[Redcloak]

Notice the missing tiles on underside of wing....that's considered to be normal I expect...

Yes, a certain amount of damage is normal. Columbia actually sustained quite a bit of tile damage on her first mission. 16 were knocked off and another 148 were damaged. (The shockwave from the SRBs igniting caused the damage. This lead to the waterfall sound suppression system that's turned on just before launch.) It would take quite a bit of damage to the tiles to cause a catastrophic failure.

[Mamzelle]

Tiles--I believe these tiles will prove to be the catalyst for this disaster. Fascinating technology, but if one too many falls off, that's it.