Orbital Mechanics
Section M 3.12.
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Landing Sites and Changing Orbital Planes

Plane change is very important for the return rendezvous. Your absolutely minimal ascent vehicle won't have enough delta-V to do a plane change into lunar orbit. If the landing site is not in the plane of the waiting Lunar Transfer Vehicle, then it will have to plane change to meet the launch vehicle. This is difficult.

The moon rotates, of course, once every 28 days. So if your landing site is in the plane of the orbit at day X, it will rotate out of that plane and not be back in that plane until day X+28. This is a bad situation; you want the option to leave the surface at any time in the mission in case you get into a situation where you can't survive until a launch window half a month away.

There are two choices for landing sites which do not rotate out from under the Lunar Transfer Vehicle's orbital plane. These are equatorial sites [which is the reason that the early Apollos landed at near equatorial sites], and polar sites.

However, polar orbit has a different orbital phasing difficulty. The transfer orbit from LMO to Trans-Earth coast uses least delta-V if the impulse is perpendicular to the Earth-moon line. For an equatorial orbit, this condition happens once every orbit. For a polar orbit, this happens twice a month. Further, if you return to LEO instead of to direct Earth surface (as Apollo did), the trans-Earth Hohmann from a minimum delta-V LMO injection is out of plane. A three-impulse transfer solves that problem (and lowers the delta-v), but at the price of longer trip time.

Conclusion: if your landing site is not equatorial, you must budget either extra delta-v or extra time.

Geoffrey Landis

Orbital Mechanics

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