Stealth technology
Stealth technology covers a range of techniques, used mostly on stealth aircraft and ships, in order to make them less visible (ideally invisible) to radar; this was most notably used during the Gulf War (1991) although it has since become less effective (due to developments in the microprocessing of data received by radar), both the United States and Russia continue to develop stealth vehicles. Stealth is not a single technology but a combination of technologies, specifically
a) Aircraft shape It has been known since at least the 1960s that aircraft shape makes a huge difference to how well an aircraft appears on radar. The BAC Vulcan, a British nuclear bomber of the 1960s, had a remarkably small appearance on radar despite its large size, and occasionally disappeared entirely. We now know that it had a fortuitously stealthy shape apart from the vertical element of the tail. On the other hand, the Tupolev 95 Russian long range bomber (NATO Reporting Name 'Bear') appeared especially well on radar. We now know that propellers (and even jet turbine blades) give a bright radar image. The Bear had 4 pairs of large (2.4 metre) contra-rotating propellers. The most efficient way to reflect radar waves back to the trasmitting radar is with two metal plates at right angles to one another, perpendicular to the incident radar wave. This configuration occurs in the tail of a conventional aircraft, where the vertical and horizontal components of the tail are set at right angles. A stealth aircraft must use a different arrangement. Often, a stealth design has the vertical element of the tail tipped at an angle, as in the F-117. The most radical approach is to eliminate the tail completely, as in the B-2 Spirit. As well as altering the tail, stealth design must bury the engines within the wing or fuselage so that the turbine blades are not visible. The shape of the aircraft must have no bumps or protuberances of any kind if it is to be stealthy. This means that all weapons, fuel tank, and other stores may not be carried on underwing pylons but must be stored internally. Furthermore, a stealth aircraft becomes unstealthy when it extends its mid-air refuellig probe.
b) Use of non-metallic materials called composites for the airframe. Composites are transparent to radar whereas metals reflect it back to the transmitting radar
c) Radar absorbing paint.
d) Technologies to reduce other signatures such as infra-red, sound, etc. Stealth aircraft need to stay subsonic to avoid being tracked by sonic boom and they need to reduce their infra-red signature. This is generally done by having a non-circular tail pipe (a slit shape)and mixing ambient air with the exhaust. The stealth aircraft must not radiate any energy which can be detected by the enemy. The F-117 uses a passive infra-red system to navigate and the F-22 has an advanced Low Probability of Interception (LPI) radar which can use to illuminate enemy aircraft without activating their radar warning receivers.
The size of a plane's image on radar is measured by the Radar Cross Section or RCS. Imagine a metal plate of area A square metres held perpendicular to the beam of radar transmitter. It reflects most of the radar energy back to the source, where it can be detected. It is said to have an RCS of A square metres. Modern stealth aircraft have an RCS of much less than 1.0 m².