How fast was the rocket going when it released the record-setting
scramjet? If the rocket was going Mach 9 in the thin atmosphere at
100,000 feet and released a stone, for example, the stone would travel
several seconds at close to Mach 9. I assume that the rocket was not
going Mach 9, but I haven't seen any information on how fast it was
going.

Regardless, it seems to me that the rocket's speed has to be
subtracted from the jet's speed to arrive at the actual jet speed when
you talk about the world's record for speed of a jet plane.

-- Don French