Bell X-2

The Bell X-2 (nicknamed "Starbuster" ) was an X-plane 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 United States 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.

Design and development
The Bell X-2 was developed to provide a vehicle for researching flight characteristics at speeds and altitudes in excess of the capabilities of the Bell X-1 and D-558 II, while investigating aerodynamic heating problems in what was then called the "thermal thicket".

The Bell X-2 had a prolonged development period due to the advances needed in aerodynamic design, control systems, materials that retained adequate mechanical properties at high temperature, and other technologies that had 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 engines in U.S. aircraft (previously demonstrated on the Me 163B during World War II) and digital flight simulation. The XLR25 rocket engine, built by Curtiss-Wright, was based on the smoothly variable-thrust JATO engine 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. For, at speeds in that region, they 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 engines, the swept-wing Bell X-2 was designed to probe the supersonic region.

Operational history
The subsequent investigation into the X-2's fatal flight raised numerous contributing factors into the crashlargely focusing on Apt's decision to turn the aircraft while still above Mach 3. Some cited his lack of experience with rocket planes, but, as historian Chris Petty notes, "he had in fact flown the complex profile almost perfectly, but this, combined with additional seconds of thrust from the XLR25 [engine], had carried the X-2 well beyond the envelope of knowledge and into the uncertain stability predicted by the GEDA [Goodyear Electronic Differential Analyzer computer]." In short, Petty suggests that Apt did his job too well and may have been pushed to exceed Mach 3 by the AFFTC and conflicting priorities within it. Petty quotes Base commander General Stanley Holtoner: "I think that every supervisory guy from me on down has criticized himself, because if we had told this boy [Apt] to stop at a specific speed, this wouldn't have happened."

One point that became clear even before the investigation was that the X-2's escape mechanism was woefully inadequate. According to The New York Times reporting on the event, Everest had criticized the relatively new detachable capsule, maintaining that "some safety had been sacrificed in preference to delaying the X-2 flight tests while the escape mechanism was modified." Another NACA research pilot, Scott Crossfield, described it more bluntly as a "way to commit suicide to keep from getting killed."

While the X-2 had delivered valuable research data on high-speed aerodynamic heat build-up and extreme high-altitude flight conditions (although it is unclear how much, as the unmanned Lockheed X-7 and IM-99 were among the winged vehicles operating at comparable or higher velocities in this era), this tragic event terminated 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 riddles 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.

Flight test program
Two aircraft completed a total of 20 flights (27 June 1952 – 27 September 1956).


 * 46-674: seven glide flights, 10 powered flights, crashed 27 September 1956, subsequently scrapped
 * 46-675: three glide flights, destroyed 12 May 1953