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Russian stealth researchers have developed materials and techniques that can reduce the head-on radar cross-section (RCS) of a Sukhoi Su-35 fighter by an order of magnitude, halving the range at which hostile radars can detect it. The research group working with Sukhoi, but based at the Institute for Theoretical and Applied Electromagnetics (ITAE) at the Russian Academy of Sciences in Moscow has performed more than 100 hours of testing on a reduced RCS Su-35 and has also experimented with the use of plasmas ionized gases to reduce RCS.
US and European aircraft manufacturers have used specially developed materials to reduce the RCS of basically non-stealthy aircraft for many years. Notable examples include the Have Glass and Have Glass II modifications to the F-16. However, Russian work in this area was undisclosed until ITAE researchers presented a paper to a conference on stealth in London in late October 2003, which was organized by the International Quality and Productivity Centre.
According to the ITAE presentation, Russian researchers have developed mathematical tools that can calculate scattering from complex configurations, such as an Su-35 carrying a full external missile load, by breaking them down into small facets and adding the effects of edge waves and surface currents. The antennas are modelled separately and then are added to the entire RCS picture.
"A problem of huge size" is how the researchers describe the Su-35 inlet, with a straight duct that provides direct visibility to the entire face of the engine compressor. The basic solution has been to apply ferro-magnetic radar absorbent material (RAM) to the compressor face and to the inlet duct walls, but this involves challenges. The researchers note: the material cannot be allowed to constrict airflow or impede the operation of anti-icing systems and must withstand high-speed airflows and temperatures up to 200°C.
The ITAE team has developed and tested coating materials that meet these standards. A layer of RAM between 0.7mm and 1.4mm thick is applied to the ducts and a 0.5mm coating is applied to the front stages of the low-pressure compressor, using a robotic spray system. The result is a 10-15dB reduction in the RCS contribution from the inlets.
The modified Su-35 also has a treated cockpit canopy which reflects radar waves, concealing the high RCS contribution from metal components in the cockpit. ITAE has developed a plasma-deposition process to deposit alternating layers of metallic and polymer materials, creating a coating that blocks radio-frequency waves, is resistant to cracking and crazing and does not trap solar heat in the cockpit. The plasma-coating process is then carried out robotically in a 22 m3 vacuum chamber.
ITAE and its partners have also developed plasma-type technology for applying ceramic coatings to the exhaust and afterburner. The conference video also showed the use of hand-held sprays to apply RAM to R-27 air-to-air missiles.
ITAE has studied at least three techniques for reducing the RCS contribution of the radar antenna, in addition to the simplest method of deflecting the antenna upwards and treating or shrouding other components. One of these is to design a radome that can be switched from RF-transparent to RF-reflective.
The interior of the radome would be coated with a cadmium sulphide or cadmium selenide thin-film semiconductor material which changes conductivity when illuminated with visible or ultra-violet light. However, the problem of making such a film has not been solved.
A second technique that is also described in Western literature is to place a frequency selective surface screen in front of the antenna. This is a foil-like metal screen etched with small apertures which allow RF energy to pass within a narrow waveband, corresponding to the radar's own operating frequency. This reduces RCS, according to ITAE, but at the expense of radar performance.
However, ITAE has flight-tested a more exotic technology: the use of a low-temperature plasma screen in front of the radar antenna. The screen hardware is mounted in front of the antenna and is transparent to the radar when switched off. When activated, the screen absorbs some incoming radar energy and reflects the rest in safe directions over all RF bands lower than the frequency of the plasma cloud. It switches on and off in tens of microseconds, according to ITAE.
In principle, this is the same as the 'plasma stealth system that was reportedly developed by the Keldysh Scientific Research Center (also part of the Academy) in 1999.
At the time, it was claimed that the system, using a 100kg generator, could reduce the RCS of any aircraft by two orders of magnitude, or 20dB. ITAE has not attempted to develop a whole-aircraft system, but researchers expressed the view that it would be difficult to apply except to a high-altitude, low-airspeed aircraft because the airstream would dissipate the plasma faster than it could be generated.
The ITAE paper also gave some indications of the direction of stealth technology for future stealth aircraft. Test facilities include large compact indoor RCS ranges for large-scale models and outdoor ground-level ranges with short pylons that can be used to test full-size aircraft (rather than the models used for US pylon tests).
In future designs, one emphasis is on large, complex skin panels, reducing the number of gaps and mechanical fasteners in the skin.
