Fuel efficiency returning to Kerbin orbit from Minmus.

[Alshain]

What is the most fuel efficient way to get back from Minmus to a station at a 90 deg inc. around Kerbin. I know how to get back from the Mun/Minmus if I'm landing and don't care about inclination but I'm having difficulty getting back into the orbit I want without expending lots of fuel.

Edited July 9, 2014 by Alshain

[Pecan]

It's most fuel-efficient to do the burn as far away as possible. Ideally combine your retrograde Minmus escape burn with an (anti)normal burn component so you get a polar Kerbin periapsis straight away. Failing that change planes as soon as you enter Kerbin SOI.

ETA: Doh! I'm not thinking about the angle to the station's orbit you'll be coming in at. For that you have three options, i) wait until Minmus is in just the right relative part of its orbit so you come in correctly, ii) get an equatorial orbit and do a huge plane-change, iii) land and re-launch into the correct orbit.

Edited July 9, 2014 by Pecan

[EdFred]

Well, you are changing your inclination by almost 90, so it's going to take fuel. You can burn due north(or south) on Minmus rather than fully retrograde when leaving.

Edited July 9, 2014 by EdFred

[Zephram Kerman]

The trick to doing maneuvers efficiently is to do them as early as possible; in this case even while you are departing Minmus. Passage of time multiplies everything.

Wait until the right day when Minmus passes over the station orbit, then do your departure as usual. But instead of setting Kerbin periapsis to whatever aerobraking altitude, you should burn a tiny bit more until Kerbin periapsis is zero. Then burn north or south to establish the polar orbit, until it comes back up to the desired periapsis.

Just FYI, I just explained it a bit differently in a similar thread.

Edited July 9, 2014 by Zephram Kerman

[EtherDragon]

The most efficient way:

First, plan your Minmus departure such that leave Minmus SoI while crossing the orbital plane for your Kerbin space-station.

Second, as soon as you leave Minmus SoI make your correction burn to get yourself essentially lined up with the space-station's orbit.

Three, normal rendezvous at this point. (Maybe a slight plane change fix after orbital circularization.)

[Alshain]

Ok, I will give these a try. I had tried to change the inclination while escaping Minmus but it had not occurred to me to wait until Minmus crosses the plane to attempt the escape. I bet that will make a big differenc. I can deal with a slight plane change but I usually ended up in a near polar orbit. I think I know why now.

Edited July 9, 2014 by Alshain

[Pecan]

But you need a polar orbit, don't you? In the OP you said "a station at a 90 deg inc. around Kerbin", which would be polar, and the only thing that makes the return different to normal.

[Alshain]

But you need a polar orbit, don't you? In the OP you said "a station at a 90 deg inc. around Kerbin", which would be polar, and the only thing that makes the return different to normal.

I misspoke. Got launch heading confused with inclination I guess. It's 0 degrees inc.

[Zephram Kerman]

I misspoke. Got launch heading confused with inclination I guess. It's 0 degrees inc.

Oh heck, well in that case it's easy. Just do your normal return from Minmus, but stop the burn when the periapsis matches your station altitude. This is almost a simple Hohman transfer. When you arrive at Kerbin periapsis, partially circularize into a "phasing orbit". In other words, interrupt that braking burn when the next orbit returns you at the same time as the station.

[Pecan]

Ok, so the question becomes, "what's the most efficient way to plane-change" and the answer to that is a) return when Minmus is at its Kerbin An/Dn, with the plane-change included in the ejection burn, return whenever you like, plane-change at An/Dn as you approach Kerbin.

[Kryxal]

Might it be more efficient to aerobrake the apoapsis down instead of going straight to the target altitude? Certainly you want to match planes ASAP, but as for the rest...

[Alshain]

True, I need to practice aerobraking anyway. I've not really done it before and I imagine one day I'm going to want to use it in a place where failure would be much more costly.

[Zephram Kerman]

If you pack wings, you can plane change while aerobraking! So cool. What could possibly go wrong?