STEALTH TECHNOLOGY

F-117 stealth attack plane

Stealth technology also termed LO technology (low observable technology) is a sub-discipline of military

tactics and passive electronic countermeasures,[1] which cover a range of techniquesused with

personnel, aircraft, ships, submarines, and missiles, to make them less visible (ideallyinvisible)

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

reflect off from the surface of planes. Such changes to shape and surface composition form stealth technology

as currently used on the Northrop Grumman B-2 Spirit "Stealth Bomber".[4] 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 throughcamouflage 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 infraredsignature. 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.

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.

The U-boat U-480 may have been the first stealth submarine. It featured a rubber coating, one layer of which

contained circular air pockets to defeat ASDIC sonar.

One of the earliest stealth aircraft seems to have been the Horten Ho 229 flying wing. It included carbon

powder in the glue to absorb radio waves.[7] Some prototypes were built, but it was never used in action.

During the 1950s, the Avro Vulcan, a British bomber, had a remarkably small appearance on radar despite its

large size, and occasionally disappeared from radar screens entirely.

In 1958, the CIA requested funding for a reconnaissance aircraft, to replace U-2 spy planes[8] 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 70,000 to 80,000 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 prior models, an optimal plane shape taking into account compactness was

developed where another "Blackbird", the SR-71, was produced, surpassing prior models in both altitude of

90,000 ft and speed of Mach 3.3.[4] The Lockheed SR-71 included a number of stealthy features, notably its

canted vertical stabilizers, the use of composite materials in key locations, and the overall finish in radar

absorbing paint.[9]

During 1970s, the U.S. Department of Defence then launched a project called Have Blue 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.

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:

Radar cross-section (RCS) reductions

Main article: Radar cross section

Almost since the invention of radar, various methods 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 means to detect 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 guidancethreats.[citation needed]

Radar cross-section

Typical RCS diagram (A-26 Invader)

Radar cross section (RCS) is a measure of how detectable an object is with aradar. A larger RCS indicates

that an object is more easily detected.

An object reflects a limited amount of radar energy. A number of different factors determine how

much electromagnetic energy returns to the source such as:

material of which the target is made;

absolute size of the target;

relative size of the target (in relation to the wavelength of the illuminating radar);

the incident angle (angle at which the radar beam hits a particular portion of target which depends upon

shape of target and its orientation to the radar source);

reflected angle (angle at which the reflected beam leaves the part of the target hit, it depends upon

incident angle);

the polarization of transmitted and the received radiation in respect to the orientation of the target

While important in detecting targets, strength of emitter and distance are not factors that affect the calculation

of a RCS because the RCS is a property of the target reflectivity.

Radar cross section is used to detect planes in a wide variation of ranges. For example, a stealth

aircraft (which is designed to have low detectability) will have design features that give it a low RCS (such as

absorbent paint, smooth surfaces, surfaces specifically angled to reflect signal somewhere other than towards

the source), as opposed to a passenger airliner that will have a high RCS (bare metal, rounded surfaces

effectively guaranteed to reflect some signal back to the source, lots of bumps like the engines, antennae, etc.).

RCS is integral to the development of radar stealth technology, particularly in applications

involvingaircraft and ballistic missiles. RCS data for current military aircraft is most highly classified.

Definition

Informally, the RCS of an object is the cross-sectional area of a perfectly reflecting sphere that would

produce the same strength reflection as would the object in question. (Bigger sizes of this imaginary

sphere would produce stronger reflections.) Thus, RCS is an abstraction: The radar cross-sectional area

of an object does not necessarily bear a direct relationship with the physical cross-sectional area of that

object but depends upon other factors.

Somewhat less informally, the RCS of a radar target is an effective area that intercepts the transmitted

radar power and then scatters that power isotropically back to the radar receiver.

More precisely, the RCS of a radar target is the hypothetical area required to intercept the transmitted

power density at the target such that if the total intercepted power were re-radiated isotropically, the

power density actually observed at the receiver is produced.[1] This is a complex statement that can be

understood by examining the monostatic (radar transmitter and receiver co-located) radar equation one

term at a time:

where

 = power transmitted by the radar (watts)

 = gain of the radar transmit antenna (dimensionless)

 = distance from the radar to the target (meters)

 = radar cross section of the target (meters squared)

 = effective area of the radar receiving antenna (meters squared)

 = power received back from the target by the radar (watts)

The   term in the radar equation represents the power density (watts per meter squared) that

the radar transmitter produces at the target. This power density is intercepted by the target with radar

cross section  , which has units of area (meters squared). Thus, the product   has the

dimensions of power (watts), and represents a hypothetical total power intercepted by the radar

target. The second  term represents isotropic spreading of this intercepted power from the

target back to the radar receiver. Thus, the product   represents the reflected power

density at the radar receiver (again watts per meter squared). The receiver antenna then collects this

power density with effective area  , yielding the power received by the radar (watts) as given by

the radar equation above.

The scattering of incident radar power by a radar target is never isotropic (even for a spherical

target), and the RCS is a hypothetical area. In this light, RCS can be viewed simply as a correction

factor that makes the radar equation "work out right" for the experimentally observed ratio of  .

However, RCS is an extremely valuable concept because it is a property of the target alone and may

be measured or calculated. Thus, RCS allows the performance of a radar system with a given target

to be analysed independent of the radar and engagement parameters. In general, RCS is a strong

function of the orientation of the radar and target, or, for the bistatic (radar transmitter and receiver

not co-located), a function of the transmitter-target and receiver-target orientations. A target's RCS

depends on its size, reflectivity of its surface, and the directivity of the radar reflection caused by

the target's geometric shape.

FACTORS THAT AFFECT RCSSize

As a rule, the larger an object, the stronger its Radar reflection and thus the greater its RCS. Also,

Radar of one band may not even detect certain size objects. For example. 10 cm (S-band Radar) can

detect rain drops but not clouds whose droplets are too small.

Material

Materials such as metal are strongly radar reflective and tend to produce strong signals. Wood and

cloth (such as portions of planes and balloons used to be commonly made) or plastic and fibreglass

are less reflective or indeed transparent to Radar making them suitable forradomes. Even a very thin

layer of metal can make an object strongly radar reflective. Chaff is often made from metallised

plastic or glass (in a similar manner to metallised foils on food stuffs) with microscopically thin layers

of metal.

Also, some devices are designed to be Radar active, such as Radar antennae and this will increase

RCS.

Radar-absorbent material, or RAM, is a class of materials used in stealth technology to disguise a

vehicle or structure from radar detection. A material's absorbency at a given frequency of radar wave

depends upon its composition. RAM cannot perfectly absorb radar at any frequency, but any given

composition does have greater absorbency at some frequencies than others; there is no one RAM that is

suited to absorption of all radar frequencies.

A common misunderstanding is that RAM makes an object invisible to radar[citation needed]. A radar absorbent

material can significantly reduce an object's radar cross-section in specific radar frequencies, but it does

not result in "invisibility" on any frequency. Bad weather may contribute to deficiencies in stealth

capability. A particularly disastrous example occurred during the Kosovo war, in which moisture[citation

needed] on the surface of an F-117 Nighthawk allowed long-wavelength radar to track and shoot it down.

RAM is only a part of achieving stealth.

The earliest forms of RAM were the materials called Sumpf and Schornsteinfeger, a coating used in

Germany during the World War II for thesnorkels (or periscopes) of submarines, to lower their reflectivity

in the 20-centimeter radar band the Allies used. The material had a layered structure and was based

on graphite particles and other semiconductive materials embedded in a rubber matrix. The material's

efficiency was partially reduced by the action of sea water.[1][2]

Germany also pioneered the first aircraft to use RAM during World War II, in the form of the Horten Ho

229. It used a carbon-impregnated plywood that would have made it very stealthy to Britain's primitive

radar of the time. However it is unknown if the carbon was incorporated for stealth reasons or because of

Germany's metal shortage.[3]

