Stealth is not a single technology, but a group of aligned disciplines. Those disciplines, in turn, need to be brought together in a workable, affordable and practical vehicle.

Bill Sweetman | Aviation Week & Space Technology

Measuring Stealth

Predicting the radar cross-section (RCS)—a measure of a target’s apparent size on radar—of a complex shape over a wide range of aspect (viewing) angles or frequencies is so difficult that it was impractical before the development of high-powered computers. In 1975, Lockheed Skunk Works engineer Denys Overholser developed a software program called Echo that could accurately model a shape composed entirely of flat surfaces and straight edges. (Russian mathematician Pyotr Ufimtsev had earlier developed formulae to compute diffraction from edges.) Within a few years, Lockheed had developed the system to deal with simple curves, and Northrop was working with complex spiral curves. More powerful computers have made it possible to model small surface details and more complex shapes and account for materials and cavities.

The first outdoor RCS test ranges—the stealth equivalent of a wind tunnel—were built in the late 1950s, including one at Groom Lake in Nevada that supported the design of the Lockheed A-12 Oxcart. A full- or sub-scale model of the vehicle, or even a complete aircraft, is mounted on a pylon and illuminated by a radar that is far enough away to approximate a far-field condition. The space between the radar and target has to be flat and free from obstacles, so outdoor ranges often resemble runways when seen from the air. Indoor or “compact” ranges—large anechoic-walled chambers with theater-screen-size parabolic reflectors to create far-field conditions—became popular in the 1980s: the world’s largest is operated by France’s DGA defense materiel agency at Bruz, in Brittany (above), and can test a complete Dassault Rafale aircraft. With better modeling, the demand for RCS ranges has declined, and at least three U.S. ranges have shut down: the former McDonnell Douglas range at Gray Butte and Northrop Grumman’s Tejon Ranch facility, both in California, and Boeing’s range at Boardman, Oregon.

The Development Of Stealth And Counterstealth

Evolution Of Stealth

Stealth is not new. The modern era of stealth, which began in the mid-1970s with Darpa’s Have Blue project, is half as old as radar itself, and precursor projects date to the 1950s and earlier. That modern era saw stealth go from a controversial, risky new technology to the center of a plan to rebuild U.S. combat air forces with more than 2,000 new bombers, fighters and heavy attack aircraft by the early 2000s. But today there are barely 200 operational manned stealth aircraft.

The Secret Pioneers Of Stealth

Sep 11, 2015 Bill Sweetman | Aviation Week & Space Technology