The primary objective of the flight program was to explore the hypersonic flight regime and compare the results against various analytical models and wind-tunnel results. The physical X-15 configuration was of only passing interest and was not an attempt to define what any future operational aircraft might look like; it was simply a means to obtain the necessary thermal environment and dynamic pressures. The researchers wanted to understand heating rates, stagnation points, laminar and turbulent flow characteristics, and stability and control issues.

Later, the X-15 would become a carrier for various experiments, and the airplane configuration would be of even less interest.

and X-15-3 (56-6672). The second airplane became X-15A-2 after North American extensively modified it following an accident midway through the flight program. The two carrier aircraft were an NB-52A (52-003) and an NB-52B (52-008); although not identical, they were essentially interchangeable.-19!

The program used a three-part designation for each flight. The first number represented the specific X-15 ("1" was for X-15-1, etc.). There was no differentiation between the original X-15- 2 and the modified X-15A-2. The second position was the flight number for that specific X-15 (this included free flights only, not captive carries or aborts); the first flight was 1, the second was 2, etc. If the flight was a scheduled captive carry, the second position in the designation was a C; if it was an aborted free-flight attempt, it was an A. The third position was the total number of times that either NB-52 had carried aloft that particular X-15, including captive carries, aborts, and actual releases. A letter from Paul Bikle established this system on 24 May 1960 and retroactively redesignated the 30 flights that had already been accomplished.-119!