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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]