Types of RAM

Iron ball paint

One of the most commonly known types of RAM is iron ball paint. It contains tiny spheres coated

with carbonyl iron or ferrite. Radar waves induce molecular oscillations from the alternating magnetic field

in this paint, which leads to conversion of the radar energy into heat. The heat is then transferred to the

aircraft and dissipated. The iron particles in the paint are obtained by decomposition of iron

pentacarbonyl and may contain traces of carbon, oxygen and nitrogen.[citation needed]

A related type of RAM consists of neoprene polymer sheets with ferrite grains or carbon black particles

(containing about 30% of crystallinegraphite) embedded in the polymer matrix. The tiles were used on

early versions of the F-117A Nighthawk, although more recent models use painted RAM. The painting of

the F-117 is done by industrial robots with the plane covered in tiles glued to the fuselage and the

remaining gaps filled with iron ball paint.[citation needed]

The United States Air Force introduced a radar absorbent paint made from both ferrofluidic and non-

magnetic substances. By reducing the reflection of electromagnetic waves, this material helps to reduce

the visibility of RAM painted aircraft on radar.[citation needed]

The Israeli firm Nanoflight has also made a radar-absorbing paint that uses nanoparticles. [4]

Foam absorber

Foam absorber is used as lining of anechoic chambers for electromagnetic radiation measurements[citation

needed]. This material typically consists of a fireproofed urethane foam loaded with carbon black, and cut

into long pyramids. The length from base to tip of the pyramid structure is chosen based on the lowest

expected frequency and the amount of absorption required. For low frequency damping, this distance is

often 24 inches, while high frequency panels are as short as 3-4 inches. Panels of RAM are installed with

the tips pointing inward to the chamber. Pyramidal RAM attenuates signal by two effects: scattering and

absorption. Scattering can occur both coherently, when reflected waves are in-phase but directed away

from the receiver, or incoherently where waves are picked up by the receiver but are out of phase and

thus have lower signal strength. This incoherent scattering also occurs within the foam structure, with the

suspended carbon particles promoting destructive interference. Internal scattering can result in as much

as 10dB of attenuation. Meanwhile, the pyramid shapes are cut at angles that maximize the number of

bounces a wave makes within the structure. With each bounce, the wave loses energy to the foam

material and thus exits with lower signal strength.[5] Other foam absorbers are available in flat sheets,

using an increasing gradient of carbon loadings in different layers.

Jaumann absorber

A Jaumann absorber or Jaumann layer is a radar absorbent device.[citation needed] When first introduced in

1943, the Jaumann layer consisted of two equally-spaced reflective surfaces and a conductive ground

plane. One can think of it as a generalized, multi-layered Salisbury screenas the principles are similar.

Being a resonant absorber (i.e. it uses wave interfering to cancel the reflected wave), the Jaumann layer

is dependent upon the λ/4 spacing between the first reflective surface and the ground plane and between

the two reflective surfaces (a total of λ/4 + λ/4 ).

Because the wave can resonate at two frequencies, the Jaumann layer produces two absorption maxima

across a band of wavelengths (if using the two layers configuration). These absorbers must have all of the

layers parallel to each other and the ground plane that they conceal.

More elaborate Jaumann absorbers use series of dielectric surfaces that separate conductive sheets. The

conductivity of those sheets increases with proximity to the ground plane.

Radar absorbent paint

The SR-71 Blackbird and other planes were painted with a special "iron ball paint". This consisted of

small metallic-coated balls. Radar energy is converted to heat rather than being reflected.

Shape, directivity and orientation

The surfaces of the F-117A are designed to be flat and very angled. This has the effect that Radar

will be incident at a large angle (to thenormal ray) that will then bounce off at a similarly high reflected

angle; it is forward-scattered. The edges are sharp to prevent there being rounded surfaces.

Rounded surfaces will often have some portion of the surface normal to the Radar source. As any ray

incident along the normal will reflect back along the normal this will make for a strong reflected signal.

From the side, a fighter plane will present a much larger area than the same plane when viewed from

the front. All other factors being equal, the plane will have a stronger signal from the side than from

the front so the orientation between the Radar station and the target is important.

