In article ,
Doug... wrote:
The first of which is, what mode do we use to get back to the Moon?
Do we use LOR? Do we use EOR and land the whole vehicle? Do we use a
combination of EOR/LOR?

Whether orbital assembly (EOR) figures into the plan depends on how big
the launchers are and how the project trades off convenience today against
growth potential tomorrow. (It is more convenient to launch everything in
one piece, but eventually, as the missions grow, you'll have to start
doing orbital assembly, so an emphasis on long-term growth says you might
as well get started now.)

Assembly has some up-front overhead costs, but at present, so does
all-in-one launch, since we have no suitable heavy launchers. (Remember,
a good bit of the money for the plan is supposed to come from closing down
shuttle operations -- that means no shuttle-derived launchers! In any
case, if you must have a heavy launcher, an EELV cluster or a fresh-start
development which *doesn't* use LC-39 is almost certainly *cheaper* than a
shuttle derivative -- yes, even including development cost -- as soon as
the number of launches goes beyond a handful.)

Whether LOR looks good depends on a number of things, notably surface stay
length and the complexity of the surface operations. The more the mission
is dominated by a heavy lander carrying a substantial crew and a lot of
surface equipment and supplies, the less you save by leaving the return
vehicle in orbit. A contributing factor is that for most landing sites,
a particular lunar orbit will be reachable only twice a month due to the
Moon's rotation, which is a problem if you want the option of doing an
emergency return on short notice.

Apollo's choice of LOR reflects Apollo's nominal mission being a short
surface excursion in the middle of a rather longer mission dominated by
cruise/orbit phases. Even the later Apollos, with mere three-day stays,
paid a price in things like constant orbit adjustments. Most recent
proposals assume that a lightweight return vehicle is taken down to the
lunar surface and launches from there.

Do we go into lunar orbit first, or do we
approach like Surveyor and most of the Soviet unmanned landers?
Don't we *need* to go into orbit first if we're going to land outside of
the classic equatorial zone?

No, not really. The only big advantage of the classic equatorial zone is
its compatibility with free-return trajectories.

A straight-in approach saves a bit of fuel and doesn't really constrain
the landing site much. Where it hurts is timing issues and backup/abort
plans: you only get one try and you'd better be ready. If you run into
even small delays, as Apollos 14 and 16 did, you've lost the landing and
you may even have trouble getting home.

On the other hand, if you haven't left anything important in lunar orbit,
there's no reason to stop in orbit on the way *back*.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |