Hypersonic Programs

Hypersonic flight is achieved at speeds at or above Mach 5, or five times the speed of sound. In the 1940s and 1950s the goal of aeronautical research was to design and build aircraft that could fly at that speed and reach altitudes at the edge of space.

Spacecraft Re-entry

When orbiting spacecraft reenter Earth's atmosphere, they are traveling at many times the speed of sound and they generate high temperatures because of friction with the air. Ballistic re-entry vehicles such as the Gemini and Apollo capsules have a thick heat shield that slows the spacecraft and dissipates heat. Aircraft designers have always considered this solution practical but primitive. They would prefer to build a spacecraft that could act like an aircraft as it reentered the atmosphere, flying through the atmosphere to a safe landing.

The X Planes

High-speed aircraft design began with rocket -powered craft. Many rocket-powered aircraft built in the 1940s and 1950s carried the X, or experimental, designation, beginning with the bullet-shaped Bell X-1, which on October 14, 1947, became the first airplane to break the sound barrier. The rocket-powered D-558 2 set an altitude record of 25,377 meters (83,235 feet) on August 21, 1953, and a speed record on November 20, 1953, when it became the first aircraft to reach Mach 2. The Bell X-2 reached a speed of Mach 3.2, but the aircraft broke up in flight, killing its pilot. Before its last flight the X-2 set an altitude record of 38,476 meters (126,200) feet on September 7, 1956.

The X-15

Flight at altitudes of 76,220 meters (250,000 feet) and above required an aircraft that was also a spacecraft and could maneuver in a near vacuum when normal control surfaces were useless. This type of aircraft required tremendous advances in aeronautical technology. Because the plane had to operate in near-vacuum conditions, it also needed advanced life support systems. The North American X-15 rocket plane was built to achieve these goals.

The X-15 was a joint program of the National Aeronautics and Space Administration (NASA), the U.S. Air Force, the U.S. Navy, and North American Aviation. This aircraft had an internal frame of titanium and a skin made from an alloy of chrome and nickel. The X-15 set many speed records, reaching Mach 6.7 on October 3, 1967. It also set many altitude records, reaching 354,200 feet (67 miles or 107 kilometers) on August 22, 1963. That achievement qualified the pilot for astronaut wings.

The X-15 was launched from under the wing of a converted B-52B Stratofortress. For high-speed flights the X-15 was flown as a conventional airplane, using aerodynamic controls. For high-altitude flights the plane flew at a steep angle until the fuel was exhausted and then coasted up for 2 or 3 more minutes.

Lifting Bodies

A lifting body is an aircraft that has a high lift-to-drag ratio. Usually, the wings are very short or nonexistent and the shape of the body of the aircraft provides lift. The impetus for the design of a lifting body came from the desire to develop a reusable launch vehicle (RLV). Such a vehicle would have to be able to operate in space and then reenter the atmosphere and operate at hypersonic, supersonic, and subsonic speeds, eventually landing on a runway as a conventional airplane does.

The first attempts to develop a controlled, recoverable spacecraft capable of landing at airfields led to the Air Force X-20 in the late 1950s. The X-20 was to be a piloted glider that could also carry a small payload and would be boosted into orbit by a Titan rocket. The X-20 would carry one pilot into orbit, complete its mission, and glide back to a runway landing. Rising costs and competition from NASA's Gemini program led to the cancellation of the X-20 in 1963.

Research and testing continued in other U.S. Air Force projects, such as the Aerothermodynamic Elastic Structural Systems Environment Tests (ASSET) and the Precision Recovery Including Maneuvering Entry (PRIME). ASSET was started in 1960 to test heat-resistant materials and investigate high-speed re-entry and glide characteristics. PRIME was started in 1966 to test unpiloted lifting bodies flown into space by Atlas rockets. The U.S. Air Force also investigated piloted lifting bodies dropped from high altitudes, proving that pilots could fly the craft to a safe landing. This research was extremely valuable in the development of the space shuttle orbiter.

The Future

The high cost of launching satellites into Earth orbit led NASA to invest in a prototype launch vehicle called the X-33. The prototype was intended to lead to a lightweight, fully reusable space plane. NASA later withdrew funding for the project, leaving it about 75 percent complete. Many of the target goals of the project had been met, including engine tests. Companies have subsequently competed for financing to design various components of RLVs under NASA's Space Launch Initiative.

There have also been joint efforts to build a hypersonic aircraft for commercial purposes. On April 18, 2001, Orbital Sciences Corporation and NASA announced plans for the development of a hypersonic test vehicle dubbed the X-43A or Hyper-X. This vehicle could be launched by a small rocket. In flight, it is expected that the plane will be powered by an engine using compressed atmospheric oxygen mixed with fuel in a "scramjet" engine. Test missions would originate from Edwards Air Force Base and fly off the coast of California. The launch vehicle and scramjet research vehicle "stack" will be air launched from NASA's B-52B carrier aircraft, the same one used for the X-15.

see also Getting to Space Cheaply (volume 1); Heat Shields (volume 3); Launch Management (volume 3); Launch Vehicles, Reusable (volume 1); Reusable Launch Vehicles (volume 4).

Elliot Richmond

Bibliography

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Bonney, Walter T. "High-Speed Research Airplanes." Scientific American 189 (October 1953):35-41.

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Dryden, Hugh L. "Fact-Finding for Tomorrow's Planes." National Geographic 104, no. 6 (December 1953):757-780.

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