Smooth surfaces

The relief of a surface could contain indentations that act as corner reflectors which would increase

RCS from many orientations. This could arise from open bomb-bays, engine intakes, ordnance

pylons, joints between constructed sections, etc. Also, it can be impractical to coat these surfaces

with radar-absorbent materials.

Measurement

Measurement of a target's RCS is performed at a radar reflectivity range or scattering range. The first

type of range is an outdoor range where the target is positioned on a specially shaped low RCS pylon

some distance down-range from the transmitters. Such a range eliminates the need for placing radar

absorbers behind the target, however multi-path interactions with the ground must be mitigated.

An anechoic chamber is also commonly used. In such a room, the target is placed on a rotating pillar

in the center, and the walls, floors and ceiling are covered by stacks of radar absorbing material.

These absorbers prevent corruption of the measurement due to reflections. A compact range is an

anechoic chamber with a reflector to simulate far field conditions.

Vehicle shape

The F-35 Lightning II offers better stealthy features (such as this landing gear door) than prior American multi-role 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. In contrast, 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[citation needed]; the Bear had 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 re-entrant 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 / SR-71).

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 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 extant 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 opens.

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, isinherently unstable, and cannot be flown without a fly-by-wire control system.

K32 HMS Helsingborg, a stealth ship

Ships have also adopted similar methods. The Skjold class patrol boat was the first stealth ship to enter

service, though the earlier Arleigh Burke class destroyer incorporated some signature-reduction features.[17]

[18] Other examples are the French La Fayette class frigate, the GermanSachsen class frigates, the

Swedish Visby class corvette, 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 goldor 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.

Shaping

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. To a first approximation, when

the diameter of a sphere is significantly larger than the radar wavelength, its radar cross section is equal to its

geometric frontal area. The return of a one-square-meter sphere is compared to that from a one-meter-square

plate at different look angles. 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. 

However, there is a little more to the story than just the so called ray tracing approach. When energy strikes a

plate that is smooth compared to wavelength, it does not reflect totally in the optical approximation sense, i.e.,

the energy is not confined to a reflected wave at a complementary angle to the incoming wave. The radiated

energy, in fact, takes a pattern like a typical reflected wave structure. The width of the main forward scattered

spike is proportional to the ratio of the wavelength to the dimension of the radiating surface, as are the

magnitudes of the secondary and tertiary spikes. The classical optical approximation applies when this ratio

approaches zero. Thus, the backscatter - the energy radiated directly back to the transmitter increases as the

wavelength goes up, or the frequency decreases. When designing a cavity for minimum return, it is important to

balance the forward scatter associated with ray tracing with the backscatter from interactions with the first

surfaces. Clearly, an accurate calculation of the total energy returned to the transmitter is very complicated,

and generally has to be done on a supercomputer.

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 a material for its application.

Types of detection/ tecnologiesAcoustics

Acoustic stealth plays a primary role in submarine stealth as well as for ground vehicles. Submarines use

extensive 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 subsoniccan 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.

[edit]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 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 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

ofactive camouflage. The original B-2 design had wing tanks for a contrail-inhibiting chemical, alleged by

some to be chlorofluorosulfonic acid,[25] but this was replaced in the final design with a contrail sensor

from Ophir that alerts the pilot when he should change altitude[26] and mission planning also considers

altitudes where the probability of their formation is minimized.

[edit]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 volume and maximize

the mixing of 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 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 are absorbed

by atmospheric carbon dioxide and water vapor, dramatically reducing the infrared visibility of the exhaust

plume.[27] 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 and so the USMC ground

combat uniform requirements document specifies infrared reflective quality standards.[28]

Infrared Radiation

There are 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 both 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.

