What is Stealth?What is Stealth?

Stealth is the technique of making a plane (or any other Stealth is the technique of making a plane (or any other object) less visible to the enemy by reducing it's radar object) less visible to the enemy by reducing it's radar and IR (infra red, heat) visibility. and IR (infra red, heat) visibility.

Reducing the IR image of a plane can be Reducing the IR image of a plane can be accomplished by directing the hot exhaust gasses to the accomplished by directing the hot exhaust gasses to the top of the plane and mix them with cold air . top of the plane and mix them with cold air .

Reducing radar visibility can be accomplished by Reducing radar visibility can be accomplished by deflecting the radar waves in such a direction that they deflecting the radar waves in such a direction that they don't go back to the emitting radar & making use of less don't go back to the emitting radar & making use of less radar reflecting materials (composite material, plastic) radar reflecting materials (composite material, plastic) and/or radar absorbing coatings and/or radar absorbing coatings

Conventional AircraftConventional Aircraft

Most conventional aircraft Most conventional aircraft have a rounded shape. have a rounded shape. This shape makes them This shape makes them aerodynamic, but it also aerodynamic, but it also creates a very efficient creates a very efficient radar reflector. The round radar reflector. The round shape means that no shape means that no matter where the radar matter where the radar signal hits the plane, signal hits the plane, some of the signal gets some of the signal gets reflected back reflected back

A flat plate at right angles to an impinging radar wave A flat plate at right angles to an impinging radar wave has a very large radar signal, has a very large radar signal,

A cavity, similarly located, also has a large return.A cavity, similarly located, also has a large return. Thus Thus she inlet and exhaust systemsshe inlet and exhaust systems of a jet aircraft of a jet aircraft

would be expected to be dominant contributors to radar would be expected to be dominant contributors to radar cross section in thecross section in the nose-on nose-on andand tail-on tail-on viewing viewing directions, and the directions, and the vertical tailvertical tail dominates the dominates the side-on side-on signature.signature.

Stealth TechnologyStealth Technology

Goal: Goal: Stealth is the technique of making a plane (or any Stealth is the technique of making a plane (or any other object) less visible to the enemy by reducing it's other object) less visible to the enemy by reducing it's radar and IR (infra red, heat) visibility. radar and IR (infra red, heat) visibility.

There are There are two different waystwo different ways to create Radar invisibility: to create Radar invisibility: The airplane can be shaped so that any radar signals it The airplane can be shaped so that any radar signals it

reflects are reflected away from the radar equipment. reflects are reflected away from the radar equipment. The airplane can be covered in materials that absorb The airplane can be covered in materials that absorb

radar signals. radar signals.

A stealth aircraft, is made A stealth aircraft, is made up of completely up of completely flat flat surfacessurfaces and very and very sharp sharp edgesedges. When a radar . When a radar signal hits a stealth plane, signal hits a stealth plane, the signal reflects away at the signal reflects away at an angle . In addition, an angle . In addition, surfaces on a stealth surfaces on a stealth aircraft can be treated so aircraft can be treated so they they absorb radar energyabsorb radar energy as well. The overall result as well. The overall result is that a stealth aircraft is that a stealth aircraft can have the radar can have the radar signature of a small bird signature of a small bird

rather than an airplane. rather than an airplane.

SHAPINGSHAPINGThere is a tremendous advantage to positioning surfaces There is a tremendous advantage to positioning surfaces

so that the radar wave strikes them at close to so that the radar wave strikes them at close to tangential anglestangential angles and far from right angles to edges. and far from right angles to edges.

To a first approximation, when the diameter of a sphere To a first approximation, when the diameter of a sphere is significantly larger than the radar wavelength, its radar is significantly larger than the radar wavelength, its radar cross section is equal to its geometric frontal area. cross section is equal to its geometric frontal area.

At normal incidence, the flat plate acts like a mirror, and At normal incidence, the flat plate acts like a mirror, and it returns 30 decibels (dB) above or 1000 times the it returns 30 decibels (dB) above or 1000 times the return from the sphere.return from the sphere.

