Vadoo a prendere il mio nuovo manuale di storia dello spazio, e cito pari pari 
When he heard the proposed solution—to glue onto the leading edges of this Shuttle small, individual tiles of a new material which would withstand the heat of reentry and could be used again—he was aghast: “How many different tiles will you require?”The answer was 31,689, and no two would match. As scientist he was satisfied that such tiles could be manufactured and that a glue could be fabricated which would hold them in place, but as an engineer he could not believe that anyone in his right mind would come up with such a complicated procedure; however, those who made the decision defended it: “Mott, we can’t use an ablating material. You pointed that out. We’ve got to have something that stays put and can be reused. So what can we use? A special copper alloy would be great, but if you covered the Shuttle with three inches of copper, there isn’t a rocket in the world that would lift it off the pad or brakes strong enough to stop it rolling when it landed. So what are you left with? We invent some new material, a new adhesive …”
“But why 31,000-odd tiles?”
“Because the Shuttle will be a living, breathing, moving thing. Its various parts will interact, and if you simply plaster our new material over its face, hundreds of feet wide, inches thick, the first creaking motion in the structure would crack the protection and make it break off in huge chunks. By using the tiles, we build in 4 x 31,000 joints … well, somewhat less because the edge of one tile makes its joint with the edge of another. You figure it out. But one hell of a lot of joints. And they give, not the whole fabric.”