Stealth technology, also termed “low–observable” technology, is a set of techniques that render military vehicles, mostly aircraft, hard to observe. Because RADAR—an acronym for RA dio D etection A nd R anging—is the primary detection technology for aircraft, most stealth technologies are directed at suppressing RADAR returns from aircraft, but stealth technology minimizes other “observables” as well, that is, energy emissions that of any kind that might be observed by an opponent. Stealth technology is deployed today on several types of aircraft and a few surface ships. Counter–stealth technologies are also under continuous development.
Development of stealth technology for aircraft began before World War I (1914–1918). Because RADAR had not been invented, visibility was the sole concern, and the goal was to create aircraft that were hard to see. In 1912, German designers produced a largely transparent monoplane; its wings and fuse–lage were covered by a transparent material derived from cellulose, the basis of movie film, rather than the opaque canvas standard in that era. Interior struts and other parts were painted with light colors to further reduce visibility. The plane was effectively invisible from the ground when flow at 900 ft (274 m) or higher, and faintly visible at lower altitudes. Several transparent German aircraft saw combat during World War I, and Soviet aircraft designers attempted the design of transparent aircraft in the 1930s.
With the invention of RADAR during World War II (1939–1945), stealth became both more needful and more feasible: more needful because RADAR was highly effective at detecting aircraft, and would soon be adapted to guiding antiaircraft missiles and gunnery at them, yet more feasible because to be RADAR–stealthy an aircraft did need to be not be completely transparent to radio waves; it could absorb or deflect them.
During World War II, Germany coated the snorkels of its submarines with RADAR–absorbent paint to making them less visible to RADARs carried by Allied antisubmarine aircraft. In 1945 the United States developed a RADAR–absorbent paint containing iron. It was capable of making an airplane less RADAR–reflective, but was heavy; several coats of the material, known as MX–410, could make an aircraft unwieldy or even too heavy to fly. However, stealth development continued throughout the postwar years. In the mid–1960s, the U.S. built a high–altitude reconnaissance aircraft, the Lockheed SR–71 Blackbird, that was extremely RADAR–stealthy for its day. The SR–71 included a number of stealth features, including special RADAR–absorbing structures along the edges of wings and tailfins, a cross–sectional design featuring few vertical surfaces that could reflect RADAR directly back toward a transmitter, and a coating termed “iron ball” that could be electronically manipulated to produce a variable, confusing RADAR reflection. The SR–71, flying at approximately 100,000 ft (30,480 m), was routinely able to penetrate Soviet airspace without being reliably tracked on RADAR.
Development of true stealth aircraft (i.e., those employing every available method to avoid detection by visible, RADAR, infrared, and acoustic means) continued, primarily in the United States, throughout the 1960s and 1970s, and several stealth prototypes were flown in the early 1970s. Efforts to keep this research secret were successful; not until a press conference was held on August, 22, 1980, after expansion of the stealth program had given rise to numerous rumors and leaks, did the U.S. government officially admit the existence of stealth aircraft. Since then, much information about the two U.S. stealth combat aircraft, the B–2 bomber and the F–117 fighter (both discussed further below), has become publicly available.
Design for stealth requires the integration of many techniques and materials. The types of stealth that a maximally stealthy aircraft (or other vehicle) seeks to achieve can be categorized as visual, infrared, acoustic, and RADAR.
Low visibility is desirable for all military aircraft and is essential for stealth aircraft. It is achieved by coloring the aircraft so that it tends to blend in with its environment. For instance, reconnaissance planes designed to operate at very high altitudes, where the sky is black, are painted black. (Black is also a llow visibility color at night, at any altitude.) Conventional daytime fighter aircraft are painted a shade of blue known as “air–superiority blue–gray,” to blend in with the sky. Stealth aircraft are flown at night for maximum visual stealth, and so are painted black or dark gray. Chameleon or “smart skin” technology that would enable an aircraft to change its appearance to mimic its background is being researched. Furthermore, glint (bright reflections from cockpit glass or other smooth surfaces) must be minimized for visual stealth; this is accomplished using special coatings.
Infrared radiation (i.e., electromagnetic waves in the 0.72–1,000 micron range of the spectrum) are emitted by all matter above absolute zero; hot materials, such as engine exhaust gases or wing surfaces heated by friction with the air, emit more infrared radiation than cooler materials. Heat–seeking missiles and other weapons zero in onthe infrared glow of hot aircraft parts. Infrared stealth, therefore, requires that aircraft parts and emissions, particularly those associated with engines, be kept as cool as possible. Embedding jet engines inside the fuselage or wings is one basic design step toward infrared stealth. Other measures include extra shielding of hot parts, mixing of cool air with hot exhausts before emission; splitting of the exhaust stream by passing it through parallel baffles so that it mixes with cooler air more quickly; directing of hot exhausts upward, away from ground observers; and the application of special coatings to hot spots to absorb and diffuse heat over larger areas. Active countermeasures against infrared detection and tracking can be combined with passive stealth measures; these include infrared jamming (i.e., mounting of flickering infrared radiators near engine exhausts to confuse the tracking circuits of heat–seeking missiles) and the launching of infrared decoy flares. Combat helicopters, which travel at low altitudes and at low speeds, are particularly vulnerable to heat–seeking weapons and have been equipped with infrared jamming devices for several decades.
Although sound moves too slowly to be an effective locating signal for antiaircraft weapons, for low–altitude flying it is still best to be inaudible to ground observers. Several ultra–quiet, low–altitude reconnaissance aircraft, such as Lockheed’s QT–2 and YO–3A, have been developed since the 1960s. Aircraft of this type are ultralight, run on small internal combustion engines quieted by silencer–suppressor mufflers, and are driven by large, often wooden propellers. They make about as much sound as gliders and have very low infrared emissions as well because of their low energy consumption. The U.S. F–117 stealth fighter, which is designed to fly at high speed at very low altitudes, also incorporates acoustic–stealth measures, including sound–absorbent linings inside its engine intake and exhaust cowlings.
RADAR is the use of reflected electromagnetic waves in the microwave part of the spectrum to detect targets or map landscapes. RADAR first illuminates the target, that is, transmits a radio pulse in its direction. If any of this energy is reflected by the target, some of it may be collected by a receiving antenna. By comparing the delay times for various echoes, information about the geometry of the target can be derived and, if necessary, formed into an image. RADAR stealth or invisibility requires that a craft absorb incident RADAR pulses, actively cancel them by emitting inverse waveforms, deflect them away from receiving antennas, or all of the above. Absorption and deflection, treated below, are the most important prerequisites of RADAR stealth.
Absorption. Metallic surfaces reflect RADAR; therefore, stealth aircraft parts must either be coated with RADAR–absorbing materials or made out of them to begin with. The latter is preferable because an aircraft whose parts are intrinsically RADAR–absorbing derives aerodynamic as well as stealth function from them, whereas a RADAR–absorbent coating is, aerodynamically speaking, dead weight. The F–117 stealth aircraft is built mostly out of a RADAR–absorbent material termed Fibaloy, which consists of glass fibers embedded in plastic, and of carbon fibers, which are used mostly for hot spots like leading wing–edges and panels covering the jet engines. Thanks to the use of such materials, the airframe of the F–117 (i.e., the plane minus its electronic gear, weapons, and engines) is only about 10% metal. Both the B–2 stealth bomber and the F–117 reflect about as much RADAR as a hummingbird
Many RADAR–absorbent plastics, carbon–based materials, ceramics, and blends of these materials have been developed for use on stealth aircraft. Combining such materials with RADAR–absorbing surface geometry enhances stealth. For example, wing surfaces can be built on a metallic substrate that is shaped like a field of pyramids with the spaces between the pyramids filled by a RADAR–absorbent material. RADAR waves striking the surface zig–zag inward between the pyramid walls, which increases absorption by lengthening signal path through the absorbent material. Another example of structural absorption is the placement of metal screens over the intake vents of jet engines. These screens—used, for example, on the F–117 stealth fighter—absorb RADAR waves exactly like the metal screens embedded in the doors of microwave ovens. It is important to prevent RADAR waves from entering jet intakes, which can act as resonant cavities (echo chambers) and so produce bright RADAR reflections.
