Stealth technology

Stealth technology, also termed low observable technology (LO technology), is a sub-discipline of military tactics and passive and active electronic countermeasures,<ref>Rao, G.A.; Mahulikar, S.P. (2002). "Integrated review of stealth technology and its role in airpower". Aeronautical Journal. 106 (1066): 629–641. doi:10.1017/S0001924000011702. S2CID 108545502.</ref> which covers a range of methods used to make personnel, aircraft, ships, submarines, missiles, satellites, and ground vehicles less visible (ideally invisible) to radar, infrared,<ref>Mahulikar, S.P.; Sonawane, H.R.; Rao, G.A. (2007). "Infrared signature studies of aerospace vehicles". Progress in Aerospace Sciences. 43 (7–8): 218–245. Bibcode:2007PrAeS..43..218M. doi:10.1016/j.paerosci.2007.06.002.</ref> sonar and other detection methods. It corresponds to military camouflage for these parts of the electromagnetic spectrum (i.e., multi-spectral camouflage).

Development of modern stealth technologies in the United States began in 1958,<ref name=national>Richelson, J.T. (10 September 2001). "Science, Technology and the CIA". The National Security Archive. The George Washington University. Retrieved 6 October 2009.</ref><ref name=learnt>Merlin, Peter W. (5–8 January 2009). Design and Development of the Blackbird: Challenges and Lessons Learned (PDF). 47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition. Orlando, Florida: American Institute of Aeronautics and Astronautics. Archived from the original (PDF) on 18 October 2013. Retrieved 6 October 2009.</ref> where earlier attempts to prevent radar tracking of its U-2 spy planes during the Cold War by the Soviet Union had been unsuccessful.<ref>Cadirci, S. "RF Stealth (or Low Observable) and Counter- RF Stealth Technologies: Implications of Counter- RF Stealth Solutions for Turkish Air Force Archived 2011-07-20 at the Wayback Machine." Naval Postgraduate School, Monterey California, PhD Thesis. March 2009. Retrieved 6 October 2009.</ref> Designers turned to developing a specific shape for planes that tended to reduce detection by redirecting electromagnetic radiation waves from radars.<ref name=b2>Yue, T. (30 November 2001). "Detection of the B-2 Stealth Bomber and a Brief History on "Stealth"". The Tech – Online Edition. Massachusetts Institute of Technology. Retrieved 5 October 2009.</ref> Radiation-absorbent material was also tested and made to reduce or block radar signals that reflect off the surfaces of aircraft. Such changes to shape and surface composition comprise stealth technology as currently used on the Northrop Grumman B-2 Spirit "Stealth Bomber".<ref name=learnt/>

The concept of stealth is to operate or hide while giving enemy forces no indication as to the presence of friendly forces. This concept was first explored through camouflage to make an object's appearance blend into the visual background. As the potency of detection and interception technologies (radar, infrared search and tracking, surface-to-air missiles, etc.) have increased, so too has the extent to which the design and operation of military personnel and vehicles have been affected in response. Some military uniforms are treated with chemicals to reduce their infrared signature. A modern stealth vehicle is designed from the outset to have a chosen spectral signature. The degree of stealth embodied in a given design is chosen according to the projected threats of detection.

History

Camouflage to aid or avoid predation predates humanity, and hunters have been using vegetation to conceal themselves perhaps as long as people have been hunting. The earliest application of camouflage in warfare is impossible to ascertain. Methods for visual concealment in war were documented by Sun Tzu in his book The Art of War in the 5th century BC, and by Frontinus in his work Strategemata in the 1st century AD.<ref>Wey, Adam Leong Kok (15 March 2014). "Principles of Special Operations: Learning from Sun Tzu and Frontinus". Comparative Strategy. 33 (2): 131–144. doi:10.1080/01495933.2014.897119. ISSN 0149-5933. S2CID 154557121.</ref>

In England, irregular units of gamekeepers in the 17th century were the first to adopt drab colours (common in 16th century Irish units) as a form of camouflage, following examples from the continent.

During World War I, the Germans experimented with the use of Cellon (Cellulose acetate), a transparent covering material, in an attempt to reduce the visibility of military aircraft. Single examples of the Fokker E.III Eindecker fighter monoplane, the Albatros C.I two-seat observation biplane, and the Linke-Hofmann R.I prototype heavy bomber were covered with Cellon. However, sunlight glinting from the material made the aircraft even more visible. Cellon was also found to degrade quickly from both sunlight and in-flight temperature changes, so the effort to make transparent aircraft ceased.<ref name="Haddow">Haddow, G.W.; Grosz, Peter M. (1988). The German Giants – The German R-Planes 1914–1918 (3rd ed.). London: Putnam. ISBN 0-85177-812-7.</ref>

In 1916, the British modified a small SS class airship for the purpose of night-time reconnaissance over German lines on the Western Front. Fitted with a silenced engine and a black gas bag, the craft was both invisible and inaudible from the ground but several night-time flights over German-held territory produced little useful intelligence and the idea was dropped.<ref>Abbott, Patrick (1989). The British Airship at War, 1914–1918. Terence Dalton. pp. 31–33. ISBN 0861380738.</ref>

Diffused lighting camouflage, a shipborne form of counter-illumination camouflage, was trialled by the Royal Canadian Navy from 1941 to 1943. The concept was followed up for aircraft by the Americans and the British: in 1945 a Grumman Avenger with Yehudi lights reached 3,000 yards (2,700 m) from a ship before being sighted. This ability was rendered obsolete by radar.<ref name="NavalMuseumQuebec">"Naval Museum of Quebec". Diffused Lighting and its use in the Chaleur Bay. Royal Canadian Navy. Retrieved 18 September 2012.^{[dead link]}</ref>

Chaff was invented in Britain and Germany early in World War II as a means to hide aircraft from radar. In effect, chaff acted upon radio waves much as a smoke screen acted upon visible light.<ref>Jones, R. V. (1978). Most Secret War: British Scientific Intelligence 1939–1945. London: Hamish Hamilton. ISBN 0-241-89746-7.</ref>

The U-boat U-480 may have been the first stealth submarine. It featured an anechoic tile rubber coating, one layer of which contained circular air pockets to defeat ASDIC sonar.<ref>"U-Boat Anti Sonar Coating". Uboataces. Retrieved 18 September 2012.</ref> Radar-absorbent paints and materials of rubber and semiconductor composites (codenames: Sumpf, Schornsteinfeger) were used by the Kriegsmarine on submarines in World War II. Tests showed they were effective in reducing radar signatures at both short (centimetres) and long (1.5 metre) wavelengths.<ref>Hepcke, Gerhard (2007). The Radar War, 1930–1945 (PDF) (Report). English translation by Hannah Liebmann (in Deutsch). Radar World. p. 45. Retrieved 1 February 2024.</ref>

In 1956 the CIA began attempts to reduce the radar cross-section (RCS) of the U-2 spyplane. Three systems were developed, Trapeze, a series of wires and ferrite beads around the planform of the aircraft, a covering material with PCB circuitry embedded in it, and radar-absorbent paint. These were deployed in the field on the so-called dirty birds but results were disappointing, the weight and drag increases were not worth any reduction in detection rates. More successful was applying camouflage paint to the originally bare metal aircraft; a deep blue was found to be most effective. The weight of this cost 250 ft in maximum altitude, but made the aircraft harder for interceptors to see.<ref name="U2+OXCART">Pedlow, Gregory W.; Welzenbach, Donald E. (1992). The Central Intelligence Agency and Overhead Reconnaissance: The U-2 and OXCART Programs, 1954–1974 (Report). Washington, DC: Central Intelligence Agency.</ref>

In 1958, the U.S. Central Intelligence Agency requested funding for a reconnaissance aircraft to replace the existing U-2 spy planes,<ref>Poteat, Gene (1998). "Stealth, Countermeasures, and ELINT, 1960–1975" (PDF). Studies in Intelligence. 48 (1): 51–59.</ref> and Lockheed secured contractual rights to produce it.<ref name=national/> "Kelly" Johnson and his team at Lockheed's Skunk Works were assigned to produce the A-12 (or OXCART), which operated at high altitude of 70,000 to 80,000 ft and speed of Mach 3.2 to avoid radar detection. Various plane shapes designed to reduce radar detection were developed in earlier prototypes, named A-1 to A-11. The A-12 included a number of stealthy features including special fuel to reduce the signature of the exhaust plume, canted vertical stabilizers, the use of composite materials in key locations, and the overall finish in radar-absorbent paint.<ref name="U2+OXCART"/>

In 1960, the USAF reduced the radar cross-section of a Ryan Q-2C Firebee drone. This was achieved through specially designed screens over the air intake, and radiation-absorbent material on the fuselage, and radar-absorbent paint.<ref>"1960s AQM-34 Ryan Firebee (USA)". PBS. Retrieved 14 January 2015.</ref>

The United States Army issued a specification in 1968 which called for an observation aircraft that would be acoustically undetectable from the ground when flying at an altitude of 1,500 feet (457 m) at night. This resulted in the Lockheed YO-3A Quiet Star, which operated in South Vietnam from late June 1970 to September 1971.<ref name="YO-3A">"YO-3A "Quiet Star" Silent Stealth Airplane". Vietnam Helicopters Museum. Retrieved 2 May 2015.</ref>

During the 1970s the U.S. Department of Defense launched project Lockheed Have Blue, with the aim of developing a stealth fighter. There was fierce bidding between Lockheed and Northrop to secure the multibillion-dollar contract. Lockheed incorporated into its bid a text written by the Soviet-Russian physicist Pyotr Ufimtsev from 1962, titled Method of Edge Waves in the Physical Theory of Diffraction, Soviet Radio, Moscow, 1962. In 1971 this book was translated into English with the same title by U.S. Air Force, Foreign Technology Division.<ref>National Air Intelligence Center, Wright-Patterson AFB, OH, 1971. Technical Report AD 733203, Defense Technical Information Center of USA, Cameron Station, Alexandria, VA, 22304-6145, USA</ref> The theory played a critical role in the design of American stealth-aircraft F-117 and B-2.<ref>Browne, M.W. "Two rival designers led the way to stealthy warplanes", The New York Times, Sci. Times Sec., 14 May 1991.</ref><ref>Browne, M.W. "Lockheed credits Soviet theory in design of F-117", Aviation Week Space Technology p. 27, December 1991.</ref><ref>Rich, Ben and L. Janos, Skunk Works, Little Brown, Boston, 1994.</ref> Equations outlined in the paper quantified how a plane's shape would affect its detectability by radar, termed radar cross-section (RCS).<ref>Knott, E.F.; Shaeffer, J.F.; Tuley, M.T. (2004). Radar cross section – Second Edition. Raleigh, North Carolina: SciTech Publishing. pp. 209–214. ISBN 1-891121-25-1. Retrieved 7 October 2009.</ref> At the time, the Soviet Union did not have supercomputer capacity to solve these equations for actual designs. This was applied by Lockheed in computer simulation to design a novel shape they called the "Hopeless Diamond", a wordplay on the Hope Diamond, securing contractual rights to produce the F-117 Nighthawk starting in 1975. In 1977 Lockheed produced two 60% scale models under the Have Blue contract. The Have Blue program was a stealth technology demonstrator that lasted from 1976 to 1979. The Northrop Grumman Tacit Blue also played a part in the development of composite material and curvilinear surfaces, low observables, fly-by-wire, and other stealth technology innovations. The success of Have Blue led the Air Force to create the Senior Trend program which developed the F-117.<ref>Kevin (14 July 2003). "F-117A Senior Trend". F-117A: The Black Jet. Retrieved 2 September 2019.</ref><ref>Goebel, Greg (1 March 2010). "Senior Trend". Vectorsite.net. Archived from the original on 3 January 2012. Retrieved 2 September 2019.</ref>

