The North American X-15 was the culmination of the rocket powered research aircraft that peppered the 1940s and 1950s beginning with the famous Bell X-1. Each of these important "x-plane" aircraft were given specific program goals and advanced as the technology of the time allowed. Early flights netted data from speeds reaching beyond Mach 1.0 up to Mach 3.0 and evolved from the straight-winged X-1 to the swept-wing Bell X-2. To further the envelope, thought shifted to data collection utilizing an advanced, highly specialized rocket plane to enter the hypersonic flight envelope (speeds beyond Mach 5.0) to reach beyond the earth's atmosphere for future space travel endeavors.
The X-15 was built by North American Aviation (best known for their war-winning P-51 Mustang fighter of World War 2) as a research platform to serve NASA, the United States Air Force (USAF) and the United States Navy (USN). The specifications for the rocket plane were ironed out by NASA (then known as NACA - the National Advisory Committee for Aeronautics) as early as 1952 and this information passed on to the USAF/USN during July of 1954. North American was then chosen to built three examples of what became the "X-15" in September of 1955.
The finalized design became a long and slender aircraft featuring a pointed nose cone, forward-set single-seat cockpit, and tubular fuselage. Short, swept (25-degrees) mid-wing mainplanes were added to the sides aft of midships and the empennage included dorsal and ventral vertical fins with a pair of side-mounted, swept-back horizontal planes. The rocket engine of choice was the Reaction Motors XLR-99 of 28,000 lb - 57,000 lb thrust (throttleable) however delays with that rocket meant that 2 x XLR-11 rocket engines took its place for the interim and dominated in early flights. Due to the intense heat generated at high-speed, high-altitude flying, the X-15's body construction consisted of a nickel-chrome alloy covering known as "Inconel X". Despite its advanced flight scheme, the X-15 still relied on conventional control systems for handling in thicker air environments. Upon reaching thinner air atmosphere, a "reaction control system" came into play utilizing hydrogen peroxide rocket thrusters found at the nose and wings. Structural dimensions of the aircraft including a length of 50 feet and a wingspan of 22 feet.
The X-15's rocket propulsion only served the aircraft during its active flight test phase - as such it was air-launched from a mothership like the X-1 before it. While it did not take off under its own power as a conventional aircraft would, it was able to glide down on its descent and land through a special undercarriage as normal. The undercarriage consisted of an unsteerable wheeled nose leg and main body steel skids. Because of this a dry lake bed was selected as the landing/recovery zone (Rogers Dry Lake near Edwards AFB). The mothership of choice for the program became a modified jet-powered Boeing B-52 Stratofortress bomber - continuing the Boeing tradition of motherships that began with the prop-driven Boeing B-29 Superfortress bomber of World War 2 and passed on to the Boeing B-50. Two flight plans were typical of the X-15 program - the first being a level altitude speed test after air launch and the second being a high-altitude, steep rate-of-climb action prior to leveling off and descending. Due to the rocket's high fuel consumption, there was a little over two minutes of powered flight available to the rocket plane. The B-52s would release their X-15 payload at roughly 45,000 feet altitude to which the X-15's rockets were then engaged. Upon burning up all of its fuel stores, unpowered flight lasted between eight and twelve minutes before the descent was required.
The initial X-15 aircraft was delivered to NASA in early 1959 and a first unpowered glide flight happened on June 8th, 1959 with pilot Scott Crossfield at the controls. Number Two aircraft, X-15A-2, arrived with some changes from the first model in February of 1964. These modifications included external, jettisonable fuel tanks for 60 additional seconds of powered flight, a removable lowered tail fin, a lengthened (28") fuselage, revised cockpit canopy, extended nose landing gear leg, and onboard storage for photo capture equipment. It was this aircraft that reached Mach 6.7 on its October 3rd, 1967 test flight.
While the X-15 line proved hugely successful during its testing career, it was a product and program always fraught with danger - accidents and forced landings were not wholly uncommon occurrences during its airborne tenure. Test pilot Jack McKay and his aircraft were flipped over when the nose leg collapsed upon touchdown. His injuries were enough to end his flying career with the X-15. Test pilot Michael Adams was killed on a November 15th, 1967 flight when his X-15A-4 (Number Three) crashed - the aircraft was thrown into a recoverable spin but Adams could not bring it out of the steep dive it had entered.
The X-15 completed its career in December of 1968 when it was faced with formal retirement, ending nearly a decade of critical work for the American space and military industry. The data collected from the X-15 proved invaluable in furthering the American race into space - rocket programs with names like Mercury, Gemini, and Apollo. Its active flight tenure covered 199 flights under the guiding hands of twelve test pilots in all. During its time aloft, the X-15 was credited with setting new altitude (354,200 feet in 1963) and speed (4,520 mph / Mach 6.7 in 1967) records its name going down in the aviation history books and its iconic shape burned into the minds of observers and aviation aficionados alike.