Gravity assists: do you use them?

[PLAD]

We can be sure that only Mun's effects are considered while the ship is in Mun's SOI, as is done in by a patched-conic approach. Back in post #63 my first test ship entered Mun's SOI going 1179.4m/s relative to Mun. 34 minutes later, at the same distance from Mun and just before leaving it's SOI it is once again going 1179.4m/s relative to Mun. If anything else was affecting it's motion during that time it would not have been going that speed. In particular it was 2000km further from Kerbin at the end of the Mun pass so if Kerbin had been pulling it towards Kerbin then it would have been going about 70m/s slower just before leaving Mun's SOI. I think the question really is, what calculation does the game use when switching SOIs? If the game just switched the reference body but kept the velocity the same then the outward motion relative to Kerbin would have been about the same as it was going in- in this case 989m/s outward (modified a bit by the Mun pulling it outward and to the left relative to Kerbin during its flyby). Instead it has been reduced roughly properly. I'd love to see the algorithm they use for SOI switches, though we could figure it out the hard way.

[EdFred]

I don't use gravity assists at all. Why? It's bad enough I have to wait for Eve to come into position just to go there. I'm not waiting around for Eve to come into position with Kerbin AND have it be in position to go from Eve to Jool. I don't bother with it in the Joolian system either. It's easier to aerobrake off Jool to intercept whichever moon you are heading for.

[arkie87]

We can be sure that only Mun's effects are considered while the ship is in Mun's SOI, as is done in by a patched-conic approach. Back in post #63 my first test ship entered Mun's SOI going 1179.4m/s relative to Mun. 34 minutes later, at the same distance from Mun and just before leaving it's SOI it is once again going 1179.4m/s relative to Mun. If anything else was affecting it's motion during that time it would not have been going that speed. In particular it was 2000km further from Kerbin at the end of the Mun pass so if Kerbin had been pulling it towards Kerbin then it would have been going about 70m/s slower just before leaving Mun's SOI. I think the question really is, what calculation does the game use when switching SOIs? If the game just switched the reference body but kept the velocity the same then the outward motion relative to Kerbin would have been about the same as it was going in- in this case 989m/s outward (modified a bit by the Mun pulling it outward and to the left relative to Kerbin during its flyby). Instead it has been reduced roughly properly. I'd love to see the algorithm they use for SOI switches, though we could figure it out the hard way.

Why wouldn't/how couldnt SOI changes be calculated as you described (switch reference body and calculate resulting velocity by subtracting velocity vector from velocity vector between new and old reference bodies)? Anything else would violate conservation of energy i.e. give you free deltaV?

[PLAD]

I just realized that the Mun's SOI is 'falling' towards Kerbin while the ship is in it, and as a result during the 34 minutes the ship is in it the Mun's speed changes 51m/s towards Kerbin (relative to the entry point) so no tricky calculation needs to be done when switching back. Doh!

I am getting a bit off the topic of the original post here, I'd like to finish by saying that in my opinion KSP does a superb job of simulating flybys, plenty accurate enough for anyone not flying real spaceships.

[BDelacroix]

I use them when I find one on accident. I don't plan them. Too much like a real job, which I have.

When I swapped station models at Duna, I noticed my older station was in a retrograde orbit. I had attached a tug to it to transfer the old crew and deorbit it, however, it would take a lot of fuel to "turn around" so when I was playing with maneuver nodes, I noticed I could cross into Ike's SOI in such a way that I ended up with a sort of figure 8 that took me around Ike, then back into a prograde orbit at Duna for a lot less fuel then trying to turn around or even land and take off again.

[TinfoilChef]

The first time I did a gravity maneuver was back in 0.16 and it was a complete accident. I had a ship returning from Minmus run out of fuel, leaving it in a highly elliptical orbit with a 65km periapsis and apoapsis somewhere between the moon and Minmus. Aerobraking would have eventually done it but only if I stayed in real time for 20 orbits (this was before physics time warp was a thing). I realized it's orbit crossed the moon's in two places. I time warped ahead a week or so and a moon encounter changed the orbit to one that had much lower AP & PE. A bit more time warp led to another encounter that lowered it even more, making an aerobraking possible that took less than 20 passes to re-enter.

Since then I try to take advantage of gravity assist if possible but I don't plan them out much ahead of time.

[hazarada]

only when I know that I won't make it to my destination otherwise, otherwise I'm too impatient as interplanetary assists can take decades to set up