"Jorge R. Frank" wrote in message
I can't agree with that unconditionally. You do have to weigh the costs
both ways. In your scenario, you have the cost of developing the
stationkeeping bus and possibly the costs of making the ascent stage
capable of long-term survival without the descent stage. It's not a
clearcut trade, and the outcome will depend on the flight rate. The
higher the flight rate, the more economical it will be to reuse.

Development funding profile will also be a player. Designing for reuse
will involve more upfront expenditures, and strikes me as one of the
first things that gets dropped when development funding gets tight.

On the flip side, this also strikes me as something that could get added
as an upgrade as the program matures, like the SIM bays and LRVs for the
J-series Apollo missions.

Yes, in a program as "trim" as the current one is trying to be, and with the
costs it would impose, I don't expect what I've described has much appeal.
But, now would be the time to discuss the possibilities. During design,
given several paths to achieve the goal, and where little additional cost is
incurred, choose one that supports reuse - or at least attempt to not
preclude it.

The related, larger, question I have is, is it better to build a few
vehicles that are single purpose, or a single vehicle for a few
purposes? The latter approach gave us the shuttle.

Multipurpose vehicles are not necessarily a bad idea - witness how the
Apollo CSM was able to serve as both a lunar vehicle and as a LEO space
station ferry. The problem with the shuttle is that it attempted to make
the jump to an operational vehicle on the very first iteration, and that
it was shoehorned into too many operational roles.

JRF

I wouldn't call the Apollo CSM a multipurpose vehicle so much as an
extensible vehicle. Same with CEV. IIRC, early concepts for CEV included
access to the lunar surface. That would have been a multipurpose vehicle.

By the way, I meant to mention that a feature of a hypothetical reusable
lunar surface / lunar orbit "shuttle" vehicle that I would also like to see
is that the vehicle is still a two-part vehicle (sort of like LM ascent and
descent stages) where the whole thing goes down and up again, but where the
"ascent stage" is really an "abort" stage, giving an option to return to
orbit if the descent stage fails at any time. I guess this would infer that
the ascent engine would use the same propellant as the descent stage, and
that the tanks would be plumbed to fuel either stage. Costly, and maybe
over-complicated, I know.

Jon