InfraRed Search & Track - IRCM

INFRARED COUNTER MEASUERS (IRCM)

IRCM can be classified into two categories passive (termed as IR suppression) & Active (Eg; decoys)

PASSIVE-(IR suppressors /Optimizers) – Minimizes the signature from aircraft
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ACTIVE-(IR flares, IR jammers, Pyrotechnic IR Decoys (IR lamp on sacrificial structures)) –Confuses the IR seeker by IR jamming, Luring away towards fake target / sacrificial structure

Aircraft / Helicopters equipped with IR counter measures is not necessarily immune to attacks by IR guided missiles as counter counter measures (CCM)are also being currently developed

PASSIVE COUNTER MEASURES

Reduction in IRSL (InfraRed Stimulated Luminance) by passive measures reduced the susceptibility of aircraft by reducing its RLo (Lock on Range). RLo approximately equal to IRSL1/2. Reduction in IRSL by a factor of 10 reduces RLo by a factor of about 3.16

                The general objectives of passive counter measure IRSS (Infra Red Signature Suppression) systems are as follows
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• To significantly reduce IRSL from almost all viewing aspects
• Maintain engine performance by minimizing back pressure penalty
• Minimize weight penalty
• Minimize cost & system configuration complexity
• Minimize external drag
• While implementing passive IRCM other signatures like RCS should not increase as a consequence

 
IRSS uses one or more of the following techniques

• Masking of hot engine parts; which includes nozzle shape alteration, jet pipe geometry modification & fitment of new assembly
• Peak temperature reduction of exhaust gases by enhancing mixing with surroundings. This can be achieved by altering the jet pipe/nozzle geometry & by fitment of separate devices in the exhaust system. The plane IRSL can also be tailored by incorporating & locating a structure that melts / ablates resulting in mixing of additives into the plume. The structure which is subjected to melting/ablation can be made from materials like magnesium, aluminum, magnesium-polytetrafluro ethylene, carbon & other energetic solid particles. The plume radiant intensity can be enhanced reduced or resonated by varying the additive material type, composition density & ablation/melting range
• Matching of IRSL by modifying the Aircraft skin temp to reduce the contrast with the surrounding by physical & chemical means. This is achieved by emissivity control, electric heating of skin, liquid evaporative cooling of surface & heat pipe cooling of the skin
• Reducing the reflecting Aircraft surfaces to mitigate the sunshine, sky shine & earth shine reflections

 
EXHAUST SYSTEM & IRSS
 
Plume IRSS can also be achieved by engine size reduction, cycle tailoring, phase/aerosol (air) mixing & nozzle shaping. Injection of carbon particles or water vapor in the plume So that it shields the plume reduces the intensity of IR emission substantially.

 The plume core stretch, its volume, temperature & concentration of IR radiation participating gases, depend on the bypass ratio (BPR-jet engines uses two ways to pass air one is through the jet engine core & this air compressed & ignited and the remaining air passed through other way called bypass and the ratio of air passing through the core & bypass is called bypass ratio). Increase in BPR from 0 to 1 rapidly decreases the exhaust gas temperature. However for BPR>1, the decreases in exhaust gas temperature is much smaller with increase in BPR. Hence BPR ~1 gives best results.
Trends in thermodynamic cycle parameters which improve cycle efficiency & lower fuel consumption also lower the heat emission from engine (so IRSL also lowered). Afterburning increases the IR emission almost 10 folds.

                A non-axi-symmetric nozzle has lesser IR emissions than an axi-symmetric nozzle because the engine hot parts are largely masked by non axi-symmetric nozzle hardware. A non axi-symmetric 2-D wedge shaped nozzle also enhances the mixing of hot exhaust gases with ambient air, thereby reducing the IRSL of the plume
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Notched nozzles also well for reducing IRSL, a 60 degree notched nozzle reduces the length of hottest part of the plume by 33%, but the reduction in spectral IR radiance occurs near 4.3 micrometer. Notched nozzle facilitate in the radial spreading of the jet & its mixing with ambient air, thereby reducing the plume length. Plume IRSL reduction can also be achieved by enhancing mixing of exhaust gases with ambient air by providing the following features at nozzle exit, corrugated surfaces, lobes, scalloped edges, turbulators & chevrons. Corrugated seals at the nozzle exit also reduces aural signature 

IRSS SYSTEMS FOR EXHAUST
 
The IRSS system that employ the specifically designed device /mixer to discharge the exhaust gas.  Some designs improve the mixing of exhaust gases with the ambient air (jet dilution). Other IRSS systems that apart from mixing relocate the discharge in the downward direction the characteristic feature of some patented IRSS systems are now elaborated.

• The exhaust gas flow channel is attached to a movable access door. This door can be closed during combat & when closed it masks the hot flow channel & hot engine parts
• Reducing the temperature of the engine by utilizing the fuel for cooling engine & exhaust nozzle, prior to feeding to the combustor.
• An elongated duct for carrying the exhaust gases, which has a baffle to obstruct the view of hot engine parts. The duct & baffle are coated with a combination of high & low emissivity materials for reducing IR emissions
• An exhaust mixer for gas turbine engine for reducing the visibility of hot engine parts. A mixer has a no of lobes; each shaped to partially block the engine exhausts which are covered in pattern selected to provide a desired degree of blockage. This device enhances the mixing with ambient air; but adds to the weight & complexity besides loss of thrust
• Method for mixing enhancement of turbofan exhaust gases with ambient air, thereby reducing the plume IR emissions. The system consists of a duct having an outer wall & inner wall forming converging-diverging-converging passage. The flow near the first converging area is equal to or greater than speed of sound & thereafter , the flow has a positive stream wise pressure gradient near the exit
• An IRSS system having a multistage mixer/ejector at each stage, the hot exhaust gases entrain cold air in the core region of the engine exhaust. This multistage mixer device also masks the hot engine parts, maintain engine performance & thoroughly diffuses the exhaust gases

to read more applications of IRST click on the button/s below

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IRST part 2

IRST Part 1

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