If we rotate the plate about one edge so that the edge is If we rotate the plate about one edge so that the edge is always normal to the incoming wave, we find that the always normal to the incoming wave, we find that the cross-section drops by a factor of 1000, equal to that of cross-section drops by a factor of 1000, equal to that of the sphere, when the look angle reaches 30 degrees off the sphere, when the look angle reaches 30 degrees off normal to the plate. normal to the plate.

Diagonal Rotation:Diagonal Rotation:

If we go back to the normal incidence case and rotate If we go back to the normal incidence case and rotate the plate about a diagonal relative to the incoming wave, the plate about a diagonal relative to the incoming wave, we see a remarkable difference.we see a remarkable difference.

In this case, the cross section drops by 30 dB when the In this case, the cross section drops by 30 dB when the plate is only 8 degrees off normal, and drops another 40 plate is only 8 degrees off normal, and drops another 40 dB by the time the plate is at a shallow angle to the dB by the time the plate is at a shallow angle to the incoming radar beam. This is a total change in radar incoming radar beam. This is a total change in radar cross section of 10 million !cross section of 10 million !

Reflection from Ducts:Reflection from Ducts:

The energy reflected from a straight duct will be reflected The energy reflected from a straight duct will be reflected in one or two bounces, while that from a curved duct will in one or two bounces, while that from a curved duct will require four or five bounces . require four or five bounces .

Number of bounces can be increased significantly Number of bounces can be increased significantly without sacrificing aerodynamic performance. without sacrificing aerodynamic performance.

A cavity might be designed with a high-cross-sectional A cavity might be designed with a high-cross-sectional aspect ratio to maximize the length-to-height ratio.aspect ratio to maximize the length-to-height ratio.

If we can attenuate the signal to some extent with each If we can attenuate the signal to some extent with each bounce, then clearly there is a significant advantage to a bounce, then clearly there is a significant advantage to a multi-bounce design. The SR-71 inlet follows these multi-bounce design. The SR-71 inlet follows these design practices design practices

The backscatter -the energy radiated directly back to the The backscatter -the energy radiated directly back to the transmitter -increases as the wavelength goes up, or the transmitter -increases as the wavelength goes up, or the frequency decreasesfrequency decreases

When designing a cavity for minimum return, it is When designing a cavity for minimum return, it is important to balance the forward scatter associated with important to balance the forward scatter associated with

ray tracing with the backscatter from interactions with the ray tracing with the backscatter from interactions with the first surfacesfirst surfaces

Although surface alignment is very important for external Although surface alignment is very important for external surfaces and inlet and exhaust edges, the return from surfaces and inlet and exhaust edges, the return from the inside of a cavity is heavily dependent on attenuating the inside of a cavity is heavily dependent on attenuating materials materials

radar-absorbing materials or RAM are coatings whose radar-absorbing materials or RAM are coatings whose electrical and magnetic properties have been altered to electrical and magnetic properties have been altered to allow absorption of microwave energy at discrete or allow absorption of microwave energy at discrete or broadband frequenciesbroadband frequencies

Principles of Operation:Principles of Operation: Microwave absorbers are produced by using existing Microwave absorbers are produced by using existing

materials and altering their dielectric and magnetic materials and altering their dielectric and magnetic properties. For purposes of analyses, the dielectric properties. For purposes of analyses, the dielectric properties of a material are categorized as its permittivity properties of a material are categorized as its permittivity and the magnetic properties as its permeability. Both and the magnetic properties as its permeability. Both terms are complex numbers with real and imaginary terms are complex numbers with real and imaginary parts. parts.

RADAR ABSORBING MATERIALS(RAM)RADAR ABSORBING MATERIALS(RAM)

Common dielectric materials used for absorbers such Common dielectric materials used for absorbers such as foams, plastics and elastomers have no magnetic as foams, plastics and elastomers have no magnetic properties, giving them permeabilities of 1.properties, giving them permeabilities of 1.

Magnetic materials such as ferrites, iron and cobalt-Magnetic materials such as ferrites, iron and cobalt-nickel alloys are used to alter the permeabilities of the nickel alloys are used to alter the permeabilities of the materials.materials.

High dielectric materials such as carbon, graphite and High dielectric materials such as carbon, graphite and metal flakes are used to modify the dielectric properties.metal flakes are used to modify the dielectric properties.

When an electromagnetic wave propagating through a When an electromagnetic wave propagating through a free-space impedance of ZO is incident upon a semi- free-space impedance of ZO is incident upon a semi- infinite dielectric or magnetic dielectric boundary of infinite dielectric or magnetic dielectric boundary of impedance Z1, a partial reflection occurs. The magnitude impedance Z1, a partial reflection occurs. The magnitude of the reflection coefficient is governed by the following of the reflection coefficient is governed by the following equation.equation.

Where,Where,

To achieve a reflection coefficient of zero: ZO = Z1 This condition is achieved when:

(3)

The perfect absorber would therefore have u1 equal to e1 and as large as possible to achieve absorption in the thinnest layer possible.

Microwave AbsorbersMicrowave Absorbers

In general, practical micro- wave absorbers are one of In general, practical micro- wave absorbers are one of two basic types: resonant or graded dielectric.two basic types: resonant or graded dielectric.

Resonant Type:Resonant Type: The simplest type of resonant absorber is the The simplest type of resonant absorber is the Salisbury Salisbury

ScreenScreen. It consists of a . It consists of a resistive sheet spaced one-resistive sheet spaced one-quarter wavelength from a conductive ground planequarter wavelength from a conductive ground plane. . The resistive sheet is as thin as possible with a The resistive sheet is as thin as possible with a resistance of resistance of 377 ohms per square inch377 ohms per square inch, matching that , matching that of free space. A wave incident upon the surface of the of free space. A wave incident upon the surface of the screen is partially reflected and partially transmitted. The screen is partially reflected and partially transmitted. The transmitted portion undergoes multiple internal transmitted portion undergoes multiple internal reflections to give rise to a series of emergent waves. At reflections to give rise to a series of emergent waves. At the design frequency, the sum of the emergent waves is the design frequency, the sum of the emergent waves is equal in amplitude to, but equal in amplitude to, but 180 degrees out of phase180 degrees out of phase with, the initial reflected portion. In theory, zero reflection with, the initial reflected portion. In theory, zero reflection takes place at this frequency; in practice, absorption of takes place at this frequency; in practice, absorption of greater than greater than 30dB 30dB (99.9 percent) may be achieved.(99.9 percent) may be achieved.

Resonant Resonant

PLASMA STEALTHPLASMA STEALTH

The Russian approach towards stealth : They are The Russian approach towards stealth : They are developing a system to make a plane invisible to radar developing a system to make a plane invisible to radar by using a sort of a plasma torch on the nose of the by using a sort of a plasma torch on the nose of the plane. The idea behind is, this 'torch' creates a plane. The idea behind is, this 'torch' creates a ionized ionized 'cloud''cloud' around the plane which will absorb radar waves. around the plane which will absorb radar waves.

PlasmaPlasma is ionized gas, which means that it's atoms have is ionized gas, which means that it's atoms have so much kinetic energy that valence electrons have been so much kinetic energy that valence electrons have been freed through atomic-level collisions.freed through atomic-level collisions.

A radio signal encountering a plasma stream would A radio signal encountering a plasma stream would easily be subject to easily be subject to dispersion.dispersion. The electromagnetic The electromagnetic radio wave encountering the plasma would become radio wave encountering the plasma would become highly energized and change directions. This would highly energized and change directions. This would cause it not to be reflected back to the radar station. It cause it not to be reflected back to the radar station. It could be possible that plasma could also entrap and could be possible that plasma could also entrap and dissipatedissipate the energy of the radio wave. the energy of the radio wave.

The The physics of plasma protectionphysics of plasma protection can be described as can be described as following. If an object is surrounded by a cloud of following. If an object is surrounded by a cloud of plasma, several phenomena are observed when the plasma, several phenomena are observed when the cloud interacts with electromagnetic waves radiated by cloud interacts with electromagnetic waves radiated by enemy radar. First, an enemy radar. First, an absorptionabsorption of electromagnetic of electromagnetic energy occurs in the cloud, since during plasma energy occurs in the cloud, since during plasma penetration it interacts with plasma charged particles, penetration it interacts with plasma charged particles, pass onto them a portion of its energy, and fades. pass onto them a portion of its energy, and fades. Second, due to specific physical processes, Second, due to specific physical processes, electromagnetic wave tends to pass around plasma electromagnetic wave tends to pass around plasma cloud. Both of these phenomena results in dramatic cloud. Both of these phenomena results in dramatic decrease of the reflected signal. decrease of the reflected signal.

