which is more expensive in terms of dV

[Sovek]

When landing or making orbit burns around the Mun, which is more expensive? prograde or retrograde orbit?

[Gaarst]

It depends on your mission.

If you do orbital manoeuvres around the Mun, both are equivalent.

If you want to land and take off, then retrograde is more expensive. Not by much though, the Mun rotates very slowly.

If you come from Kerbin orbit, then I'd say prograde is slightly better as the Mun rotates Kerbin the same way it rotates on itself. To be confirmed though.

Edit: anyway, the dV spared will not be over a few dozen of m/s, so it's not that important, and you most likely won't fail your mission if you decide to land while orbiting retrograde

[GoSlash27]

Sovek,

It doesn't matter one way or the other. The Mun's rotation is so slow that the difference is negligible.

Best,

-Slashy

[ajburges]

Prograde is cheaper, but a sidereal rotational velocity of 9.0416 m/s means you only save 32 m/s round trip.

Retrograde allows for free-return trajectory which is an interesting abort mode and cheaper for fly-by returns. Unfortunately free-return offers no net benefit if you capture.

Hmm...

I think I may have just had an idea for a challenge. Lunar rendezvous Mun landing. Mother ship can not thrust more than 5-30 m/s between lunar injection burn and lander return (so it can't capture and needs a trajectory that would establish a Mun resonant, elliptical orbit for a second flyby). To the sandbox!

... not sure what the judging criteria would be though

[Alshain]

I think I may have just had an idea for a challenge. Lunar rendezvous Mun landing. Mother ship can not thrust more than 5-30 m/s between lunar injection burn and lander return (so it can't capture and needs a trajectory that would establish a Mun resonant, elliptical orbit for a second flyby). To the sandbox!

... not sure what the judging criteria would be though

I don't think it would be that hard to an experienced player IF you can ensure the mothership passes on the same side of the Mun each time. You just need to have enough fuel in the lander and then take off to meet it as it passes, similar to doing a direct launch to rendezvous from the KSC. Once you get that closest approach it would be just like any other rendezvous to match it's trajectory away from the Mun, but your lander would need a good TWR because it will be a lot of delta V in a very short time span.

EDIT: Actually, thinking more on it, if you can see which side of the planet it will pass, you can just land on that side. So even then it doesn't seem like it would be that hard. You just need a lander with a bunch of fuel. It would be fun to try, but It's not incredibly practical since a lander capable of that would be capable of returning on it's own anyway.

Edited October 14, 2015 by Alshain

[Mastikator]

The Mun's rotation speed is something like 9m/s at the equator. Meanwhile the escape velocity is 807m/s, that's like +/-1% difference. The only way you could even notice is if you're doing perfect burns all the way.

[ajburges]

I don't think it would be that hard to an experienced player IF you can ensure the mothership passes on the same side of the Mun each time. You just need to have enough fuel in the lander and then take off to meet it as it passes, similar to doing a direct launch to rendezvous from the KSC. Once you get that closest approach it would be just like any other rendezvous to match it's trajectory away from the Mun, but your lander would need a good TWR because it will be a lot of delta V in a very short time span.

EDIT: Actually, thinking more on it, if you can see which side of the planet it will pass, you can just land on that side. So even then it doesn't seem like it would be that hard. You just need a lander with a bunch of fuel. It would be fun to try, but It's not incredibly practical since a lander capable of that would be capable of returning on it's own anyway.

Ya, learned of the Aldrin cycler concept today. Fairly close to my idea.

Such cyclers only become practical once there is a good use for heavy, reuseable station parts (like self sustained life support).

Still, plotting and executing a resonant flyby 3 patches ahead (with fp error and no correction) sounds like quite the navigational achievement.