Gravity Assist leaving Jool, How do I calculation Kerbin intercept?

[davidpsummers]

I have completed a tour of Jool's moons and I want to bring my data back to Kerbin. I want to use one of the moons to fling me out of the Jool system and save fuel. The problem is that I will then need to burn to intercept Kerbin. Any ideas on how I calculate that?

[The_Rocketeer]

You would be better off/save more fuel using a moon to sling you into a low periapsis of Jool and use Jool's gravity to assist for Kerbin. Of course you want to make sure your Jool exit trajectory is retrograde to Jool's orbit. As for calculating an intercept, I'm not aware of a way to do this in a single burn yet. If someone does know, I wanna know too!

[EdFred]

You change your patched conics number limit in your settings.cfg file in the main KSP folder from three to something higher. I have mine currently set for six. It will then show your current SOI trajectory, plus your next five. So, leaving Bop or Pol, and you wanted to use a Tylo --> Jool assist, it would show your trajectory in Bop or Pol SOI, Jool's SOI, Tylo's SOI, Jool's again, Kerbol's SOI, and also into Kerbin's.

[cantab]

Tylo is the leading pick to boot you out of the Joolian system. Laythe and Vall are options but have less "power". With Tylo it should be possible to get enough of a gravity assist to put you right into an intersection of Kerbin's orbit, though actually getting the Kerbin encounter requires proper timing. If you can't quite get there, make a burn during your assist.

[davidpsummers]

The timing the issue, isn't it? I mean, a gravity assist that crosses Kerbin's orbit is pretty doable. But the odds that Kerbin is in the right spot when the gravity assist is available aren't that big, are they?

[dzikakulka]

@david

Depends on what path you'd like to take. Stepping aside form OP, just for example: if you would like to transfer from Kerbin to Duna with Mun gravity assist, alignment is usually not a huge problem because transfer window (I mean quite optimal one) is like 20 days and sidereal period of Mun is few days. So you can easily wait and do it.

While I can't recall or check how exactly Joolian system looked like, I think the case with closest moons would be fairly similiar.

[cantab]

In theory it's the same as for a normal departure, you need the right phase angle between Jool and Kerbin. In practice it will be somewhat trickier to do, but provided you have at least a reasonably close approach to Kerbin you can probably make a course correction at some point.

[Kryxal]

I'd say the best way to make it all line up would be to aim for the same AP/PE around Kerbol as best you can. That is, you use Alexmoon's launch window planner to see what you want to end up with, then try to get an assist to do just that.

[davidpsummers]

So the answer seems to be to wait until the phase angle between Jool and Kerbin is correct, slingshot yourself out with as much retrograde as possible, and then try and adjust until you intersect Kerbin?

[metaphor]

Starting from Jool's orbit, you can get a Kerbin intercept anytime pretty easily by just adding some radial in/out component to your burn. Radial in shortens the time until periapsis, and radial out lengthens the time until periapsis. Since Kerbin's orbit is so much smaller than Jool's, a small radial burn can change the time you encounter Kerbin's orbit (near solar periapsis) by a lot. By tweaking that time, you can always get a Kerbin encounter. (By the way, this also works in other situations like going from Kerbin to Moho.)

If you're in orbit around Jool, an easy way to add a radial in/out component to your burn is to move the maneuver node forwards or backwards along your orbit. You want to exit Jool's SOI mostly going retrograde, but with some radial in/out (and maybe normal north/south) component. So assuming you're going counterclockwise around Jool, and make a maneuver node going straight towards solar retrograde, if you move that maneuver node a little forwards you're going to get a radial out component (lengthening the time until Kerbin encounter), and if you move the maneuver node a little backwards you're going to get a radial in component (shortening the time until Kerbin encounter). If you use a Tylo flyby, just make sure it's in the right position to take you out of the SOI in the right direction. It takes about a 1300 m/s burn from low Tylo orbit to encounter Kerbin, which is about 400 m/s above Tylo escape velocity, so make sure you have that amount of velocity as you pass Tylo periapsis.

[el_coyoto]

If you like porkchop plots, graphs and all that, I find KSP Trajectory Optimization Tool to be an excellent companion to mission planning.

I used it a few times to do gravity assists because I find the maneuver node system a bit limited in those cases, but it can do so many other things... (including multi flybys in a "Grand Tour" fashion)

[tsotha]

So the answer seems to be to wait until the phase angle between Jool and Kerbin is correct, slingshot yourself out with as much retrograde as possible, and then try and adjust until you intersect Kerbin?

Ideally you would sling yourself out at the optimal ejection angle, which the launch window calculator will give you.

[Ruedii]

You change your patched conics number limit in your settings.cfg file in the main KSP folder from three to something higher. I have mine currently set for six. It will then show your current SOI trajectory, plus your next five. So, leaving Bop or Pol, and you wanted to use a Tylo --> Jool assist, it would show your trajectory in Bop or Pol SOI, Jool's SOI, Tylo's SOI, Jool's again, Kerbol's SOI, and also into Kerbin's.

Just remember one thing, though: The more patched conics you have, the worse the "conics lines are twitchy" and "conics predictions are innacurate" bugs you encounter. However, I think this is a good trade-off.

Additionally, increasing patched conics to insane levels will result in performance loss, but I don't think anyone will set their conics that high.

[Yakky]

Starting from Jool's orbit, you can get a Kerbin intercept anytime pretty easily by just adding some radial in/out component to your burn. Radial in shortens the time until periapsis, and radial out lengthens the time until periapsis. Since Kerbin's orbit is so much smaller than Jool's, a small radial burn can change the time you encounter Kerbin's orbit (near solar periapsis) by a lot. By tweaking that time, you can always get a Kerbin encounter. (By the way, this also works in other situations like going from Kerbin to Moho.)

I believe tweaking your orbit using this strategy and other orbit-tweaking strategies is the way to go. As long as you can eject into a transfer orbit having its periapsis at Kerbin's orbit, you can often manufacture a Kerbin encounter without too much delta-V. You can use small radial burns and also mess around with when and where the burn takes place.

Another great, nearly foolproof technique is to be prepared to "go once around" in your transfer orbit. Just enter your transfer orbit from Jool back to Kerbin at any point, without worrying about where Kerbin will be when you get there. Then, when you get to periapsis (i.e. near Kerbin's orbit), a small prograde or retrograde burn will usually create an encounter the next time around. This works because your transfer orbit is so much bigger/slower than Kerbin's orbit that even a fairly small delta-V adjustment can lengthen or contract it by a substantial fraction of a Kerbin year. Just watch the target encounter markers as you add or remove thrust from the maneuver node you're setting up... they will rapidly converge to an encounter. The only drawback here is that your Kerbals are stuck in interplanetary space a little longer.

Edited October 2, 2014 by Yakky

[Yakky]

You would be better off/save more fuel using a moon to sling you into a low periapsis of Jool and use Jool's gravity to assist for Kerbin. Of course you want to make sure your Jool exit trajectory is retrograde to Jool's orbit. As for calculating an intercept, I'm not aware of a way to do this in a single burn yet. If someone does know, I wanna know too!

This is especially true if you're going to do a thrust-assisted gravity slingshot, i.e. augment the gravity slingshot with a prograde burn. Dropping in close to Jool will get you going really fast and will therefore greatly amplify the Oberth Effect benefit of any prograde burn you do. A thrust-assisted slingshot also lets you use your burn to refine the departure angle even if it's not quite ideal without thrusting.