1980s and 1990s
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.
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.
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."
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.
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.
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.
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.
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.
The X-40A was an 80 per cent subscale version of the X-37 experimental autonomous spaceplane technology demonstrator, but lacking propulsion or thermal protection systems. The X-40A flew seven approach and landing test flights at NASA Dryden in 2001 to reduce risk for the X-37 program, including in-flight evaluation of guidance, navigation and control software for its autonomous flight controls. The craft was towed aloft by an Army CH-47D Chinook helicopter to an altitude of 15,000 feet, and then released to fly an autonomously controlled 75-second descent to a landing on the main runway at Edwards Air Force Base. Boeing built the X-40A originally for the Air Force as part of that service's Space Maneuver Vehicle program. It made one free flight in 1998 at Holloman Air Force Base in New Mexico before being loaned to NASA to aid the X-37 effort. The X-37 is being developed by Boeing for NASA to demonstrate advanced integrated technologies that would reduce the cost and risk of future reusable space launch vehicle systems.
Four decades of supersonic-combustion ramjet propulsion research culminated in a successful flight of the X-43A hypersonic technology demonstrator in March 2004, the first time a scramjet-powered aircraft had flown freely. After being launched by Dryden's venerable B-52B mothership off the coast of Southern California, a modified first-stage Pegasus booster rocketed the X-43A to 95,000 feet before the X-43A separated and flew under its own scramjet power at an airspeed of Mach 6.8, or about 5,000 mph, for about 11 seconds. On Nov. 16, another identical scramjet-powered X-43A did it again, this time reaching hypersonic speeds above Mach 9.6, or about 6,800 mph, in the final flight of the X-43A project. Both flights set world airspeed records for an aircraft powered by an air-breathing engine, and proved that scramjet propulsion is a viable technology for powering future space-access vehicles and hypersonic aircraft. .