It is not known whether the plasma stealth system It is not known whether the plasma stealth system developed by the Russians employs a plasma laser or developed by the Russians employs a plasma laser or some other method for creating a plasma field. some other method for creating a plasma field.

B2 STEALTH BOMBERB2 STEALTH BOMBER

The B-2 bomber, commonly known as the stealth The B-2 bomber, commonly known as the stealth bomber, was an ambitious project, to say the least. In bomber, was an ambitious project, to say the least. In the 1970s, the U.S. military wanted a replacement for the the 1970s, the U.S. military wanted a replacement for the aging B-52 bomber. They needed a plane that could aging B-52 bomber. They needed a plane that could carry carry nuclear bombsnuclear bombs across the globe, to the Soviet across the globe, to the Soviet Union, in only a few hours. And they wanted it to be Union, in only a few hours. And they wanted it to be nearly invisible to enemy sensors. nearly invisible to enemy sensors.

A 172-foot wide flying wing that looks like an insect to A 172-foot wide flying wing that looks like an insect to radar scanners! The craft is also revolutionary from an radar scanners! The craft is also revolutionary from an aeronautics perspective: It doesn't have any of the aeronautics perspective: It doesn't have any of the standard stabilizing systems found on a conventional standard stabilizing systems found on a conventional airplaneairplane, but pilots say it flies as smoothly as a fighter jet. , but pilots say it flies as smoothly as a fighter jet.

Basic Design:Basic Design:The B-2 bomber has a completely different design: It's The B-2 bomber has a completely different design: It's

one big wing, like a one big wing, like a boomerangboomerang. .

This flying wing design is much more efficient than a This flying wing design is much more efficient than a conventional plane. Instead of separate wings supporting conventional plane. Instead of separate wings supporting all the weight of the fuselage, the entire craft works to all the weight of the fuselage, the entire craft works to generate lift. Eliminating the tail and fuselage also generate lift. Eliminating the tail and fuselage also reduces reduces dragdrag -- the total force of air resistance acting on -- the total force of air resistance acting on

the planethe plane. . Greater efficiency helps the B-2 travel long distances in Greater efficiency helps the B-2 travel long distances in

a short period of time. It's not the fastest craft around – a short period of time. It's not the fastest craft around – It’s high subsonic, meaning its top speed is just under It’s high subsonic, meaning its top speed is just under the speed of sound (around 1,000 ft/sec or 305 m/s) -- the speed of sound (around 1,000 ft/sec or 305 m/s) -- but it can go 6,900 miles (11,000 km) without refueling but it can go 6,900 miles (11,000 km) without refueling and 11,500 miles (18,500 km) with one in-flight refueling. and 11,500 miles (18,500 km) with one in-flight refueling.

Fly ControlsFly ControlsThe B-2 has four General Electric F-118-GE-100 The B-2 has four General Electric F-118-GE-100 jet jet

enginesengines, each of which generates 17,300 pounds of , each of which generates 17,300 pounds of thrust. Just as in an ordinary plane, the pilot steers the thrust. Just as in an ordinary plane, the pilot steers the B-2 by moving various parts of the wings. The B-2 has B-2 by moving various parts of the wings. The B-2 has elevons and rudderselevons and rudders along the trailing edge of the along the trailing edge of the plane. Just like the elevators and ailerons on a plane. Just like the elevators and ailerons on a conventional plane, the conventional plane, the elevons change the plane's elevons change the plane's pitchpitch (up and down movement) (up and down movement) and rolland roll (rotation along (rotation along the horizontal axis). The the horizontal axis). The elevons and rudders also elevons and rudders also control the plane's yawcontrol the plane's yaw (rotation along the vertical (rotation along the vertical axis). axis).

