WASHINGTON — STEALTH," the ability to avoid radar, will surely be one of the most important military technologies of coming decades. But it's not a magic cloak rendering aircraft undetectable to adversarial eyes.The art and science of stealth aims at making a plane's radar return small enough to create chinks in the fence of overlapping radar defenses, according to engineers who work on stealth projects. By the time a stealth plane is spotted, it should be too late to do anything about it. "We've never claimed to be invisible," said Paul Martin of Lockheed, program manager for the F-117 Stealth fighter. The wedge-shaped black Lockheed F-117 became famous during the Gulf war for its ability to attack highly defended targets with impunity. In recent weeks, however, the Air Force has admitted that a few F-117 missions received backup support from EF-111 electronic warfare aircraft, which actively jam radars with powerful bursts of energy. At a recent symposium on stealth technology, Lockheed officials claimed the F-117's radar-avoiding attributes are extremely effective against all electronic wavelengths of interest, typically 100 megahertz up to 20 gigahertz. This covers everything from early-warning radars to air-to-air missiles. The plane has to be in full stealth mode, however, with various antennas retracted, for maximum effectiveness. If by chance it flies too close to a ground radar that's suddenly turned on, the plane could be detected. "I have no doubt good radar systems can pick us up under a lot of conditions," said Paul Martin. The F-117 suffered no hits or losses during the Gulf war. Lockheed claimed that its performance proved the value of stealth technology for all military aircraft of the future. Although the F-117 and its larger cousin, the troubled B-2 Stealth bomber program, have been unclassified for only a relatively short time, the basic geometry of radar avoidance is well-known.
Enormous electronic energy A flat plate standing upright relative to the incoming radar wave reflects enormous amounts of electronic energy; tilt it backwards by 30 degrees, and its radar image drops by a factor of 1,000. Rotate the plate so that, to the radar wave, it appears to be a diamond shape standing on a point, and the reflected image drops by further large factors. The right combination of tilt and twist can reduce the radar cross-section of a single plate by 10 million. Thus, the design of the F-117, with its many facets of flat, slanting diamonds arranged for the best combination of radar avoidance from many angles. Increases in computing power allow newer designs, such as the B-2 and Lockheed's F-22 fighter, to be curved, reducing any resemblance to Darth Vadar's helmet. A curved design "doesn't really reduce radar cross-section, but allows better aerodynamic performance with the same radar cross-section," said Alan Brown, Lockheed director of engineering.
Shape easiest design aspect But overall shape is only the first, and perhaps easiest, aspect of stealth design. Reducing radar return from jet engine intakes and ducts is also crucial, and not easy. These large boxes of metal are unavoidable in modern aircraft, and so electronically "hot" that for all practical purposes the entire radar return of the F-15 fighter comes from its engine inlets, said Brown. For F-117 intakes, radar return is muffled by covering them with what one engineer called a "high-tech tennis racket" of strung steel, whose openings are precisely calibrated to absorb electromagnetic waves. For the engine ducts themselves, the interior is coated with radar-absorbent material so that as few waves as possible that make it in are reflected out. Curving the ducts causes the straight-traveling waves to bounce more, and thus be absorbed more. The absorbent coatings themselves are the third major component of stealth design, and that most fraught with technical jargon such as "attenuation by cancellation," and so forth. To oversimplify, stealth coatings are basically plastic-like materials that allow electromagnetic waves to enter. Once the waves have penetrated, they are scattered and absorbed by lots of tiny little ferromagnetic bits embedded in the coating. Coatings cover the F-117. "They're electro-magnetic shock absorbers," said Alan Brown. They're even used on the canopy - pilot's heads would otherwise be a major source of radar return. Stealth designers even worry about the radar return from a jet's wake. Normally, this isn't a problem, but when the pilot sticks throttles full forward and a plane goes on afterburner, extra ion density in the wake makes it show up strongly on radar. Perhaps this is one reason the F-117 is a relatively slow, sub-sonic airplane. The new F-22 fighter will be the first Air Force plane able to cruise at supersonic speeds without afterburners, apparently eliminating this worry.