Stealth provides an aircraft the edge to early detect, localize and lock onto the target aircraft, without painting the enemy’s air interception radar. The aircraft’s frame is built to reflect away the majority of radar waves and absorb a fraction of it as well, when painted with radar absorbent materials. Active stealth is another technique improvised in the 5th generation and later jets to avoid reflections back to the receivers. The mentioned techniques collectively reduce the radar signature of an aircraft, but there are other passive measures as well to detect an aircraft from miles away including acoustic signature, thermal signature etc., Acoustic signature can’t help in localizing the aircraft precisely, rather detect it when it’s nearby, whereas the thermal signature measuring and support instruments integrated within an aircraft can help to localize and find co-ordinates of an aircraft precisely. The main sensor is a thermal camera, and the whole system is known as Infrared Search and Track in short IRST.
An IRST is a passive system, which reads the thermal signature of the bodies present in its sight cone. A hot object will have a significant difference in is signature against the cold background or environment. For instance, an aircraft’s body gets heated, due to air drag it faces while flying, additionally the engines produce heat, which is more significant than the overall body frame of the aircraft itself. Engines have exhaust nozzles facing rearwards, producing the thrust force, to product lift for the aircraft, the exhaust gases produce heat in the exhaust nozzles, increasing the thermal signature for any relative instrument. In stealth aircrafts, not only radar signature is suppressed, but thermal signature as well. For instance, the exhaust nozzles of B2 Spirit bomber, F117 Night Hawk and YF-23 Black Widow-II are hidden from any ground based thermal systems, whereas in F22, there are 2 degrees of freedom available to the V-Shaped exhaust nozzles, these nozzles help reduce the reduction in exhaust cross-section area, which means that less heat will encounter those nozzles, additionally it uses air coolants in its airframe to reduce the thermal signature. In very modern 5th Generation Stealth aircrafts, e.g., F-35 and J-20, the exhaust nozzles are serrated to reduce the exhaust cross-section area, as well as specific materials have also been used having low thermal constant, which means that the nozzles will not absorb much heat. The outer surface of the exhaust nozzles in F-35 has special coating of low thermal signature materials, further reducing the thermal signature. Modern stealth aircrafts use the liquid cooling of the aircraft body more effectively. They have fins and surfaces present alongside the nozzles to hide the exhausts from IRSTs & incoming heat seeking missiles from sides, this is very prominent in J-20, where horizontal stabilizers are present to hide the exhausts.
Being said that, the question raises, are IRSTs stealth busters?
The answer is NO, although IRSTs are claimed to have ranges up to 100 Kms, but these are in ideal situations, such as against large targets facing away from the sensors and favorable weather conditions. As mentioned earlier, the stealth aircrafts have reduced thermal signature, in a quest to detect, localize and lock onto a stealth aircraft to fire an Infrared Guided Missiles, will bring too close to the lion’s den. Generally, the within visual range missiles, WVRs for short, have maximum ranges up to 35-50 Kms, and are effective against an agile maneuvering target up to 20-30 Kms. Considering an ideal situation, where a stealth aircraft facing away from an onboard IRST of an enemy’s fighter jet with its radar turned off, the airborne AWACs and AEW&Cs, will detect an incoming aircraft from six of the homie jet, and will warn it. One on One situations with are very unlikely to happen, and if they happen, the modern stealthy aircrafts having super cruise ability added with counter measures, will evade and counter attack in such scenarios. IRSTs do provide some information about the target, where the passive searching is an icing on the cake. But, even if an aircraft is detected in the sensor, its exact location, speed and acceleration data can only be found using Laser Range Finders, which have maximum ranges in a couple of 10s Kms. IRSTs are a good addition to an aircraft, but they are not alternative of a radar or can be called stealth busters due to their own limitations.