The B-2 has a sophisticated The B-2 has a sophisticated fly-by-wire systemfly-by-wire system. Instead . Instead of adjusting the flaps through mechanical means, the of adjusting the flaps through mechanical means, the pilot passes commands on to a computer, which adjusts pilot passes commands on to a computer, which adjusts the flaps. In other words, the pilot controls the computer the flaps. In other words, the pilot controls the computer and the computer controls the steering system. and the computer controls the steering system.

Safety MeasuresSafety Measures

The computer also does a lot of work independent of the The computer also does a lot of work independent of the pilot's input. It constantly monitors pilot's input. It constantly monitors gyroscopicgyroscopic sensors to sensors to keep track of the keep track of the plane's attitudeplane's attitude -- its position relative -- its position relative to the airflow. If the plane starts to turn unexpectedly, the to the airflow. If the plane starts to turn unexpectedly, the computer automatically moves the rudders to counteract computer automatically moves the rudders to counteract the turning force. The corrections are so precise that the the turning force. The corrections are so precise that the pilot usually won't feel any shift at all. pilot usually won't feel any shift at all.

The B-2 also has a small wedge-shaped flap in the The B-2 also has a small wedge-shaped flap in the middle of the trailing edge. The computer adjusts this middle of the trailing edge. The computer adjusts this flap, called the flap, called the gust load alleviation system (GLAS),gust load alleviation system (GLAS), to to counteract air turbulence forces. counteract air turbulence forces.

Stealth Measures in B2Stealth Measures in B2The B-2's flat, narrow shape and black coloration help it The B-2's flat, narrow shape and black coloration help it

fade into the night. Even in the daytime, when the B-2 fade into the night. Even in the daytime, when the B-2 stands out against blue sky, it can be hard to figure out stands out against blue sky, it can be hard to figure out which way the plane is going. The B-2 emits minimal which way the plane is going. The B-2 emits minimal exhaust, so it doesn't leave a visible trail behind it. exhaust, so it doesn't leave a visible trail behind it.

The B-2's engines are buried inside the plane. This helps The B-2's engines are buried inside the plane. This helps muffle the noise. The efficient aerodynamic design helps muffle the noise. The efficient aerodynamic design helps keep the B-2 quiet as well, because the engines can keep the B-2 quiet as well, because the engines can operate at lower power settings. operate at lower power settings.

The engine system also works to minimize the plane's The engine system also works to minimize the plane's infrared (heat) signature. Infrared sensors, including those infrared (heat) signature. Infrared sensors, including those on on heat-seeking missilesheat-seeking missiles, typically pick up on hot engine , typically pick up on hot engine exhaust. In the B-2, all of the exhaust passes through exhaust. In the B-2, all of the exhaust passes through cooling vents before flowing out of the rear ports. Putting cooling vents before flowing out of the rear ports. Putting the exhaust ports on the top of the plane further reduces the exhaust ports on the top of the plane further reduces the infrared signature the infrared signature

Radar Avoidance:Radar Avoidance:The stealth bomber's peculiar shape deflects radio beams The stealth bomber's peculiar shape deflects radio beams

in both ways. The large flat areas on the top and bottom of in both ways. The large flat areas on the top and bottom of the plane are like tilted mirrors. These flat areas deflect the plane are like tilted mirrors. These flat areas deflect most radio beams away from the station, presuming the most radio beams away from the station, presuming the station isn't directly beneath the plane. station isn't directly beneath the plane.

The plane itself also works like a curved mirror, particularly The plane itself also works like a curved mirror, particularly in the front section. The entire plane has no sharp, angled in the front section. The entire plane has no sharp, angled edges -- every surface is curved in order to deflect radio edges -- every surface is curved in order to deflect radio waves. The curves are designed to bounce almost all waves. The curves are designed to bounce almost all radio waves away at an angle. radio waves away at an angle.

The B-2's body is mainly composed of composite The B-2's body is mainly composed of composite material -- combinations of various lightweight substances. material -- combinations of various lightweight substances. The composite material used in the B-2 bomber is The composite material used in the B-2 bomber is specifically designed to absorb radio energy with optimum specifically designed to absorb radio energy with optimum efficiency. Parts of the B-2, such as the leading edge, are efficiency. Parts of the B-2, such as the leading edge, are also covered in advanced radio-absorbent paint and tape. also covered in advanced radio-absorbent paint and tape.