OK, I'll bite. "Unpainted"? What material is, in its natural unpainted state, that bilious orange colour?

I'd tend to believe its coated with something, but don't intuitively go to the idea the dingus is hammered of Victorian iron left to oxide on the pad.

As far as Challenger, it was the loss of tiles, not the tiles themselves, that were the issue. I don't see a better solution than a tile array properly adhered. One great big shield would thermally expand such that the integrity of the fastening around complex curves would be stress concentrations; a monolithic shield is a far more difficult thing to pull off over those thermal ranges than a composite one.