Stealth aircraft are aircraft that use stealth technology to make it harder to be detected by radar and other means than conventional aircraft by employing a combination of features to reduce visibility in the visual, audio, infrared and radio frequency (RF) spectrum. They were first developed by the Germans at the end of World War Two, with the Horten Ho 229. Well known examples include the United States' F-117 Nighthawk (1980s-2008), B-2 Spirit, and the F-22 Raptor.
While no aircraft is totally invisible to radar, stealth aircraft limit current conventional radar's abilities to detect or track them effectively enough to prevent an attack. Stealth is accomplished by using a complex design philosophy to reduce the ability of an opponent's sensors to detect, track and attack an aircraft.
Modern stealth aircraft first became possible when a mathematician working for Lockheed Aircraft during the 1970s adopted a mathematical model developed by Pyotr Ufimtsev, a Russian scientist, to develop a computer program called Echo 1. Echo made it possible to predict the radar signature an aircraft made with flat panels, called facets. In 1975, engineers at Lockheed Skunk Works found that an airplane made with faceted surfaces could have a very low radar signature because the surfaces would radiate almost all of the radar energy away from the receiver. Lockheed built a model called "the Hopeless Diamond". It was named that because it looked like a squat diamond and looked too hopeless to ever fly. For the first time, designers realized that it might be possible to make an aircraft that was virtually invisible to radar.
Reduced radar cross section is only one of five factors that designers addressed to create a truly stealthy design such as the F-22. The F-22 has also been designed to disguise its infrared emissions to make it harder to detect by infrared homing ("heat seeking") surface-to-air or air-to-air missiles. Designers also addressed making the aircraft less visible to the naked eye, controlling radio transmissions, and noise abatement.
The first combat use of stealth aircraft was in December 1989 during Operation Just Cause in Panama. On December 20, 1989 two USAF F-117s bombed a Panamanian defense Force Barracks in Rio Hato, Panama. In 1991, F-117s were tasked with attacking the most heavily fortified targets in Iraq and were the only jets allowed to operate inside Baghdad's city limits.
Stealth aircraft are designed with a focus on minimal radar cross section (RCS) rather than aerodynamic performance. Highly stealth aircraft (the F-117 Nighthawk and B-2 Spirit) are aerodynamically unstable in all three axes and require constant flight corrections from the fly-by-wire system to maintain controlled flight. Most modern non-stealth fighter aircraft (F-16, Su-27, Gripen, Rafale) are unstable on one or two axes only.
Earlier full stealth aircraft (such as the F-117 and B-2) lack afterburners, because the hot exhaust would increase their radar cross section and infrared footprint. As a result their performance in air combat maneuvering required in a dogfight would never match that of a dedicated fighter aircraft, but that did not matter since they are both designed to be bombers. More modern techniques however allow for stealthy designs such as the F-22 without compromising aerodynamic performance. Even though stealth aircraft have to compromise ultimate performance due to stealth considerations, some new aircraft, like the F-22 and F-35, have performance characteristics that meet or exceed those of current jet fighters due to advances in other technologies such as flight control systems, engines, airframe construction and materials.
The high level of computerization and large amount of electronic equipment found inside stealth aircraft are often claimed to make them vulnerable to passive detection. This is highly unlikely and certainly systems such as Tamara and Kolchuga, which are often described as counter-stealth radars, are not designed to detect stray electromagnetic fields of this type. Such systems are designed to detect intentional, higher power emissions such as radar and communication signals. Stealth aircraft are deliberately operated to avoid or reduce such emissions.
Stealth aircraft are still vulnerable to detection immediately before, during, and after using their weaponry. Since stealth payload (reduced RCS bombs and cruise missiles) are not yet generally available, and ordnance mount points create a significant radar return, stealth aircraft carry all armament internally. As soon as weapons bay doors are opened, the plane's RCS will be multiplied and even older generation radar systems will be able to locate the stealth aircraft. While the aircraft will reacquire its stealth as soon as the bay doors are closed, a fast response defensive weapons system has a short opportunity to engage the aircraft.
This vulnerability is addressed by operating in a manner that reduces the risk and consequences of temporary acquisition. The B-2's operational altitude imposes a flight time for defensive weapons that makes it virtually impossible to engage the aircraft during its weapons deployment. New stealth aircraft designs such as the F-22 can release munitions and return to stealthy flight in less than a second.
Some weapons require that the weapon's guidance system acquire the target while the weapon is still attached to the aircraft. This forces relatively extended operations with the bay doors open. In the absence of official information, it is likely that the use of such weapons in stealth aircraft is reduced as much as possible.
In case of 4th and 5th generation "reduced RCS" (semi-stealth) fighter-bomber designs, air-to-ground armament is mainly carried on external pylons, accepting the higher risk of detection. The internal weapon bays are reserved for various anti-aircraft missiles.
Fully stealth aircraft carry all armament internally, which limits the payload. By way of comparison, the F-117 carries only two laser or GPS guided bombs, while a non-stealth attack aircraft can carry several times more. This requires the deployment of additional aircraft to engage targets that would normally require a single non-stealth attack aircraft.
This apparent disadvantage could be offset by the fact that fewer supporting aircraft are required to provide air cover, air-defence suppression and electronic counter measures.
Stealth aircraft are high-maintenance equipment, as their stealth capability requires detail-oriented care. The most obvious aspect is the aircraft's skin, that has a specific shape to reflect radar impulses away from the emission source, and a coating to absorb electromagnetic waves using materials such as graphite-ferrite microspheres. All openings and edges are electromagnetically shielded. The cockpit windows are shielded with delicate gold and indium foil layers.
