JohnB wrote:
If it's tough enough to withstand the high temps of re-entry, I'd guess that they're made out of something a bit stronger than our typical aircraft-grade aluminum.
Yes, and no. IIRC there was a situation a few years back where one of the orbiters had tile-related issues due to being caught outdoors in a rainstorm following reentry. When the ships are prepped for flight, there's some kind of clear coating applied so they can sit out on the pad at Canaveral rain or shine ... but it ablates off during the flight. Without the coating the tiles apparently absorb and retain moisture and water. They had to put the orbiter into a hangar under heat lamps to get the tiles dried out.
Enterprise is a weird duck when it comes to the shuttles anyways. She never had a TPS installed, from the looks of her she has some kind of compound (stucco-ish) coating over her skin that NASA carved panel lines into in order to make it look like she had tiles when she doesn't. There are also sections of her that have some kind of thick canvas-like blankets (NOT the thermal blankets that Discovery, Atlantis and Endeavour have). Even her OMS pods are fibreglas - they removed them and had to send them out for a comprehensive refurbishment when they put her into the then-new Dulles NASM.
OTOH she does have a space-rated airframe*, landing gear, landing gear doors and payload bay doors. But not a space-rated crew cabin, engine/thruster plumbing, etc. The original plan was to refit her to be the second spaceflight-capable shuttle, but the costs of tearing her down and putting all the rest of the components in was prohibitive** relative to building a new shuttle from an existing static test article (STA-99 which became OV-099 Challenger)
(* and ** assuming one doesn't buy the rumor that Enterprise suffered significant airframe damage when Fred Haise bounced her down the runway at Edwards at the end of ALT-5)