Principles

Stealth technology (or LO for low observability) is not one technology. It is a set of technologies, used in combinations, that can greatly reduce the distances at which a person or vehicle can be detected; more so radar cross-section reductions, but also acoustic, thermal, and other aspects.

Radar cross-section (RCS) reductions

Almost since the invention of radar, various methods have been tried to minimize detection. Rapid development of radar during World War II led to equally rapid development of numerous counter radar measures during the period; a notable example of this was the use of chaff. Modern methods include Radar jamming and deception.

The term stealth in reference to reduced radar signature aircraft became popular during the late eighties when the Lockheed Martin F-117 stealth fighter became widely known. The first large scale (and public) use of the F-117 was during the Gulf War in 1991. However, F-117A stealth fighters were used for the first time in combat during Operation Just Cause, the United States invasion of Panama in 1989.<ref>Crocker, H. W. III (2006). Don't Tread on Me. New York: Crown Forum. p. 382. ISBN 978-1-4000-5363-6.</ref>

Vehicle shape

Aircraft

The possibility of designing aircraft in such a manner as to reduce their radar cross-section was recognized in the late 1930s, when the first radar tracking systems were employed, and it has been known since at least the 1960s that aircraft shape makes a significant difference in detectability. The Avro Vulcan, a British bomber of the 1960s, had a remarkably small appearance on radar despite its large size, and occasionally disappeared from radar screens entirely. It is now known that it had a fortuitously stealthy shape apart from the vertical element of the tail. Despite being designed before a low radar cross-section (RCS) and other stealth factors were ever a consideration,<ref name="newscientist82">Sweetman, Bill. "The Bomber that radar cannot see." New Scientist, 4 March 1982.</ref> a Royal Aircraft Establishment technical note of 1957 stated that of all the aircraft so far studied, the Vulcan appeared by far the simplest radar echoing object, due to its shape: only one or two components contributing significantly to the echo at any aspect (one of them being the vertical stabilizer, which is especially relevant for side aspect RCS), compared with three or more on most other types.<ref>Dawson 1957, p. 3.</ref><ref>Writing for the American Institute of Aeronautics and Astronautics, J. Seddon and E. L. Goldsmith noted that "Due to its all-wing shape, small vertical fin, and buried engines, at some angles [The Avro Vulcan] was nearly invisible to radar".<ref>Seddon and Goldsmith 1999, p. 343.</ref></ref> While writing about radar systems, authors Simon Kingsley and Shaun Quegan singled out the Vulcan's shape as acting to reduce the RCS.<ref>Kingsley and Quegan 1999, p. 293.</ref> In contrast, the Tupolev 95 Russian long-range bomber (NATO reporting name 'Bear') was conspicuous on radar. It is now known that propellers and jet turbine blades produce a bright radar image;^{[citation needed]} the Bear has four pairs of large (5.6-meter diameter) contra-rotating propellers.

Another important factor is internal construction. Some stealth aircraft have skin that is radar transparent or absorbing, behind which are structures termed reentrant triangles. Radar waves penetrating the skin get trapped in these structures, reflecting off the internal faces and losing energy. This method was first used on the Blackbird series: A-12, YF-12A, Lockheed SR-71 Blackbird.

The most efficient way to reflect radar waves back to the emitting radar is with orthogonal metal plates, forming a corner reflector consisting of either a dihedral (two plates) or a trihedral (three orthogonal plates). This configuration occurs in the tail of a conventional aircraft, where the vertical and horizontal components of the tail are set at right angles. Stealth aircraft such as the F-117 use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them. A more radical method is to omit the tail, as in the B-2 Spirit. The B-2's clean, low-drag flying wing configuration gives it exceptional range and reduces its radar profile.<ref name="croddy 341-2">Croddy and Wirtz 2005, pp. 341–342.</ref><ref>Siuru 1993, pp. 114–115.</ref> The flying wing design most closely resembles a so-called infinite flat plate (as vertical control surfaces dramatically increase RCS), the perfect stealth shape, as it would have no angles to reflect back radar waves.<ref>"B-2: The Spirit of Innovation" (PDF). Northrop Grumman Corporation. Retrieved 15 October 2023.</ref>

In addition to altering the tail, stealth design must bury the engines within the wing or fuselage, or in some cases where stealth is applied to an extant aircraft, install baffles in the air intakes, so that the compressor blades are not visible to radar. A stealthy shape must be devoid of complex bumps or protrusions of any kind, meaning that weapons, fuel tanks, and other stores must not be carried externally. Any stealthy vehicle becomes un-stealthy when a door or hatch opens.

