Brian Thorn writes:

The propulsion and power systems to get the racks to and from the
Space Station. Even if this requires only half the mass of the Apollo
SM (and I think that's a realistic ballpark figure), we're still
talking two or three times the mass of the CM alone. All in a capsule
only a couple of feet greater in diameter than Apollo.

You're being grossly mislead by a system designed for lunar travel.
The Apollo SM was grossly oversized for LEO missions. As such, you're
comparing apples and oranges. For reasonable sized service modules, I
suggest you look at vehicles (existing and proposed) that serve this
role in LEO.

I'd look at the delta-V of Progress, Soyuz, ATV, HTV, and the like.
You'll be pleasantly surprised that these don't require nearly the
delta-V needed to brake (the CSM and LM) into lunar orbit and later
put the CSM on an earth return trajectory.

I have no objection to a large capsule used for crew and cargo
transport. It's the proposal that the crew (or cargo) AND the
service systems (propulsion, life support -- more than a few hours
worth -- and electrical power) can be all compressed into a 15-ft
diameter capsule that I have difficulty accepting. My argument is
that if we want to reuse the service systems in addition to the crew
cabin, then the capsule is less suited for it than a winged or
lifting-body shape.

Where did you get 15 feet? You've got a 16.4 foot diameter limit, if
you're talking about using Delta IV or Atlas V as your launch vehicle.
That extra 1.4 feet buys you quite a bit of extra volume, since it
also lets the capsule be longer and still maintain an Apollo CM shape.

If you grossly simplify the problem as a cube, you see:

16.4ft ^ 3 = 4411ft^3
15ft ^ 3 = 3375ft^3

So that extra 1.4 feet of length increases your volume by nearly 1/3
of the smaller volume. I'll bet your LEO specified replacement SM
equipment will fit in that space.

Whether we should try to reuse the service systems is another matter.
But I think if we're going to aim for eventual reusability of the
command module, we should instead aim from the beginning for complete
reusability... command and service module... with an airframe designed
for it from the start.

No doubt. However, the devil in the details isn't as bad as you
suspect. We're trying to reuse the shape of the Apollo CM, the SM is
largely irrelevant. The systems the SM contains ought to be replaced
with more modern systems anyway. Especially the propulsion system,
where I'd want to get away from toxic propellants and switch to
something a bit more benign.

Jeff
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