Chuck Yeager almost died flying a rocket powered F-104 Starfighter (actually an NF-104A) to an altitude of 104,000 feet that later went into a flat spin during descent. The problem with flying an aircraft to extremely high altitude, is it losses control authority while in a near vacuum. The aircraft then descends back into the denser atmosphere while in an off nominal attitude. This results in the vehicle going into a flat spin that is almost impossible to recover from. I understand that SpaceShipOne's designers have done some clever tricks with its wings to avoid this specific problem. Never the less, transitioning from free molecular to continuum aerodynamics at supersonic speed is very difficult to model (lots of unknowns). The pilot doing these test flights on SpaceShipOne is very courageous.

Gary