back to models
North American X-15
History of the X-15
The North American X-15 rocket-powered aircraft/spaceplane was part of the X-series of experimental aircraft, initiated with the Bell X-1, that were made for the USAAF/USAF, NACA/NASA, and the USN. The X-15 set speed and altitude records in the early 1960s, reaching the edge of outer space and returning with valuable data used in aircraft and spacecraft design. As of 2012, it holds the official world record for the fastest speed ever reached by a manned rocket-powered aircraft.
During the X-15 program, 13 different flights by eight pilots met the USAF spaceflight criteria by exceeding the altitude of 50 miles (80 km) thus qualifying the pilots for astronaut status. The USAF pilots qualified for USAF astronaut wings, while the civilian pilots were awarded NASA astronaut wings in 2005, 35 years after the last X-15 flight.
Of all the X-15 missions, two flights (by the same pilot) qualified as space flights per the international (Fédération Aéronautique Internationale) definition of a spaceflight by exceeding 100 kilometers (62.1 mi, 328,084 ft) in altitude.
Design and Development
The X-15 was a research aircraft, and there were changes to it over the course of the program and between the different airframes. The X-15 had to be operated under several different situations including the time attached to a carrier aircraft, drop, main engine start and acceleration, a ballistic flight into thin air/space, re-entry into thicker air, and an unpowered glide to landing. Alternatively, if the main engine was not started the pilot needed to go directly to a landing. The main rocket engine only operated for a relatively short part of the flight, but was capable of boosting the X-15 to its high-speeds and altitudes. Without the main engine on, the X-15's instruments and control surfaces remained functional, but the plane could not maintain altitude.
Because the X-15 also had to be controlled in a region where there was too little air for aerodynamic surfaces, it had a reaction control system (RCS) that used rocket thrusters. There were two different X-15 pilot controls setups: one type basically used three joysticks, and another type used one joystick.
The X-15 type with multiple control sticks for the pilot, included a traditional rudder and stick, and another joystick on the left which gave commands to the reaction control system. A third joystick on the right side was used during high G maneuvers to augment the center stick. In addition to pilot input the X-15 "Stability Augmentation System" (SAS) gave extra inputs to the aerodynamic controls to help the pilot maintain control. The reaction control system could be operated in two modes: manual and automatic. The automatic mode used a feature called "Reaction Augmentation System" (RAS) that helped stabilize the vehicle during high altitudes. The RAS was typically used for around three minutes of a X-15 flight before being automatically turned off.
The other setup used the MH-96 flight control system which allowed one joystick in place of three and simplified pilot inputs. The MH-96 could automatically blend aerodynamic and rocket controls depending on how effective each system was at controlling the X-15.
The X-15 had an ejection seat that allowed ejection at speeds up to Mach 4 and/or 120,000 feet (~36.6 km) altitude, although it was not used during the program. In the event of ejection, the seat had deployable fins which were used until it reached a safer speed/altitude, where it could deploy its main parachute. Pilots wore a pressure suit, which could be pressurized with nitrogen gas. Above 35,000 ft (~10.7 km) altitude, the cockpit was pressurized to 3.5 psi (0.24 atm) with nitrogen gas, and oxygen for breathing was fed separately to the pilot.
Engines and fuel
Early flights used two Reaction Motors XLR11 engines. Later flights were undertaken with a single Reaction Motors Inc XLR99 rocket engine generating 57,000 pounds-force (250 kN) of thrust that powered the aircraft. This engine used ammonia and liquid oxygen for propellant and hydrogen peroxide to drive the high-speed turbopump that delivered fuel to the engine. It could burn through 15,000 pounds (6804 kg) of fuel in 80 seconds. The XLR99s could be throttled, and were the first such controllable engines that were man-rated.
The XLR11 used ethyl alcohol and liquid oxygen, and the XLR99 used ammonia and liquid oxygen as fuel. The X-15 reaction control system (RCS), for maneuvering in low-pressure/density environment, used hydrogen peroxide as a monopropellant. More specifically, it was high-test peroxide, which decomposes into water and oxygen in the presence of a catalyst, and can give an ISP of 140 seconds. The HTP also fueled a turbopump for the main engines and auxiliary power units (APUs). Additional tanks for helium and liquid nitrogen performed other functions, for example the fuselage interior was purged with helium gas and the liquid nitrogen was used as coolant for various systems.
The X-15 had a thick wedge tail for stability at hypersonic speeds. However, this produced a lot of drag at slower speeds. In fact, the blunt end of the X-15 could produce as much drag as an entire F-104 Starfighter.
A wedge shape was used because it is more effective than the conventional tail as a stabilizing surface at hypersonic speeds. A vertical-tail area equal to 60 percent of the wing area was required to give the X-15 adequate directional stability.
Additionally, stability at hypersonic speeds was aided by side panels that could extend out from the tail to further increase area, and these panels doubled as air-brakes.
Three X-15s were built, flying 199 test flights, the last on 24 October 1968. The first X-15 flight was an unpowered test flight by Scott Crossfield, on 8 June 1959; he also piloted the first powered flight, on 17 September 1959, with his first XLR-99 flight on 15 November 1960. Twelve test pilots flew the X-15; among them were Neil Armstrong (first man to walk on the moon) and Joe Engle (later a space shuttle commander). In July and August 1963, pilot Joe Walker crossed the 100 km altitude mark, joining the NASA astronauts and Soviet Cosmonauts as the first humans to have crossed the barrier into outer space. Other missions included captive flights and glide flights.
On 15 November 1967, U.S. Air Force test pilot Major Michael J. Adams was killed on X-15 Flight 191 when the (X-15-3) entered a hypersonic spin while descending, then oscillated violently as aerodynamic forces increased after re-entry. As his aircraft's flight control system operated the control surfaces to their limits, acceleration built to 15 g vertical and 8 g lateral. The airframe broke apart at 60,000 ft (18,000 m) altitude, scattering the X-15's wreckage for 50 square miles (130 km2). On 8 June 2004, a monument was erected at the cockpit's locale, near Randsburg, California. Major Adams was posthumously awarded Air Force astronaut wings for his final flight in X-15-3, which had reached 81.1 km (50.4 mi, 266,000 ft) of altitude. In 1991, his name was added to the Astronaut Memorial.
The second X-15A was rebuilt after a landing accident. It was lengthened 2.4 feet (0.73 m), a pair of auxiliary fuel tanks attached under the fuselage, and a heat-resistant surface treatment applied. Re-named the X-15A-2, it first flew on 28 June 1964, reaching 7,274 km/h (4,520 mph, 2,021 m/s).
The altitudes attained by the X-15 aircraft do not match that of Alan Shepard's 1961 NASA space capsule flight nor subsequent NASA space capsules and space shuttle flights. However, the X-15 flights did reign supreme among rocket-powered aircraft until the second spaceflight of Space Ship One in 2004.
Five aircraft were used for the X-15 program: three X-15s, two B-52 bombers:
X-15A-1 – 56-6670, 82 powered flights
A 200th flight over Nevada was slated for 21 November 1968, piloted by William J. Knight. Technical problems and bad weather delayed the flight six times, and on 20 December 1968, the 200th flight was finally canceled. The X-15 was detached from the NB-52A wing and prepared for indefinite storage.
History and images courtesy of www.wikipedia.org, all other info Copyright 2006, JPLANES.COM