Jool slingshots?

[Firemetal]

Ok so lets say I have a spacecraft that I want to go to Jool with. I have seen a lot of people trying moon gravity assists like Laythe and Tylo since burning retrograde at pe takes too much DV and aero-capturing is way too dangerous. I have tried this myself but gravity assists never seem to get me into an orbit. It always just slingshots me into a different escape trajectory. Is there a certain way to do this?

Edited July 8, 2016 by Firemetal

[Plusck]

  On 7/8/2016 at 3:46 PM, Firemetal said:

Ok so lets say I have a spacecraft that I want to go to Jool with. I have seen a lot of people trying moon gravity assists like Laythe and Tylo since burning retrograde at pe takes too much DV and aero-capturing is way too dangerous. I have tried this myself but gravity assists never seem to get me into an orbit. It always just slingshots me into a different escape trajectory. Is there a certain way to do this?

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Simplest way (for me) is via Tylo, since it leaves less chance of getting an unpredictable set of encounters afterwards.

So for Tylo, you want to enter the system slightly ahead* and sun-side**, and just graze Tylo's orbit heading prograde.

While you are still a couple of hundred days away, you need to set a maneuvre node and use a combination of prograde/retrograde and radial-in/out to change the time you arrive, while still just grazing Tylo's orbit. With a burn for a few dozen m/s, you should be able to time the encounter so that Tylo sends you deeper into the Jool system. This change in angle means you are no longer on an escape trajectory: the deeper you go (i.e. the closer you pass Tylo), the tighter your resulting orbit will be around Jool. You need to place a fake 0m/s node after the encounter so that you can see where it will go.

This is an example with a close pass by Tylo (and apologies for this being 1.0.5, before the change in shading for orbit lines):

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*"slightly ahead": Jool is going to be catching up with you at the encounter anyway, so you can't help but enter Jool's SOI from the "ahead" (prograde) direction.

** "sun-side": you do NOT want to enter Jool going the wrong way around, or you'll never get anything done. On the pic above, the sun is just off to the top-left.

Edited July 8, 2016 by Plusck

[Firemetal]

Ok thanks so its all timing?

[Plusck]

  On 7/8/2016 at 4:13 PM, Firemetal said:

Ok thanks so its all timing?

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Absolutely. Just a combination of prograde/retrograde and radial-in/out burns can have a huge effect on the time you hit Jool's SoI, while still ending up with exactly the same trajectory inside the system. It's a bit fastidious to set up, but the aim is to time it so that Tylo is where you want it when you get there. And where you want it is more or less what you see on that Tylo encounter in my screenshot.

Using Laythe is the same. In a sense it's easier to set up (Laythe's orbit is so short that it takes very little to correct your arrival time), but on the other hand you end up going very low and fast by Jool and therefore the post-encounter corrections to go where you want are harder to set up (imo).

[Firemetal]

Ok sending a probe there right now! Thanks!