UAVs bring their own set of stealth challenges
Four decades after the first truly stealthy air vehicles were secretly flown in the U.S., the technology has matured and proliferated. Many countries could now design a stealthy, small unmanned aerial vehicle, but since uncompromised stealth capabilities bring penalties, developers and operators are still wrestling with multiple trade-offs.
These issues were discussed at the Stealth Conference, conducted by military/ defense conference organizer Defence IQ (www.defenceiq.com) and held recently in London. And, although BAE Systems Advanced Technology Centre (exhibiting here in Singapore at Stand No. A721) made a presentation on using new radar techniques to detect targets with a low radar cross section (RCS), Dr. Bob Haffa, director of the Northrop Grumman Analysis Center (Chalet F1/2), voiced the consensus that “the advantage remains with the stealthy.”
The new generation of unmanned combat air vehicles (UCAVs) emerging in the U.S. and Europe promise a step-reduction in RCS, according to reports. Shape is the key to these new designs, and Jan Ritter of EADS Military Aircraft (Stand No. A701) described how his company has been designing its yet-to-be-unveiled UCAV. He also reported how EADS has amassed significant expertise in computational electromagnetics (CEM), “which is far more difficult to master than computational aerodynamics.”
Although some software used in RCS modeling is commercially available, it is no substitute for in-house development, unless stealth is not your core business, Ritter said. As ever, the techniques to counter low- versus high-frequency radars are different, and he noted that this also applies to the CEM process. “No single mathematical model is available to handle all details in the interesting frequency domains,” he added.
But what about visual stealth? The first generation of stealth warplanes relies on darkness for cover, but the latest concept of operations revealed by the U.S. for its J-UCAS program includes up-to-24-hour operation over enemy territory, to plug what Northrop Grumman’s Haffa described as “the emerging persistent surveillance-attack gap.” Some industry observers speculate that new techniques for visual camouflage are under development, such as a Boeing-developed fiberoptic fabric coating from which light can be emitted on a directional basis.
Another technique that apparently remains classified in the West is so-called “plasma stealth” in which the electrical field surrounding part or the whole of an air vehicle is altered to reduce the RCS. The Russians reportedly have been conducting research in this field recently.
But a retired aerospace engineer has described to Aviation International News an experimental program which–40 years ago–protected the inlets of Lockheed’s SR-71 Blackbird spyplane and proved to work in flight. At the time, the space and power requirements were too great for operational deployment. More recently, there has been speculation that the leading edge of the B-2’s wing is so equipped.
Another engineer from EADS, Jurgen Kruse, discussed the interplay between passive stealth and active ECM. He said a reduction in the RCS of a non-stealthy warplane, such as the F-16 or the Tornado, will enhance the effectiveness of the ECM systems onboard those aircraft. “There is a linear dependency between RCS and jamming power,” he noted.
A “hitlist” of the radar “hotspots” on a non-stealthy platform that are most worthy of treatment can be developed by accurate measurement followed by modeling. Kruse described how EADS mounted an operational Tornado on RaSigma3, the company’s full-scale radar signature measurement facility at Manching in southern Germany. It can handle aircraft weighing up to 70 tons. The new Airbus A400M military transport is scheduled for measurement there.
On a fighter, the most promising areas for treatment are cockpits, radomes, antennas and engine inlets. Kruse said that such treatment can yield overall radar cross section reductions of up to 20dB. He also noted that canopies with an optically transparent coating of indium tin oxide suppress reflections from within the cockpit. Radomes made with frequency-selective layers will reflect radar energy at threat frequencies, while remaining transparent for the working frequency of the antenna within, he said.
Kruse said radar absorbent material can be added to engine inlets and elsewhere, such as leading edges, weapons and pylons, and that a typical thermoplastic broadband radar absorbent material will be 3mm thick and cover 6 to 16 GHz. Radar absorbent material also comes in “sandwich” form with a foam or honeycomb core, he said.
Julian Barber of Thales UK (Chalet F3) supported the efforts to measure existing equipment for stealth. “Know your signatures and let them drive your mission,” he advised. But he warned that retrospective stealth is costly. Decoys will often prove more cost-effective, he said, and he described his company’s recent work on small active and passive retro-reflectors.
Barber and others acknowledged the role of good mission planning, using tactical routing to evade known threats. U.S. Air Force F-117 pilot Major Doug Downey said this procedure normally takes 12 hours for a Stealth Fighter sortie. Formerly, a large mission planning facility accompanied each F-117 deployment, but now it’s all done back at home base in New Mexico using satellite communications to and from the theater of operations.
A spokesman for Lockheed Martin said the F-35 Joint Strike Fighter will carry a multispectral, all-aspect model of its signatures, for comparison with, and evasion of, enemy threats. However, such a system might not be exportable, leading to further problems over technology transfer between the U.S. and international customers for the F-35.