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Stealthy Chinese J-20 Vulnerable
This article first appeared in Aviation Week & Space Technology.
China's newest combat aircraft prototype, the J-20, will require an intense development program if it is going to catch up with fast-moving anti-stealth advances.
In fact, anti-stealth will bring into question all stealth designs: How much invulnerability will current low-observability techniques offer as air defense systems adopt larger and more powerful active, electronically scanned array (AESA) radars? From the early days of AESA development, a key goal was to build a radar that could detect very small objects -- such as a cruise missile at a distance great enough to target and shoot it down -- or a larger object like a fighter with a very low-observable treatment.
Airborne detection of stealth aircraft may already be an operational capability. In a series of tests at Edwards AFB, Calif., in 2009, Lockheed Martin's CATbird avionics testbed -- a Boeing 737 that carries the F-35 Joint Strike Fighter's entire avionics system -- engaged a mixed force of F-22s and Boeing F-15s and was able to locate and jam F-22 radars, according to researchers. Raytheon's family of X-band airborne AESA radar -- in particular, those on upgraded F-15Cs stationed in Okinawa -- can detect small, low-signature cruise missiles.
Moreover, Northrop Grumman's lower-frequency, L-band AESA radar on Australia's Wedgetail airborne early warning and control aircraft is larger and potentially more capable of detecting stealth aircraft at longer ranges.
Lockheed Martin also hinted at a JSF anti-stealth capability in 2009 in a reference to combat with sophisticated, foreign aircraft. "The F-35's avionics include onboard sensors that will enable pilots to strike fixed or moving ground targets in high-threat environments, day or night, in any weather, while simultaneously targeting and eliminating advanced airborne threats," said Dan Crowley, then-executive vice president and F-35 program general manager.
Better images emerging from China point clearly to the J-20's use of stealth technology, but major uncertainties and questions remain unresolved.
The overall shape resembles that of the F-35 and F-22, which have a single "chine line" uniting the forebody, upper inlet lips, and wing and canard edges with a curved surface above that line and flat, canted body surfaces below it. The wing and canard edges are aligned: The wing and canard leading edges are parallel and the trailing edge of each canard is aligned with the opposite wing's trailing edge. The same basic philosophy also has been adopted in British, Swedish and Japanese studies for stealth fighters.
The aim in all cases is to endow a practical, agile fighter configuration with a "bow-tie" radar signature, with the smallest signature around the nose and the greatest (still much lower than that of a conventional aircraft with curved or vertical-slab sides) to the side. The fighter's mission planning system, using a database of known radar locations, then derives a "blue line" track that weaves between radars and avoids exposing the side-on signature to those radars more than transiently.
The "diverterless" supersonic inlet avoids a signature problem caused by a conventional boundary layer diverter plate. For example, the F-22 has a conventional inlet, which is likely to require extensive radar absorbent material (RAM) treatment.
The biggest uncertainty about the Chinese design concerns the engine exhausts, which as seen on the prototype are likely to cause a radar cross-section (RCS) peak from the rear aspect. One possibility is that a stealthier two-dimensional nozzle will be integrated later in the program; however, the nozzles on the current aircraft show some signs of RCS-reducing sawtooth treatment, suggesting that the People's Liberation Army has accepted a rear-aspect RCS penalty rather than the much greater weight and complexity of 2D nozzles.
Other features are less clear. Stealth development has been dogged by detail-design challenges. All the antennas on the aircraft have to be flush with the skin and covered with surfaces that retain stealth properties while being transparent in a specific frequency. Maintainability becomes a complex tradeoff: Some systems requiring frequent attention will be accessed via landing gear and weapon bays, and others by latched and actuated doors that can open and close without affecting RCS -- but the latter involves a weight penalty.
Perhaps the toughest hurdle is managing radio-frequency surface currents over the skin. Early stealth designs used heavy, maintenance-intensive RAM. The F-22 introduced a much lighter surface treatment, but it has proven unexpectedly difficult to maintain, causing corrosion issues. Lockheed Martin now asserts that the F-35 will be robust and affordable to maintain in service, with a combination of a high-toughness, sprayed-on topcoat and a conductive layer cured into composite skin panels.
The Chengdu J-20 design has struck many analysts and observers as familiar and somewhat different from the F-22, F-35 or Sukhoi T-50.
"The J-20 is reminiscent of the Russian MiG-1.42 both in terms of planform and also with regard to the rear fuselage configuration," says Douglas Barrie, senior fellow for military aerospace at London's International Institute for Strategic Studies. "The most obvious difference is the greater forward fuselage shaping as the basis for low-observable characteristics, along with the different engine intake configuration. The MiG program was canceled by the Russian government around 1997," he notes. However, the similarity to the MiG concept may suggest some collusion with the Russian aviation industry.
The J-20 made its first flight shortly before 1 p.m. Beijing time on Jan. 11. The flight ended three weeks of anticipation that began in late December when the new design started taxi tests.
The discussion about the program will now shift to the aircraft's mission (fighter or, more likely, long-range strike), sensors (strike missions would require a high-resolution, long-range radar) and communications (which would demand high-speed data links and sophisticated integration).
Conventional radars have only one-half to one-third of the range of an AESA radar. Moreover, the movement of a conventional, mechanically scanned radar antenna provides a tell-tale glint of radio-frequency reflections to enemy aircraft with advanced radars. Such reflections undercut the effectiveness of a stealth airframe. China is known to be pursuing newer radar technology.
"It's too early to tell the true status of the Chinese AESA program," says a Washington-based intelligence official. "We've seen lots of press and air show information on the program, but that doesn't automatically translate into a robust development or give us an accurate look at where [China] is as far as fielding one anytime soon.
"Like the [high-performance] engine, it'll be a challenge to take the step from older radars to one designed for a fifth-generation fighter," he says. "Again, though, the J-20 is just the first or second -- depending on whom you believe -- prototype in a very long development program."
If the Chinese conduct a few months of flight tests and there are no more aircraft involved in the program, this might indicate that the J-20 is a proof-of-concept or technical demonstrator. If there are several aircraft eventually, a prototype program would be a more likely conclusion.
The flight occurred during a visit to China by U.S. Defense Secretary Robert Gates, who says Chinese President Hu Jintao confirmed the event to him in talks. However, Gates still believes the U.S. will retain a preponderance of stealth fighters through 2025.
Photo Credit: Internet via Airpower Australia