Aircraft capable of flying at speeds greater than five times the speed of sound.
Although they are advertised as being able to circle the globe in less than four hours, hypersonic aircraft do not offer much promise to weary airline passengers in the near future. Most applications of hypersonic aircraft are either in the context of warfare or spaceflight. With the exception of the terminal stage of certain missiles, hypersonic flight is conducted exclusively at very high altitudes, where the air density and pressure are a fraction of their values at sea level.
In space, any controlled maneuver requires the expenditure of fuel, whereas inside the atmosphere, aerodynamic forces can be used by deflecting control surfaces. Because the speed required for Earth orbit at low altitudes is approximately 18,000 miles per hour, spacecraft reenter the atmosphere at extremely high Mach numbers, ranging typically from 25 on the space shuttle to over 36 on the Apollo capsules. In reentry flight, the craft spends only a few minutes at hypersonic speed before decelerating to supersonic speeds, which allow more controlled maneuvering and gliding to selected landing sites.
During ascent into space, modern hypersonic aircraft ride on rocket boosters, spending the shortest possible time in the dense lower regions of the atmosphere. This situation will have to change when aircraft use air-breathing engines for propulsion at hypersonic speeds. Air-breathing engines take the oxygen needed for combustion from the atmosphere, reducing the amount lifted from the ground. The advantage of mastering this technology may be easily seen. In a hydrogen-oxygen propulsion system, which is the most efficient known means of chemical propulsion, 89 percent of the total weight of fuel and oxidizer is oxygen. However, air-breathing hypersonic flight poses several difficult problems.
The air flowing around a vehicle moving at hypersonic speeds has several interesting features. In front of the vehicle nose stands an extremely strong shock wave. This shock is like the blast wave from an explosion, heating the air enough to make oxygen and nitrogen molecules vibrate at high frequencies, dissociate into atoms, radiate large amounts of heat, and even ionize. The air becomes compressed to values as high as ten to one hundred times its normal density, and the extremely high pressure imposes very high loads on the vehicle. Over the upper surface of the vehicle, the air accelerates to supersonic speeds, and the density and temperature fall so quickly that the dissociated air does not have time to recombine. Around the vehicle, shock waves lie very close to the surface. Air friction heats the surface and increases the drag on the vehicle. Within a thin layer, the flow changes properties through a very large range. The reliable design of such vehicles is extremely difficult, because accurate, full-scale aerodynamic prediction is difficult and expensive, and approximate methods do not provide enough accuracy.
Refining the prediction methods through experimentation is also not easy because of the sheer difficulty of conducting flow experiments under hypersonic conditions. Hypersonic wind tunnels require extreme pressures, temperatures, and flow rates and can operate under steady conditions only for milliseconds. In the early 1990’s, when President Ronald Reagan’s National AeroSpace Plane initiative resumed the development of technology for hypersonic flight, the total experience of wind-tunnel testing at hypersonic speeds from all tests conducted to that date was estimated to be less than one second.
The inherent difficulties of hypersonic travel have not prevented the development of hypersonic vehicles. In 1933, German rocket expert Eugen Sänger published his concept for an “antipodal bomber” (antipodes are two points on opposite sides of the earth), a crewed hypersonic glider launched on a large rocket that would deliver bombs to distant targets across the globe, by skipping in and out of the atmosphere. This project was canceled in 1942.
In February, 1949, the U.S. Army launched the V-2 WAC Corporal rocket from the White Sands Missile Range in New Mexico. The rocket reached a speed of 3,500 miles per hour and an altitude of 100 miles before the WAC Corporal stage ignited and reached an altitude of 244 miles. The vehicle reentered the atmosphere at a speed of more than 5,000 miles per hour.
On April 12, 1961, Soviet flight major Yuri Gagarin returned to Earth after an orbital flight during which he traveled at hypersonic speeds that charred the surface of his spherical space capsule. Since then, rockets with or without human crew have routinely flown in the hypersonic regime.
The first hypersonic research airplane, which used aerodynamic lift to stay aloft, was the North American X-15, developed by the National Aeronautics and Space Administration (NASA). Air-launched from a B-52 bomber, the X-15 first flew on June 8, 1959. It was 50.75 feet long with a wingspan of 22.25 feet. Its Thiokol XLR-99 throttleable rocket engine burned a mixture of anhydrous ammonia and liquid oxygen to reach Mach 6. By the end of August, 1963, the X-15 piloted by NASA’s Joseph A. Walker had reached a record altitude of 354,200 feet.
X-15 flight tests revealed a number of interesting facts about hypersonic flight, including the existence of turbulent hypersonic boundary layers, and that turbulent heating rates were lower than predicted by theory, but that hot spots developed on the surface, causing material failures. The flights demonstrated piloted transition from aerodynamic to reaction controls and back again, including hypersonic/supersonic reentry at angles of attack up to 26 degrees and glide to precise landings.
The third X-15, which set a number of records, was lost, along with its pilot, Michael J. Adams, on November 15, 1967. The program was canceled after this fatal accident.
The X-15A-2 vehicle was designed to pursue the idea of hypersonic flight using an air-breathing engine instead of a rocket. The plan was to test a ramjet engine using supersonic combustion, although it was never flight-tested. The challenges of hypersonic air-breathing propulsion were numerous, but two fundamental problems dominated.
First, theoretical research showed that drag is incurred when a supersonic flow is slowed down to subsonic speeds, as through a shock, and when heat is added to a flow at a high Mach number. At low supersonic speeds, the drag due to the shock is less than the drag that would be incurred if heat were added to a supersonic flow. Thus, ramjet engines for flight at less than Mach 4 use shocks to slow the flow to subsonic speeds before adding heat by burning fuel in the combustion chamber of the ramjet engine. At speeds above Mach 4, it is more efficient to add heat at supersonic speeds than it is to slow down the flow to subsonic conditions.
Second, in supersonic combustion there is an extremely short time available in which to add fuel, mix it with the air moving at supersonic speed, and complete the combustion before the flow exits the engine. The X-30, dubbed the National AeroSpace Plane, was built to develop supersonic combustion ramjets, or scramjets. In the 1990’s, this program was canceled without any test flights.
NASA’s Langley Research Center at the turn of the millennium described a program called Hyper X to study hypersonics technology at speeds from Mach 5 to Mach 10. The NASA/Boeing X-43 was designed to study scramjet-powered flight at speeds from Mach 6 to Mach 10, following launch using a Pegasus booster rocket from a B-52 over the Pacific Ocean. Released at 20,000 feet, the 12-foot-long X-43 was designed to be accelerated by the rocket to a speed of Mach 6 and an altitude of 90,000 feet. The scramjet engine was designed to operate for seven to ten seconds, accelerating the X-43 to a speed of Mach 10. The engine had an oval-shaped air intake and burned hydrogen with air in a copper combustion chamber at supersonic speeds. Lacking landing gear, the vehicle was designed to transmit data before expending its energy and falling into the ocean.
The U.S. Air Force and NASA have continued to study hypersonic lifting bodies for hypersonic reentry. The X-20 Dyna-Soar hypersonic boost glide vehicle was designed to launch into orbit on a Titan III solid-fuel rocket, reenter the atmosphere, and glide at hypersonic speeds to deliver a nuclear weapon. However, the Dyna-Soar was shelved without ever flying.
The X-23 lifting body flew in 1966, demonstrating maneuvering during reentry. The X-24A and X-24B craft investigated low-speed characteristics of lifting bodies. The NASA/Boeing X-37, part of NASA’s Hyper X program, investigates technologies for orbit-on-demand, including hypersonic glide reentry. The vehicle, built by Boeing Phantom Works, is 27.5 feet long, with a wingspan of 15 feet, a weight of about 6 tons, and a payload bay 7 feet long and 4 feet in diameter. The Boeing X-40 maneuverable spaceplane integrated technology demonstrator is a predecessor to planned vehicles for flight at Mach 16 up to 300,000 feet, sending 1,000 to 3,000 pounds of payload into orbit for military missions.