By way of example, until the relatively recent introduction of improved sealing products, on the B-2 it would often take more hours of work to reseal access panels that were opened for maintenance, than the required maintenance itself. Stealth aircraft skin must also be protected from foreign object damage, as imperfections in the skin can dramatically increase the radar cross section.
In short, stealth depends on maintaining a high level of detail in every aspect of aircraft maintenance. This makes them a serious economic burden on stealth aircraft operators.
The B-2 Stealth Bomber has a skin made with highly specialized thermoplastics and composites which are radar-absorbent. A 1997 report by the US Congress' General Accounting Office found that these materials however are susceptible to damage from extreme temperatures, rain and moisture.
Stealth aircraft are typically more expensive to develop and manufacture. An example is the B-2 Spirit that is many times more expensive to manufacture and support than conventional bomber aircraft. The B-2 program cost the U.S. Air Force almost $45 billion.
Both Australia and Russia have announced that they have developed processing techniques that allow them to detect the turbulence of aircraft at reasonably long ranges (possibly negating the stealth technology). However, there is no indication that such detections are accurate enough to allow engagement or even cueing of an engagement system. Position data might be used to direct air defense fighter aircraft and most stealth designs are no match for interceptor aircraft.
Passive (multistatic) radar, bistatic radar and especially multistatic systems are believed to detect some stealth aircraft better than conventional monostatic radars, since first-generation stealth technology (such as the F117) reflects energy away from the transmitter's line of sight, effectively increasing the radar cross section (RCS) in other directions, which the passive radars monitor. Such a system typically uses either low frequency broadcast TV and FM radio signals (at which frequencies controlling the aircraft's signature is more difficult). Later stealth approaches do not rely on controlling the specular reflections of radar energy and so the geometrical benefits are unlikely to be significant.
Researchers at the University of Illinois at Urbana-Champaign with support of DARPA, have shown that it is possible to build a synthetic aperture radar image of an aircraft target using passive multistatic radar, possibly detailed enough to enable Automatic Target Recognition (ATR).
In December 2007, SAAB researchers also revealed details a system called Associative Aperture Synthesis Radar (AASR) that would employ a large array of inexpensive and redundant transmitters and a few intelligent receivers to exploit forward scatter to detect low observable targets. The system was originally designed to detect stealthy cruise missiles and should be just as effective against aircraft. The large array of inexpensive transmitters also provides a degree of protection against anti-radar (or anti-radiation) missiles or attacks.
Some analysts claim infra-red search and track (IRST) systems can be deployed against stealth aircraft, because any aircraft surface heats up due to air friction. These analysts also point to the resurgence in such systems in several Russian designs in the 1980s, such as those fitted to the MiG-29 and Su-27. The latest version of the MiG-29, the MiG-35, is equipped with a new Optical Locator System that includes even more advanced IRST capabilities.
The Dutch company Thales Nederland, formerly known as Holland Signaal, have developed a naval phased-array radar called SMART-L, which also is operated at L-Band and is claimed to offer counter stealth benefits. However, as with most claims of counter-stealth capability, these are unproven and untested. True resonant effects might be expected with HF sky wave radar systems, which have wavelengths of tens of metres. However, in this case, the accuracy of the radar systems is such that the detection is of limited value for engagement.
Over-the-horizon radar is a design concept that increases radar's effective range over conventional radar. It is claimed that the Australian JORN Jindalee Operational Radar Network can overcome certain stealth characteristics. It is claimed that the HF frequency used and the method of bouncing radar from ionsphere overcomes the stealth characteristics of the F-117A. In other words, stealth aircraft are optimized for defeating much higher-frequency radar from front-on rather than low-frequency radars from above.
There are unsubstantiated rumours that Australian civilian air traffic controllers have tried to contact, in-air, high-altitude, F-117A pilots crossing the Australian continent without airspace clearance due to datafeed from the JORC. Apparently there was no response from the pilots.
To date, stealth aircraft have been used in several low- and moderate-intensity conflicts, including Operation Desert Storm, Operation Allied Force and the 2003 invasion of Iraq. In each case they were employed to strike high-value targets that were either out of range of conventional aircraft in the theater or were too heavily defended for conventional aircraft to strike without a high risk of loss. In addition, because the stealth aircraft do not have to evade surface-to-air missiles and anti-aircraft artillery over the target they can aim more carefully and thus are more likely to hit the target and cause less collateral damage. In many cases they were used to hit the high value targets early in the campaign (or even before it), before other aircraft had the opportunity to degrade the opposing air defense to the point where other aircraft had a good chance of reaching those critical targets.
Stealth aircraft in future low- and moderate-intensity conflicts are likely to have similar roles. However, given the increasing prevalence of excellent Russian-built surface-to-air missile systems on the open market (such as the SA-10, SA-12 and SA-20 (S-300P/V/PMU) and SA-15 (9K331/332)), stealth aircraft are likely to be very important in a high-intensity conflict in order to gain and maintain air supremacy, especially to the United States who is likely to face these types of systems. It is possible to cover one's airspace with so many air defences with such long range and capability that conventional aircraft would find it very difficult "clearing the way" for deeper strikes. For example, China license-builds all of the previously mentioned SAM systems in large quantities and would be able to heavily defend important strategic and tactical targets in the event of a conflict. Even if anti-radiation weapons are used in an attempt to destroy the SAM radars of such systems, or stand-off weapons are launched against them, these modern surface-to-air missile batteries are capable of shooting down weapons fired against them.