Re: tiles?
I thought this was helpful.
I learned from a regular at the space-talk board that the original design of the shuttle had a titanium skin. (No luck finding titanium info on the net.)
The idea was scrapped because of cost and weight.
The two post-production weight reductions on the external tank (17,000 lbs total) amount to more than 3 times the weight of the 'deprecated' titanium skin.