The third approach is masking. This is clearly much easier to do when the majority of the power is taken off by

the turbine, as in a propjet or helicopter application, than when the jet provides the basic propulsive force. The

former community has been using this approach to infrared suppression for many years, but it is only recently

that the jet-propulsion crowd has tackled this problem. The Lockheed F 117A and the Northrop B 2 both use a

similar approach of masking to prevent any hot parts being visible in the lower hemisphere. In summary,

infrared radiation should be tackled by a combination of temperature reduction and masking, although there is

no point in doing these past the point where the hot parts are no longer the dominant terms in the radiation

equation. The main body of the airplane has its own radiation, heavily dependent on speed and altitude, and the

jet plume can be a most significant factor, particularly in afterburning operation. Strong cooperation between

engine and airframe manufacturers in the early stages of design is extremely important. The choice of engine

bypass ratio, for example, should not be made solely on the basis of performance, but on a combination of that

and survivability for maximum system effectiveness. The jet-wake radiation follows the same laws as the

engine hot parts, a very strong dependency on temperature and a multiplicative factor of emissivity. Air has a

very low emissivity, carbon particles have a high broadband emissivity, and water vapour emits in very specific

bands. Infrared seekers have mixed feelings about water vapour wavelengths, because, while they help in

locating jet plumes, they hinder in terms of the general attenuation due to moisture content in the atmosphere.

There is no reason, however, why smart seekers shouldn't be able to make an instant decision about whether

conditions are favourable for using water-vapour bands for detection.

Detection

Theoretically there are a number of methods to detect stealth aircraft at long range.

[edit]Reflected waves

Passive (multistatic) radar, bistatic radar[19] and especially multistatic radar systems are believed to detect

some stealth aircraft better than conventional monostatic radars, since first-generation stealth technology

(such as the F117) reflects energy away from the transmitter's line of sight, effectively increasing

the radar cross section (RCS) in other directions, which the passive radars monitor. Such a system

typically uses either low frequency broadcast TV and FM radio signals (at which frequencies controlling

the aircraft's signature is more difficult). Later stealth approaches do not rely on controlling the specular

reflections of radar energy and so the geometrical benefits are unlikely to be significant.

Researchers at the University of Illinois at Urbana-Champaign with support of DARPA, have shown that it

is possible to build a synthetic aperture radar image of an aircraft target using passive multistatic radar,

possibly detailed enough to enable automatic target recognition(ATR).

In December 2007, SAAB researchers also revealed details for a system called Associative Aperture

Synthesis Radar (AASR) that would employ a large array of inexpensive and redundant transmitters and

a few intelligent receivers to exploit forward scatter to detect low observable targets.[20] The system was

originally designed to detect stealthy cruise missiles and should be just as effective against aircraft. The

large array of inexpensive transmitters also provides a degree of protection against anti-radar (or anti-

radiation) missiles or attacks.

[edit]Infrared (heat)

Some analysts claim Infra-red search and track systems (IRSTs) can be deployed against stealth aircraft,

because any aircraft surface heats up due to air friction and with a two channel IRST is a CO2 (4.3 µm

absorption maxima) detection possible, through difference comparing between the low and high channel.

[21][22] These analysts also point to the resurgence in such systems in several Russian designs in the

1980s, such as those fitted to the MiG-29 and Su-27. The latest version of the MiG-29, the MiG-35, is

equipped with a new Optical Locator System that includes even more advanced IRST capabilities.

In air combat, the optronic suite allows:

Detection of non-afterburning targets at 45-kilometre (28 mi) range and more;

Identification of those targets at 8-to-10-kilometre (5.0 to 6.2 mi) range; and

Estimates of aerial target range at up to 15 kilometres (9.3 mi).

For ground targets, the suite allows:

A tank-effective detection range up to 15 kilometres (9.3 mi), and aircraft carrier detection at 60 to 80

kilometres (37 to 50 mi);

Identification of the tank type on the 8-to-10-kilometre (5.0 to 6.2 mi) range, and of an aircraft carrier

at 40 to 60 kilometres (25 to 37 mi); and

Estimates of ground target range of up to 20 kilometres (12 mi).

[edit]Longer Wavelength Radar

VHF radar systems have wavelengths comparable to aircraft feature sizes and should exhibit scattering in

the resonance region rather than the optical region, allowing most stealth aircraft to be detected. This has

prompted Nizhniy Novgorod Research Institute of Radio Engineering (NNIIRT) to develop

VHF AESAs such as the NEBO SVU, which is capable of performing target acquisition for SAM batteries.

