Responding to Larry J's questions:

>1. Would a design suitable for reentry from Mars also be suitable for
>reentry from the Moon?

Yes. A vehicle designed for Mars return would be significantly safer for Lunar return due to the larger entry corridor and lower G loading.

>I understand that a heat shield designed for a 13 km/sec reentry
>wouldn't have any trouble handling a 11 km/sec reentry.

Not correct. It boils down to entry corridor width and peak deceleration. The Apollo CM was at the very edge of acceptability for lunar return (the Apollo astronauts were courageous men).

>I'm asking about the geometry. My gut feeling is that it would be
>fine, just as the Apollo CM shields worked for reentry from Earth
>orbit as well as for lunar return.

The business end of the Apollo CM is really just a spherical section. It's a very old design (actually came out in the late 1950s). NASA went with the Apollo CM because it was statically stable for all Mach number and was very simple to model. A classic KISS (Keep It Simple Stupid) design. A Mars return CEV needs to be either a biconic or raked conic shape for improved Lift/Drag ratio.

>2. While having a common vehicle is desirable to the greatest
>possible extent, would it be better to go primarily for commonality
>of the really expensive parts like systems and software and be
>less concerned about the less costly parts like structure, thermal
>control systems, and the heat shield.

The heat shield is integral to the vehicle's structure. Using a vehicle with the greatest possible commonality is sound engineering and should be the basis for the CEV's design. However the CEV should also be designed for the worst case entry condition which is Mars Return. If the CEV is based upon an Apollo CM then Mars return is not an option until a new vehicle is designed.

>For the CEV, the reentry and thermal requirements are much
>more rigorous for the moon as compared to the Mars
>environment. Like I said, a vehicle optimized for the Mars mission
>might be well suited for the moon mission as well, but it might
>also be overbuilt and considerably more expensive.

There is no question this is true. A CEV designed for Mars return would require a significantly different shape than the Apollo CM. This new shape would have to be recertified in hypersonic facilities that are now semi-abandoned junk piles and then subjected to flight tests before being manned rated. ***IF*** the objective is to go only to the Moon and never to Mars then reusing the Apollo CM is without question the cheapest option. However if Mars return is part of the agenda then designing the CEV for the worst case would be much cheaper in the long run and also safer for lunar return.

>Since the moon missions are intended to precede the Mars
>mission by several years, they could serve as a good proving
>ground for the critical systems while continued development
>goes on for the more challenging mission.

The Apollo CM is old technology and has been studied to death. The basic concept dates to around 1959. There's nothing new to be learned here. A Mars return CEV represents a "new" design. I say "new" because people have known that we needed something beyond the Apollo CM back in the mid 1960s. There was a manned Mars mission study done at NASA Ames around 1965. The problems of entry corridor and peak G loading were recognized back then. The old Ames study proposed a Mars return vehicle based upon a ranked conic as an alternative to the Apollo CM.

>Besides, this would give you the opportunity to design 2 heat
>shields instead of 1! Can you tell that I'm a defense contractor
>who works on space systems?

I know you're pulling my leg but beware of killing the golden goose. There's barely enough money to do anything in Space Exploration.

Thank God that Kerry lost! Six months from now in an alternative future with Kerry as President, I would be earning my salary as a mechanic at the local gas station or a plumber snaking out clogged toilets. Following an imploded Space Program, there are not too many job opportunities for spacecraft heat shield designers (the profession is a bit limiting).

Gary