Infra-Red Signature Reduction

Improvements in low observables have been made in all areas with the exception of Infra-Red (IR) suppression devices, which are limited in efficiency. Infra-Red weapons systems are prolific worldwide both air-to-air and surface to air. As a measure of their impact, approximately 90 percent of all combat losses over the 15 years 1985-2000 are attributable to infra-red missiles.

Hot aircraft gas turbine engine exhaust nozzles emit infrared radiation (IR) which is highly undesirable for military combat aircraft. Such aircraft engines include variable area axisymmetric, axisymmetric vectoring, and two dimensional convergent/divergent (CD) nozzles. Convergent and divergent flaps and seals confine hot exhaust flow and typically are used to provide variable throat area and exit area nozzles. These flow confining elements get hot and the divergent flaps and seals provide an unwanted infrared radiation (IR) signature for the engine and aircraft. Infrared radiation from gas turbine engines is conventionally suppressed by shielding and cooling the hot metal structures of the engine. Nozzles may also require or make use of cooling for structural reasons. Cooling air is conventionally drawn from the fan section or a compressor section of the gas turbine engine which is expensive in terms of fuel and power consumption. Nozzles including cooling air ejectors, such as the type used on some General Electric J79 engine models, have employed slot type ejectors to induct ambient cooling air from the atmosphere to supplement the engine supplied cooling air in order to reduce the use of the more expensive engine air.

Axial flow engines of the type concerned are well known and generally comprise an outer housing containing a centrally located fuel burning turbine that produces a central stream of hot "core" gasses and drives an air induction fan in the forward part of the housing. A portion of the air induced by the fan bypasses the turbine as an annular fan stream formed between the turbine and the outer housing so as to surround the core gas stream from the turbine. In afterburner or augmentor operation, additional fuel is added for burning in an afterburner section behind the turbine to produce additional thrust. It is well known that it is advantageous to effect mixing of the two streams during afterburner or augmentor operation for reasons of burning efficiency, noise abatement, infra-red signature reduction, and to permit use of a shorter augmentor structure.

The mixing has been accomplished by the use of convoluted mixing chutes or crossover passages, sometimes called "daisy chutes" because of their cross-sectional configuration. In these devices the chuted portions of the fan (cool) and core (hot) streams are circumferentially arranged in alternating segments around the unchuted portion of the core stream. To promote mixing between the alternating segments of fan and core streams, the augmentors have been provided with perforated diffusion panels and with turbulance producing vanes or turbulators that can be actuated from stream aligned positions during normal non-augumented operation to angular positions during augmentor operation.

Electromagnetic radiation, emitted or reflected by the gas turbine exit nozzle contributes significantly to the "signature" of a vehicle. By mounting the exit nozzle within a suitably shaped shroud this signature is reduced. Radar signals are absorbed or diffused by the shroud and infra-red emissions are masked, either by the shroud itself or by cool air ducted by the shroud to pass around the nozzle and the exhaust stream issuing from it. Such an arrangement, by its nature, defines a constraining envelope within which the nozzle must lie in order to be effective. This imposes constraints upon the use of reheat or thrust vectoring of the engine exhaust stream.

One type of two dimensional nozzle is a single expansion ramp nozzle referred to as a SERN nozzle. SERN was developed as a variable area non-axisymmetric nozzle with a unique installed performance characteristic of low weight and frictional drag because there is no or a smaller lower cowl. Low observable (LO) exhaust nozzle technology is being developed for current and future fighter/attack aircraft. LO nozzles should be integrated cleanly with the aircraft airframe and not degrade the aircraft's performance due to weight and drag penalties. Exhaust systems for combat aircraft should possess characteristics to enhance aircraft survivability, including high internal performance, reduced radar cross section (RCS), low infrared (IR) signatures, low installed weight, low installation drag and, in some cases, thrust-vectoring capabilities.