Departing Mun from Polar Orbit

[Dan Kerman]

So I'm in a polar orbit around the Mun. I have plenty of dv to work with but, how to leave the Mun SOI and return to Kerbin? Do you burn to get coplanar with Kerbin first and then burn to leave the Mun and return to Kerbin...  or do a burn prograde on the backside of the Mun to leave it's SOI heading towards Kerbin and then burn retrograde in reference to Kerbin to return? Or...  a combination of both? Is there an efficiency to combining it into one burn and then fine tuning one out of the Mun's SOI?

[steve_v]

3 minutes ago, Dan Kerman said:

burn prograde on the backside of the Mun to leave it's SOI

Pretty much. Inclination change is cheap once you leave Mun SOI, as you are then close to Kerbin apoapsis.

[GoatRider]

You get a much better orbit around Kerbin if you wait until the plane of your polar orbit is tangent to the Mun orbit around Kerbin. 

[Dan Kerman]

Yes...  that does makes sense... and just missed that window... about 20 deg off that line at the moment.

[Zhetaan]

There is an efficient way to do it without needing to correct inclination.

Mainly, you need to understand that the object of a good return is to leave the Mun in a direction that is parallel to the Mun's own retrograde vector.  That's easy enough to see when everything is coplanar:  burn on the prograde side to raise the apoapsis on the retrograde side so that you leave along a path that ultimately shoots out the back side of the Mun's sphere of influence.  There is nuance to that in the form of ejection angles and other tricks, but that's the idea.

It can be done in three dimensions as well, but there are, as these things go, fewer solutions.  In other words, it will take time.

The key is to wait until your polar orbit's axis points through Kerbin.  Then eject when your rocket is above the prograde point of the Mun as normal.  This will happen twice in any given Mun orbit (of Kerbin--not your rocket's orbit of the Mun).

However, this isn't an interplanetary transfer window, either; there's a lot of room for error.  Remember that your polar orbit of the Mun is from Kerbin's perspective equal to the Mun's orbit of Kerbin plus minor perturbations:  everything is a matter of scale.  Your polar orbit of the Mun, if you were to leave the Mun at the exact north pole of its sphere of influence, constitutes an inclination of a bit over eleven degrees with respect to Kerbin.  Close polar orbit of the Mun is less than a degree.  Imagine that, for example, you wanted to go from Jool to the Mun:  that trajectory is essentially the same as going to Kerbin until you get very close to (or even within) Kerbin's sphere of influence.  With scale in mind, you can get a satisfactory Mun return from quite a wide selection of axis angles.

Edited February 1, 2019 by Zhetaan

[Dan Kerman]

10 minutes ago, Zhetaan said:

The key is to wait until your polar orbit's axis points through Kerbin.  Then eject when your rocket is above the prograde point of the Mun as normal.  This will happen twice in any given Mun orbit (of Kerbin--not your rocket's orbit of the Mun).

OK... that makes a lot of sense as it gets me out of the Mun's influence, departing it on a plane with respect to Kerbin, if I do it right,  while also burning retrograde to Kerbin.

[Zhetaan]

3 minutes ago, Dan Kerman said:

OK... that makes a lot of sense as it gets me out of the Mun's influence, departing it on a plane with respect to Kerbin, if I do it right,  while also burning retrograde to Kerbin.

Correct.  And giving credit where it's due, that is the same thing that @GoatRider said, but with a different mathematical definition.

I will caution you that because it is a polar orbit, you will have some inclination with respect to Kerbin; that's unavoidable.  However, as I said before, it's probably going to be less than a degree unless you're in a really high orbit.

[Dan Kerman]

Thanks to you both! It worked nicely....  Back in Kerbin orbit... used a quite a bit less dv than I originally planned for. I was a little late making my burn from the Mun..  once back to Kerbin, 12 deg inclination, but quite acceptable.  definitely clean that up for next time. And I see how that would work for any polar orbit departure...  great feeling when the light bulb goes on.

[bewing]

Additionally, you're only talking about something on the order of 300m/s of deltaV as the difference between a "good" return trajectory vs. a fairly "bad" one. And your craft is probably less than half full of fuel, and a lot of players throw away a bunch of fuel along with the whole bottom of their craft when they reenter at Kerbin anyway.

 

[Dan Kerman]

This vessel is a refuel/delivery vessel...  with it still being about 1/3 full..  just going to leave it in orbit around Kerbin as an "just in case" fuel stop. Just has a scientist in a module that I need Jeb to go up and "rescue" for the 6,000 science points he's carrying. Then I can close out the stock tech tree.  Yippee!

[Duck McFuddle]

If you have enough Dv then just burn to exit the Mun and then go retrograde until you are heading towards Kerbin. There is probably a more efficient way to doing this, so don't take me too seriously, but this is how I would do it. 

[bamslamkabam]

If you have the DV just escape the mun and then coast out. Then burn retrograde to bring the periapsis lower