Actually, the hot gases also torched the aft attach structure, resulting in the righthand SRB pivoting so the front end slammed into the LO2 tank. Both LH2 and LO2 flashed to vapor in the reduced atmosphere, followed by the combined gases ignition. It was not a detonation per se, but a low-order flashover. The crew cabin separated from the rest of the orbiter, which it was designed to do, and, sadly, had a recovery system been included, many of the astronauts would have survived. Larry Milloy browbeat Morton-Thiokol's program manager into okaying the launch. (See "Truth, Lies and O-rings") Could have been avoided if he had listened to MacDonald' engineering recommendation, rather than being told to think like a manager! RIP!