North American X-15

The North American X-15 was a hypersonic, rocket-powered research aircraft jointly operated by the United States Air Force and NASA as part of the American X-plane research program during the 1960s. Carried aloft under the wing of a modified B-52 bomber and released at high altitude before igniting its own rocket engine, the X-15 was designed to explore the extreme conditions of very high-speed, high-altitude flight at the edge of the atmosphere, gathering data that informed the design of later crewed spacecraft and reentry vehicles. On October 3, 1967, pilot William J. Knight flew an X-15 to a speed of 4,520 miles per hour, Mach 6.7, setting the official world speed record for a crewed, powered aircraft that remains unbroken. Several X-15 flights also exceeded the U.S. Air Force's definition of the boundary of space, at 50 miles altitude, and the pilots who flew those missions were awarded astronaut wings, with the highest flight reaching 102,100 feet in altitude.