Getting Ready for Maximum Speed

The general cautiousness that was beginning to permeate NASA was also affecting the X-15 program, and the buildup to the maximum speed flight was unusually conservative. Although the program had never been "wild and crazy," it had previously taken reasonable risks when it understood the problems and their consequences. This was not the case during preparations for the maximum speed flight, which really did not represent that large an increment over the Mach 6+ speeds already attained. Nevertheless, in preparation, the program dealt with each individual piece separately.

Pete Knight flew the next X-15A-2 flight (2-49-86) with the ventral on, primarily to familiarize himself with the handling qualities, since all of his previous flights had been with the ventral off. All future X-15A-2 flights would use either the ventral or the ramjet. Flight 2-50-89 was the first flight where the external tanks operated (knowingly) successfully, including the improved instrumentation that let the ground crew and pilot know the propellants were transferring correctly.[319]

The NB-52s required a modification to strengthen the wing in order to carry the X-15A-2 and its external tanks. On 27 June 1066, Bob Rushworth was in the cockpit of a scheduled captive-carry flight (2-C-80) to test the X-15A-2 with full external tanks. (NASA)

Edwards in early April, in time for flight 2-51-92. In addition, NASA relocated the thermocouple recording system from the center-of-gravity compartment to the main instrument bay since it had failed to operate on the previous two flights because of the cold environment. By May 1967, three dummy ramjet shapes had arrived at Edwards, and wind-tunnel tests in the JPL 21-inch hypersonic tunnel had verified the mated ramjet configuration. One of the three dummy ramjets was sent to Inglewood to have a thermal protection system installed, and engineers at the FRC instrumented the other two in preparation for flight. Researchers had already calibrated the flow – field cone probes at Mach numbers of 3.5 and 4.4 in the Ames 1 by 3-foot wind tunnel, with additional tests scheduled at Mach numbers of 5.0 and 7.4.[320]

Pete Knight evaluated the handling qualities of the X-15A-2 with the dummy ramjet installed under the fixed portion of the ventral stabilizer on flight 2-51-92. This flight did not include the external tanks and reached Mach 4.8. Knight jettisoned the ramjet just before landing, much like the ventral rudder, to provide the necessary clearance for the landing gear. Next up for X-15A-2 was a flight with the ablative coating and dummy ramjet, but without the external tanks.[321]