Neil Armstrong, amongst others, has called the X-15 “the nutst successful research airplane in history.” That might be stretching a point, but it was certainly the most successful of the highspeed X-planes.
It had taken 44 years to go from Kitty Hawk to Air Force Captain Charles E. Yeager’s first supersonic flight in the Bell X-l on 14 October 1947. Six more years were required before NACA test pilot Scott Crossfield got to Mach 2 in the Navy-Douglas D-5S8-2 Skyrocket. A remarkably short three years had passed when Captain Milburn G. Apt coaxed the X-2 above Mach 3, before tumbling out of control to his death. There, progress stalled, awaiting the arrival of the three small, black North American X-15 research airplanes that would more than double the speed and L altitude milestones.
The X-15 flight program began slowly, mostly because the million-horsepower XLR99 engine was 1 not ready. This undoubtedly worked in the program’s favor since it forced the engineers and pilots [ to gain experience with the airplane and its systems prior to pushing the envelope too lar.
The first 20 months took the X-15 from Crossfield’s glide flight to essentially duplicating the performance of the X-2: Mach 3.5 and 136,500 feet. Then the XLR99s arrived and things got serious. Six days after the last flight with the interim XLRlls, Major Robert M. White took X-15-2 past Mach 4, the first time a piloted aircraft had flown that fast. Mach 5 fell, also to Bob \Y bite, four months later. Mach 6, again to White, took six more months. Once it began flying with the ultimate engine, it took only 15 flights to double the maximum Mitch number achieved by the X-2.
Altitude was a similar story. Captain Iven C. Kincheloe, Jr. was the first person to fly above 100,000 feet, in the X-2 on 7 September 1956. Thirteen flights with the big engine allowed Bob White to fly above 200,000 feet for the first time. Three months later, he broke 300,000 feet. Once it began flying with the ultimate engine, the X-15 took only 19 months to double the maximum altitude achieved by the X-2. Ultimately, during its 199 flights, the X-15 recorded a maximum altitude of 354,200 feet and a maximum speed of 4,520 mph (Mach 6.7). They were stunning achievements.