Retrograde Mun orbit

[Falkenherz]

I recently accepted three rescue missions, two of them around Mun, one in a Kerbin orbit. Nice synergy, I thought, for a good pile of cash!

But nothing in Kerbal ever just works out fine, does it?

The Kerbal orbit turned out to be halfway out to Minmus. One wreck around Mun was orbiting prograde, the other retrograde.

Sigh. I have 1800 dV left after achieving LKO.

So I burned prograde to Mun, rendez-vous´d the wreck in prograde orbit, burned prograde outwards towards the wreck in HKO. Easy.

But, I did not manage to burn back into a retrograde orbit around Mun without expending heaps of deltaV for normal/antinormal burns. Is there a specific trick on how to achieve this?

I will have to launch another ship and will try to do this starting again from LKO. I read somewhere that the Apollo missions did a "8" pattern and entered retrograde orbit. How do I burn for this kind of "8" pattern?

Edited May 6, 2015 by Falkenherz

[KerbMav]

Move your maneuver node along your orbit and maybe try the "plus one orbit" button on the node - or use PreciseNode for easier handling here - until you get an encounter with Mun the usual way (by only lowering your orbit) instead of trying to force one with weird burns - it might help to align your plane with the Mun's.

Try to find a maneuver that will have you arrive at your new PE on Mun's orbit to meet Mun, instead of burning "left and right" to get the orbit that you want, try burning at your AP more or less to get your PE "above or below" the Mun in its orbit.

If nothing works out, the least fuel consumption to reverse your orbit will be with a very low PE and the highest possible AP before leaving Mun's SOI; burn retrograde at your AP then, because you will be going at the slowest possible speed there.

The "8" was for a free return trajectory, they only did a flyby to slow down and lower their PE at earth.

To get into a specific orbit you want to arrive before or after the moon, so either start your burn earlier/later or burn more/less - just play around with the maneuver node until you get what you want.

[technion]

If you fail to get the right orbit next time - using a radial burn from far out, the as soon as you enter the SOI, will turn a "20km prograde" to a "20km prograde on the retrograde side of the planet" for virtually no dv.

[Falkenherz]

Ah, I should have used a google-forum search, this for sure was not a new question...

http://forum.kerbalspaceprogram.com/threads/115673-Ending-hohmann-transfer-in-prograde-vs-retrograde-orbits/page2

[KerikBalm]

but to answer one question, for a sufficiently large plane change, it is most efficient to do a burn to set your apoapsis out to the edge of the SOI, then do the plane change at apoapsis, and then lower apoapsis again at perapsis.

Reversing direction is nothing more than a 180 degree plane change.

From low mun orbit, you need about 550 m/s to cancel your orbital velocity, and another 550 to orbit the other way... 1100 m/s dV needed.

In contrast, for <200 m/s, you can send your apopasis way out there to nearly escape from the Mun, and then at apoapsis, reverse direction for <100 m/s, and then lower your PE again for the same cost as when you raised your apoapsis... in the end, you use about half as much dV as if you did the plane change in low orbit.

The smaller the plane change, the less likely this is to work.

It works for a 180 degree change... but you wouldn't come out ahead for a 1 degree plane change

[Falkenherz]

And there I thought I had learned enough about orbital mechanics... Back to watching videos, I guess! I love this game!