STAYING INVISIBLE IN THE AIR THE first thing to note about stealth technology is that it isn’t a technology per se but rather a concept of a broad series of technologies and design features that reduce the “detectability” of a particular object. Generally stealth technology is mentioned in relation to aircraft but it can also be applied to surface ships, submarines, helicopters, missiles, ground vehicles and even people. In order to make an aircraft “stealthy,” it has to reduce its signature in the following five areas: Radar cross section (RCS), sound, visibility, heat and radar emission. All five areas need to be addressed together, as it is not enough to reduce an RCS signature while emitting a high signature in the other areas. However, trade-offs have to be made as it is presently virtually impossible to cover all five requirements. Plane invisible RCS is basically how an object reflects a radar beam off it. Here, it does not necessarily mean that a large object will have a larger RCS signature in contrast to a smaller one. Instead it depends on the shape of the object and the type of material the object is made of. To reduce the RCS of an aircraft, a combination of purpose-shaped, radar- absorbing and non-radar reflective material is used. Purpose-shaping is designing an aircraft so that it prevents a radar signal from returning to its source, or to trap the signal within the airframe since radar detection depends upon the radar beam being deflected back to its source. Both the F-117 Nighthawk fighter and the B-2 Spirit bomber lack any significant tailfins because tailfins have high radar reflection. Similarly, any bombs and missiles must be carried internally as the protrusions caused by mounting them externally would result in a higher RCS. Even then there is a risk of detection as an opening bomb bay door increases the RCS of a stealth aircraft. Purpose shaping has its limitations because the shapes that reduce RCS have generally very poor aerodynamics capabilities. The F-117 for instance, can only fly with the aid of computers, and even then its performance is poor compared to other jet aircraft of similar size. Radar absorbing material normally takes the form of paint used to coat the metal parts of an aircraft (metal being highly reflective to radar) and is classified into resonant and non-resonant material. The former creates an interference field from a radar signal and thus results in the radar signal cancelling itself out rather than being reflected. However, it is limited because the material has to be somewhat tailored to the radar frequency that is expected to be encountered. This is not the case for non-resonant material, which dissipates the radar signal over its surface. However this increases the aircraft’s surface temperature and thus a reduced RCS is gained at the expense of an increased heat signal. The final way of reducing RCS is through using non-radar reflective material in the form of composite materials. Unlike metals, composite materials do not reflect radar signals. However the problem is composite materials are still not durable enough to replace metal in some parts of an aircraft. Sonic and sight reducer Reducing sound is done by having an aircraft fly at subsonic speed. This allows it to avoid being detected by the sonic boom created when travelling at supersonic speed. Decreasing visibility is done via both design and operational means. Painting an aircraft in a particular colour, for instance, can make it difficult to see. The F-117 is painted black because it is designed to operate at night, while the B-2 is painted in a different scheme that makes it less noticeable in both day and night. Decreasing visibility via operational means calls for an aircraft follow a flight path through remote areas, flying at night or through bad weather. A sound signature can be reduced by having the stealth aircraft fly through an area where its sound would be drowned out by other noise. If you look at the engines on the F-117 and B-2, you would notice that the engine exhausts are smaller and slit-shaped. This is done to reduce both the heat and visibility signatures of the aircraft. The design minimises the exhaust area and maximises the mixing of the hot exhaust with cool ambient air. This lowers the heat signature and also causes the jet exhaust to be less visible at night. Minimising emissions Finally a stealth aircraft has to ensure that it has a minimal radar emission profile. This is because anyone using a radar system can be detected. Since aircrafts use several radars systems to operate, scrapping radar on airplanes altogether is not a practical solution. To this end, much work is being done with the US Air Force’s latest fighter, the F- 22 Raptor. It has a “Low Probability of Interception” radar that allows it to track enemy aircraft without triggering their radar warning receivers. Evolving silently Stealth technology is a continuously evolving field where there are constant counter measures as soon as one advancement is made. Some current counter-measure efforts involve detecting stealth aircraft via the turbulence created by a flying aircraft using low frequency radars (which can detect almost anything but cannot determine the target’s exact location) and the disruption caused to radio and television signals created by aircraft. There are also ongoing efforts in developing unmanned air combat vehicles with stealth capabilities, though these will still take several years before they take to the skies.
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