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When the XS-1 was being developed for history's first supersonic aircraft flights, plans were already underway for the XS-2 (later redesignated the X-2), which would fly at speeds and altitudes vastly greater than its predecessor. It was to be a Mach 3 rocket plane, capable of flights over 100,000 feet.
The X-2's structure had to be able to withstand aerodynamic heating never-before encountered, and the life support systems for the pilot needed to address the dangers of extremely high-altitude, high-airspeed flight. The challenges of developing a unique stainless steel/nickel alloy structure, a new throttleable rocket engine ranging in thrust from 2,500 to 15,000 lbs. of thrust, a jettisonable cockpit capsule, and control system difficulties delayed the program for years. The contract was signed in 1945, the first glide flight was in 1952, and the first powered flight occurred in 1955. Ironically, by that time the X-3, the X-4, and the X-5 had all flown.
If the development of the X-2 was unusually time consuming, the flight test program was tragically brief. Two of the X-2s were built; both were destroyed in accidents.
Ship number two only made only three test glides, and exploded in 1953 during a captive test flight with its B-50 carrier aircraft. It was attached to the mothership when a leather gasket on the X-2's liquid oxygen tank exploded as they were flying at about 30,000 feet. The X-2 was a total loss. Bell test pilot Jean Ziegler and B-50 crewman Frank Walco were killed as the explosion ripped the X-2 out of the bomb bay. The B-50 was landed safely, then scrapped.
Ship number one's first flight at Edwards Air Force Base was on August 5 of the following year. Its first powered flight was made on November 18, 1955. During the next ten months the X-2 was finally showing its potential. On July 23, 1956 Air Force Capt. Frank Everest set a new unofficial world's speed record of Mach 2.87 (1,900 mph). On September 7 USAF Capt. Iven Kincheloe set a new altitude record of 125,906 feet. Then on the aircraft's 13th powered flight, USAF Capt. Milburn Apt (making his first flight in the X-2) was killed after the plane went out of control after successfully breaking Mach 3 (Mach 3.196).
Apt's plane became uncontrollable shortly after a routine engine shut down at 65,500 feet. He jettisoned the cockpit capsule at about 40,000 feet, but was unable to leave the capsule and parachute to safety before it impacted the ground. The rest of the airplane crashed about five miles away.
The X-2 program advanced the technologies of heat resistant alloys and rocket engines, but the grim lessons learned about the dangers of inertial coupling, which caused the aircraft to go out of control, and the requirements of flight safety became its legacy. The National Advisory Committee for Aeronautics' High-Speed Flight Station (predecessor of today's NASA Dryden Flight Research Center), which expected to receive the airplane for flight research, never did because of the accident, but it had supported the Air Force with advice and data analysis during its flight testing.