In the 1990’s, the X-38 crew return vehicle (CRV) extended the work on the X-23 and X-24, in the development of a lifting body that would be attached to the International Space Station (ISS) as an emergency escape system. Separated from the ISS using rocket thrusters and able to carry an incapacitated crew of up to seven, the X-38 was to navigate using the Global Positioning System (GPS) and glide through hypersonic reentry at angles of attack of up to 38 degrees, with the heating taken by thermal tiles on the vehicle. Following supersonic maneuvering using flaperons, the X-38 would deploy first parachutes and then a large parafoil. An on-board automatic control system would guide the parafoil-suspended X-38 to a soft landing into the wind. In 2001, the X-38 project was canceled, but a parallel project conducted by the French and the European Space Agency aimed to develop a reusable crew taxi or crew rescue vehicle.
NASA’s space shuttle uses aerodynamic lift at high angles of attack and hypersonic speeds of up to Mach 25 during its reentry and descent into the atmosphere. It uses heat-shield tiles to protect critical parts of the fuselage and the wings during reentry. By using aerodynamic lift in the upper atmosphere, the shuttle stays at high altitudes, where the air is much thinner, until much of its orbital kinetic energy has been dissipated before sinking into the denser parts of the atmosphere and gliding to a runway landing.
The NASA/Lockheed Martin X-33 reusable launch vehicle, a smaller predecessor of the Lockheed VentureStar concept, tested the idea of achieving single-stage boost to low-Earth orbit using ultra-lightweight composite fuel and oxidizer tanks and a rocket engine that used an “aerospike” external expansion nozzle. The X-33 was canceled in 2001, along with the launch-on-demand, glide-to-landing NASA/Orbital Sciences X-34 vehicle.
The Buran (Snowstorm) Soviet space shuttle had its first orbital flight in November, 1988, on an Energia booster. It circled Earth twice between 247 and 256 kilometers above the surface before reentering and landing at Tyuratum. The French-European Hermes spaceplane project was conceived as a mini-shuttle, carrying four to six crew members and 4,500 kilograms of cargo into orbit atop an Ariane-5 booster. The project was canceled in 1992 but may have been replaced by the continuing European Space Agency (ESA) crew rescue vehicle project.
Hypersonic air-breathing vehicle designs including a scramjet engine were reported to have been tested in the 1990’s by Russia on top of a surface-to-air missile and by the Indian Space Research Organization using a solid rocket booster. The Japanese Hope-X space shuttle and the British horizontal takeoff and landing (HOTOL) concepts do not appear to have progressed beyond small-scale wind-tunnel models. As of mid-2001, there was no reusable hypersonic aerodynamic vehicle in operation other than NASA’s space shuttle.
Aircraft configurations optimized for aerodynamic flight at hypersonic speeds are generally thin and flat, with a highly swept fuselage and short wings. At hypersonic cruise, the upper surface of the vehicle stays essentially parallel to the flight direction, minimizing the disturbance to the flow there. The oblique shock formed under the vehicle stays very close to the slanted surface, providing a lifting cushion of high pressure. Such vehicles are called hypersonic waveriders. Waverider configurations generally exhibit rudders and elevator-aileron combinations (elevons) as primary control surfaces. Tip flaps improve lift-to-drag ratio and rudder effectiveness. Such vehicles are unstable in pitch, like many modern fighter planes, and require fly-by-wire, stability-augmented computer control. Concepts for reducing the shock drag and heating at the nose include the Russian idea of injecting ionized gas jets into the shock, and the American idea of ionizing the gas ahead using plasma or laser beams.
Miller, Jay. The X-Planes: X-1 to X-29. Specialty Press, 1983. A history of the experimental aircraft programs of NASA and its predecessor, the National Advisory Committee for Aeronautics (NACA), with good coverage of the X-15 program and many illustrations of successful and unsuccessful concepts. Smith, Terry. “The Dyna-Soar X-20: A Historical Overview.” Quest 3, no. 4 (Winter, 1994): 13-18. An article on the lifting-body program that has gained new relevance with the advent of CRVs and hypersonic guided weapons, developed by NASA, the U.S. Air Force, and the ESA.
Air Force, U.S.
Forces of flight
National Aeronautics and Space Administration
The X-15, shown here with pilot Neil Armstrong, was the first hypersonic aircraft developed by the United States.