Some comments concerning Raoul's post:

>All the energy is dissipated as heat.

Actually most of the energy is dissipated as heat. A lot of energy is dissipated through molecular dissociation into atomic species, e.g. N2 : N + N, and ionization if the entry is fast enough.

>A returning spacecraft does not get hot through friction with the atmosphere.
>A layer of air builds up around it and between the layer and the surrounding >atmosphere is a shockwave, where the heat is generated.

Friction does play a role but the main heating processes are:

convection
surface catalysis
radiation

Atmospheric gas is superheated when it passes through the bow shock. This heated gas (actually a plasma) is called the "shock layer". Some of the shock layer's heat is convected through the boundary layer to the vehicle's surface. The gas is chemically broken down in the shock layer. When the dissociated gas touchs the vehicle's surface, it recombines chemically and releases heat (this is called "surface catalysis"). If the vehicle is going really fast (greater than 10 km/sec), then the shock layer will emit a hellish light that can also heat the vehicle. This sort of heating is called "radiative heating". For the Space Shuttle, convective heating dominated. For the Galileo Probe that entered Jupiter's atmosphere, radiative heating dominated.

>Early Mercury capsules were in some ways much safer because they
>descended more steeply than the Shuttle and thus not much of the heat ended
>up in the heatshield.

The Mercury and Gemini capsules used "ballistic entry" to reenter the atmosphere. Ballistic entry means "no lift". The Space Shuttle and Apollo Command Module both used lift. Lifting entry requires active control and is technically more complex (more can go wrong). The advantage to lifting entry is the G-loading is much less. The more normal the entry angle to the atmosphere, the greater the peak heat flux and decelleration loading. However for a shallower entry angle the peak heat flux will be less but the integrated heat flux is usually greater (more time in the atmosphere at high speed). If the heat shield works through ablation (the heat shield breaks down chemically and out gasses to blow away the shock layer from the vehicle's surface) then it tends to protect better against a high heat flux rather than against an integrated heat flux (also called a "heat load"). The Mercury capsule used an ablator heat shield. The Space Shuttle employs a reuseable nonablating heat shield (reenforced carbon-carbon, silica foam tiles and silica blankets).

>The blunter the capsule and the faster it moves,the farther away the shockwave >occurs.

Not true. The blunter the capsule the greater the shockwave standoff distance. Some of aerospace's greatest scientists have studied shockwave standoff distance.

>Most of the heat of the Mercury capsule was swept away into the atmosphere.

That's the secret of spacecraft heat shields. Put the heat into the atmosphere and not in the spacecraft.

>Max Faget wanted the Orbiter to re-enter like a Mercury capsule, with its nose
>held high. The US Air Force insisted on a flyable spaceplane, with a slower
>re-entry, the nose no higher than 40 degrees to the horizontal.

40 degress is a fairly high angle-of-attack. Both Faget and the Air Force got their wish. Also, Max Faget is an aerospace god on the same pedistal with von Braun and Goddard/

>As a result, the Shuttle would be subjected to 4 times as much heat for twice as
>long.

Again this is heat load (integrated heat flux). The heat load was a necessary requirement for having a low peak heat flux and decelleration. The economics of the Space Shuttle stinks to high heaven but the basic engineering of the Space Shuttle is excellent and a tribute to its engineers given the crappy politics that surrounded it. Max Faget was one of the Shuttle's designers. It was not his preferred design (his earlier designs were much, much better). Had the politicians left Max Faget alone to design the Shuttle correctly, we'd probably now have commerical factories in LEO and colonies on Mars.

Gary