NUMBER OF FLIGHTS CONDUCTED

In all, 199 flights were conducted over a nine-year period from June 1959 to October 1968. Three airplanes were built, repaired, and rebuilt during that period. The third airplane was a significant modification. This longer version included external fuel tanks to extend the flight time, the range of altitude, and the Mach number to be investigated. Most of the initial objectives for the airplane were reached in the early years. But because the X-15 could fly in the hypersonic regime, NASA wanted to conduct many experiments, some examining various materials using the airplane as a test bed.

One of the thermal protection techniques used to protect hypersonic vehicles from the intense aerodynamic heating environment is the covering of the vehicle surface with an ablative material. This material would directly absorb the heat and burn away (ablate), thus protecting the surface underneath. Some of the later X-15 test flights tested a specific ablative material, namely MA-25S developed by Martin Marietta. This silicon-based material was sprayed on the surface of the X-15. After several hours of curing, it was sprayed with a coating of Dow Corning DC90-090, a silicon – based sealer, which gave the X-15 a white color.

Some of these caused problems in flight. For example, for some flights an ablative material was put on the airplane for testing purposes and for additional heat protection. As the material vaporized, it coalesced on the windshield, making it opaque, seriously affecting the visibility of the pilot. For further tests of the ablating material, the engineers had to install an external shield on half the windshield that could be moved away after ablation had obscured the other side in order to allow the pilot to have clear vision for the remainder of the flight.