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F-117 stealth attack plane

Stealth technology also known as LO technology (low observable technology) is a sub-discipline of military tactics and passive electronic countermeasures,[1] which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, in order to make them less visible (ideally invisible) to radar, infrared,[2] sonar and other detection methods.

Development in the United States occurred in 1958,[3][4] where earlier attempts in preventing radar tracking of its U-2 spy planes during the Cold War by the Soviet Union had been unsuccessful.[5] Designers turned to develop a particular shape for planes that tended to reduce detection, by redirecting electromagnetic waves from radars.[6] Radar absorbent material was also tested and made to reduce or block radar signals that "bounced" off from the surface of planes. Such changes to shape and surface composition form stealth technology as currently deployed on the B-2 Spirit "Stealth Bomber".[4] Billions of dollars have also been spent in developing stealth over a number of decades but the U.S. has been the only country economically able to do this.[6][7]

The concept of stealth is to operate or hide without giving enemy forces any indications as to the presence of friendly forces. This concept was first explored through camouflage by blending into the background visual clutter. As the potency of detection and interception technologies (radar, IRST, surface-to-air missiles etc.) have increased over time, 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 will generally have been designed from the outset to have reduced or controlled signature. Varying degrees of stealth can be achieved. The exact level and nature of stealth embodied in a particular design is determined by the prediction of likely threat capabilities.

Contents

History

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

Yehudi lights were successfully employed in World War II by RAF Shorts Sunderland aircraft in attacks on U-boats. In 1945 a Grumman Avenger with Yehudi lights got within 3,000 yards (2,700 m) of a ship before being sighted. This ability was rendered obsolete by the radar of the time.

One of the earliest stealth aircraft seems to have been the Horten Ho 229 flying wing. This included carbon powder in the glue to absorb radio waves.[8] However, it was never deployed in any quantity.

In 1958, the CIA requested funding for a reconnaissance aircraft, to replace U-2 spy planes[9] in which Lockheed secured contractual rights to produce the aircraft.[3]. "Kelly" Johnson and his team at Lockheed's Skunk Works were assigned to produce the A-12 or OXCART the first of the former top secret classified Blackbird series which operated at high altitude of 70000 to 80000 ft and speed of Mach 3.2 to avoid radar detection. Radar absorbent material had already been introduced on U-2 spy planes, and various plane shapes had been developed in earlier prototypes named A1 to A11 to reduce its detection from radar.[4] Later in 1964, using previous models an optimal plane shape taking into account compactness was developed where another "Blackbird", the SR-71, was produced, surpassing previous models in both altitude of 90 000 ft and speed of Mach 3.3.[4]

During 1970s, the U.S. Department of Defence then launched a project called Have Blue the project to develop a stealth fighter. Bidding between both Lockheed and Northrop for the tender was fierce to secure the multi billion dollar contract. Lockheed incorporated in its program paper written by a Soviet/Russian physicist Pyotr Ufimtsev in 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 (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. This theory played a critical role in the design of American stealth-aircraft F-117 and B-2.[10][11][12] The paper was able to find whether a plane's shape design would minimise its detection by radar or its radar cross-section (RCS) using a series of equations[13] could be used to evaluate the radar cross section of any shape. Lockheed used it to design a shape they called the Hopeless Diamond, securing contractual rights to mass produce the F-117 Nighthawk.

The F-117 project began with a model called "The Hopeless Diamond" (a wordplay on the Hope Diamond) in 1975 due to its bizarre appearance. 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 success of Have Blue lead the Air Force to create the Senior Trend[14][15] program which developed the F-117.

Stealth principles

Stealth technology (or LO for "low observability") is not a single technology. It is a combination of technologies that attempt to greatly reduce the distances at which a person or vehicle can be detected; in particular radar cross section reductions, but also acoustic, thermal, and other aspects:

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Radar cross-section (RCS) reductions

Almost since the invention of radar, various techniques have been tried to minimize detection. Rapid development of radar during WWII led to equally rapid development of numerous counter radar measures during the period; a notable example of this was the use of chaff.

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.[16] Increased awareness of stealth vehicles and the technologies behind them is prompting the development of techniques for detecting stealth vehicles, such as passive radar arrays and low-frequency radars. Many countries nevertheless continue to develop low-RCS vehicles because they offer advantages in detection range reduction and amplify the effectiveness of on-board systems against active radar guidance threats.[citation needed]

Vehicle shape

The F-35 Lightning II offers better stealthy features (such as this landing gear door) than previous American fighters, such as the F-16 Fighting Falcon

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. On the other hand, the Tupolev 95 Russian long range bomber (NATO reporting name 'Bear') appeared especially well on radar. It is now known that propellers and jet turbine blades produce a bright radar image; the Bear had four pairs of large (5.6 meter diameter) contra-rotating propellers.

Another important factor is the internal construction. Behind the skin of some aircraft are structures known as re-entrant triangles. Radar waves penetrating the skin of the aircraft get trapped in these structures, bouncing off the internal faces and losing energy. This approach was first used on the F-117.

The most efficient way to reflect radar waves back to the transmitting 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 approach is to eliminate the tail completely, as in the B-2 Spirit.

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 existing aircraft, install baffles in the air intakes, so that the turbine 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 is opened.

Planform alignment is also often used in stealth designs. Planform alignment involves using a small number of surface orientations in the shape of the structure. For example, on the F-22A Raptor, the leading edges of the wing and the tail surfaces are set at the same angle. Careful inspection shows that many small structures, such as the air intake bypass doors and the air refueling aperture, also use the same angles. The effect of planform alignment is to return a radar signal in a very specific direction away from the radar emitter rather than returning a diffuse signal detectable at many angles.

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 use of re-entrant triangles and planform alignment, this time on the external airframe.

Shaping requirements have strong negative influence on the aircraft's aerodynamic properties. The F-117 has poor aerodynamics, is inherently unstable, and cannot be flown without a fly-by-wire control system.

The stealth ship, K32 HMS Helsingborg

Ships have also adopted similar techniques. The Visby corvette was the first stealth ship to enter service, though the earlier Arleigh Burke class destroyer incorporated some signature-reduction features [1]. Other examples are the French La Fayette class frigate, the USS San Antonio amphibious transport dock, and most modern warship designs.

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, bounce off an object (the inside of the cockpit has a complex shape, with the pilot's helmet itself providing 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 the pilot's vision.

Non-metallic airframe

Dielectric composites 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 the material for its application.

Radar absorbing material

Radar absorbent material (RAM), often as paints, are used especially on the edges of metal surfaces. While the material and thickness of RAM coatings is classified, the material seeks to absorb radiated energy from a ground or air based radar station into the coating and convert it to heat rather than reflect it back.

Radar stealth countermeasures and limitations

Low frequency radar

Shaping does not offer 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 identify it, and the location information lacks sufficient weapon targeting accuracy. Noise poses another problem, but that can be efficiently addressed using modern computer technology; Chinese "Nantsin" radar and many older Soviet-made long-range radars were modified this way. It has been said that "there's nothing invisible in the radar frequency range below 2 GHz". [2]

Multiple transmitters

Much of the stealth comes from reflecting the transmissions in a different direction other than a direct return. Therefore detection can be better achieved if the sources are spaced from the receivers, known as bistatic radar, and proposals exist to use reflections from sources such as civilian radio transmitters, including cellular telephone radio towers.[17]

Moore's law

By Moore's law the processing power behind radar systems is expected to improve over time, which will erode the ability of physical stealth to hide an aircraft, but that same level of improvement will boost the stealth aircraft's own electronic warfare equipment, which will always have a quieter return signal to mask than a non-stealth aircraft would return.[18][19]

Acoustics

Acoustic stealth plays a primary role in submarine stealth as well as for ground vehicles. Submarines have extensive usage of rubber mountings to isolate and avoid mechanical noises that could 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, although the Blackbird relied more on its extremely high speed and altitude.

Visibility

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

Most stealth aircraft use matte paint and dark colors, and operate only at night. Lately, interest on 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 mask shadows in the airframe (in daylight, against the clear background of the sky, dark tones are easier to detect than light ones) 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 chlorofluorosulphonic acid[3], but this was replaced in the final design with a contrail sensor from Ophir that alerts the pilot when he should change altitude[20] and mission planning also considers altitudes where the probability of their formation is minimized.

Infrared

An exhaust plume contributes a significant infrared signature. One means of reducing the IR signature is to have a non-circular tail pipe (a slit shape) in order to minimize the exhaust cross-sectional volume and maximize the mixing of the hot exhaust with cool ambient air. Often, cool air is deliberately injected into the exhaust flow to boost this process. Sometimes, the jet exhaust is vented above the wing surface in order to shield it from observers below, as in the B-2 Spirit, and the unstealthy A-10 Thunderbolt II. To achieve infrared stealth, the exhaust gas is cooled to the temperatures where the brightest wavelengths it radiates on are absorbed by atmospheric carbon dioxide and water vapor, dramatically reducing the infrared visibility of the exhaust plume. [4] 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.

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 stealth

The size of a target's image on radar is measured by the radar cross section or 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.[21] 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, though this varies widely depending on aircraft and radar.[citation needed]

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 absorbing it altogether, the target achieves a smaller radar cross section.[5]

Stealth tactics

Stealthy strike aircraft such as the F-117, designed by Lockheed Martin's famous Skunk Works, 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, so that 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. Many ground-based radars exploit Doppler filter to improve sensitivity to objects having a radial velocity component with respect to the radar. Mission planners use their knowledge of the 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. In order to be able to fly these "safe" routes, it is necessary to understand the enemy's radar coverage (see Electronic Intelligence). Airborne or mobile radar systems such as AWACS can complicate tactical strategy for stealth operation.

List of stealth aircraft

Manned

Fully stealth designs

Retired
In service
Under development
Cancelled
Technology demonstrators

Reduced RCS designs

Unmanned (full stealth)

List of stealth ships

Fully Stealth Designs

Reduced RCS Designs

See also

References

  1. ^ Rao, G.A., & Mahulikar, S.P., (2002) Integrated review of stealth technology and its role in airpower, Aeronautical Journal, v. 106(1066): 629-641.
  2. ^ Mahulikar, S.P., Sonawane, H.R., & Rao, G.A., (2007) Infrared signature studies of aerospace vehicles, Progress in Aerospace Sciences, v. 43(7-8): 218-245.
  3. ^ a b Richelson, J.T. (10 September 2001). "Science, Technology and the CIA". The National Security Archive. The George Washington University. http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB54/. Retrieved 6 October 2009. 
  4. ^ a b c d Merlin, P.W. "Design and Development of the Blackbird: Challenges and Lessons Learned" American Institute of Aeronautics and Astronautics 47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition 5–8 January 2009, Orlando, Florida. Accessed 2009-10-06.
  5. ^ Cadirci, S. "RF Stealth (Or Low Observable) and Counter- RF Stealth Technologies: Implications of Counter- RF Stealth Solutions for Turkish Air Force." Naval Postgraduate School, Monterey California, Ph.D. Thesis. March 2009. Accessed 6 October 2009.
  6. ^ a b 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. http://tech.mit.edu/V121/N63/Stealth.63f.html. Retrieved 5 October 2009. 
  7. ^ "Military Airplanes". Hawkeye Engineer. College of Engineering, University of Iowa. Fall 2001. http://www.engineering.uiowa.edu/~hawkeng/fall01/jsf3.shtml. Retrieved 5 October 2009. 
  8. ^ Myhra 2009, p. 11.
  9. ^ Poteat, Gene (1998). "Stealth, Countermeasures, and ELINT, 1960-1975". Studies in Intelligence 48 (1): 51–59. http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB54/st08.pdf. 
  10. ^ Browne, M.W. "Two rival designers led the way to stealthy warplanes", New York Times, Sci. Times Sec., May 14, 1991.
  11. ^ Browne, M.W. "Lockheed credits Soviet theory in design of F-117", Aviation Week Space Technology p. 27, December 1991.
  12. ^ Rich, Ben and L. Janos, Skunk Works, Little Brown, Boston, 1994.
  13. ^ 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. http://books.google.com/books?id=0WuGjb8sqCUC&pg=PA210&dq=ufimtsev+finite+equation#v=onepage&q=&f=false. Retrieved 7 October 2009. 
  14. ^ F-117A Senior Trend
  15. ^ "Senior Trend". Vectorsite.net, 1 April 2008.
  16. ^ Crocker, H. W. III (2006). Don't Tread on Me. New York: Crown Forum. p. 382. ISBN 9781400053636. 
  17. ^ MIT's "The Tech - online edition" article Detection of the B-2 Stealth Bomber And a Brief History on “Stealth” by Tao Yue published November 30, 2001 in (Volume 121 >> Issue 63)
  18. ^ Global Opposition Movement Challenges JSF
  19. ^ The Naval Institute guide to world naval weapon systems By Norman Friedman, Introduction page x
  20. ^ Air and Space mag: Why contrails hang around.
  21. ^ Knott, Eugene; Shaeffer, John, and Tuley, Michael (1993). Radar Cross Section, 2nd ed. Artech House, Inc.. pp. 231. ISBN 0-89006-618-3. 

Bibliography

Notes

External links


Simple English

File:F-117 Nighthawk
F-117 stealth attack plane

Stealth technology also known as LOT (Low Observability Technology) is a sub-discipline of military electronic countermeasures which covers a range of techniques used with aircraft, ships and missiles, in order to make them less visible.


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