Scott:"With that said, the tiles are fairly large even still"

Actually, it depends on where along the underside of the orbiter. The tiles I have seen with my eyes are actually quite small. And (many of them) are not perfectly square. Think about it, perfect squares will not tile a curved and non-square surface. Each tile is more-or-less unique to its location on the surface. Think of it more as tailoring and less like covering.

Also, there is more to thermal protection than the tiles. Less vulnerable/superheated areas are covered with a quartz-fiber blanket, and the really critical highly heated and buffeted parts (on and just under the leading edge wing surface) are covered with a different material.

scott: "Is it really necesserry to have this degree of thermal protection? Perhaps it would be possible to design a tile or other system that offered slightly less thermal protection, but offered much greater reliability."

Nope. This really is an all or nothing issue. Letting in just, say, 2% of the peak heat would turn the aluminum skin of the orbiter into slag.

Think of the problem of thermal protection as being defined and bounded by all the other design choices made in constructing the shuttle. Within that space of possible solution states, there really ane not very many 'optimal' or goal states. The tiles were the most optimal choice in the 1970's, and frankly they are still the best solution to "thermal protection for *the* space shuttle design".

The shuttle spends about as much energy as a low-yield nuke getting into orbit, and it has to throw off about that much energy on the way in, and the main mechanism to throw off that energy is to slam into the atmosphere tiles-first. Now note that there has been only one full-scale failure of this system after over 100 uses.

Complexity happens, systems fail. Remember that next time you have a flat. in a bad part of town. ;-)

Davin E. Lafon