The inherently high cost of RADAR–absorbent, airframe–worthy materials makes stealth aircraft expensive; each B–2 bomber costs approximately $2.2 billion, while each F–117 fighter costs approximately $45 million; the U.S. fields 21 B–2s and 54 F–117s. The Russian Academy of Sciences, however, according to a 1999 report by Jane’s Defense Weekly, claims to have developed a low–budget RADAR–stealth technique, namely the cloaking of aircraft in ionized gas (plasma). Plasma absorbs radio waves, so it is theoretically possible to diminish the RADAR reflectivity of an otherwise non–stealthy aircraft by a factor of 100 or more by generating plasma at the nose and leading edges of an aircraft and allowing it flow backward over the fuselage and wings. The Russian system is supposedly lightweight (≌220 lb [100 kg]) and be retrofittable to existing aircraft, making—if the stealth capability available at far less cost to virtually any airforce. A disadvantage of the plasma technique that it would probably make the aircraft glow in the visible part of the spectrum.
Deflection. Most RADARs are monostatic, that is, for reception they use either the same antenna as for sending or a separate receiving antenna colocated with the sending antenna; deflection therefore means reflecting RADAR pulses in any direction other than the one they came from. This in turn requires that stealth aircraft lack flat, vertical surfaces that could act as simple RADAR mirrors. RADAR can also be strongly reflected wherever three planar surfaces meet at a corner. Planes such as the B–52 bomber, which have many flat, vertical surfaces and RADAR–reflecting corners, are notorious for their RADAR–reflecting abilities; stealth aircraft, in contrast, tend to be highly angled and streamlined, presenting no flat surfaces at all to an observer that is not directly above or below them. The B–2 bomber, for example, is shaped like a boomerang.
A design dilemma for stealth aircraft is that they need not only to be invisible to RADAR but to use RADAR; inertial guidance, the Global Positioning System, and laser RADAR can all help aircraft navigate stealthily, but an aircraft needs conventional RADAR to track incoming missiles and hostile aircraft. Yet the transmission of RADAR pulses by a stealth aircraft wishing to avoid RADAR detection is self–contradictory. Furthermore, RADAR and radio antennas are inherently RADAR–reflecting.
At least two design solutions to this dilemma are available. One is to have moveable RADAR–absorbent covers over RADAR antennas that slip aside only when the RADAR must be used. The antenna is then vulnerable to detection only intermittently. Even short–term RADAR exposure is, however, dangerous; the only stealth aircraft known to be have been shot down in combat, an F–117 lost over Kosovo in 1999, is thought to have been tracked by RADAR during a brief interval while its bomb–bay doors were open. The disadvantage of sliding mechanical covers is that they may stick or otherwise malfunction, and must remain open for periods of time that are long by electronic standards. A better solution, presently being developed, is the plasma stealth antenna. A plasma stealth antenna is composed of parallel tubes made of glass, plastic, or ceramic that are filled with gas, much like fluorescent light bulbs. When each tube is energized, the gas in it becomes ionized, and can conduct current just like a metal wire. A number of such energized tubes in a flat, parallel array, wired for individual control (a “phased array”), can be used to send and receive RADAR signals across a wide range of angles without being physical rotated. When the tubes are not energized, they are transparent to RADAR, which can be absorbed by an appropriate backing. One advantage of such an array is that it can turn on and off very rapidly, and only act as a RADAR reflector during the electronically brief intervals when it is energized.
