3What is Stealth?Stealth is the technique of making a plane (or any other object) less visible to the enemy by reducing it's radar and IR (infra red, heat) visibility.Reducing the IR image of a plane can be accomplished by directing the hot exhaust gasses to the top of the plane and mix them with cold air .Reducing radar visibility can be accomplished by deflecting the radar waves in such a direction that they don't go back to the emitting radar & making use of less radar reflecting materials (composite material, plastic) and/or radar absorbing coatings
4Conventional Aircraft Most conventional aircraft have a rounded shape. This shape makes them aerodynamic, but it also creates a very efficient radar reflector. The round shape means that no matter where the radar signal hits the plane, some of the signal gets reflected back
5A flat plate at right angles to an impinging radar wave has a very large radar signal, A cavity, similarly located, also has a large return.Thus she inlet and exhaust systems of a jet aircraft would be expected to be dominant contributors to radar cross section in the nose-on and tail-on viewing directions, and the vertical tail dominates the side-on signature.
6Stealth TechnologyGoal: Stealth is the technique of making a plane (or any other object) less visible to the enemy by reducing it's radar and IR (infra red, heat) visibility.There are two different ways to create Radar invisibility:The airplane can be shaped so that any radar signals it reflects are reflected away from the radar equipment.The airplane can be covered in materials that absorb radar signals.
7A stealth aircraft, is made up of completely flat surfaces and very sharp edges. When a radar signal hits a stealth plane, the signal reflects away at an angle . In addition, surfaces on a stealth aircraft can be treated so they absorb radar energy as well. The overall result is that a stealth aircraft can have the radar signature of a small bird rather than an airplane.
8SHAPINGThere 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.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.At normal incidence, the flat plate acts like a mirror, and it returns 30 decibels (dB) above or 1000 times the return from the sphere.If we 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 1000, equal to that of the sphere, when the look angle reaches 30 degrees off normal to the plate.
9Diagonal Rotation:If we go back to the normal incidence case and rotate the plate about a diagonal relative to the incoming wave, we see a remarkable difference.In this case, the cross section drops by 30 dB when the plate is only 8 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 million !
10Reflection from Ducts: 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 .Number of bounces can be increased significantly without sacrificing aerodynamic performance.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
11The backscatter -the energy radiated directly back to the transmitter -increases as the wavelength goes up, or the frequency decreasesWhen designing a cavity for minimum return, it is important to balance the forward scatter associated withray tracing with the backscatter from interactions with the first surfacesAlthough surface alignment is very important for external surfaces and inlet and exhaust edges, the return from the inside of a cavity is heavily dependent on attenuating materials
12RADAR ABSORBING MATERIALS(RAM) radar-absorbing materials or RAM are coatings whose electrical and magnetic properties have been altered to allow absorption of microwave energy at discrete or broadband frequenciesPrinciples of Operation:Microwave absorbers are produced by using existing materials and altering their dielectric and magnetic properties. For purposes of analyses, the dielectric properties of a material are categorized as its permittivity and the magnetic properties as its permeability. Both terms are complex numbers with real and imaginary parts.
13Common dielectric materials used for absorbers such as foams, plastics and elastomers have no magnetic properties, giving them permeabilities of 1.Magnetic materials such as ferrites, iron and cobalt-nickel alloys are used to alter the permeabilities of the materials.High dielectric materials such as carbon, graphite and metal flakes are used to modify the dielectric properties.When an electromagnetic wave propagating through a free-space impedance of ZO is incident upon a semi- infinite dielectric or magnetic dielectric boundary of impedance Z1, a partial reflection occurs. The magnitude of the reflection coefficient is governed by the following equation.
14Where,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.
15Microwave Absorbers Resonant Type: In general, practical micro- wave absorbers are one of two basic types: resonant or graded dielectric.Resonant Type:The simplest type of resonant absorber is the Salisbury Screen. It consists of a resistive sheet spaced one-quarter wavelength from a conductive ground plane. The resistive sheet is as thin as possible with a resistance of 377 ohms per square inch, matching that of free space. A wave incident upon the surface of the screen is partially reflected and partially transmitted. The transmitted portion undergoes multiple internal reflections to give rise to a series of emergent waves. At the design frequency, the sum of the emergent waves is equal in amplitude to, but 180 degrees out of phase with, the initial reflected portion. In theory, zero reflection takes place at this frequency; in practice, absorption of greater than 30dB (99.9 percent) may be achieved.
