An F-117 Nighthawk taxis down the runway at Aviano Air Base, Italy. The stealth fighters are deployed as part of the 31st Air Expeditionary Wing, supporting NATO's Operation Allied Force. (Photo by Senior Airman Mitch Fuqua)
All four Air Force Flight Test Center F-117s fly in formation over the Mojave Desert.
The first B-2 rolled out of its hangar at Air Force Plant 42, Palmdale, Calif., in November 1988. Its first flight was July 17, 1989
Stealth refers to the act of trying to hide or evade detection. It is not so much a technology as a concept that incorporates a broad series of technologies and design features. As a concept, stealth is nothing new, having been invented by the first caveman to cover himself with leaves so that he could sneak up on a dim-witted antelope. Soldiers hid behind trees. Submarines hid under the waves to sneak up on ships, and it was submarines that first used special coatings on their periscopes to avoid radar detection during World War II.
For airplanes, stealth first meant hiding from radar. After World War II, various aircraft designers and strategists recognized the need to design planes that did not have large radar signatures (a radar signature is how big the airplane appears on radar from a specific angle and distance; it is often referred to as the "radar cross section"). But their ability to hide from radar was limited for many years for several reasons. One major limitation was aircraft designers' inability to determine exactly how radar reflected off an airplane.
In the nineteenth century, Scottish physicist James Clerk Maxwell developed a series of mathematical formulas to predict how electromagnetic radiation would scatter when reflected from a specific geometric shape. His equations were later refined by the German scientist, Arnold Johannes Sommerfield. But for a long time, even after aircraft designers attempted to reduce radar signatures for aircraft like the U-2 and A-12 OXCART in the late 1950s, the biggest obstacle to success was the lack of theoretical models of how radar reflected off a surface. In the 1960s, Russian scientist Pyotr Ufimtsev began developing equations for predicting the reflection of electromagnetic waves from simple two-dimensional shapes. His work was regularly collected and translated into English and provided to U.S. scientists. By the early 1970s, a few U.S. scientists, mathematicians, and aircraft designers began to realize that it was possible to use these theories to design aircraft with substantially reduced radar signatures. Lockheed Aircraft, working under a contract to the Defense Advanced Research Projects Agency, soon began development of the F-117 stealth fighter.
Aircraft designers generally describe an airplane's radar cross section in terms of "decibel square meters," or dBsm. This is an analogy that compares the plane's radar reflectivity to the radar reflectivity of an aluminum sphere of a certain size. The B-2 reportedly has a radar signature of an aluminum marble. The F-22 Raptor interceptor is roughly the same, and the F-117 is only slightly less stealthy. The newer Joint Strike Fighter has the signature of an aluminum golf ball. The older B-1 bomber, designed during the 1970s and 1980s, is about the size of a three-foot (one-meter)-diameter sphere, whereas the 1950s-era B-52 Stratofortress, a monstrously non-stealthy airplane, has an enormous radar cross section of a 170-foot (52-meter)-diameter sphere. The size of an aircraft has little relationship to its radar cross section, but its shape certainly does.
When designing a stealth aircraft, engineers try to either absorb radar energy or deflect it away from the radar receiver. They absorb it with special materials or "trap" it within the airplane's structure. They deflect it by carefully designing the structure. Certain parts of an aircraft structure are notorious for reflecting radar energy. Cockpits, for instance, bounce radar straight back to the source, so they must be carefully designed and coated with special materials. Engine inlets are often designed so that the radar energy cannot go straight into them and reach the face of the turbine blades. Instead, the radar energy is bounced back and forth inside the inlet. Tail surfaces are sharply angled, rather than vertical, so that they bounce radar in a different direction.
Stealth does not always refer to radar. Reducing an aircraft's heat signature is also important. This is usually done by channeling the engine exhaust through long tubes and mixing it with cooler outside air.
Because radar can still detect very small radar signatures, stealth aircraft are also operated in a careful manner and assisted by other aircraft. For instance, they try to avoid certain radars and operate in conjunction with aircraft designed to jam enemy radar. They try to hide in the electromagnetic "noise" of the battlefield.
While stealth was a major effort of aircraft designers of the 1980s and 1990s, the widespread availability of powerful computers and knowledge of stealth techniques has meant that it is no longer difficult to design an aircraft with some stealth characteristics, although achieving the degree of stealth incorporated into the F-117 or the B-2 is still difficult. Today, the research emphasis has shifted to developing various systems that can be used with a stealth aircraft, such as radar and weapons that will not be easily detected. Naturally, there is also an effort among missile and radar designers to develop systems that can detect stealth aircraft. Low-frequency radar will spot virtually any stealthy aircraft but is bad at determining its exact location. Communications networks enabling a defensive system to combine information and locate a target also connect these and other radars. Other systems attempt to pick up radio and television signals that may bounce off a stealthy airplane.
The development of stealthy airplanes teaches several important lessons about technology. The first is that often many different technologies must be combined to achieve a desired outcome. An advance in one field, such as materials or aerodynamics, must be accompanied by advances in other fields, such as computing or electromagnetic theory. The second lesson is that sometimes trial and error techniques are insufficient and advances in mathematical theory are necessary in order to achieve significant advances. Finally, stealth teaches the lesson that technology is never static - a "stealth breakthrough" may only last for a few years before an adversary finds a means of countering it.
--Dwayne A. Day
Sources and Further Reading:
Aronstein, David C., and Piccirillo, Albert C. Have Blue and the F-117A: Evolution of the Stealth Fighter. Washington, DC: American Institute of Aeronautics and Astronautics, 1997.
Boyne, Walter J. Beyond the Horizons: The Lockheed Story. New York: St. Martin's Press, 1998.
Crickmore, Paul, and Crickmore, Allison. "F-117 Nighthawk Story – The Early Years." Air Forces Monthly, August 1999, 62-69.
Fulghum, David A. "Stealth Retains Value, But Its Monopoly Wanes." Aviation Week & Space Technology, February 5, 2001, 53-55.
Fulghum, David A. "Counterstealth Tackles U.S. Aerial Dominance." Aviation Week & Space Technology, February 5, 2001, 55-57.
Lake, Jon. "Stealth: Staying One Step Ahead." Combat Aircraft, Oct.-Nov. 1999, 380-390.
Rich, Ben and Janos, Leo. Skunk Works: A Personal Memoir of My Years at Lockheed. Boston: Little Brown and Co., 1996.
Sweetman, Bill. Stealth Aircraft. Osceola, Washington: Motorbooks International, 1986.