I was just flipping through the must-read Easterbrook article mentioned below when I came across this passage ...

The external fuel tank, for instance, is full of oxygen and hydrogen cooled to -400ƒ F. to make the gases flow as liquids. Ice will form on the tank. When Columbia's tiles started popping off in a stiff breeze, it occurred to engineers that ice chunks from the tank would crash into the tiles during the sonic chaos of launch: Goodbye, Columbia. So insulation was added to the tank. But while thermal cladding solves the ice problem, it adds weight. The entire vehicle, loaded, weighs 4.5 million pounds. Say you add one percent. Doesn't sound like much. One percent comes to 45,000 pounds. That's almost all of the payload.

Ugh ...