The jet which cannot be detected by a radar is called stealth jet. It uses stealth technology. Stealth technology, also known as LOT (Low Observability Technology), is a sub-discipline of military electronic countermeasures which covers a range of techniques used with aircraft, ships and missiles, in order to make them less visible (invisible) to radar, infrared and other detection methods. A ’stealth’ jet will generally have been designed from the outset to have reduced or controlled signature. It is possible to have varying degrees of stealth. The exact level and nature of stealth embodied in a particular jet is determined by the prediction of likely threat capabilities and the balance of other considerations, including the raw unit cost of the system. The material used for the ’stealth jet’ should be either transparent to the radar or it should absorb the radar signals so that it cannot be identified.
The principles of ‘STEALTH JET’ are
1. Radar cross section reductions
2. Visibility
3. Infra red
4. Reducing radio frequency emissions
1. In the radar cross section reductions, the shape of the stealth jet is designed in such a way that the radar signals which penetrate through this will become weak. An important factor in the design is the internal construction. Behind the skin of the aircraft are structures known as re-entrant triangles. Radar waves penetrating the skin of the aircraft get trapped in these structures, bouncing off the internal faces and losing energy. In reflecting the radar back to the transmitting end there are many techniques. One of them is with orthogonal metal plates,forming a corner reflector consisting of either two plates or three plates.
Stealth design must bury the engines within the wing or fuselage, or in some cases where stealth is applied to an existing aircraft, install baffles in the air intakes, so that the turbine blades are not visible to radar. Planform alignment is also often used in stealth designs. Planform alignment involves using a small number of surface orientations in the shape of the structure. The leading edges of the wing and the tail surfaces are set at the same angle. The effect of planform alignment is to return a radar signal in a very specific direction away from the radar emitter rather than returning a diffuse signal detectable at many angles. Shaping requirements have strong negative influence on the aircraft’s aerodynamic properties and it needs computer assistance for balancing it.
2. For less visibility, most stealth aircraft use matte paint and dark colors, and operate only at night. Lately, interest on daylight Stealth has emphasized the use of gray paint in disruptive schemes, and it is assumed that Yehudi lights could be used in the future to mask shadows in the airframe. The ’stealth jet’ has wing tanks for a contrail-inhibiting chemical, alleged by some to be chlorofluorosulphonic acid, and mission planning also considers altitudes where the probability of their formation is minimized.
3. An exhaust plume contributes a significant infrared (IR) signature. One means of reducing the IR signature is to have a non-circular tail pipe in order to minimize the exhaust cross-sectional volume and maximize the mixing of the hot exhaust with cool ambient air. Often, cool air is deliberately injected into the exhaust flow to boost this process. Sometimes, the jet exhaust is vented above the wing surface in order to shield it from observers below. To achieve infrared stealth, the exhaust gas is cooled to the temperatures where the brightest wavelengths it radiates on are absorbed by atmospheric carbon dioxide and water vapor, dramatically reducing the infrared visibility of the air craft. Another way to reduce the exhaust temperature is to circulate coolant fluids such as fuel inside the exhaust pipe, where the fuel tanks serve as heat sinks cooled by the flow of air along the wings.
4. Reducing radio frequency emissions is that a stealth vehicle must avoid radiating any other detectable energy, such as from on board radars, communications systems, or RF leakage from electronics enclosures. Some ’stealth’ jets use passive infra-red and “low light level TV” sensor systems to aim its weapons and some use an advanced LPI radar which can illuminate enemy aircraft without triggering a radar warning receiver response.
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