18PLASMA STEALTHThe Russian approach towards stealth : They are developing a system to make a plane invisible to radar by using a sort of a plasma torch on the nose of the plane. The idea behind is, this 'torch' creates a ionized 'cloud' around the plane which will absorb radar waves.Plasma is ionized gas, which means that it's atoms have so much kinetic energy that valence electrons have been freed through atomic-level collisions.A radio signal encountering a plasma stream would easily be subject to dispersion. The electromagnetic radio wave encountering the plasma would become highly energized and change directions. This would cause it not to be reflected back to the radar station. It could be possible that plasma could also entrap and dissipate the energy of the radio wave.
19The physics of plasma protection can be described as following The physics of plasma protection can be described as following. If an object is surrounded by a cloud of plasma, several phenomena are observed when the cloud interacts with electromagnetic waves radiated by enemy radar. First, an absorption of electromagnetic energy occurs in the cloud, since during plasma penetration it interacts with plasma charged particles, pass onto them a portion of its energy, and fades. Second, due to specific physical processes, electromagnetic wave tends to pass around plasma cloud. Both of these phenomena results in dramatic decrease of the reflected signal.It is not known whether the plasma stealth system developed by the Russians employs a plasma laser or some other method for creating a plasma field.
20B2 STEALTH BOMBERThe B-2 bomber, commonly known as the stealth bomber, was an ambitious project, to say the least. In the 1970s, the U.S. military wanted a replacement for the aging B-52 bomber. They needed a plane that could carry nuclear bombs across the globe, to the Soviet Union, in only a few hours. And they wanted it to be nearly invisible to enemy sensors.A 172-foot wide flying wing that looks like an insect to radar scanners! The craft is also revolutionary from an aeronautics perspective: It doesn't have any of the standard stabilizing systems found on a conventional airplane, but pilots say it flies as smoothly as a fighter jet.
22Basic Design:The B-2 bomber has a completely different design: It's one big wing, like a boomerang.This flying wing design is much more efficient than a conventional plane. Instead of separate wings supporting all the weight of the fuselage, the entire craft works to generate lift. Eliminating the tail and fuselage also reduces drag -- the total force of air resistance acting on the plane.Greater efficiency helps the B-2 travel long distances in a short period of time. It's not the fastest craft around – It’s high subsonic, meaning its top speed is just under 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 and 11,500 miles (18,500 km) with one in-flight refueling.
25Fly ControlsThe B-2 has four General Electric F-118-GE-100 jet engines, each of which generates 17,300 pounds of thrust. Just as in an ordinary plane, the pilot steers the B-2 by moving various parts of the wings. The B-2 has elevons and rudders along the trailing edge of the plane. Just like the elevators and ailerons on a conventional plane, the elevons change the plane's pitch (up and down movement) and roll (rotation along the horizontal axis). The elevons and rudders also control the plane's yaw (rotation along the vertical axis).The B-2 has a sophisticated fly-by-wire system. Instead of adjusting the flaps through mechanical means, the pilot passes commands on to a computer, which adjusts the flaps. In other words, the pilot controls the computer and the computer controls the steering system.
27Safety MeasuresThe computer also does a lot of work independent of the pilot's input. It constantly monitors gyroscopic sensors to keep track of the plane's attitude -- its position relative to the airflow. If the plane starts to turn unexpectedly, the computer automatically moves the rudders to counteract the turning force. The corrections are so precise that the pilot usually won't feel any shift at all.The B-2 also has a small wedge-shaped flap in the middle of the trailing edge. The computer adjusts this flap, called the gust load alleviation system (GLAS), to counteract air turbulence forces.
28Stealth Measures in B2The B-2's flat, narrow shape and black coloration help it fade into the night. Even in the daytime, when the B-2 stands out against blue sky, it can be hard to figure out which way the plane is going. The B-2 emits minimal exhaust, so it doesn't leave a visible trail behind it.The B-2's engines are buried inside the plane. This helps muffle the noise. The efficient aerodynamic design helps keep the B-2 quiet as well, because the engines can operate at lower power settings.The engine system also works to minimize the plane's infrared (heat) signature. Infrared sensors, including those on heat-seeking missiles, typically pick up on hot engine exhaust. In the B-2, all of the exhaust passes through cooling vents before flowing out of the rear ports. Putting the exhaust ports on the top of the plane further reduces the infrared signature
30Radar Avoidance:The stealth bomber's peculiar shape deflects radio beams in both ways. The large flat areas on the top and bottom of the plane are like tilted mirrors. These flat areas deflect most radio beams away from the station, presuming the station isn't directly beneath the plane.The plane itself also works like a curved mirror, particularly in the front section. The entire plane has no sharp, angled edges -- every surface is curved in order to deflect radio waves. The curves are designed to bounce almost all radio waves away at an angle.The B-2's body is mainly composed of composite material -- combinations of various lightweight substances. The composite material used in the B-2 bomber is specifically designed to absorb radio energy with optimum efficiency. Parts of the B-2, such as the leading edge, are also covered in advanced radio-absorbent paint and tape.