A supersonic aircraft flies faster than Mach 1, or the speed of sound, whereas a hypersonic aircraft is a plane capable of flying at Mach 5, or five times the speed of sound. At sea-level atmospheric pressure, with air temperatures of 59°F (15°C), the speed of sound is about 760 miles per hour (1,225 kph). Hypersonic flight has been possible since the late 1950s, but before it can become practical, designers will have to address some of the physical challenges associated with ultra-high-speed flight.
The X–15. On October 14, 1947, Major Charles E. "Chuck" Yeager broke the sound barrier in a Bell XS–1 rocket-powered research plane. Five years later, in 1952, officials at the National Advisory Committee for Aeronautics (NACA) set out to develop a craft capable of hypersonic flight. That craft was the X–15, designed by North American Aviation. The X–15 debuted on October 15, 1958, and between June 8, 1959, and October 24, 1968, more than a dozen pilots in three X–15s flew 199 missions, successively passing Mach 3 (1960), Mach 4 and 5 (1961), and Mach 6 (1963).
Had the X–15 program continued, it might have provided the model, not only for hypersonic flight on Earth, but also for space flight. However, a number of circumstances brought an end to the program. One was a change in leadership as NACA, founded in 1917, gave way in 1958 to the National Aeronautics and Space Administration (NASA). Another change was the urgent political goal of beating the Soviets in the space race after the surprise launch of the Sputnik satellite in 1958. Desirous of putting the first man on the Moon, U.S. leaders bypassed the X–15 flight model in favor of rockets.
The X–15 was also challenged by the physical constraints of hypersonic flight. On October 3, 1967, pilot Peter Knight reached Mach 6.7, and nearly incinerated the tail of his craft. Six weeks later, on November 15, the inflight breakup of the third X–15 claimed the life of pilot Mike Adams. The X–15 made its final flight on October 24, 1968.
Other hypersonic studies. By the time the X–15 ceased operation, the United States had already developed two other extraordinary aircraft, the U2 and the SR–71. The latter, introduced in December 1964, was capable of attaining Mach 3—a speed that, while fast, was not hypersonic. During the early 1990s, the SR–71 was taken off-line for a few years due to the high cost of keeping it aloft, and this hiatus fanned reports that the U.S. Air Force (USAF) and Department of Defense (DOD) were developing a replacement.
Since 1979, there had been talk of this putative SR-–71 successor, identified as "Aurora" by a code name accidentally included in a 1985 Pentagon budget request. However, the USAF and DOD repeatedly denied that they were developing a replacement for the SR–71, which went back into commission in 1995.
Research on hypersonic flight has continued, however. Aerospace engineers have promoted the concept of the HyperSoar hypersonic Global Range Recce/Strike Aircraft, which could attain speeds up to Mach 10 and carry a payload nearly twice as large as that of a subsonic craft. Flying at an altitude of approximately 130,000 feet (39,624m), it would skip across the top layer of Earth's atmosphere like a rock skipping across the surface of water.
In June 2001, NASA tested the X–43A, a hypersonic craft with a special engine called a scramjet, which brought together features of both a conventional turbojet and a
rocket. It was to be launched by a Pegasus rocket, but unfortunately the rocket failed during the test flight. NASA has continued to work on hypersonic craft, but before such planes can be made operational, engineers will have to develop a means of controlling temperatures so as to keep the craft from bursting into flame as it reenters the atmosphere.
█ FURTHER READING:
Godwin, Robert. X-15: The NASA Mission Reports, Incorporating Files from the USAF. Burlington, Ontario: Apogee Books, 2000.
Henne, P. A. Applied Computational Aerodynamics. Washington, D.C.: American Institute of Aeronautics and Astronautics, 1990.
Noor, Ahmed Khairy, and Samuel L. Venneri. Future Aeronautical and Space Systems. Reston, VA: American Institute of Aeronautics and Astronautics, 1997.
Grier, Peter. "Hypersonic Aircraft Test Fails." Air Force Magazine 84, no. 8 (August 2001): 17.
Leary, Warren E. "Test of Revolutionary Jet Promises to Transform Flight." New York Times. (May 22, 2001): F4.
Aurora/Senior Citizen. Federation of American Scientists. < http://www.fas.org/irp/mystery/aurora.htm > (March 4,2003).
HyperSoar Hypersonic Global Range Recce/Strike Aircraft. Federation of American Scientists. < http://www.fas.org/man/dod-101/sys/ac/hypersoar.htm > (March 4, 2003).
X–15—Hypersonic Research at the Edge of Space. National Aeronautics and Space Administration. < http://www.hq.nasa.gov/office/pao/History/x15/cover.html > (March 4, 2003).