encounter Jool rather than Jool's orbit.

[Snark]

13 hours ago, Snark said:

I think it's more trouble than it's worth. Hard and finicky to set up, limited in how much oomph it can give, and hard enough to aim that there's a good chance that you lose more dV to lack of precision than you gain from the gravity boost.

Actually, I should qualify that.  There's one place that I've found gravity assists to be actually helpful and worthwhile, and that's capturing to Jool.  A reverse gravity assist (i.e. to lower your speed, rather than increase it) is pretty easy to set up, using Tylo or Laythe (they both work pretty well-- which one's "better" depends on your intended mission profile, but Tylo's probably more common for most cases).

A well-executed Tylo reverse gravity assist can get you several hundred meters per second of dV; it's enough that when you set it up right, you can directly capture to Jool on an interplanetary trip from Kerbin without needing to burn at all-- just the assist is enough.  (You'll almost certainly want to do a burn afterwards to adjust your orbit and set a course for whichever moon you're heading to, but it's still quite a lot of dV savings.)

Detailed step-by-step instructions for setting up a Tylo reverse assist here, for the curious: