NASA - History - HIstoric Aircraft

The "X" designation, originally "XS" for eXperimental Supersonic, applied to a family of experimental aircraft not intended for production beyond a limited number built solely for flight research. The D-558-1 and -2 did not bear the "X" label but were clearly intended for the same purpose. This was also true of the non-X-designated lifting bodies, whereas the XB-70 was originally intended to be a production bomber, and the XF-92A was expected to be a production fighter. Neither of the two latter aircraft actually went into production, and both models became significant research aircraft, hence their inclusion here.

Aircraft	Remarks
X-1	Bell Aircraft built three of the original X-1s, plus an X-1A and X-1B, an X-1D. There was also an X-1E rebuilt from the X-1 #2. They flew a total of 214 flights between 1946-1958. This was a joint program among the NACA, the Air Force, and Bell Aircraft. The bullet-shaped, rocket-powered aircraft became the first airplane to break the sound barrier on Oct. 14, 1947. Flight research by the NACA continued through such advanced models as the X-1B and X-1E, providing a wealth of information for use in correlating from the X-1 #2 wind-tunnel data with actual flight data and for designing later high-performance aircraft.
D-558-I	Douglas manufactured three airframes. They flew a total of 225 flights between 1947-1953. This was a joint program among the NACA, the Navy-Marine Corps,. and Douglas Aircraft. The straight-winged, turbojet-powered "Skystreak" collected data in the transonic region about stability, control, loads, buffeting, and handling qualities. D-558-I Fact Sheet
D-558-II	Douglas manufactured three airframes. They flew a total of 312 flights between 1948-1956. This was a joint program among the NACA, the Navy-Marine Corps,. and Douglas Aircraft. The swept-wing aircraft flown with both turbojet and rocket power set an altitude record of 83,235 ft. on Aug. 21, 1953, and a speed record on Nov. 20, 1953, when it became the first aircraft to reach Mach 2. The "Skyrocket" collected data about handling qualities, wing loads, and stability and control, especially pitch-up. D-558-II Fact Sheet
XF-92A	Convair manufactured one airframe. The XF-92A flew a total of 325 NACA flights between 1948-1953. Other flights were flown by Convair and the Air Force. This was a joint program among the NACA, the Air Force, and Convair to test the country's first delta-wing air-craft. Stability and control, pitch-up, and lift-over- drag measurements obtained from this program contributed to the technology used to develop the F-102, F-106, XF2Y-1 Sea Dart, and B-58 aircraft.
X-2	Bell manufacture two airframes. They flew a total of 17 flights between 1954-1956. This was a joint program with the Air Force, although the NACA never flew the swept-wing, rocket-powered aircraft designed to fly Mach 3. The NACA supported the Air Force with advice and data analysis. The X-2 did become the first aircraft to reach Mach 3, recording a Mach 3.2 speed on its last flight, which destroyed the aircraft and killed the Air Force pilot because of inertial coupling. It collected data on aerodynamic heating and stability and control effectiveness at high speeds and altitudes. The X-2 was also the first aircraft to fly higher than 100,000 feet on Sept. 7, 1956, when it reached 126,200 feet 20 days before the aircraft reached Mach 3.2
X-3	Douglas manufactured one airframe. The X-3 flew a total of 26 flights between 1952-1955. This slender, jet-powered aircraft tested such new materials as titanium and collected data on stability and control, pressure distribution, and flight loads. The X-3 failed to achieve the high speeds for which it was designed but pioneered in the use of titanium and contributed to the development of aircraft tire technology.
X-4	Northrop manufactured two airframes. They flew a total of 90 flights between 1948-1953. In a joint program with the Air Force and Northrop, the NACA conducted most of the flights in this semi-tailless aircraft (which had no horizontal stabilizer). Powered by two turbojet engines and featuring swept wings, the X-4 helped demonstrate that tail surfaces are important for proper control effectiveness but that a properly configured semi- tailless airplane was a viable platform for research on dynamic stability and also provided data (from tufts) on airflow anomalies.
X-5	Bell manufactured two airframes, only one of which was flown by the NACA. It flew a total of 133 Air Force - NACA flights between 1951-1953. The X-5 completed all of the research goals originally set for the first aircraft capable of variably sweeping its wings in flight. Demonstrating wing sweep from 20 to 60 degrees, the aircraft verified NACA wind-tunnel predictions of reduced drag and improved performance resulting from increased wing sweep as it approached Mach 1. Even vicious spinning characteristics of the X-5 yielded a wealth of data for determining poor aircraft spin design.
X-15	North American Aviation manufactured 3 airframes. They flew a total of 199 flights between 1959-1968. This joint program by NASA, the Air Force, the Navy, and North American operated the most remarkable of all the rocket research aircraft. Composed of an internal structure of titanium and a skin surface of a chrome-nickel alloy known as Inconel X, the X-15 first set speed records in the Mach 4-6 range with Mach 4.43 on Mar. 7, 1961; Mach 5.27 on June 23, 1961; Mach 6.04 on Nov. 9, 1961; and Mach 6.7 on Oct. 3, 1967. The airplane also set an altitude record of 354,200 feet (67 miles) on Aug. 22, 1963, and provided an enormous wealth of data on hypersonic air flow, aerodynamic heating, control and stability at hypersonic speeds, reaction controls for flight above the atmosphere, piloting techniques for reentry, human factors, and flight instrumentation of relevance not only to aeronautics but to spaceflight. X-15 Fact Sheet
Lifting Bodies	DFRC (M2-F1), Northrop (M2-F2), Northrop (M2-F3), Northrop (HL-10), Martin (X-24A), Martin (X-24-B). There was one of each lifting body type manufactured. They flew a total of 223 flights between 1963-1975 (not including nearly 400 car tows of the M2-F1). This joint program between the Air Force and NASA demonstrated the ability of pilots to maneuver and safely land a wingless vehicle designed to fly back to Earth from space and be landed like an aircraft at a pre-determined site. The information generated by the lifting body program contributed to the data base that led to development of today's space shuttle program, especially its approach and landing techniques. The rocket-powered lifting bodies (all but the unpowered M2-F1) have also contributed to the upcoming X-33 space technology demonstrator and the X-38 (on both of which, see below). Lifting Bodies Fact Sheet
XB-70	North American manufactured two airframes. They flew a total of 129 flights between 1964-1969. The joint program among North American Rockwell, the Air Force, and NASA featured the world's largest experimental aircraft with a delta wing and hinged wing tips that could be folded down to a 65° angle to improve stability at the aircraft's supersonic speeds of up to Mach 3, a speed at which the Valkyrie was designed to ride its own shock wave. The program used the Valkyrie to conduct fundamental flight research at high speeds for use in designing future supersonic aircraft, both military and civilian. The aircraft produced a significant quantity of information on supersonic flight at up to Mach 3 speeds in areas such as noise (including sonic booms), potential flight corridors, validation of wind-tunnel data, flight control, operational problems, and clear-air turbulence.
X-29	Grumman manufactured two airframes. They flew a total of 437 flights between 1984-1992. In a joint program involving the Defense Advanced Research Projects Agency, the Air Force, NASA, Grumman, and other contractors, this single-engine, jet-powered aircraft investigated the use of advanced composite materials, a forward-swept wing with a thin supercritical airfoil, a variable- incidence canard, a computerized fly-by-wire flight control system to overcome the aircraft's inherent instability, behavior at high angles of attack, and a vortex flow-control system (among other technologies). On Dec. 13, 1985, the X-29 became the first forward-swept-wing airplane in the world to exceed Mach 1 in level flight, and flight results showed that a highly unstable aircraft with forward-swept wings could be flown safely with excellent maneuverability and high G-loads. It could also be flown with good control response up to about 40° angle of attack. The flight research also added to engineers' understanding of advanced composites, used increasingly in aircraft construction, and of digital flight-control systems. X-29 Fact Sheet
X-30	Only a 1/3-scale concept demonstrator was built, "flown" only in a high-temperature tunnel between 1986-1994. This joint effort by NASA, the Department of Defense, and five major contractors explored development of technologies for a new generation of aerospace vehicles for hypersonic cruise in the atmosphere or single-stage-to-orbit using air breathing primary propulsion and horizontal takeoff and landing. Although a full-scale aircraft was never built because Congress ended funding in 1994, the program had expected such a vehicle to fly at Mach 25. The program developed significant advances in high-temperature, carbon-carbon materials, lightweight titanium and beryllium alloys, and high strength, corrosion-resistant titanium-alloy composites. These technologies and the program's work with supersonic-combustion ramjet propulsion will all be useful to subsequent U.S. aerospace efforts in the hypersonic area.