Stealth technology is most effective when combined with other measures for avoiding detection. For example, the F–117 and B–2 are both designed to fly at night, the most obvious visual stealth measure. Further, the F–117 is designed to fly close to the ground (i.e., at less than 500 ft [152 m]). Normal ground–based RADAR cannot see oncoming targets until they are in a line of direct sight, which, for a fast, low–flying aircraft approaching through hilly terrain, may not occur until the aircraft is almost above the RADAR. Even down–looking RADARs carried on aircraft have more difficulty tracking craft that are flying near ground–level, mingling their reflections with the noisy pattern of echoes from the ground itself (the “ground clutter”). The F–117 therefore can fly close to the ground, swerving under computer control to avoid obstacles such as hills or towers. This flight style is known as jinking, snaking, or terrain following. (An aircraft such as the B–2 is too large to perform the rapid maneuvers required for jinking, and so flies at higher altitudes.)
At the opposite extreme from jinking flight, ultra–high altitudes have also been used for stealth purposes. Reconnaissance aircraft deployed by the United States since the 1950s, including the U–2 and the SR–71, have set most of the altitude records for “air–breathing” craft (i.e., craft that do not, like rockets, carry their own oxygen). Such planes fly near the absolute limit of aerodynamic action; if they went any higher, there would be not be enough air to provide lift.
An aircraft cannot be made truly invisible. For example, no matter how cool the exhaust vents of an aircraft are kept, the same amount of heat is always liberated by burning a given amount of fuel, and this heat must be left behind the aircraft as a trail of warm air. Infrared–detecting devices might be devised that could image this heat trail as it formed, tracking a stealth aircraft.
Furthermore, every jet aircraft leaves swirls of air—vortices—in its wake. Doppler RADAR, which can image wind velocities, might pinpoint such disturbances if it could be made sufficiently high–resolution.
Other antistealth techniques could include the detection of aircraft–caused disturbances in the Earth’s magnetic field (magnetic anomaly detection), networks of low–frequency radio links to detect stealth aircraft by interruptions in transmission, the use of specially–shaped RADAR pulses that resist absorption, and netted RADAR. Netted RADAR is the use of more than one receiver, and possibly more than one transmitter, in a network. Since stealth aircraft rely partly on deflecting RADAR pulses, receivers located off the line of pulse transmission might be able to detected deflected echoes. By illuminating a target area using multiple transmitters and linking multiple receivers into a coordinated network, it should be possible to greatly increase one’s chances of detecting a stealthy target. No single receiver may record a strong or steady echo from any single transmitter, but the network as a whole might collect enough information to track a stealth target.
Stealthy jet aircraft have been used for surveil–lance since the 1950s, but dedicated–design stealth warplanes were not used in combat prior to the first Gulf War (1991). In that war, F–117s—which first became operational in 1982—made some 1,300 sorties and were the only aircraft to bomb targets in downtown Baghdad. B–2 bombers were first used in combat in the Kosovo conflict in 1999, flying bombing sorties from Missouri to Yugoslavia (with midflight refueling over the Atlantic). F–117s were also used in the Kosovo conflict; one was shot down and two were damaged by enemy fire. The first overseas combat deployment of B–2 bombers occurred in 2003, during Operation Iraqi Freedom.
Stealth technology is also employed in United States cruise missiles such as the Tomahawk and the AGM–129A. The Tomahawk, a tactical weapon that can carry either nuclear or conventional warheads, has been deployed in four versions, including air–, sea–, and ground–launched types, and was used extensively in combat in both Gulf Wars and in Afghanistan in 2002. The AGM–129A is stealthier than the 1970s–vintage Tomahawk; it carries the W80 250–kiloton nuclear warhead and is designed to be fired from under the wings of the B–52H Stratofortress strategic bomber. The AGM–129A has not been used in combat.
See also Electromagnetic spectrum.
Hume, Andrew L., and Christopher Baker. “Netted RADAR Sensing,” in proceedings of the CIE International Conference on RADAR IEEE, 110–114, 2001.