In June 1959 a smooth black aircraft was released from a Boeing B-52 mother plane from an altitude of 37,550 ft., thus beginning the first flight of the North American X-15 rocket plane. During the course of this blog, we'll follow the X-15 from the drawing board to the frontier of outer space.
During the early days of powered flight, aviation pioneers envisioned the concept of a spaceplane, one which could both fly in earth's atmosphere while transcending the realm of space. The X-15 program was a natural progression of aviation technology and application since the time of the Wrights. The biplane yielded to the streamlined monoplane, and by the late 1930's the first experimental jet engines appeared, ushering in an era of high-speed flight. But as an airplane flew closer to the speed of sound, it encountered compressibility, the bunching up of air around it as it neared Mach 1, causing high drag, buffeting, changes in structural loads, and even loss in control and in-flight breakups. Until Chuck Yeager flew the Bell X-1 to Mach 1.06 in October 1947, it appeared the speed of sound might well be a barrier to future flight. Once the sound barrier was broken, aviation accelerated rapidly into the supersonic era, with Scott Crossfield breaking Mach 2 in November 1953 flying the Douglas D-558-2 Skyrocket with Captain Milburn Apt breaking Mach 3 in the first Bell X-2 in September 1956. Sadly, Apt was killed when the plane went out of control during its return to Edwards.
Later that year, Bell had produced advanced variants capable of exceeding Mach 2 at 90,000 ft., while the sweptwing X-2 could climb above 125,000 ft. In 1952 North American Aviation entered the rocket plane competition. Their design, the X-15 was built with a mission of achieving an altitude of 250,000 ft.at near Mach 5, or 4,000 mph., which was a grueling task at the time. The project was led by the U.S. Air Force and the National Advisory Committee for Aeronautics (NACA), which later became NASA in 1958. While the project was geared toward the acquisition of scientific and flight data, the Cold War loomed as a factor in the background. Since the X-15 was actually a rocket with a cockpit, it wasn't designed to take off from a runway, but rather it had to be carried aloft by a mother plane, which was a specifically modified B-52 bomber. At 50 feet long, the X-15 was suspended from a pylon underneath the wing of the B-52 with the optimum launch point at an altitude of 45,000 ft. at a speed of 600 mph. At that point the X-15 pilot would ignite the rocket engine and start climbing out of the Earth's atmosphere and into space. Propelled by a mixture of liquid oxygen and ammonia, the fuel burn lasted under two minutes, though the aircraft climbed drastically during the burn, buffeting the pilot in the process. Once the desired altitude was reached, the pilots would conduct experiments in space, gathering valuable flight data. With the X-15 designed for high altitude flight, where the air is so thin that conventional aerodynamic controls could not function, it was equipped with a reaction control system, similar to that later used on space shuttles and the International Space Station.
Flying at thousands of miles per hour put its share of stress on the outer shell of the X-15 with aerodynamic friction generating temperatures as high as 1,200 degrees fahrenheit. To overcome the heat effect from the friction, the X-15's outer shell was made of a special nickel-chromium alloy called Inconel X. Conversely, the aircraft's reaction control system spewed bursts of cold hydrogen peroxide, emitted at a very high concentration, creating small amounts of thrust from which to steer the plane in the upper atmosphere. Once the pilot turned off the engine or the aircraft ran out of fuel, it became a glider, and a very heavy and fast one at that. This put the pilot in a position of trading altitude and speed for distance in order to reach the destination with a safe landing. The nose wheel lacked steering with the main landing gear consisting of skids, two retractable steel beams which the X-15 slid on while landing. This arrangement was necessary due to the speed of the plane when landing, since no existing tire could withstand the heat and friction generated upon landing, which was absorbed by the beams. Because of this, the X-15 had to land on a dry lake bed rather than a conventional runway. While a conventional aircraft could make its landing approach at less than 200 mph., the X-15 began its approach at 20,000 ft. at speeds greater than 1,500 mph. Being an experimental aircraft, the X-15 was subject to a number of problems. In spite of this, the pilots were usually able to land the plane without issue. The fuel tanks had to be nearly empty upon landing, since the skids would dig in to the soil, producing drag and vibrations.
The X-15 program involved more than designing a new aircraft. With new flight and environmental controls, as well as a pilot protection system and test range, the X-15 required a new infrastructure to both test and produce. In 1954 a preliminary design team headed by John Becker initiated an initial study of a Mach 6 rocket-boosted hypersonic research plane. Becker's study foresaw a number of features built into the X-15. Becker also encouraged the military to participate in the X-15 project, forming the NACA-the National Advisory Committee for Aeronautics in October of that year. A joint program directive issued in December 1954 gave technical oversight to NACA and design and construction authority to the Air Force. The Navy and Air Force would jointly fund the effort. The Air Force supervised a design competition in 1955 among Bell, Republic, Douglas and North American for the proposed rocket plane. While Bell appeared to be a good candidate, based upon the performance of its X-1, X-2 and X-5 rocket planes, the Air Force awarded the contract to North American in 1956. Though the North American design was about twenty million more than the Bell, Republic and Douglas designs, which had technical flaws, the Air Force believed a more rocketlike design was necessary. The X-15 was powered by the XLR-99 rocket engine, which, at 57,000 lbs., could deliver three times the thrust of the Bell X-2 and eight times that of the X-1.
After much testing of both engine and flight controls, the X-15 began its first program flight on June 8, 1959. While the more potent XLR-99 had teething problems until November of 1960, the X-15 flew during the interim with the less powerful XLR-11 engine, having a number of successful flights. The X-15 was a craft built for records, for which it had several notable ones. On March 7, 1961 Air Force Major Robert M. White became the first pilot to achieve Mach 4. He exceeded Mach 5 on June 23 of that year and Mach 6 on November 9. On August 22, 1963 NASA research pilot Joseph Walker attained an altitude record of 354,200 ft. And finally on October 3, 1967 Air Force Major William J. Knight reached a speed of Mach 6.7-the fastest flown by a piloted aircraft to date-a tribute to the X-15 design team, pilots and ground crew keeping the planes at the leading edge of flight.