[Thud]

The detached foam was deaccelerating in the air blast at Mach 1.5 while the shuttle was continuing to accelerate up towards the foam. The relative velocity is the difference between the foam's velocity lost in the air stream and the velocity gained by the shuttle as it accelerated, all in the interval between the foam detaching from the fuel tank and it striking the shuttle.

[John Jamieson]

That's true, but what does it have to do with what I posted?

[John Jamieson]

It's interesting that NASA assumed a 10 to 16 degree angle of attack for foam hitting the tiles. The foam could have hit the RCC at the full 90 degrees.

I didn't catch the stuff about the possibility of two pieces, but if so, a leading edge hit would seem to be the best fit.