maybe it's just a brain cramp, but since the X-33 is a Professional and Flight Qualified, shouldn't it have a hesalite crystal instead of the sapphire?
Interesting question... The original theory was that since hesalite doesn't shatter it would be safer, on the other hand synthetic sapphire crystals were only invented in the 1960s, and the early ones were likely flat, thin, and fragile compared to modern ones. Interesting thing to consider, I've seen Rolex sapphire crystals shatter, mainly as they are flat with sharp edges, but on the other hand, Omega sapphire crystals are domed, which would make each point of the crystal vastly stronger and more resistant to impact than a flat crystal. I've never seen a domed crystal shatter or chip. The X33 does have a domed crystal, so maybe that helps to a degree.