Despite the advantages offered by VHF radar, their longer wavelengths result in poor resolution

compared to comparably sized X-band radar array. As a result, these systems must be very large before

they can have the necessary resolution for an engagement radar.

The Dutch company Thales Nederland, formerly known as Holland Signaal, have developed a naval

phased-array radar called SMART-L, which also is operated at L-Band and is claimed to offer counter

stealth benefits.

[edit]OTH radar (over-the-horizon radar)

Over-the-horizon radar is a design concept that increases radar's effective range over conventional radar.

It is claimed that the Australian JORN Jindalee Operational Radar Network can overcome certain stealth

characteristics.[23] It is claimed that the HF frequency used and the method of bouncing radar from

ionosphere overcomes the stealth characteristics of the F-117A. In other words, stealth aircraft are

optimized for defeating much higher-frequency radar from front-on rather than low-frequency radars from

above.

[edit]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 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.[29] 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,[30] 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.[31]

[edit]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 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 AWACS can complicate tactical strategy for stealth operation.

[edit]Research

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.[32] 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 (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 RF more complex laser

discharges, but these may be difficult in practice.[33]

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.[34][35][36] The X-53 Active Aeroelastic Wing was a US

Air Force, Boeing, and NASA effort.

In fluidics, fluid injection 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 fluidic systems, in which larger forces in fluids are diverted by smaller jets or flows of fluid

intermittently, to change the direction of vehicles.

In circulation control, near the trailing edges of wings, aircraft flight control systems are replaced by slots

which emit fluid flows.[37][38][39]

In thrust vectoring, in jet engine nozzles, swiveling parts are replaced by slots which inject fluid flows into

jets to divert thrust.[40] Tests show that air forced into a jet engine exhaust stream can deflect thrust up to

15 degrees. The U.S. FAA has conducted a study about civilizing 3D military thrust vectoring to help

jetliners avoid crashes. According to this study, 65% of all air crashes can be prevented by deploying

thrust vectoring means.[41][42]

Limitations

There is no one optimum stealth design, but rather each mission requirement generates an appropriate mix of techniques. Implementation of stealth is not without penalties. Some of the materials used require special and costly maintenance. The maneuverability of an aircraft can be compromised by the introduction of stealth design features. As was the case with the F-117A, each B-2 bomber will have its own covered maintenance facility, since the B-2's low observable features require frequent performance of structural and maintenance activities.Stealth requires not only design compromises, it also imposes operational compromises. Sensors to locate targets pose a particular problem for stealth aircraft. The large radars used by conventional aircraft would obviously compromise the position of a stealth aircraft. Air-to-air combat would rely on passive detection of transmissions by hostile aircraft, as well as infrared tracking. However, these techniques are of marginal effectiveness against other stealth aircraft, explaining the limited application of stealth to the Advanced Tactical Fighter.

Aircraft for attacking targets on the ground face a similar problem. FLIR can be used for precise aiming at targets whose general location is known, but they are poorly suited for searching for targets over a wide area. A radar on the aircraft to scan for potential targets would compromise its position. In order to locate targets, stealth aircraft may rely on an airborne laser radar, although such a sensor may prove of limited utility in poor weather. A more promising approach would be to use data from reconnaissance satellites,

either transmitted directly from the satellite or relayed through communications satellites from processing centers in the United States.

There are limits to the utility of stealth techniques. Since the radar cross-section of an aircraft depends on the angle from which it is viewed, an aircraft will typically have a much smaller RCS when viewed from the front or rear than when viewed from the side or from above. In general stealth aircraft are designed to minimize their frontal RCS. But it is not possible to contour the surface of an aircraft to reduce the RCS equally in all directions, and reductions in the frontal RCS may lead to a larger RCS from above. Thus while a stealth aircraft may be difficult to track when it is flying toward a ground-based radar or another aircraft at the same altitude, a high-altitude airborne radar or a space-based radar may have an easier time tracking it.

