The X-15 is a famous and significant part of aviation history known as the North American rocket-powered research aircraft that closed the gap between manned flight within the atmosphere and manned flight to outer space.
In the joint X-15 hypersonic research program that NASA conducted with the Air Force, the Navy, and North American Aviation, the purpose of the aircraft was to fly high and fast, testing the machine and subjecting pilots to conditions that future astronauts would face.
The aircraft made the first manned flights to the edge of space and was the world’s first piloted aircraft to reach hypersonic speeds. Three X-15s were built, they made 199 flights between 1959-1968.
The X-15 became the first winged aircraft to attain hypersonic speeds, the velocities of Mach 4, 5, and 6 (four, five, and six times the speed of sound, 761.2 mph/1,225 kph).
Except for number two X-15 modified as the X-15A-2, the X-15s were roughly 50 feet (15 meters) long, with a 22-feet (6.7 meters) wingspan. The wedge-shaped vertical tail was 13 feet (3.9 meters) high.
Although the number two aircraft was later modified, the basic X-15 was a single-seat, mid-wing monoplane designed to explore the areas of high aerodynamic heating rates, stability and control, physiological phenomena, and other problems relating to hypersonic flight (above Mach 5).
The outer skin of the X-15 consisted of a nickel-chrome alloy called Inconel X, employed in a heat sink structure to withstand the results of aerodynamic heating when the aircraft was flying within the atmosphere. The cabin was made of aluminum and was isolated from the outside to keep it cool.
The early flights of the aircraft, the X-15 initially flew with two XLR-11 engines, producing a thrust of 16,380 lbs (72,861 newtons). Once the XLR-99 was installed, the thrust became 57,000 lbs (253,548 newtons).
Because of the large fuel consumption of its rocket engine, the X-15 was air-launched from a B-52 aircraft at about 45,000 feet (13.7 km) and speeds upward of 500 mph (800 kph).
Once its powerful rocket ignited, the X-15 streaked upward to the limits of the atmosphere, then glided unpowered to land on a dry lake bed. Its typical flights lasted about 10 minutes.
X-15A-2 was the second of the three X-15s. North American modified it for even greater speed, adding the large orange and white propellant tanks and lengthening the fuselage by about 18 inches (45 cm).
X-15A-2 set the world's unofficial speed and altitude records of 4,520 mph (7,274 kph/Mach 6.7) and 354,200 feet (108 km) in October 1967. It was delivered to the museum in 1969.
Depending on the mission, the rocket engine provided thrust for the first 80 to 120 seconds of flight. The remainder of the normal 8- to 12-minute flight was without power and ended in a 200-mph (320 km) glide landing.
There were 10 other pilots in the program for a total of 12: five from NASA, including Neil Armstrong, who later became the first man to walk on the moon, five from the Air Force, one from the Navy, and one, Crossfield, from North American.
Pilots usually used one of two types of flight profiles, a speed profile that maintained a level altitude until time for the descent to a landing, and a high-altitude flight plan that required maintaining a steep rate of climb until reaching altitude and then descending.
The information gathered from the highly successful X-15 program contributed to the development of the Mercury, Gemini, and Apollo piloted spaceflight programs as well as the Space Shuttle program.
The distinguished Langley aeronautical researcher John Becker identified 25 specific accomplishments of the X-15 program. These included:
- The first application of hypersonic theory and wind tunnel work to an actual flight vehicle.
- The first use of reaction controls for attitude control in space.
- The first reusable superalloy structure capable of withstanding the temperatures and thermal gradients of hypersonic reentry.
- The development of (a servo-actuated ball) nose flow direction sensor for operation over an extreme range of dynamic pressure and a stagnation air temperature of 1,900° Fahrenheit (1037 Celsius)
- The development of the first practical full pressure suit for pilot protection in space.
- The development of inertial flight data systems capable of functioning in a high dynamic pressure and space environment.
- The discovery that hypersonic boundary layer flow is turbulent and not laminar.
- The discovery that turbulent heating rates are significantly lower than had been predicted by theory.
- The first direct measurement of hypersonic aircraft skin friction and discovery that skin friction is lower than had been predicted.
- The discovery of hot spots generated by surface irregularities.