Gravity brake: Kerbin -> Eve -> Moho

[valenti_scott]

Hello-
I was wondering if it is possible to fly from Kerbin to Moho, using Eve (or Kerbin itself) as a gravity brake. I think the answer is "no," because of the theory of what happens in a gravity assist. Or at least my understanding of the theory...

In theory what is happening is that you are adding the velocity of the assisting planet to your own. That being the case, there is no way to use Eve or Kerbin to slow you down (and send you on your way to Moho).

On the other hand, maybe you could use the Mun, because you could catch it when it is moving retrograde compared to Kerbin's orbit. But I think the effect would be small, and there would be no way to do it again (after the vehicle is in an orbit around the Sun).

But maybe I'm thinking about this all wrong. Let me know what you think.
-Scott

[Quizzical]

It's possible, but not at all easy. The bigger problem is that the orbital planes of Kerbin, Eve, and Moho are all different. Moho is about 7 degrees away from Kerbin, and that difference is a large chunk of the difficulty in getting to Moho. You want to adjust your orbital plane while at as high of an altitude as possible, as it takes less delta-v that way. If you use a gravity assist off of Eve to lower your orbit, and then try to adjust your plane, you could easily end up using more delta-v than if you just did it the straightforward way.

It's also possible to use the gravity assist off of Eve to change your orbital plane to be much closer to Moho's, but this requires that the gravity assist be done while Eve is very near the intersection between Eve's plane of orbit and Moho's. Lining up a gravity assist at all is tricky enough, without needing to make it happen in a particular place that it is unnatural to do it.

[valenti_scott]

Thanks for the reply; interesting ideas; but I'm still bothered by my understanding (in theory), that in a gravity assist you are adding the velocity of the assisting planet to your own.

Assuming that is true, I can see how it can get you to higher orbits (like Jool), but not lower orbits (like Moho). But I suspect there is something I'm not seeing here...

[Jupiter3927]

The momentum transfer works both ways.
Aim for the other side of the planet and you'll lose momentum instead of gaining some from the planet.
Eve is one of the best planets to use for a gravity assist due to the huge mass.
It's possible to use Moho for an assist to get there if you fly past it a few times before going for orbit.

Plane changes can be done very cheap if you have a very elliptical orbit and change at the apoapsis.

[Quizzical]

Think of it this way. When you zip by some planet, the planet's gravitational pull is strongest on you at periapsis. When you're at periapsis, the planet pulls you in some particular direction. That direction is basically the average direction that the planet pulls you in for the time that you're in its sphere of influence, so on net, the planet will pull your velocity in that particular direction. It doesn't have to be the direction that the planet is moving, and usually isn't, unless your periapsis is directly behind the planet.

[valenti_scott]

OK, that makes sense. Thanks everyone. I did watch a vid which talked about how you want to make your periapsis minimal, and yet safe (not running into a mountain on the Mun, for instance).

And at periapsis that is when you have the greatest acceleration due to gravity (in the radial direction), so I guess your "assist acceleration" is in that direction.

I'm wondering if you're final "assist velocity" is effected by the velocity of the planet or moon (in the larger SOI). E.g., in the case of the Mun maybe you add in the velocity of the Mun in its orbit around Kerbin?

[Quizzical]

Your velocity relative to Kerbin is (your velocity relative to Mun) + (Mun's velocity relative to Kerbin). That's just how vector addition works and has nothing to do with gravity assists.

[MechBFP]

Is it possible? Absolutely. Is it practical to do in the game? Pretty close to impossible. You can't plan that many gravity assists in advance, and even if you could the burn accuracy would need to be 99.999% accurate every time otherwise all your calculations would go out the window.

[valenti_scott]

Let's say you're getting a gravity assist from the Mun. Let's propose a hypothetical situation in which the Mun is moving the opposite way around Kerbin. You get a gravity assist from the Mun.

Then let's compare that to the usual situation, in which the Mun is moving at its usual velocity around Kerbin. You do the gravity assist again; the only difference being that the Mun is moving in its usual direction.

I feel that the resulting gravity assist would have to be different, although the difference may be small.

[MechBFP]

Originally posted by valenti_scott:

Let's say you're getting a gravity assist from the Mun. Let's propose a hypothetical situation in which the Mun is moving the opposite way around Kerbin. You get a gravity assist from the Mun.

Then let's compare that to the usual situation, in which the Mun is moving at its usual velocity around Kerbin. You do the gravity assist again; the only difference being that the Mun is moving in its usual direction.

I feel that the resulting gravity assist would have to be different, although the difference may be small.

If you mean you are going the same direction as the Muns orbit in one case, and the opposite direction in the other case, then yes they would be different. In the first case (same direction) you get a boost in speed (gravity assist), in the second case (opposite direction) you get slowed down (gravity brake).

If you meant always going the same direction as the Mun, but with the Mun orbiting Kerbin in opposite directions, then yes you would also see a difference, although at that distance it would be relatively minor because Kerbin's gravity is significantly weaker than the Muns at that distance.

[GunsForBucks]

Think of it like this.

If you launch straight up at sunset that makes your ship aligned retrograde to kerbins orbit around kerbol the sun.

This will slow you down relative to Kerbin in its orbit and you will fall in toward the sun.

Launching straight up at sunrise makes your ship aligned prograde to kerbins orbit and thus you accelerate outward from the sun.

Basically if you are doing a gravity assist slingshot around a moon you just have to align your exit in the same way. Retrograde to kerbins movement around the sun to go in, prograde to go out. Basically going around in orbit aligned with rotation you want the moon between you and the sun to go in and kerbin between the mun and the sun to go out.

Hope that makes sense... probably a case of drawing it would be a lot more clear.

[valenti_scott]

I was thinking you could take the hyperbolic shape of your gravity assist, and then draw a line (a ray actually) from the periapsis through the center of the planet and on to infinity, wouldn't that define the vector which is applied to your velocity?

In the case of the Mun moving one way and then the opposite way, I meant you'd keep your course relative to the background stars the same. I just think the Mun would give you a small extra kick one way or another, depending on its velocity around Kerbin.

[valenti_scott]

Hey, given my breakthrough understanding of gravity assist, I decided to embark on a rendezvous and science-return of a near Kerbin object.

I am using a Mun fly-by to help me reach Kerbin escape velocity with a little less fuel.

Thanks again everyone for your help.

[Jupiter3927]

Use the Mun on the way back too and you can save even more fuel.
Gravity assists are really going to help out on a Jool mission.
It's possible to pinball off Tylo, Laythe and Vall for a free capture into a reasonably stable orbit.
Or just throw a ton of tiny probes near Tylo all on slightly different trajectories and watch gravity do its thing...

[valenti_scott]

Ah, nice.