Return from elliptic Laythe orbit with 4k dV

[tibbe]

I'm currently in an elliptic Laythe orbit (Pe: 117km, Ap: 3,168km) and I have 4k dV left. I'm having trouble plotting a course back to Kerbin. I'm trying to escape when Laythe is moving Jool retrograde to get an extra boost, but I can't get my escape trajectory to be Jool retrograde. When should I burn when returning from Laythe to Kerbin?

Edited October 22, 2015 by tibbe

[Starhawk]

You will want to line up both your ejection from Laythe into Jool orbit and your ejection from Jool into solar orbit in the correct direction. They should be in approximately the same direction when you make your burn. You should need much, much less than 4000 m/s to return to Kerbin.

Happy landings!

[Snark]

It's not totally clear from your post where your Laythe periapsis is-- i.e. when your ship is at Laythe periapsis, is its orbital velocity around Laythe pointing in the same direction as Jool's orbit around the Sun, in the opposite direction, or somewhere else?

In any case: Some options below. You could go with any of these, because 4000 m/s of dV is a lot and you will easily go home to Kerbin with plenty to spare, so you have a lot of choices.

Option #1: Direct ejection. Eject when Laythe is moving retrograde relative to Jool's orbit around the sun (i.e. when Laythe is on the sunward side of Jool), and also when your ship is orbiting Laythe retrograde relative to Jool's orbit around the sun (i.e. when the ship is on the opposite side of Laythe from Jool). This will allow you to add Laythe's Jool-relative velocityand your ship's Laythe-relative velocity to your burn, which will save some dV.

Option #1a: Same as #1, but time it so that you get a slingshot boost from Tylo. This can make a big difference-- Tylo is big and heavy, it packs a pretty good wallop as a slingshot fulcrum. It means waiting for a few Laythe orbits until Tylo is in the right position.

Option #2: Jool slingshot. Eject when Laythe is on the side of Jool that's away from the sun, and your ship is on the side of Laythe towards Jool (i.e. when your ship is orbiting Laythe in the retrograde direction of Laythe's orbit around Jool). Give it enough dV so that when it ejects from Laythe, its periapsis takes it down as low as possible to Jool, i.e. just above the top of the atmosphere. Coast down to Jool periapsis, then do a burn there to eject from Jool solar-retrograde to take you home again.

Option #2a: Same as #2, but with a Tylo slingshot.

Ideally, to make maximum use of Oberth effect, you'd want to do your Laythe ejection burn when you're at Laythe periapsis. However, you're limited there by the fact that the periapsis orientation is stuck in one particular direction that you can't change, unless you're willing to wait until Jool orbits around to the other side of the Sun.

But the cold fact of the matter is that with a honking great load of dV like 4000 m/s, you have easily enough for pretty much any trajectory that it works-- you've got plenty of dV to spare, so you don't need to be too concerned with optimizing Oberth unless you want to do so just for the challenge of it.

[Yasmy]

You should need much, much less than 4000 m/s to return to Kerbin.

Happy landings!

Yup, about 1100 m/s, plus some extra for a plane change, from low Laythe orbit back to Kerbin. If your current elliptical Laythe orbit is pointing in the wrong direction, you could just burn suboptimally in the correct non-prograde direction for departure and still have lots of fuel left when you get home.

[Kryxal]

I'd say lower the orbit if you're aligned the wrong way ... don't make things more complicated than they need to be.

[Red Iron Crown]

Go to map mode, focus on Jool. Adjust the camera so that you are looking down on its North pole (i.e. from "above"). Zoom out until you can see Jool's orbit line, rotate the camera so it runs up and down with Jool's dark side on the right. Zoom back in so Laythe is clearly visible. Timewarp until Laythe is in the 10 o'clock position, roughly. Then eject in Laythe's prograde direction.