Parallel alignment of edges or even surfaces is also often used in stealth designs. The technique involves using a small number of edge orientations in the shape of the structure. For example, on the F-22A Raptor, the leading edges of the wing and the tail planes are set at the same angle. Other smaller structures, such as the air intake bypass doors and the air refueling aperture, also use the same angles. The effect of this is to return a narrow radar signal in a very specific direction away from the radar emitter rather than returning a diffuse signal detectable at many angles. The effect is sometimes called "glitter" after the very brief signal seen when the reflected beam passes across a detector. It can be difficult for the radar operator to distinguish between a glitter event and a digital glitch in the processing system.

Stealth airframes sometimes display distinctive serrations on some exposed edges, such as the engine ports. The YF-23 has such serrations on the exhaust ports. This is another example in the parallel alignment of features, this time on the external airframe.

The shaping requirements detracted greatly from the F-117's aerodynamic properties. It is inherently unstable, and cannot be flown without a fly-by-wire control system.

Similarly, coating the cockpit canopy with a thin film transparent conductor (vapor-deposited gold or indium tin oxide) helps to reduce the aircraft's radar profile, because radar waves would normally enter the cockpit, reflect off objects (the inside of a cockpit has a complex shape, with a pilot helmet alone forming a sizeable return), and possibly return to the radar, but the conductive coating creates a controlled shape that deflects the incoming radar waves away from the radar. The coating is thin enough that it has no adverse effect on pilot vision.

Ships

Ships have also adopted similar methods. Though the earlier Arleigh Burke-class destroyer incorporated some signature-reduction features.<ref>"DDG-51 Arleigh Burke-class". FAS website. Federation of American Scientists. Archived from the original on 24 December 2013. Retrieved 2 February 2011.</ref><ref>Benson, Robert (November 1998). "The Arleigh Burke: Linchpin of the Navy". Asia-Pacific Defense Forum. Federation of American Scientists. Retrieved 2 February 2011.</ref> the Norwegian Skjold-class corvette was the first coastal defence and the French La Fayette-class frigate the first ocean-going stealth ship to enter service. Other examples are the Dutch De Zeven Provinciën class frigates, the Taiwanese Tuo Chiang stealth corvette, German Sachsen-class frigates, the Swedish Visby-class corvette, the USS San Antonio amphibious transport dock, and most modern warship designs.

Materials

Non-metallic airframe

Dielectric composite materials are more transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface. Composites may also contain ferrites to optimize the dielectric and magnetic properties of a material for its application.

Radar-absorbent material

Radiation-absorbent material (RAM), often as paints, are used especially on the edges of metal surfaces. While the material and thickness of RAM coatings can vary, the way they work is the same: absorb radiated energy from a ground- or air-based radar station into the coating and convert it to heat rather than reflect it back.<ref>Arora, Swayam; KaurResearch, Ramanpreet (December 2013). "Stealth Technology And Counter Stealth Radars: A Review" (PDF). Inventy: International Journal of Engineering and Science. 3 (12): 15–19. eISSN 2278-4721. ISSN 2319-6483.</ref> Current technologies include dielectric composites and metal fibers containing ferrite isotopes. Ceramic composite coating is a new type of material systems which can sustain at higher temperatures with better sand erosion resistance and thermal resistance.<ref>"How a Tougher Skin Could Change the Shape of Stealth Aircraft". 18 May 2021.</ref> Paint comprises depositing pyramid-like colonies on the reflecting superficies with the gaps filled with ferrite-based RAM. The pyramidal structure deflects the incident radar energy in the maze of RAM. One commonly used material is called iron ball paint.<ref>Wolchover, Natalie (21 January 2011). "How Do Stealth Planes Evade the Enemy?". Live Science. Bath, England. Retrieved 1 July 2019.</ref> It contains microscopic iron spheres that resonate in tune with incoming radio waves and dissipate most of their energy as heat, leaving little to reflect back to detectors. FSS are planar periodic structures that behave like filters to electromagnetic energy. The considered frequency-selective surfaces are composed of conducting patch elements pasted on the ferrite layer. FSS are used for filtration and microwave absorption.

Radar stealth countermeasures and limits

Low-frequency radar

Shaping offers far fewer stealth advantages against low-frequency radar. If the radar wavelength is roughly twice the size of the target, a half-wave resonance effect can still generate a significant return. However, low-frequency radar is limited by lack of available frequencies (many are heavily used by other systems), by lack of accuracy of the diffraction-limited systems given their long wavelengths, and by the radar's size, making it difficult to transport. A long-wave radar may detect a target and roughly locate it, but not provide enough information to identify it, target it with weapons, or even to guide a fighter to it.<ref>Maj. Gen. Carlsons briefing on Stealth Fighters, Tuesday, April 20, 1999</ref>

Multiple emitters

Stealth aircraft attempt to minimize all radar reflections, but are specifically designed to avoid reflecting radar waves back in the direction they came from (since in most cases a radar emitter and receiver are in the same location). They are less able to minimize radar reflections in other directions. Thus, detection can be better achieved if emitters are in different locations from receivers. One emitter separate from one receiver is termed bistatic radar; one or more emitters separate from more than one receiver is termed multistatic radar. Proposals exist to use reflections from emitters such as civilian radio transmitters, including cellular telephone radio towers.<ref>MIT's "The Tech – online edition" article Detection of the B-2 Stealth Bomber and a Brief History on "Stealth" by Tao Yue published 30 November 2001 in (Volume 121, Issue 63)</ref>

Moore's law

By Moore's law the processing power behind radar systems is rising over time. This will eventually erode the ability of physical stealth to hide vehicles.<ref>Global Opposition Movement Challenges JSF</ref><ref>The Naval Institute guide to world naval weapon systems By Norman Friedman, Introduction page x</ref>

Ship wakes and spray

Synthetic aperture sidescan radars can be used to detect the location and heading of ships from their wake patterns.<ref>Reed, Arthur M.; Milgram, Jerome H. (1 January 2002). "Ship Wakes and Their Radar Images". Annual Review of Fluid Mechanics. 34 (34): 469–502. Bibcode:2002AnRFM..34..469R. doi:10.1146/annurev.fluid.34.090101.190252.</ref> These are detectable from orbit.<ref>Graziano, Maria; Grasso, Marco; d'Errico, Marco (2017). "Performance Analysis of Ship Wake Detection on Sentinel-1 SAR Images". Remote Sensing. 9 (11): 1107. Bibcode:2017RemS....9.1107G. doi:10.3390/rs9111107.</ref> When a ship moves through a seaway it throws up a cloud of spray which can be detected by radar.<ref>Raizer, Victor (2013). "Radar backscattering from sea foam and spray". 2013 IEEE International Geoscience and Remote Sensing Symposium - IGARSS. pp. 4054–4057. doi:10.1109/IGARSS.2013.6723723. ISBN 978-1-4799-1114-1. S2CID 32858575.</ref>

Acoustics

Acoustic stealth plays a primary role for submarines and ground vehicles. Submarines use extensive rubber mountings to isolate, damp, and avoid mechanical noises that can reveal locations to underwater passive sonar arrays.

Early stealth observation aircraft used slow-turning propellers to avoid being heard by enemy troops below. Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. The presence of supersonic and jet-powered stealth aircraft such as the SR-71 Blackbird indicates that acoustic signature is not always a major driver in aircraft design, as the Blackbird relied more on its very high speed and altitude.

One method to reduce helicopter rotor noise is modulated blade spacing.<ref name="Global mod blade">"Stealth Helicopter: MH-X Advanced Special Operations Helicopter". GlobalSecurity.org. Retrieved 28 April 2012.</ref> Standard rotor blades are evenly spaced, and produce greater noise at a given frequency and its harmonics. Using varied spacing between the blades spreads the noise or acoustic signature of the rotor over a greater range of frequencies.<ref name="NASA mod blade">Edwards, Bryan (May 2002). Psychoacoustic Testing of Modulated Blade Spacing for Main Rotors – NASA/CR-2002-211651 (PDF). NASA. p. 1.2. CiteSeerX 10.1.1.15.3782. Retrieved 1 July 2019.</ref>

Visibility

The simplest technology is visual camouflage; the use of paint or other materials to color and break up the lines of a vehicle or person.

Most stealth aircraft use matte paint and dark colors, and operate only at night. Lately, interest in daylight Stealth (especially by the USAF) has emphasized the use of gray paint in disruptive schemes, and it is assumed that Yehudi lights could be used in the future to hide the airframe (against the background of the sky, including at night, aircraft of any colour appear dark<ref name=NDRC>Bush, Vannevar; Conant, James; Harrison, George (1946). "Camouflage of Sea-Search Aircraft" (PDF). Visibility Studies and Some Applications in the Field of Camouflage. Office of Scientific Research and Development, National Defence Research Committee. pp. 225–240. Archived from the original (PDF) on 23 October 2013. Retrieved 12 February 2013.</ref>) or as a sort of active camouflage. The original B-2 design had wing tanks for a contrail-inhibiting chemical, alleged by some to be chlorofluorosulfonic acid,<ref>"Military Power". Aeronautics.ru. January 2000. Archived from the original on 14 January 2004.</ref> but this was replaced in the final design with a contrail sensor that alerts the pilot when he should change altitude<ref>Gosnell, Mariana (July 2007). "Flight Lines: Why contrails hang around". Air and Space. Retrieved 1 July 2019.</ref> and mission planning also considers altitudes where the probability of their formation is minimized.

In space, mirrored surfaces can be employed to reflect views of empty space toward known or suspected observers; this approach is compatible with several radar stealth schemes. Careful control of the orientation of the satellite relative to the observers is essential, and mistakes can lead to detectability enhancement rather than the desired reduction.

Infrared

An exhaust plume contributes a significant infrared signature. One means to reduce IR signature is to have a non-circular tail pipe (a slit shape) to minimize the exhaust cross sectional area and maximize the mixing of hot exhaust with cool ambient air (see Lockheed F-117 Nighthawk). Often, cool air is deliberately injected into the exhaust flow to boost this process (see Ryan AQM-91 Firefly and Northrop Grumman B-2 Spirit). The Stefan–Boltzmann law shows how this results in less energy (Thermal radiation in infrared spectrum) being released and thus reduces the heat signature. In some aircraft, the jet exhaust is vented above the wing surface to shield it from observers below, as in the Lockheed F-117 Nighthawk, and the unstealthy Fairchild Republic A-10 Thunderbolt II. To achieve infrared stealth, the exhaust gas is cooled to the temperatures where the brightest wavelengths it radiates are absorbed by atmospheric carbon dioxide and water vapor, greatly reducing the infrared visibility of the exhaust plume.<ref>Kopp, Carlo (November 1989 – January 1990). "Optical Warfare – The New Frontier". Australian Aviation. Australian Aviation (original). 1989 (November). Retrieved 23 June 2019.</ref> Another way to reduce the exhaust temperature is to circulate coolant fluids such as fuel inside the exhaust pipe, where the fuel tanks serve as heat sinks cooled by the flow of air along the wings.^{[citation needed]}

Ground combat includes the use of both active and passive infrared sensors. Thus, the United States Marine Corps (USMC) ground combat uniform requirements document specifies infrared reflective quality standards.<ref>Junek, Larry; Cameron, Meghan; Ditto, Susan; Morris, Elizabeth; Shaughnessy, Michael (28 May 2010). "Warfighter Support: Observations on DOD's Ground Combat Uniforms; GAO-10-669R". Government Accountability Office. United States Federal Government. Retrieved 23 June 2019.</ref>

Reducing radio frequency (RF) emissions

In addition to reducing infrared and acoustic emissions, a stealth vehicle must avoid radiating any other detectable energy, such as from onboard radars, communications systems, or RF leakage from electronics enclosures. The F-117 uses passive infrared and low light level television sensor systems to aim its weapons and the F-22 Raptor has an advanced LPI radar which can illuminate enemy aircraft without triggering a radar warning receiver response.

Measuring

The size of a target's image on radar is measured by the radar cross section (RCS), often represented by the symbol σ and expressed in square meters. This does not equal geometric area. A perfectly conducting sphere of projected cross sectional area 1 m² (i.e. a diameter of 1.13 m) will have an RCS of 1 m². Note that for radar wavelengths much less than the diameter of the sphere, RCS is independent of frequency. Conversely, a square flat plate of area 1 m² will have an RCS of σ=4π A² / λ² (where A=area, λ=wavelength), or 13,982 m² at 10 GHz if the radar is perpendicular to the flat surface.<ref>Knott, Eugene; Shaeffer, John; Tuley, Michael (1993). Radar Cross Section, 2nd ed. Artech House, Inc. p. 231. ISBN 0-89006-618-3.</ref> At off-normal incident angles, energy is reflected away from the receiver, reducing the RCS. Modern stealth aircraft are said to have an RCS comparable with small birds or large insects,<ref>"F-22 Raptor Stealth". GlobalSecurity.</ref> though this varies widely depending on aircraft and radar.

If the RCS was directly related to the target's cross-sectional area, the only way to reduce it would be to make the physical profile smaller. Rather, by reflecting much of the radiation away or by absorbing it, the target achieves a smaller radar cross section.<ref>Sweetman, Bill (January 2008). "Unconventional Weapon: What we learned about stealth technology from the combat career of the F-117". Smithsonian Air & Space Magazine. Archived from the original on 19 July 2012.</ref>

Tactics

Stealthy strike aircraft such as the Lockheed F-117 Nighthawk, are usually used against heavily defended enemy sites such as command and control centers or surface-to-air missile (SAM) batteries. Enemy radar will cover the airspace around these sites with overlapping coverage, making undetected entry by conventional aircraft nearly impossible. Stealthy aircraft can also be detected, but only at short ranges around the radars; for a stealthy aircraft there are substantial gaps in the radar coverage. Thus a stealthy aircraft flying an appropriate route can remain undetected by radar. Even if a stealth aircraft is detected, fire-control radars operating in C, X and Ku bands cannot paint (for missile guidance) low observable (LO) jets except at very close ranges.<ref>Majumdar, Dave (8 November 2018). "How Russia Could Someday Shootdown an F-22, F-35 or B-2 Stealth Bomber". The National Interest.</ref> Many ground-based radars exploit Doppler filter to improve sensitivity to objects having a radial velocity component relative to the radar. Mission planners use their knowledge of enemy radar locations and the RCS pattern of the aircraft to design a flight path that minimizes radial speed while presenting the lowest-RCS aspects of the aircraft to the threat radar. To be able to fly these "safe" routes, it is necessary to understand an enemy's radar coverage (see electronic intelligence). Airborne or mobile radar systems such as airborne early warning and control (AEW&C, AWACS) can complicate tactical strategy for stealth operation.

Research

After the invention of electromagnetic metasurfaces, the conventional means to reduce RCS have been improved significantly.<ref name="A. Modi 19 2">A. Y. Modi; M. A. Alyahya; C. A. Balanis; C. R. Birtcher, "Metasurface-Based Method for Broadband RCS Reduction of Dihedral Corner Reflectors with Multiple Bounces," in IEEE Transactions on Antennas and Propagation, vol.67, no.12, pp. -, Dec. 2019. doi:10.1109/TAP.2019.2940494</ref><ref name="A. Modi 19">Modi, A. Y.; Balanis, C. A.; Birtcher, C. R.; Shaman, H. (January 2019). "New Class of RCS-Reduction Metasurfaces Based on Scattering Cancellation Using Array Theory". IEEE Transactions on Antennas and Propagation. 67 (1): 298–308. Bibcode:2019ITAP...67..298M. doi:10.1109/TAP.2018.2878641. S2CID 58670543.</ref><ref name="A. Modi 17">Modi, A. Y.; Balanis, C. A.; Birtcher, C. R.; Shaman, H. (October 2017). "Novel Design of Ultra-Broadband Radar Cross Section Reduction Surfaces using Artificial Magnetic Conductors". IEEE Transactions on Antennas and Propagation. 65 (10): 5406–5417. Bibcode:2017ITAP...65.5406M. doi:10.1109/TAP.2017.2734069. S2CID 20724998.</ref> As mentioned earlier, the main objective in purpose shaping is to redirect scattered waves away from the backscattered direction, which is usually the source. However, this usually compromises aerodynamic performance.<ref>Li, Yongfeng; Zhang, Jieqiu; Qu, Shaobo; Wang, Jiafu; Chen, Hongya; Xu, Zhuo; Zhang, Anxue (5 June 2014). "Wideband radar cross section reduction using two-dimensional phase gradient metasurfaces". Applied Physics Letters. 104 (221110): 221110. Bibcode:2014ApPhL.104v1110L. doi:10.1063/1.4881935.</ref> One feasible solution, which has extensively been explored in recent time, is to use metasurfaces which can redirect scattered waves without altering the geometry of a target.<ref name="A. Modi 19 2"/><ref name="A. Modi 19"/><ref name="A. Modi 17"/> Such metasurfaces can primarily be classified in two categories: (i) checkerboard metasurfaces, (ii) gradient index metasurfaces. Similarly, negative index metamaterials are artificial structures for which refractive index has a negative value for some frequency range, such as in microwave, infrared, or possibly optical.<ref>Shelby, R. A.; Smith, D.R; Shultz, S. (2001). "Experimental Verification of a Negative Index of Refraction". Science. 292 (5514): 77–79. Bibcode:2001Sci...292...77S. doi:10.1126/science.1058847. PMID 11292865. S2CID 9321456.</ref> These offer another way to reduce detectability, and may provide electromagnetic near-invisibility in designed wavelengths.

Plasma stealth is a phenomenon proposed to use ionized gas, termed a plasma, to reduce RCS of vehicles. Interactions between electromagnetic radiation and ionized gas have been studied extensively for many purposes, including concealing vehicles from radar. Various methods might form a layer or cloud of plasma around a vehicle to deflect or absorb radar, from simpler electrostatic to radio frequency (RF) more complex laser discharges, but these may be difficult in practice.<ref>Adamovich, I.V.; Rich, J. W.; Chernukho, A.P.; Zhdanok, S.A. (2000). "Analysis of the Power Budget and Stability of High-Pressure Nonequilibrium Air Plasmas" (PDF). Proceedings of 31st AIAA Plasmadynamics and Lasers Conference, 19–22 June 2000. pp. Paper 00–2418. Archived from the original (PDF) on 10 September 2006.</ref>

Several technology research and development efforts exist to integrate the functions of aircraft flight control systems such as ailerons, elevators, elevons, flaps, and flaperons into wings to perform the aerodynamic purpose with the advantages of lower RCS for stealth, via simpler geometries and lower complexity (mechanically simpler, fewer or no moving parts or surfaces, less maintenance), and lower mass, cost (up to 50% less), drag (up to 15% less during use), and inertia (for faster, stronger control response to change vehicle orientation to reduce detection). Two promising approaches are flexible wings, and fluidics.

In flexible wings, much or all of a wing surface can change shape in flight to deflect air flow. Adaptive compliant wings are a military and commercial effort.<ref>Scott, William B. (27 November 2006). "Morphing Wings". Aviation Week & Space Technology.</ref><ref>"FlexSys Inc.: Aerospace". Archived from the original on 16 June 2011. Retrieved 26 April 2011.</ref><ref>Kota, Sridhar; Osborn, Russell; Ervin, Gregory; Maric, Dragan; Flick, Peter; Paul, Donald. "Mission Adaptive Compliant Wing – Design, Fabrication and Flight Test" (PDF). Ann Arbor, Michigan; Dayton, Ohio; United States: FlexSys Inc., Air Force Research Laboratory. Archived from the original (PDF) on 22 March 2012. Retrieved 26 April 2011.</ref> The X-53 Active Aeroelastic Wing was a US Air Force, Boeing, and NASA effort.

In fluidics, fluid injection into airflows is being researched for use in aircraft to control direction, in two ways: circulation control and thrust vectoring. In both, larger more complex mechanical parts are replaced by smaller, simpler, lower mass fluidic systems, in which larger forces in fluids are diverted by smaller jets or flows of fluid intermittently, to change the direction of vehicles. Mechanical control surfaces that must move cause an important part of aircraft radar cross-section.<ref name="IDST1">Uppal, Rahesh (3 March 2022). "Active Flow Control for Stealth Aircraft and Drones". International Defense, Security & Technology (IDST). Retrieved 30 May 2023.</ref><ref name="NATO">Making aircraft less detectable (video). Europe, United States: North Atlantic Treaty Organization (NATO). 3 August 2018. Retrieved 30 May 2023.</ref><ref name="NatInt2019">Axe, David (13 February 2019). "The F-22 and B-2 Bomber Are Old: A New Generation of Super Stealth Is Coming". The National Interest. Center for the National Interest. Retrieved 21 June 2019.</ref> Omitting mechanical control surfaces can reduce radar returns.<ref name="NatInt2019"/><ref name="UAV2010">Christopher, Dombrowski (5 October 2010). "New test plane flies without control surfaces". Ars Technica. Wired Media Group. Retrieved 21 June 2019.</ref><ref name="UAV2017">"Successful first flight trial completion of unmanned aerial vehicle, MAGMA". BAE Systems. 13 December 2017. Retrieved 21 June 2019.</ref> As of 2023^[update], at least two countries are known to be researching fluidic control. In Britain, BAE Systems has tested two fluidically controlled unmanned aircraft, one starting in 2010 named Demon,<ref name="UAV2010"/><ref name="NatInt2019"/> and another starting in 2017 named MAGMA, with the University of Manchester.<ref name="UAV2017"/> In the United States, the Defense Advanced Research Projects Agency (DARPA) program named Control of Revolutionary Aircraft with Novel Effectors (CRANE) seeks "... to design, build, and flight test a novel X-plane that incorporates active flow control (AFC) as a primary design consideration. ... In 2023, the aircraft received its official designation as X-65."<ref>Wlezien, Richard. "Control of Revolutionary Aircraft with Novel Effectors (CRANE)". Defense Advanced Research Projects Agency (DARPA). Retrieved 4 October 2023.</ref><ref>Trimble, Steve (16 May 2023). "DARPA Receives X-65 Designation for Active Flow Experiment". Aviation Week & Space Technology. Retrieved 4 October 2023.</ref> In winter 2024, construction began, at Boeing subsidiary Aurora Flight Sciences.<ref name="AFS">Smith, Carmen (3 January 2024). "Aurora Begins Building Full-Scale Active Flow Control X-Plane". Aurora Flight Sciences, Boeing (Press release). Retrieved 1 February 2024.</ref> In summer 2025, flight testing is to start.<ref name="AFS"/>

In circulation control, near the trailing edges of wings, aircraft flight control systems are replaced by slots which emit fluid flows.<ref>John, Philip (2010). "The flapless air vehicle integrated industrial research (FLAVIIR) programme in aeronautical engineering". Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering. London: Mechanical Engineering Publications. 224 (4): 355–363. doi:10.1243/09544100JAERO580. hdl:1826/5579. ISSN 0954-4100. S2CID 56205932. Archived from the original on 23 June 2018.</ref><ref>"Showcase UAV Demonstrates Flapless Flight". BAE Systems. 2010. Archived from the original on 7 July 2011. Retrieved 22 December 2010.</ref><ref>"Demon UAV jets into history by flying without flaps". Metro. UK: Associated Newspapers Limited. 28 September 2010.</ref>

List of stealth aircraft

F-117 Nighthawk
B-2 Spirit
F-22 Raptor
F-35 Lightning II
J-20
Su-57
B-21 Raider<ref>"New stealth bomber contract likely to be boon for Antelope Valley". Los Angeles Times. 8 February 2015. Retrieved 17 March 2023.</ref>
FC-31
SU-75 Checkmate

List of reduced-signature ships

Navy ships worldwide have incorporated signature-reduction features, mostly for the purpose of reducing anti-ship missile detection range and enhancing countermeasure effectiveness rather than actual detection avoidance. Such ships include:

List of stealth helicopters

References

Bibliography

Dawson, T.W.G., G.F. Kitchen and G.B. Glider. Measurements of the Radar Echoing Area of the Vulcan by the Optical Simulation Method. Farnborough, Hants, UK: Royal Aircraft Establishment, September 1957 National Archive Catalogue file, AVIA 6/20895
Ufimtsev, Pyotr Ya., "Method of edge waves in the physical theory of diffraction," Moscow, Russia: Izd-vo. Sov. Radio [Soviet Radio Publishing], 1962, pages 1–243.
Doucet, Arnaud; Freitas, Nando de; Gordon, Neil (2001) [2001]. Sequential Monte Carlo Methods in Practice. Statistics for Engineering and Information Science (1st ed.). Berlin: Springer-Verlag. ISBN 978-0-387-95146-1. Retrieved 11 March 2009.
Analogues of Stealth – Northrop Grumman Archived 12 August 2012 at the Wayback Machine
Countering stealth
How "stealth" is achieved on F-117A
United States Patent No.6,297,762. 2 October 2001. Electronic countermeasures system (Apparatus for detecting the difference in phase between received signals at two spaced antennas and for then retransmitting equal amplitude antiphase signals from the two spaced antennas is disclosed.)
Gal-Or, Benjamin (1990). "Multiaxis Thrust Vectoring Flight Control vs Catastrophic Failure Prevention". Vectored Propulsion, Supermanoeuvreability, and Robot Aircraft. Reports to U.S. Dept. of Transportation/FAA, Technical Center, ACD-210, FAA X88/0/6FA/921000/4104/T1706D, FAA Res. Grant-Award No: 94-G-24, CFDA, No. 20.108, 26 December 1994. Springer Verlag. ISBN 0-387-97161-0. and 3-540-97161-0.
Suhler, Paul A. (2009). From Rainbow to Gusto: Stealth and the Design of the Lockheed Blackbird. American Institute of Aeronautics and Astronautics. ISBN 978-1-60086-712-5.

External links

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