Another limitation of stealth aircraft is their vulnerability to detection by bi-static radars. The contouring of a stealth aircraft is designed to avoid reflecting a radar signal directly back in the direction of the radar transmitter. But the transmitter and receiver of a bi-static radar are in separate locations — indeed, a single transmitter may be used by radar receivers scattered over a wide area. This greatly increases the odds that at least one of these receivers will pickup a reflected signal. The prospects for detection of stealth aircraft by bi-static radar are further improved if the radar transmitter is space-based, and thus viewing the aircraft from above, the direction of its largest radar cross section.

Several analysts claim stealth aircraft such as the ATF will be vulnerable to detection by infrared search and track systems (IRST). The natural heating of an aircraft's surface makes it visible to this type of system. The faster and aircraft flies, the warmer it gets, and thus, the easier to detect through infrared means. One expert asserts "if an aircraft deviates from its surroundings by only one degree centigrade, you will be able to detect it at militarily useful ranges." In fact, both the Russian MiG-29 and Su-27 carry IRST devices, which indicates that the Russians have long targeted this as a potential stealth weakness.

Stealth aircraft are even more vulnerable to multiple sensors used in tandem. By using an IRST to track the target and a Ladar (laser radar), or a narrow beam, high-power radar to paint the target superior data is provided.

The most basic potential limitation of stealth, is its vulnerability to visual detection. Since the ATF is 25-30 percent larger than the F-15 and 40 percent larger than the F-18, for example, it will be much easier to detect visually from ranges on the order of 10 miles. When one considers that stealth characteristics will drastically reduce the effectiveness of several types of guided air-to-air missiles, fighter engagements will probably move back to the visual range arena. In this context, the cumbersome F-22 would be at a distinct disadvantage.

Another potential "limitation" of stealth technology has little to do with its capabilities. Rather, some question the effect the pursuit of such hi tech aircraft will have on the US aerospace industry as a whole. These aircraft would not be available for foreign export until well into the next century. During that time, competitors such as the Gripen, Rafale and EFA will be peddled aggressively by European exporters. One analyst estimated that US foreign sales saved the Pentagon "about $2.8 billion through surcharges to recover part of their development costs and perhaps another $4 billion through the learning curve effect of higher production runs." Thus, America's stealth success could actually backfire, on its larger aerospace industry by causing it to forfeit sales to a new generation of top-of-the-line, although less formidable, European fighter aircraft.

DESIGN

General design

The general design of a stealth aircraft is always aimed at reducing radar and thermal detection. It is the

designer's top priority to satisfy the following conditions; some of which are listed below, by using their

skills, which ultimately decides the success of the aircraft:-

Reducing thermal emission from thrust

Reducing radar detection by altering some general configuration (like introducing the split rudder)

Reducing radar detection when the aircraft opens its weapons bay

Reducing infra-red and radar detection during adverse weather conditions

[edit]Limitations

B-2 Spirit stealth bomber of the U.S Air Force

[edit]Instability of design

Early stealth aircraft were designed with a focus on minimal radar cross section (RCS) rather than

aerodynamic performance. Highly-stealth aircraft like the F-117 Nighthawk are aerodynamically unstable

in all three axes and require constant flight corrections from a fly-by-wire (FBW) flight system to maintain

controlled flight.[13] Most modern non-stealth fighter aircraft are unstable on one or two axes only.[citation

needed] However, in the pursuit of increased maneuverability, most 4th and 5th-generation fighter aircraft

have been designed with some degree of inherent instability that must be controlled by fly-by-wire

computers.[citation needed] As for the B-2 Spirit, based on the development of the flying wing aircraft[14] by Jack

Northrop since 1940, design allowed creating stable aircraft with sufficient yaw control, even without

vertical surfaces such as rudders.