X-31	Rockwell Aerospace, North American Aircraft, and Deutsche Aerospace manufactured two airframes. They flew a total of 555 flights between 1990-1995. In a joint program involving the Defense Advanced Research Projects Agency, the U.S. Navy, German Federal Ministry of Defense, Deutsche Aerospace, Rockwell International, the U.S. Air Force, and NASA, this Enhanced Fighter maneuverability demonstrator showed the value of using thrust vectoring (by means of carbon-carbon paddles) coupled with advanced flight control systems to provide high maneuverability and controlled flight at high angles of attack. Featuring a delta-shaped, composite, twisted camber wing and strakes on the rear fuselage, the X-31 achieved stabilized flight at 70° angle of attack. With nose strakes added to increase stability, the aircraft exhibited remarkable "post-stall" maneuverability, such as a 180° turn at an extremely high angle of attack, known as the "Herbst maneuver." X-31 Fact Sheet
X-33	NASA selected Lockheed Martin to design, build, and fly the X-33 Advanced Technology Demonstrator test vehicle between March and December 1999. The X-33, a half-scale vehicle, was expected to feature a lifting-body shape, a new "aerospike" rocket engine, and a rugged metallic thermal protection system. It was expected to demonstrate in flight the new technologies needed for a Reusable Launch Vehicle (RLV). The X-33 was to be an unpiloted vehicle, launched vertically like a rocket but landed horizontally like an airplane, and was expected to be capable of reaching an altitude of approximately 50 miles and speeds of more than Mach 11. A full-scale RLV would increase reliability dramatically and lower the cost of putting a pound of payload into space from the current figure of $10,000 to $1,000. However, in 2001 NASA ceased funding the program. X-33 Fact Sheet
X-34	Orbital Sciences Corp. manufactured three airframes. They flew a total of three flights between 1999-2001. In the summer of 1996, NASA contracted with Orbital Sciences Corp. to design, build, and test-fly the X-34, a small, reusable technology demonstrator for a launch vehicle. The X-34 was a single-engine rocket that was to be carried aloft and launched from an Orbital Sciences L-1011 aircraft. On June 29, 1999, the prototype test version of the X-34 made its first captive-carry flight attached to the belly of its newly modified L-1011 carrier aircraft. A second captive-carry flight occurred successfully on Sept. 3, and a third and final flight occurred on Sept. 14. The X-34 was intended to fly at Mach 8 and to reach an altitude of 250,000 feet, but in 2001 NASA ceased funding the program. X-34 Fact Sheet
X-36	Boeing Phantom Works manufactured two airframes. They flew a total of 33 flights between 1997-1998. This Tailless Fighter Agility Research Aircraft demonstrated the feasibility of future tailless fighters to achieve levels of agility superior to today's best military fighter aircraft. These 28-percent-scale, remotely-piloted X-36s participate in a program featuring a tailless configuration to reduce the radar signature of the aircraft. In a follow-on effort, the Air Force Research Lab (AFRL) contracted with Boeing to fly AFRL's Reconfigurable Control for Tailless Fighter Aircraft (RESTORE) software as a demonstration of the adaptability of the neural-net algorithm to compensate for in-flight damage or malfunction of effectors, i.e., flaps, ailerons and rudders. Two RESTORE research flights were flown in December 1998, proving the viability of the software approach. X-36 Fact Sheet
X-37	The X-37 is projected to be an orbital experimental vehicle to be lifted to orbit by the Space Shuttle or a reusable launch vehicle and returned to Earth under its own power. To be built by the Boeing Phantom Works under a cooperative agreement signed in July 1999, it is projected to be 27.5 feet long, about half the length of the Shuttle payload bay. It is expected to weigh about six tons and to have a wingspan of 15 feet. On-orbit propulsion will be provided by the AR-2/3, a highly reliable engine with a legacy stretching back to the 1950s.
X-38	Two airframes were manufactured. They have flown a total of 15 flights between 1997-2001. This Advanced Technology Demonstrator for a Crew Return Vehicle from the International Space Station completed four captive flights beneath B-52 0008 during 1997, three in 1998, and then performed its first drop test on March 12, 1998, using a steerable, parafoil parachute. During 1999, the X-38 had successful free flights on Feb. 6, Mar. 5, July 9 with two separate vehicles, one with and one without flight control surfaces. A captive-carry flight of Vehicle 132 attached to the B-52 mothership took place on Sept. 13, with most flight objectives reached, followed by another captive-carry flight on Nov. 18. Employing a lifting-body concept, the X-38 is expected to be developed for a fraction of the costs of previous human space vehicles. X-38 Fact Sheet
X-40A	The X-40A is an experimental vehicle shaped like the X-37 but lacking its advanced thermal protection materials. It is also 20 percent smaller than the X-37. Built by Boeing for the Air Force, it was released from a helicopter and glide-tested in 1998. Plans call for it to be drop tested from helicopters in 2000 to reduce risk before the X-37 is flight-tested.
X-43	MicroCraft Inc. manufactured three airframes. With a contract award in early 1997, X-43A (part of the Hyper-X Program) is projected to be a hypersonic, experimental research vehicle about 12 feet long with a wing span of about 5 feet. There will be 3 expendable vehicles, each of which will feature a supersonic combustion ramjet (scramjet), air breathing engine in which the airflow through the engine is supersonic. Each X-43A will ride to the initial test condition on the first stage of a Pegasus booster rocket. The booster/research vehicle "stack" will be launched by NASA's B-52 from altitudes of 17,000 to 43,000 feet. For each flight, the X-43A will be boosted to successive speeds of Mach 7 (two flights) and then Mach 10. The booster will carry the X-43A to about 100,000 feet, where it will separate from Pegasus and fly under its own scramjet power. The flight tests will originate within the Sea Test Range off the coast of Southern California. X-43 Fact Sheet