Returning to Kerbin from Laythe

[Jodo42]

EDIT: So metaphor was apparently right; I can easily get to Kerbin orbit from low Laythe orbit. The problem appears to be timing/positioning.

That's my planned trajectory. Well under my dV budget. As you can see, Kerbin is almost half an orbit away from me when I reach its altitude.

Alexmoon's transfer planner said I should leave at 18y 136d 0h, but I figured being off by 2 hours wouldn't hurt, since I've never burned at the exact time for my other interplanetary missions.

Green is Laythe prograde, red is retro.

Same thing, except Jool relative. (you'll notice the prograde delta-v went down; I had to move the node 1 orbit ahead because the one I placed was less than a minute away and I don't have that kind of TWR. Doing that moved my orbit to between Kerbin and Eve, so I retracted some prograde to get back inline with Kerbin's orbit.)

What am I doing wrong?

Oh, and I have another vessel which is in a very similar orbit, but is orbiting Laythe-relative retrograde. Will I have to do anything differently for its ejection?

Edited October 4, 2014 by Jodo42

[EdFred]

Make sure Tylo is moving retrograde to Jool's orbit when you encounter it, encounter Tylo on it's retrograde side and get flung inwards.

If looking from above the Kerbol system, and Jool is at 12 o clock position, Kerbin should be somewhere in the 3-4 o clock position. Encounter Tylo while it is at 6 o clock relative to Jool, and encounter it on the retro grade side (Tylo's 9 o clock). These are approximate positions, Tylo could be 30 degrees either side of 6 o clock, depending how much of an assist you get. You can also use Jool for the assist instead of Tylo.

Edited October 4, 2014 by EdFred

[metaphor]

You could leave straight from Laythe orbit to Kerbin, it should take less than 1500 m/s. Just burn when Laythe is in the right place in its orbit so that you exit Jool's SOI going solar retrograde (with a small radial in/out component so you can tweak the Kerbin intercept).

[Jodo42]

-removed for redundancy-

Edited October 4, 2014 by Jodo42

[Kryxal]

Being off by two hours won't hurt with a Jool transfer ... being off by two DAYS isn't such a huge deal. Oh, and for the retrograde orbit, the only difference is that you'll want to make your ejection burn on the Jool side of Laythe, you'll still be moving in the Laythe-prograde direction.

[Kulebron]

Depending on what you want to achieve. If you're trying to get Tylo assist, you have to time it, and with the maneuver nodes you show you don't encounter Tylo. Try to see where Tylo is at the time you'll depart (make it target). Maybe you'll need to skip an orbit or two to get perfect alignment. Then you need depart from Laythe in a steeper orbit (relative to Jool) and make Tylo turn you to closer to Jool retrograde.

You have plenty of delta-v, why do you want to reduce it with Tylo assist?

[Kulebron]

Also note that Kerbin is very far at the closest approach. Either depart from Jool in steeper orbit (to fall down on Sun quicker) or depart "upwards" a bit, to come later.

[cantab]

Launch windows not seeming to match the game is usually a time system issue. Why is the game time window indicating Y18 and Precisenode indicating Y5? That seems very odd.

[Kulebron]

Game may show time in Kerbin units (6 hour days, 400 days/year), precise node may do it in Earth units.

[Jodo42]

I'm on Kerbin time, alexmoon is on Kerbin time. I have no idea why PreciseNode is saying year 5. If it were using Earth time I'd think it would have a different day, but they're both day 135. It's not using MET either, since that's only at 4 and something years.

[Redshift OTF]

I'm not sure what the question is or if it has been answered but I will give my opinion in simple terms.

Getting from Laythe to Kerbin is really easy in terms of DeltaV. If you are looking down on the solar system and Jool is at the bottom and the Sun is near the middle then you need to burn left to get towards Kerbin. If Laythe is going left and you ship is also orbiting left then both of these things will add up and give you a boost meaning you don't need a lot of DeltaV to perform the manoeuvre. To be more accurate you perform the burn a bit before Laythe and your ship in orbit are both going fully left as Jools gravity will pull you round a bit. That's a simple description

OP. In your case Kerbin is not in the correct place when you do your burn. This is not a problem because the trip takes so long you only need to add a bit of a radial adjustment to your manoeuvre node that Kerbin will be in a different place by the time you get there. Adding a slight radial adjustment to your burn will make the trip longer and if you give it enough time Kerbin will have come right around by the time you get to the intersection of Kerbins orbit. Does this make sense?

[Jodo42]

The radial solution worked. I barely ever use radial burns, since I heard long ago that they're the least efficient way to change your orbit. Luckily, this ship had lots of spare dV. I'm still unsure as to what caused the time discrepancy, but it looks like it doesn't matter at this point.

I'm curious as to how radial burns make a trip take longer. Perhaps something to ask in the science labs. Regardless, this was a learning experience in more ways than one. Thanks for all of your help, everyone, especially Redshift.

[Redshift OTF]

Sweet! Glad it worked. I don't have a complete explanation for how it works but if you are far away from a target or moving slowly with respect to it then the radial burn becomes a lot more useful than normal. When I'm entering the SOI of a moon or planet it is easier to adjust the periapsis using radial burns I think.