In article ,
Doug... wrote:
OK -- this is one of the things I wanted to get discussed. The mode
decision has a huge impact on the launchers you want to use...

Mostly the part about whether to do orbital assembly -- specifically, a
decision *not* to do substantial orbital assembly would mean major new
launcher requirements, and the details would become very dependent on the
spacecraft design. (That alone is a strong argument for orbital assembly,
which largely separates spacecraft and launcher issues.)

So -- we can get pretty much anywhere we want at pretty much any time,
as long as we're not worried about free return trjectories? OK... so,
you think NASA (in its current gutless state) will ever commit to a TLI
with no free return capability?

I think it likely that the preferred approach would be the same used for
Apollo: start with free-return trajectories, but gradually move away from
them -- to open up the choice of landing site -- as experience builds up.
Apollo 11 went to the Moon in a free-return trajectory, Apollo 12 started
out in one but moved away from it en route, and Apollo 17 was not in one
at any time. Even the later Apollos stayed relatively *close* to free-
return trajectories for the sake of abort options... but just what "close"
means depends on your performance margins. Many aspects of such missions
get easier if you invest in more generous margins than what Apollo had.

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*.

True. Then again, there's a lot to be gained from man-tended orbital
exploration, too, don't you think?

Correct, but anything that's designed to shift for itself for a while in
lunar orbit doesn't need to be revisited by the *same mission*. There is
little reason to put people in lunar orbit if all they are going to do is
look at displays and push buttons, which is about all that normally needs
doing for remote-sensing operations; the earlier lunar-orbit assets can
reasonably be unmanned for normal operation. An occasional servicing
visit can be done either by a dedicated mission, or by a landing mission
on the way down.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |