Stealth is the technique of making a plane (or any other object) less visible to the enemy by reducing its radar and IR (infra red, heat) visibility. Reducing the IR image of a plane can be accomplished by directing the hot exhaust gasses to the top of the plane and mix them with cold air (the B-2 uses this technique). Reducing radar visibility can be accompli^ed by deflecting the radar waves in such a direction that they don't go back to the emitting radar (F-117 uses faceting, the B-2 uses continues curving) or making use of less radar reflecting materials (composite material, plastic) and/or radar absorbing coatings. A study of radar absorbing materials is also made. The seminars also deals with plasma stealth technology
Stealth is the technique of making a plane (or any other object) less visible to the enemy by reducing its radar and IR (infra red, heat) visibility. Reducing the IR image of a plane can be accomplished by directing the hot exhaust gasses to the top of the plane and mix them with cold air (the B-2 uses this technique). Reducing radar visibility can be accompliced by deflecting the radar waves in such a direction that they don't go back to the emitting radar (F-117 uses faceting, the B-2 uses continues curving) or making use of less radar reflecting materials (composite material, plastic) and/or radar absorbing coatings (e.g. B-2). The active substance in these coatings are mostly metal ions. Because of this most of the radar absorbing coatings aren't very water-resistant.
Stealth technology, designs and materials engineered for the military purpose of avoiding detection by radar or any other electronic system. Stealth, or anti detection, technology is applied to vehicles (e.g., tanks), missiles, ships, and aircraft with the goal of making the object more difficult to detect at closer and closer ranges. Since radar is the most difficult form of detection to elude, avoidance is generally accomplished by reducing the radar cross section (RCS) of the object to within the level of background noise; for example, the reported goal of U.S. military designers is to make a fighter plane with an RCS the size of a bird. The RCS is the area of an imaginary perfect reflector that would reflect the same amount of energy back to the receiving radar antenna, as does the actual target, which may be much larger or even smaller than the RCS. A pickup truck, for example, with its flat surfaces and sharp edges has an RCS of approximately 200 sq m, but a smooth-edged fighter jet has an RCS of only 2 to 4 sq m. The RCS of any given object, however, differs at various angles and radar frequencies. Much about stealth technology remains classified, but among the anti detection techniques used in the U.S. Air Force F-117 Stealth fighter plane (which probably has an RCS of 1 sq m or less) are a low profile with no flat surfaces to reflect radar directly back, the intensive substitution of radar opaque composites in place of metals, and an overall coating of radar absorbing
material. The implementation of stealth technology usually requires such compromises as reduced payload capacity, aerodynamic instability, and high design, production, and maintenance expenses.
II. STEALTH DESIGN
i. REDUCING RADAR VISIBILITY
There are two basic approaches to passive radar cross-section reduction: shaping to minimize backscatter, and coating for energy absorption and cancellation. Both of these approaches have to be used coherently in aircraft design to achieve the required low observable levels over the appropriate frequency range in the electromagnetic spectrum.
Most conventional aircraft have a rounded shape. This shape makes them aerodynamic, but it also creates a very efficient radar reflector. The round shape means that no matter where the radar signal hits the plane, some of the signal gets reflected back
A stealth aircraft, on the other hand, is made up of completely flat surfaces and very sharp edges. When a radar signal hits a stealth plane, the signal reflects away at an angle, like this:
In addition, surfaces on a stealth aircraft can be treated so they absorb radar energy as well. The overall result is that a stealth aircraft like an F-l 17A can have the radar signature of a small bird rather than an airplane. The only exception is when the plane banksâ€there will often be a moment when one of the panels of the plane will perfectly reflect a burst of radar energy back to the antenna. There is a tremendous advantage to positioning surfaces so that the radar wave strikes them at close to tangential angles and far from right angles to edges, as will now be illustrated.One case to consider is a rotation of the plate from normal incidence to a shallow angle, with the radar beam at right angles to a pair of edges. The other is with the radar beam at 45 degrees to the edges. The frequency is selected so that the wavelength is about 1/10 of the length of the plate, in this case very typical of acquisition radars on surface to air missile systems. At normal incidence, the flat plate acts like a mirror, and its return is 30 decibels (dB) above
(or 1,000 times) the return from the sphere. If we now rotate the plate about one edge so that the edge is always normal to the incoming wave, we find that the cross section drops by a factor of 1,000, equal to that of the sphere, when the look angle reaches 30 degrees off normal to the plate. As the angle is increased, the locus of maxima falls by about another factor Of 50, for a total change of 50,000 from the normal look angle. Now if you go back to the normal incidence case and rotate the plate about a diagonal relative to the incoming wave, there is a remarkable difference. In this case, the cross section drops by 30 dB when the plate is only eight degrees off normal, and drops another 40 dB by the time the plate is at a shallow angle to the incoming radar beam. This is a total change in radar cross section of 10,000,000! From this, it would seem that it is fairly easy to decrease the radar cross section substantially by merely avoiding obviously high-return shapes and attitude angles. However, multiple-reflection cases have not yet been looked at, which change the situation considerably. It is fairly obvious that energy aimed into a long, narrow, closed cavity, which is a perfect reflector internally, will bounce back in the general direction of its source. Furthermore, the shape of the cavity downstream of the entrance clearly does not influence this conclusion. However, the energy reflected from a straight duct will be reflected in one or two bounces, while that from a curved duct will require four or five bounces. It can be imagined that with a little skill, the number of bounces can be increased significantly without sacrificing aerodynamic performance. For example, a cavity might be designed with a high-cross-sectional aspect ratio to maximize the length-to-height ratio. If we can attenuate the signal to some extent with each bounce, then clearly there is a significant advantage to a multi-bounce design. The SR-71 inlet follows these design practices. One of the main efforts taken by designers of the stealth aircraft of today is to carry the weapons payload of the aircraft internally. This has shown that carrying weapons internally can considerably decrease the radar cross-section of the aircraft. Bombs and Missiles have a tendency to reflect the incoming radar waves to a higher extent. Providing missiles with RAM and RAS is an impossible by the cost of these things. Thus the missiles are carried in internal bombays which are opened only when the weapons are released. Aircraft has used another method of avoiding detection for a very long time. Radars can use the radar waves or electro-magnetic energy of planes radar and locate it. An aircraft can remain undetected just by turning the radar off.
COATINGS AND ABSORBERS
It is fairly clear that although surface alignment is very important for external surfaces and inlet and exhaust edges, the return from the inside of a cavity is heavily dependent on attenuating materials. It is noted that the radar-frequency range of interest covers between two and three orders of magnitude. Permeability and dielectric constant are two properties that are closely associated with the effectivity of an attenuating material. They both vary considerably with frequency in different ways for different materials. Also, for a coating to be effective, it should have a thickness that is close to a quarter wavelength at the frequency of interest.
When the basic aircraft signature is reduced to a very low level, detail design becomes very important. Access panel and door edges, for example, have the potential to be major contributors to radar cross-section unless measures are taken to suppress them. Based on the discussion of simple flat plates, it is clear that it is generally unsatisfactory to have a door edge at right angles to the direction of flight. This would result in a
noticeable signal in a nose on aspect. Thus, conventional rectangular doors and access panels are unacceptable. The solution is not only to sweep the panel edges, but also to align those edges with other major edges on the aircraft. The pilot's head, complete with helmet, is a major source of radar return. It is augmented by the bounce path returns associated with internal bulkheads and frame members. The solution is to design the cockpit so that its external shape conforms to good low radar cross section design rules, and then plate the glass with a film similar to that used for temperature control in commercial buildings. Here, the requirements are more stringent: it should pass at least 85% of the visible energy and reflect essentially all of the radar energy. At the same time, a pilot would prefer not to have noticeable instrument-panel reflection during night flying. On an unstable, fly by wire aircraft, it is extremely important to have redundant sources of aerodynamic data. These must be very accurate with respect to flow direction, and they must operate ice-free at all times. Static and total pressure probes have been used, but they clearly represent compromises with stealth requirements. Several quite different techniques are in various stages of development. On board antennas and radar systems are major potential sources of high radar visibility for two reasons. One is that it is obviously difficult to hide something that is designed to transmit with very high efficiency, so the so-called in band radar cross section is liable to be significant. The other is that even if this problem is solved satisfactorily, the energy emitted by these systems can normally be readily detected. The work being done to reduce these signatures cannot be described here.
ii. REDUCING IR VISIBILITY
There is two significant sources of infrared radiation from air breathing propulsion systems: hot parts and jet wakes. The fundamental variables available for reducing radiation are temperature and emissivity, and the basic tool available is line of sight masking. Recently some interesting progress has been made in directed energy, particularly for multiple bounce situations, but that subject will not be discussed further here. Emissivity can be a double-edged sword, particularly inside a duct. While a low emissivity surface will reduce the emitted energy, it will also enhance reflected energy that may be coming from a hotter internal region. Thus, a careful optimization must be made to determine the preferred emissivity pattern inside a jet engine exhaust pipe. This pattern must be played against the frequency range available to detectors, which typically covers a band from one to 12 microns. The short wavelengths are particularly effective at high temperatures, while the long wavelengths are most effective at typical ambient atmospheric temperatures. The required emissivity pattern as a function of frequency and spatial dispersion having been determined, the next issue is how to make materials that fit the bill. The first inclination of the infrared coating designer is to throw some metal flakes into a transparent binder. Coming up with a transparent binder over the frequency range of interest is not easy, and the radar coating man probably won't like the effects of the metal particles on his favorite observable. The next move is usually to come up with a multi layer material, where the same cancellation approach that was discussed earlier regarding radar suppressant coatings is used. The dimensions now are in angstroms rather than millimeters.
The big push at present is in moving from metal layers in the films to. metal oxides for radar cross-section compatibility. Getting the required performance as a function of frequency is not easy, and it is a significant feat to get down to an emissivity of 0.1, particularly over a sustained frequency range. Thus, the biggest practical ratio of emissivities is liable to be one order of magnitude. Everyone can recognize that all of this discussion is meaningless if engines continue to deposit carbon (one of the highest emissivity materials known) on duct walls. For the infrared coating to be effective, it is
not sufficient to have a very low particulate ratio in the engine exhaust, but to have one that is essentially zero. Carbon buildup on hot engine parts is a cumulative situation, and there are very few bright, shiny parts inside exhaust nozzles after a number of hours of operation. For this reason alone, it is likely that emissivity control will predominantly be. employed on surfaces other than those exposed to engine exhaust gases, i.e., inlets and aircraft external parts. The other available variable is temperature. This, in principle, gives a great deal more opportunity for radiation reduction than emissivity, because of the large exponential dependence. The general equation for emitted radiation is that it varies with the product of emissivity and temperature to the fourth power. However, this is a great simplification, because it does not account for the frequency shift of radiation with temperature. In the frequency range at which most simple detectors work (one to five microns), and at typical hot-metal temperatures, the exponential dependency will be typically near eight rather than four, and so at a particular frequency corresponding to a specific detector, the radiation will be proportional to the product of the emissivity and temperature to the eighth power. It is fairly clear that a small reduction in temperature can have a much greater effect than any reasonably anticipated reduction in emissivity.
Another main aspect that reduces the IR signature of a stealth aircraft is to place the engines deep into the fuselage. This is done in stealth aircraft like the B-2, F-22 and the JSF. The IR reduction scheme used in F-l 17 is very much different from the others. The engines are placed deep within the aircraft like any stealth aircraft and at the outlet, a section of the fuselage deflects the exhaust to another direction. This is useful for deflecting the hot exhaust gases in another direction.
III. STEALTH MATERIALS AND COATINGS
Typical materials for reduced-observable treatments include, but are not limited to, the following categories:
i.There are two kinds of conductive fillers: conductive fibers, which look like very light whiskers 2 to 6 mm long, are made of carbon, metals, or conductive-material coated glass fibers; and conductive-material coated particles, which may look like colored sand.
ii.Sprays include conductive inks or paints, which normally contain silver,
copper, zinc, bronze, or gold as the base ingredient. They appear black, metallic gray,
copper, bronze, or gold in color.
iii.Small cell foams, both open and closed, are painted, or loaded, with absorbing inks and paints. These foams resemble flexible foam rubber sheetsor air conditioning filters. They can be single-layered or noticeably multi-layered, with glue lines separating the strata. A ground plane, if applied, can consist of a metallic paint, a metallic sheet (aluminum foil or metalized thin plastic), or undetectable sprayed inks. Some manufacturers may mark the front of these foams with lettering saying "front" or with serial numbers if the ground plane is not obvious. Some foam may contain composite fiber to make them more rigid or even structural.
iv.Magnetic Radar Absorbing Material (MAGRAM), as applied to vehicles, may appear in forms such as surface coverings, molded edges, or gapfillers. It consists of very fine grained ferromagnetic or ferrite particles suspended in a variety of rubber, paint, or plastic resin binders. At least one commercially available version uses a silicon-based binder.lt may be applied as sprays, sheets, molded or machined parts, or putties. Because of the general colors of typical binders and Ferro-magnetic particles, the natural colors of MAGRAM range from light gray to nearly black; however, with additional pigments added for other reasons (e.g., visual camouflage or manufacturing / maintenance - aid coding), almost any color
is possible. Thin films of plastic or paper material may cover one or both sides of sheets for identification coding or maintaining preapplication surface cleanliness.
v.Resistive Cards (R-Cards) consist of a sheet of fiber paper or very thin plastic covered with a continuous coat of conductive ink, paint, or extremely thin metallic film. The surface electrical resistivity of the coating may be constant or may vary continuously in one or two directions. The conductive ink versions are likely to be dark gray to black. The metallic coated versions may vary in color depending on both the specific metals used and the thick nesses involved, but black, yellow, green, and gold tints are common.
vi.Loaded ceramic spray tiles are sprayed-on and fired ceramic coatings heavily loaded with electrically conductive fillers or ferromagnetic particles. They are likely to range from dark gray to black in color. Depending on the specific filler and surface-sealing glaze used, they may range from smooth to abrasive in surface texture. Sprayed-on coatings may range from a few millimeters to tens of centimeters in thickness, vii.Absorbing honeycomb is a lightweight composite with open cells normally 3 to 12 mm in diameter and 25 to 150 mm maximum thickness. It is treated with partially conductive inks, paints, or fibers. The honeycomb core may be shipped without being loaded, in which case it might be indistinguishable from
materials used solely for structural purposes. The conductive inks and paints for subsequent loading are likely to come from an entirely different source than the core itself.
vii.Transparent RAM (T-RAM) looks like sheet polycarbonate. It is normally 75 to 85 percent transparent in the visible spectrum. Absorbing materials can vary from fibers or spheres spread throughout the material to thin coatings, which look like yellow/green metallic window tinting.
viii.Infrared (IR) Treatments usually consist of paints and coatings. Often these coatings are customized to tailor reflectance and/or radiation of IR energy.
Because of the wide spectrum (0.8 to 14 microns wavelength) of IR energy and the variety of applications, IR coatings may either be reflective (low emissivity) or designed to absorb (high emissivity). Coatings used for IR treatment include specially designed military paints in camouflage colors or commercial paints designed to reflect solar heat. Some of these products have a noticeable metal content in the paint/binder due to the IR pigments used. Others are designed to have high emissivity and as such, contain pigments that absorb IR. These high emissivity coatings contain carbon-based or other highly emissive particle-based pigments (normally nearly black). In either case, these IR pigments are sometimes shipped separately from the paint/binder.
SMALL CELL FOAMS
IV. PLASMA STEALTH
Plasma stealth technology is what can be called as "Active stealth technology" in scientific terms. This technology was first developed by the Russians. It is a milestone in the field of stealth technology. The technology behind this not at all new. The plasma thrust technology was used in the Soviet / Russian space program. Later the same engine was used to power the American Deep Space 1 probe.
In plasma stealth, the aircraft injects a stream of plasma in front of the aircraft. The plasma is ionized gas particles. The plasma will cover the entire body of the fighter and will absorb most of the electromagnetic energy of the radar waves, thus making the aircraft difficult to detect. The same method is used in Magneto Hydro Dynamics. Using Magneto Hydro Dynamics, an aircraft can propel itself to great speeds.
Plasma stealth will be incorporated in the MiG-35 "Super Fulcrum / Raptor Killer". This is a fighter which is an advanced derivative of the MiG-29 "Fulcrum / Baaz". Initial trials have been conducted on this technology, but most of the results have proved to be fruitful.
i. Less maneuverability
ii. Poor air to air combatability
iii. Aerodynamic instability
iv. Below sonic velocity
v. Reduced pay load capacity
vi. High expenses : Fighters in service and in development for the United States
Air Force (USAF) cost are:
B-2 ($2 billion) F-117 ($70 million) F-22($100 million)
V. STEALTH AIRCRAFTS OF YESTERDAYS, TODAY AND
Stealth technology is a concept that is not at all new. During the Second World War, allied aircraft used tin and aluminum foils in huge numbers to confuse German radar installations. This acted as a cover for allied bombers to conduct air raids. This method was later used as chaffs by aircrafts to dodge radar guided missiles.
The first stealth aircraft was the F-117 developed by Lockheed Martin. It was a top-secret project developed by its Skunk Works unit. The F-117 was only revealed during the late 80s and then saw action in the Persian Gulf.
In due course of time the B-2 was developed as a successor to the B-l. Though both of them serve different purposes, the B-2 went a step ahead of the F-117. The B-2 was developed to deliver nuclear weapons and other guided and unguided bombs. On the other hand the F-117 was developed to deliver its precision laser guided bombs.
Another stealth aircraft, which made a lot of promises and in the end ended up in a trashcan, was the A-12. It was a fighter that was designed to replace the F-14 and F-l 8 in the future. The capabilities of this aircraft were boasted to such an extent that the project ended up in a big mess. Billions of dollars were wasted for nothing.
Stealth technology became famous with the ATF contest. The Boeing-Lockheed YF-22 and the McDonell Douglas-Grumman YF-23 fought for the multi-billion contract to build the fighter that would take the USAF into the fifth generation fighter era. The Boeing-Lockheed won the contract and the F-22 was approved to be the replacement for the F-15 "Eagle" interceptor.
America now has a competitors, Russia decided to respond to the development of the F- . 22 by making the Su-47 (S-37) "Berkut" and the MiG-35 "Super Fulcrum / Raptor Killer". These fighters were developed by the two leading aviation firms in Russia
Sukhoi and Mikhoyan Gurevich (MiG). The future of these projects totally depends on the funding which will be provided to the Russian defense sector. There are some hopes of increase in the funding to these projects as countries like India have started providing funds and technical assistance for these projects.
Another competition that soon came into the spotlight after the ATF competition was the JSF. This time Boeing developed the X-32 and the Lockheed martin its X-35. With the experience gained from developing the F-22, they were tasked with making a replacement for the F-16. This saw great technological advances, as they had to make the first operational supersonic VSOL aircraft. Lockheed martin took the technical assistance of Russian scientists who developed the Yak-141. The Yak-141 is the first supersonic VSTOL aircraft. In the end the Lockheed team with its X-35 won the contract and the fighter was re-designated as the F-35.
Many projects remain over the horizon that will use stealth technology as its primary capability. They come from some of the most unlikely contenders. These projects include the Euro JSF, which will be designed by the team that developed the EF-2000. Russia is stepping forward with its LFS project with the S-54 and other designs. Two new entries into this field will be India and China. India will be introducing its MCA, which is a twin engine fighter without vertical stabilizers. This fighter will use thrust vectoring instead of rudders. China will be introducing the J-12 (F-12/XXJ). This is a fighter that is similar to the F-22.
The development of stealthy airplanes teaches several important lessons about technology. The first is that often many different technologies must be combined to achieve a desired outcome. An advance in one field, such as materials or aerodynamics, must be accompanied by advances in other fields, such as computing or electromagnetic theory. The second lesson is that sometimes trial and error techniques are insufficient and advances in mathematical theory are necessary in order to achieve significant advances. Finally, stealth teaches the lesson that technology is never static - a "stealth breakthrough" may only last for a few years before an adversary finds a means of countering it.
II. Stealth design
i. Reducing RADAR visibility
ii. Reducing IR visibility
III. Stealth materials and coatings
IV. Plasma stealth
V. Stealth aircrafts of yesterdays,today and tomorrow