Gravity assist to reduce delta v?

[Turvok]

Has anyone worked out the timings or if it's even possible to use the gravity of other planets to get to the outer planets using less delta v? Eg. Swing around Eve or Duna to use less delta v to get to jool or eeloo?

Edited November 20, 2013 by Turvok

[TechnicalK3rbal]

It is possible with moons, and I would imagine it is also with planets, but it seems like it would be to impractical to use another planet for a gravity assist.

[KvickFlygarn87]

Well, it is indeed possible, but you need to be very skilled to pull it off.

[Supernovy]

Yes, with liberal abuse of the manoeuvre node system I managed to use a Kerbin assist to get to Duna.

There are two key things to keep in mind when attempting interplanetary gravity assists: patience and vector mathematics. Generally, you either want to come out of an encounter prograde, retrograde, or either of those two with a bit of inclination (although gravity assists for plane changes require even more timing). This also means it can be beneficial to come into an encounter at an angle.

Secondly, you want to get those encounters without burning much fuel, and this is where the manoeuvre nodes come into play. Target the planet you want to slingshot off, and note how far away the position indicators are. Then place a node after the encounter with 0 delta-v, this makes the target system show what happens after one orbit. If the position indicators are still not close enough to get an encounter with a small burn, place another node between the last node and the encounter. The node will (hopefully) go onto the previous node's orbit and make the target system show the positions after two orbits.

Keep doing this until you're satisfied with the closeness of the positions, burn a little to get a good encounter, then either repeat or burn to your final destination. Eve is handy for assists as it's the most massive inner planet, but be careful not to hit the atmosphere.

[Turvok]

Thanks Supernovy. That helps that it is possible. But is there any savings in it over top of going directly to a planet.

Ie if it takes 6000 delta v to get to jool but only 3000 to get to eve and a slight burn of 2000 from there to jool u would be saving 1000. (I'm on my cell at the moment sorry I can't look up the actual numbers)

I'm more theory crafting here but mostly wondering if you can get a craft to one of the outer planets using less then the generally accepted amount of delta v or a ship that can't technically get there. Nasa does it all the time with real world physics sort of Curious if the engine allows that complex of maneuver

[Supernovy]

Yes there is, you can get to Jool using only the stock Kerbal X (which doesn't have enough delta-v to do a Mun landing and return).

In fact, here's a video Scott Manley did on it:

[Specialist290]

I can confirm that gravity assists can be extremely helpful if you manage to get a good one. Just ran the numbers using Arrowstar's Trajectory Optimization Tool (extremely handy for this sort of thing) and compared the delta-v numbers for a direct burn to Moho vs. a Kerbin -> Eve flyby -> Moho trajectory. From what the trajectory planner tells me, you can shave off ~1000 m/s from the departure burn by waiting for a good time to set up a gravity assist.

[arq]

Gravity assists can be used to move around the system on fumes. I haven't done much between planets, but I've gone all over the Jool system (planets work the same, it's just a bigger scale) using them. Also, a few times my Munlanders didn't quite have enough dV to get back to Kerbin so I burned to escape Mun's SOI, then with a few assists off of Mun I was able to get my peri back into Kerbin's atmo.

[allmhuran]

Boy do I have some content for you!

First, a quick look at the basic mechanics.

Now a more detailed explanation of how this works using some terrible animation and in game footage:

Finally, evidence of just how useful this can be: 5528 science points earned on a first career mode launch using starting parts only:

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[Turvok]

Wow thanks guys. That's what I wanted. + rep allmhuran! In the top picture if I understand it right up is the direction of the planets travel so that would be from eve's North pole facing down on the planet and kerbol would be on the left of the picture.?

I'm going to study the gifs when I get to my pc at home next week. Special thanks to specialist290 for linking the trajectory optimization tool. I was kinda looking for something along those lines with Google already.

[allmhuran]

In the top picture if I understand it right up is the direction of the planets travel so that would be from eve's North pole facing down on the planet and kerbol would be on the left of the picture.?

You got it.