[edit]Dogfighting ability

Earlier stealth aircraft (such as the F-117 and B-2) lack afterburners, because the hot exhaust would

increase their infrared footprint, and breaking the sound barrier would produce an obvious sonic boom, as

well as surface heating of the aircraft skin which also increased the infrared footprint. As a result their

performance in air combat maneuvering required in a dogfight would never match that of a dedicated

fighter aircraft. This was unimportant in the case of these two aircraft since both were designed to be

bombers. More recent design techniques allow for stealthy designs such as the F-22 without

compromising aerodynamic performance. Newer stealth aircraft, like the F-22, F-35 and the Sukhoi T-50,

have performance characteristics that meet or exceed those of current front-line jet fighters due to

advances in other technologies such as flight control systems, engines, airframe construction and

materials.[4][15]

[edit]Electromagnetic emissions

The high level of computerization and large amount of electronic equipment found inside stealth aircraft

are often claimed to make them vulnerable to passive detection. This is highly unlikely and certainly

systems such as Tamara and Kolchuga, which are often described as counter-stealth radars, are not

designed to detect stray electromagnetic fields of this type. Such systems are designed to detect

intentional, higher power emissions such as radar and communication signals. Stealth aircraft are

deliberately operated to avoid or reduce such emissions.[citation needed]

Current Radar Warning Receivers look for the regular pings of energy from mechanically swept radars

while fifth generation jet fighters useLow Probability of Intercept Radars with no regular repeat pattern.[16]

[edit]Vulnerable modes of flight

Stealth aircraft are still vulnerable to detection during, and immediately after using their weaponry. Since

stealth payload (reduced RCS bombs and cruise missiles) are not yet generally available, and ordnance

mount points create a significant radar return, stealth aircraft carry all armament internally. As soon as

weapons bay doors are opened, the plane's RCS will be multiplied and even older generation radar

systems will be able to locate the stealth aircraft. While the aircraft will reacquire its stealth as soon as the

bay doors are closed, a fast response defensive weapons system has a short opportunity to engage the

aircraft.

This vulnerability is addressed by operating in a manner that reduces the risk and consequences of

temporary acquisition. The B-2's operational altitude imposes a flight time for defensive weapons that

makes it virtually impossible to engage the aircraft during its weapons deployment. All stealthy aircraft

carry weapons in internal weapons bays. New stealth aircraft designs such as the F-22 and F-35 can

open their bays, release munitions and return to stealthy flight in less than a second.

Some weapons require that the weapon's guidance system acquire the target while the weapon is still

attached to the aircraft. This forces relatively extended operations with the bay doors open.

Also, such aircraft as the F-22 Raptor and F-35 Lighting II Joint Strike Fighter can also carry additional

weapons and fuel on hardpoints below their wings. When operating in this mode the planes will not be

nearly as stealthy, as the hardpoints and the weapons mounted on those hardpoints will show up on radar

systems. This option therefore represents a trade off between stealth or range and payload. External

stores allow those aircraft to attack more targets further away, but will not allow for stealth during that

mission as compared to a shorter range mission flying on just internal fuel and using only the more limited

space of the internal weapon bays for armaments.

[edit]Reduced payload

In a 1994 live fire exercise near Point Mugu, California, a B-2 Spirit dropped forty-seven 500 lb (230 kg) class Mark 82

bombs, which represents about half of a B-2's total ordnance payload in Block 30 configuration

Fully stealth aircraft carry all fuel and armament internally, which limits the payload. By way of

comparison, the F-117 carries only two laser or GPS guided bombs, while a non-stealth attack aircraft

can carry several times more. This requires the deployment of additional aircraft to engage targets that

would normally require a single non-stealth attack aircraft. This apparent disadvantage however is offset

by the reduction in fewer supporting aircraft that are required to provide air cover, air-defense suppression

and electronic counter measures, making stealth aircraft "force multipliers".

[edit]Sensitive skin

The B-2 has a skin made with highly specialized materials such as Polygraphite.[17]

[edit]Cost of operations

Stealth aircraft are typically more expensive to develop and manufacture. An example is the B-2

Spirit that is many times more expensive to manufacture and support than conventional bomber aircraft.

The B-2 program cost the U.S. Air Force almost $45 billion.[18]