effect of retrograde mun orbit for apollo-styled mission?

[deepspacecreeper]

hello, i plan to conduct an apollo-styled mission. i would like to ask if it is in retrograde orbit around the mun , does it need more delta-v for the mun ascent? how much does it affect the required delta-v?

[UmbralRaptor]

There is an effect, but it doesn't matter. << 100 m/s for TMI, and costs all of 18 m/s for ascent.

[Temstar]

Yes retrograde orbit insertion cost slightly more delta-V than prograde since you're paying that angular velocity twice landing and lifting off. But the rotation of the Moon/Mun is so slow that the amount is very marginal. The reason why Apollo went for a retrograde orbit mission profile is so that in case service module engine failed to fire behind the moon to place them in lunar orbit, the moon will slingshot them back to Earth. Even small burns using the lunar module engine will place the craft back onto the re-entry corridor if they are not already on free return trajectory.

If you try to insert around Mun/Moon in prograde orbit and your service module engine fail to fire, then the spacecraft will get a prograde slingshot and be thrown into a solar orbit. For the Apollo spacecraft that will be fatal.

[Klingon Admiral]

Didn't the later Apollo missions use a cislunar free-return trajectory by going past the moons orbit and encountering it past Apogee? A cismunar trajectory would result in a prograde orbit after being captured. A circumlunar trajectory (the one shaped like an eight) results in a retrograde orbit.

[RoboRay]

I don't believe a cislunar free-return trajectory has ever been used. On later Apollo missions, I think the S-IVB placed the spacecraft on a "free return" was actually just a highly elliptical Earth orbit that didn't quite reach the Moon at all, and then returned to perigee within the Earth's atmosphere. Once the crew knew they had had a good LM and was ready to commit to orbiting the Moon, the CSM's SPS motor shifted the spacecraft into a trajectory that reached the moon but was no longer a free return.

This was considered acceptable from a safety standpoint. If the SPS didn't work, they'd still be on a free return and if the SPS did work to get them off the free return, they had two engines (the SPS and the LM's descent motor) capable of getting them back onto a free return. Independent failures of both would be extremely unlikely, and a single catastrophic failure that damaged both would almost certainly be unsurvivable by the crew.

The reason for it was to facilitate higher inclinations for the lunar orbit, as a pure free-return must be equatorial. This let the crew visit landing sites further north or south of the equator.

I'm not certain if the later missions captured into a prograde or retrograde orbit, though. I did a quick search for trajectory variations by mission and didn't see anything promising. Maybe somebody can turn up something with details.

Edited October 15, 2013 by RoboRay

[Evrion]

Because the Mun is tidally locked to Kerbin, it's rotation speed is very low, so orbital direction has very little effect on require mission dV.

This is also true for other tidally locked moons. Approaching planets, I always make sure to get to the prograde side, but for tidally locked moons, I'm not so concerned. Unless I'm trying to dock with a station!

[Starwhip]

The amount of Delta-V would be so small as to be almost nonexistent. You could probably offset the difference with RCS.

[Kerbart]

Unless I'm trying to dock with a station!

There's no reason why retrograde is a problem when you're docking with a station.

As long as the station has a retrograde orbit too.

Otherwise the time frame to line up the docking ports is indeed very short.