X-2

The X-2 (nicknamed "Starbuster") was a research aircraft built to investigate flight characteristics in the Mach 2–3 range. The X-2 was a rocket-powered, swept-wing research aircraft developed jointly in 1945 by Bell Aircraft Corporation, the U.S. Air Force and the National Advisory Committee for Aeronautics (NACA) to explore aerodynamic problems of supersonic flight and to expand the speed and altitude regimes obtained with the earlier X-1 series of research aircraft.
The X-2 had a prolonged development period due to the advances needed in aerodynamic design, control systems, high-temperature resistant materials and other technologies that had yet to be developed. Not only did the X-2 push the envelope of manned flight to speeds, altitudes and temperatures beyond any other aircraft at the time, it pioneered throttleable rocket motors in U.S. aircraft (only previously demonstrated on the Me 163B during World War II) and a rudimentary digital flight control system. The XLR25 rocket engine, built by Curtiss-Wright, was based on the smoothly variable-thrust JATO units built by Robert Goddard in 1942 for the Navy.
Providing adequate stability and control for aircraft flying at high supersonic speeds was only one of the major difficulties facing flight researchers as they approached Mach 3. At speeds in that region, engineers knew they would also begin to encounter a "thermal barrier" - severe heating effects caused by aerodynamic friction. Constructed of stainless steel and a copper-nickel alloy (K-Monel), and powered by a liquid propellant (alcohol and oxygen) two-chamber XLR25 2,500 to 15,000 lbf (11 to 67 kN) sea level thrust, continuously throttleable rocket engine, the swept-wing Bell X-2 was designed to probe the that unknown supersonic region.
Following a drop launch from a modified B-50 bomber, Bell test pilot Jean "Skip" Ziegler completed the first unpowered glide flight of an X-2 at Edwards Air Force Base on 27 June 1952. Ziegler and aircraft #2 (46-675) were subsequently lost on 12 May 1953, in an explosion during a captive flight intended to only check the aircraft's liquid oxygen system.
Lt. Col. Frank K. "Pete" Everest completed the first powered flight in the #1 airplane (46-674) on 18 November 1955. By the time of his ninth and final flight in late July 1956, the project was years behind schedule, but he had established a new speed record of Mach 2.87 (1,900 mph, 3050 km/h).
About this time, the Lockheed YF-104A was demonstrating speeds of Mach 2.2 or 2.3 in a pure fighter configuration.
The X-2 was living up to its promise, but not without difficulties. At high speeds, Everest reported the X-2's controls were only marginally effective. High speed center of pressure shifts along with vertical fin aeroelasticity were major factors. Moreover, simulation and wind tunnel studies, combined with data from his flights, suggested the aircraft would encounter very severe stability problems as it approached Mach 3.
A pair of less experienced but excellent pilots, Captains Iven C. Kincheloe and Milburn G. "Mel" Apt, were assigned the job of further expanding the envelope of the aircraft and, on 7 September 1956, Kincheloe became the first pilot ever to climb above 100,000 ft (30,500 m) as he flew the X-2 to a peak altitude of 126,200 ft (38,466 m).
Just 20 days later, on the morning of 27 September, Apt was launched from the B-50 for his first flight in a rocket powered aircraft. He had been instructed to follow the "optimum maximum energy flight path" and to avoid any rapid control movements beyond Mach 2.7. With nozzle extenders and a longer than normal motor run, Apt flew an extraordinarily precise profile; he became the first man to exceed Mach 3, reaching Mach 3.2 (2,094 mph, 3,370 km/h) at 65,500 ft (19,960 m). The flight had been flawless to that point, but, for some reason, shortly after attaining top speed, Apt attempted a banking turn while the aircraft was still above Mach 3 (lagging instrumentation may have indicated he was flying at a slower speed or perhaps he feared he was straying too far from the safety of his landing site on Rogers Dry Lake). As a result, the X-2 tumbled violently and he found himself struggling with the same problem of "inertia coupling" which had overtaken Chuck Yeager in the X-1A nearly three years before. Unable to recover control, Apt fired the ejection capsule, which was itself only equipped with a relatively small drogue parachute. Apt was probably disabled by the severe release forces. As the capsule fell for several minutes to the desert floor, he never egressed to use his personal parachute.
While the X-2 had delivered valuable research data on high-speed aerodynamic heat build-up and extreme high-altitude flight conditions, this tragic event caused the termination of the program before the National Advisory Committee for Aeronautics could commence detailed flight research with the aircraft.
The search for answers to many of the unknowns of high-Mach flight had to be postponed until the arrival, three years later, of the most advanced of all the experimental rocket aircraft - the North American X-15.