Prograde vs. Retrograde Orbit for Lunar Space Stations

[pattersoncr]

I know Apollo used retrograde lunar orbits. I have heard that this was because a retrograde orbit leads to lower dV requirements for the LM. A retrograde orbit also seems to better lend itself to a free return trajectory (this is speculation on my part. You can probably set up a perfectly good free return trajectory in a prograde direction, but I haven't tried). I've been doing some thinking about the development of lunar and cislunar infrastructure (mostly inspired by Greg Burch's addons). It seems to me that all of this space infrastructure (stations in lunar orbit, craft entering & leaving lunar orbit, landers, etc.). Ought to be orbiting the moon in the same direction (at least where rendezvous is intended).

What are some of the advantages/disadvantages of prograde vs. retrograde orbits? Is one clearly superior for the support of space infrastructure? If neither is clearly superior, it seems to me that one direction ought to be picked "by convention".

Does anyone have any thoughts on this?

 

[Zatnikitelman]

If I was doing a lunar infrastructure, I'd use a retrograde orbit as you approach Earth slower than if you were in a prograde orbit. But that's just what I'd do, I don't know if there's any technical advantages one way or the other.

 

[pattersoncr]

...I'd use a retrograde orbit as you approach Earth slower than if you were in a prograde orbit...

This would reduce the dV for the susequent EOI (Earth Orbit Insertion), assuming you're talking about a purely Earth orbit to lunar orbit sort of craft.

This is exactly the type of info I was looking for.

Thanks

 

[Andy44]

Retrograde is the way I"d go, for the Apollo-related reasons you cited. A lunar-bound spacecraft in a free return trajectory will pass over the moon from east to west, which is retrograde. If the station is also retrograde, the spacecraft requires much less delta-V to enter a lunar orbit and rendezvous with the station. Since the lunar surface only moves like 4 m/s, about the the speed of a bicycle, you can land or launch from the surface in any direction with almost no delta-V penalty.

I have heard of exotic free-return trajectories that pass the moon prograde, maybe it was flytandem who found one, but I suspect they take extra time to make a round-trip to the moon and back, and if they were worth the trouble I'm sure NASA would've used them at some time.

All of this goes out the window, of course, if you don't care about free-return. Also, you may put the station in a high-inclination orbit to get access to landing sites at high lunar latitudes. In this case, you have to come up with a different scheme, and there will be a delta-V penalty to manage, probably by restricting transfers to a time window which opens about every 2 weeks or so.

 

[pattersoncr]

I was planning on a station in equatorial orbit along with a main base on the equator (a la Greg's Heinlein Base). The equatorial base would serve as a hub for settlements sited further north and south. That seems to me a better solution than a high inclination orbit and its resultant 2 launch windows a month.

Are there any adantages to a prograde orbit for stations?

 

[joeybigO]

There would be no advantage to going prograde for station, however if you want to rendezvous for your spacecraft and return via shortest route home, I would think that having a retrograde would be a lot more simple design.

The fact would be if you chose say a LaGrange point to hold an orbit, if you choose the prograde, you would have to be doing some major stationkeeping, or orbital maintenance. However if you chose the retro option, it would keep you in check, and you can have a one time ejection directly to earth, otherwise you would have to go around the moon for a prograde stop (that i'm not sure of, i'd have to do the math to check if thats right) I'll stop there and check that last statement, because i've been wrong and don't want to lead you around the wrong path there.