After almost three decades of focusing almost exclusively on radar stealth, the US aerospace industry is exploring new technologies to achieve stealth in the visible spectrum.
Since famed Skunk Works engineer Denys Overholser developed the equations in the mid-1970s to design the Lockheed Have Blue stealth demonstrator, aircraft makers have played a cat-and-mouse game with technological advances in air defence systems.
The first objective has been focused on defeating detection by airborne and surface radars. So the state of the art in stealth technology evolved quickly from the faceted shape of the Lockheed Martin F-117 to the blended curves of the Northrop Grumman B-2 bomber to the Lockheed F-22's unique combination of high speed, agility and very low observability to radars.
© US Air Force/Tim Bicheno-Brown
Despite only one confirmed kill in the history of a radar stealthy aircraft - Serbia's shootdown of a US Air Force F-117 over Kosovo on 27 March 1999 - the focus on improving radar stealth continues behind a secret curtain of "black" programmes.
When Boeing unveiled the "stealthy" F-15 Silent Eagle in March, programme manager Brad Jones faced a barrage of sceptical questions from journalists. Government and industry officials from the rival Lockheed F-35 Joint Strike Fighter programme also have questioned Boeing's claims on the F-15SE. But stealth technology has "come a long way" since even the early 1990s, Jones said, although he declined to elaborate.
Achieving radar stealth is a function of shaping features that reflect radar waves away from the receiver, special materials and coatings that absorb radar energy and electronic systems that can jam or disrupt the radar from transmitting signals in the first place.
A recent lawsuit against Lockheed brought by a former F-22 stealth engineer provides a detailed glimpse into this specific application. Lockheed denies the claims by the former employee, Darrol Olsen, that the company knowingly supplied defective stealth coatings to the US Air Force during the development phase of the programme. But court records show that Lockheed's goal is to make the F-22 appear as the size of a "bumblebee" to a radar detector.
The F-22's contoured shape achieves the majority of the desired radar cross-section objective, court documents say. In addition, the F-22 also has numerous layers of coatings applied to the aircraft skin.
"The initial requirements were for three layers of coating to be stacked together in the following order," the court documents show. "The first coating is a primer designed to smooth and seal the surface of the skin and promote adhesion of the conductive coating.
"The second is a conductive coating consisting of silver flakes mixed with polyurethane materials and intended to conduct, dissipate and reflect the radar waves away. The third is the topcoat, which has other metallic materials and infrared properties to reduce heat and thus radar detection. In addition, other coatings such as [radar absorbent materials], gap fillers, and adhesives were applied on various specific areas of the aircraft."
Similar treatments in some form apply to all applications of radar stealth technology. The forms of such projects have proliferated from manned fighters and bombers, to recoverable unmanned aircraft and expendable missiles. During the last decade, radar stealth technology has also expanded from a US monopoly to an international pursuit, with the F-35 introducing very low observable stealth design and materials to the mainstream international fighter market.
© Rex Features
The only confirmed kill of a radar stealthy aircraft - Serbia's shootdown of a USAF F-117
The USAF has revealed plans to consider stealth technology for a next-generation airlifter and aerial tanker to possibly appear after 2020. Perhaps the only aerospace sector untouched by advanced stealth technology has been the rotorcraft market, since the cancellations in 2004 of the Sikorsky/Boeing RAH-66 Comanche and the Defense Advanced Research Project Agency's unmanned combat armed rotorcraft.
But how long radar stealth can continue to be an effective antidote to foiling air defence systems remains a serious and open question. As recently as 2006, an F-22 programme manager said that aircraft still had the advantage. Despite the advances of integrated air defence systems made in Russia and the former Eastern Bloc, the "physics are still against you" if you want to intercept an F-22 from the ground, the Lockheed official said.
But some signs suggest the primacy of radar-only stealth is starting to fade, albeit slowly and incrementally. Serdar Cadirci, a Turkish air force officer, published his master's thesis last March for the US naval postgraduate school, focusing on the current state of stealth and counter-stealth technologies.
"Stealth technology mostly focuses on defeating conventional monostatic radar systems that cover the microwave band," Cadirci writes. "Thus, the success of counter stealth endeavours is focused mostly on novel and unique air defence infrastructure configurations. These designs include either radars operating in other bands [high frequency or far above those of traditional microwave radars], specially designed receiver units and sensors able to detect low observables or new applied techniques and smart tactics."
However, as the sophistication of air defence systems rises to manage the current state of radar stealth technology, there will be an inevitable response from the attackers. The secrets of next-generation stealth concepts remain well-kept within the aerospace industry, but a clear trend is beginning to emerge. The next wave of progress in stealth technology will be to reduce an aircraft's signature simultaneously across all bands of the electromagnetic spectrum, from microwave waves to foil radars to visible light to deceive even the naked eye.
Lockheed is understood to have launched the pursuit of visual stealth technology for next-generation combat aircraft, to include a future version of the F-35.
Asked to clarify its studies, the company replies: "Lockheed Martin Aeronautics and our advanced development programmes are continuously working to mature technologies and capabilities to improve all aspects of stealth. All [Lockheed] stealth technologies and research in this area are sensitive and/or classified and are co-ordinated with the appropriate US government agencies."
Just as wing-mounted lamps were used in the Second World War to obscure Grumman TBM-3D Avenger dive-bombers from ground observation, the next generation of US combat aircraft are likely to feature techniques to make them invisible to a naked human eye - or electro-optical sensor.
In his thesis, Cadirci says the Avengers were modified in a crude attempt to eliminate the shadowy underwing. By thus decreasing the contrast between the aircraft and the sky, the Avengers could gain several precious seconds of invisibility as they approached surfaced German submarines. The technology was shelved after the Germans installed radars on submarines, rendering visual stealth irrelevant.
The USAF used a similar technique on McDonnell Douglas F-4s during the Vietnam War under the Compass Ghost programme, which succeeded in reducing their visibility to enemy MiG-21 fighters by 30%, Cadirci says.
But recent developments for sensors and lighting technology hold the promise of revolutionising this old idea. Sensors embedded in an aircraft skin can now precisely measure the brightness of the air as an aircraft moves through the sky.
In a modern update of Project Yehudi, Kevin Dowling, an engineer for Philips Electronics North America, has patented a concept to embed sensors and light emitting diode-based lighting into the skin of a stealthy aircraft.
Dowling says the technology exists in the lighting industry to sense the contrast between the colour of the sky as the aircraft passes through it, then adjust the colour radiated by LED panels to match it. From a certain distance, an observer would see only a shimmery, blurry object moving through the air, he says.
In recent weeks, a major West Coast-based US defence contractor has contacted Philips to develop the concept for naval applications, he says. Philips is not in a position to develop and sell a visual stealth system, Dowling adds, but would supply lighting components to a systems integrator.
More recently, a joint team of researchers at Duke University and the Southeast University of Nanjing unveiled a composite material whose properties are invisible to certain microwave bands, which is the first step in developing a structure capable of cloaking.
Source: Flight International