SpArK
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How to get Stealthy
1.0 How to get Stealthy
1.1 Ingredients of Stealth technology
To make a stealthy aircraft, designers had to consider five key ingredients:
- reducing the imprint on radar screens / stifling radio transmissions
- turning down the heat of its infrared picture
- Improve aerodynamics
- making the plane less visible.
- muffling noise
To understand more about each ingredient, here is some theory first.
1.2 Radar Cross section (RCS)
The first goal is to cut down the size of the aircraft's radar image, called its "radar cross section," or RCS. This normally involves using radical design features and some nonmetallic materials.
A conventional fighter aircraft has an Radar Cross Section (RCS) in the region of 6 square metres. The much larger B-2B bomber, using the latest stealth technology, displays an RCS of only 0.75 square metres. By comparison, a bird in flight displays an RCS of 0.01 square metres.
Stealth plane designers have to take in account that the used materials (for instance composites) may not be transparant to radar, but they are also not completely reflective. In other words, the parts behind the skin of the plane may be invisible for the eye, but they are not for radar waves, thus causing echos.
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2.0 Getting invisible
This section explains more about what radar echos look like and how they can be prevented to reach the radar receiver again after hitting the plane.
2.1 Echo scattering
Curving surfaces on conventional aerodynamic bodies act as scatterers, reflecting radar waves from any angle and giving the radar operator a clear signal. The right-angled surfaces at the wing and tail roots also reflect radar signals straight back to their source.
Scintillation is a measure of how rapidly the size of the return varies with the angle. The greater this variation, the more difficult a target is to track. The lower the number of lobes and the narrower the lobes, the lower the probability of detecting any return.
Panels on planes are angled so that radar is scattered and no signal goes back to base.
The F-117 airframe for instance has a large number of faceted surfaces, not unlike a crystal.
The facets are presumed to
reflect radar energy away from the aircraft in any other direction than that of the radar emitter.
A flat plate at right angles to an impinging radar wave has a very large radar signal, and a cavity, similarly located, also has a large return. Thus the inlet and exhaust systems of a jet aircraft would be expected to be dominant contributors to radar cross section in the nose-on and tall-on viewing directions, and the vertical tail dominates the side-on signature.
2.2 Radar absorbtion
A second way of stopping radar reflections is by coating the plane with material that soaks up radar energy.
These typically consist of carbon, carbon fibre componsites, or magnetic ferrite-based substance.
The result is that for instance the B-2 is reported to have the same RCS as a child's tricycle!
Flight-control surface can be made from honeycombed materials which reflect incoming radar waves internally rather than back to the radar. Radar-absorbing coatings can be applied to the surface of the body which effectively drain the energy of the radar signal.
2.3 Echo cancellation
The key dimension of a quarter wavelength can vary in practice from millimeter to one meter. Although the coating designer will frequently try to use materials whose dielectric constant varies in a way that maintains a constant wavelength independent of frequency, the reality is that a number of different coatings and absorbers are needed to cover the required bandwidth.
Imagine a low frequency absorber that might be made of glass fiber hex-cell material. Its resistance is graded from front to back so that the edge is initially electro-magnetically soft and gradually becomes more attenuating as the wave passes through. This approach is particularly taken when, for practical reasons, the layer cannot be as deep as a quarter wavelength. The inner absorber is covered by a high-frequency ferromagnetic coating, which completes the frequency coverage.
Metal components such as the engine, which produce significant radar reflections, can be shielded using a metal and plastic sandwich whose layers are spaced in such a way as to create a standing wave, cancelling out any radar reflections.
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3.0 Heat radiation reduction
Infrared radiation (heat) should be minimized 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.
The jet-wake radiation follows the same laws as the engine hot parts. Various ways have been developed and tested to cool down the engine exhaust gasses. The ilustration above shows how the hot exhaust gasses can be surrounded by cooler air, significantly reducing the IR signature of the plane.
Air has a very low emissivity, carbon particles have a high broadband emissivity, and water vapor emits in very specific bands. Infrared seekers have mixed feelings about water-vapor 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 were favorable for using water-vapor bands for detection.
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4.0 Turbulence reduction
By optimizing the aerodynamics of the stealth plane, the for the eye invisible turbulence trail in the air, can be kept to a minimum. This way it becomes harder for the very special laser equipment to detect the trail and trace it back all the way to the plane which created it.
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5.0 Visual detection reduction
5.1 Hiding smoke contrails (jet wake)
Reducing smoke in the exhaust is accomplished by improving the efficiency of the combustion chambers. Getting rid of contrails - that distinct white line in the sky caused by high flying jets - is a harder task.
Tests have been done using exotic chemicals to be inserted into the engine outlet gases to modify infrared signature as well as to force water molecules in the exhaust plume to break up into much finer particles, thus reduce or even eliminate contrails. One of the chemical used for this was chloro-fluoro-sulphonic acid. Several other acids were tested too, but the result was that the chemicals were too corrosive and the system was waved.
5.2 Low visibility
An aircraft at low to medium altitudes tends to be a black dot against the background of the sky. To avoid this, the plane a given a special medium gray color.
The gray, when combined with light scattering at low to medium altitudes ensures about as low observability as can be possible, or a reduction to 30% in visibility.
5.3 Low level flight
Another technique used by aircraft to avoid radar is to fly at very low levels where there is a great deal of 'ground clutter' ... radar reflections given off by buildings and other objects. Low-level aircraft can go undetected by most radar systems.
The latest ground-defence systems however are designed to discriminate between ground-clutter and hostile planes. In addition, ground-clutter is partly avoided by using 'look down' radar systems, which track aircraft from other aircraft flying above.
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