North American X-15 Cockpit (photo)

The North American X-15s were designed to explore the problems of flight at very high speeds and altitudes. They became the first winged aircraft to attain velocities of Mach 4, 5, and 6 (four, five, and six times the speed of sound). Because of its high-speed capability, the X-15 had to be designed to withstand aerodynamic temperatures on the order of 1,200 degrees F. As a result, the aircraft was fabricated using a special high-strength nickel alloy named Inconel X. Three X-15 research aircraft were built and flown, completing a total of 199 research flights. The National Air and Space Museum has the historic X-15 #1, Air Force serial 56-6670.