Counter-stealth performance
It is a general wisdom that IRST is of limited usefulness due to its sensitivity to adverse weather conditions. However, most modern stealth fighters (excepting the F-35 and J-31 tactical bombers) are intended to operate at high altitudes – above 50.000 ft – where ambient temperatures range from -30 to -60 degrees Celsius, which helps provide excellent contrast. Air at this altitude is also very dry, with 99,8% of the atmospheric water being below 45.000 ft. Combined with low air density and low aerosoil content, this means that there is very little atmospheric absorption of IR radiation. This applies especially to the longwave band, but detection capability is significantly improved in most bands as can be seen from the image below.
Stealth aircraft are designed to have certain IR signature reduction measures, but effectiveness of these is rather limited due to basic physics. To fly, aircraft has to overcome two basic forces: gravity and drag. Drag is created due to friction with air, compressibility effects and lift. To overcome gravity, aircraft needs lift. To generate lift, aircraft has to move forward and overcome drag. As a result, aircraft has to perform work – which creates heat. Indeed the largest IR sources on the fighter aircraft are its engines. Jet engines work by burning fuel in order to heat up huge quantities of air, which is then propelled out of the rear in order to push the aircraft forward. This leads to significant heat – engine itself is very hot (especially turbines), as is the exhaust nozzle.Engine heats up airframe surrounding it, which can be detected. Exhaust plume is also very hot, though much of the radiation is typically absorbed by the atmosphere (this depends on the altitude – refer to the image before this paragraph).
Other than the engines themselves and their exhaust, there are other sources of IR radiation. Any moving objects have to push the air out of the way. If object is fast – for example, an aircraft flying at high subsonic or supersonic speeds – air cannot move out of the way quickly enough. This leads to compression of the air in front of the aircraft, which in turn leads to heating of said air. At Mach 1,7, a supercruising fighter generates shock cones with stagnation temperature of 87 degrees Celsius. As the air moves out of the way for the aircraft, it also creates significant friction with the aircraft itself, leading to heating of the aircraft’s skin. In a jet fighter, hottest parts of the airframe other than the engine nozzles are tip of the nose, front of the canopy, as well as leading edges (of wings, tail(s) and air intakes).
As mentioned before, MiG-31 would heat up to 760 degrees Celsius during intercepts due to aerodynamic heating alone. Airframe temperature due to friction can reach 54,4 degrees Celsius at Mach 1,6 and 116,8 degrees Celsius at Mach 2,0. F-22 has two pitot tubes – one at each side of the nose – which are heated to 270* C during flight operations to prevent them from icing at high altitude. Avionics have to be cooled – especially radar. Heat exhaust is typically located at fighter’s upper surface – just behind the cockpit in Gripen, and about one canopy length behind it for the F-22. F-35 is in even worse situation since it uses fuel as a coolant, and said fuel completely surrounds its engine. This has the effect of increasing its IR signature as well as thepossibility of bursting into flames if hit.
These temperatures can be compared to the ambient air (Standard US Atmosphere). F-22 achieves maximum cruise speed of Mach 1,72 at ~38.000 ft without afterburner, and maximum speed of Mach 2,0 at between 38.000 and 58.000 ft with afterburner. Above cca 53.000 ft it requires afterburner to fly, and can achieve maximum altitude of ~64.000 ft, where it is limited to maximum speed of Mach 1,6-1,8. Ambient temperature is -44,4 *C at 30.000 ft, -54,2 *C at 35.000 ft, -56,5 *C at 40.000 ft to 60.000 ft, and -55,2 *C at 70.000 ft. That is to say, difference between shock cone of a M 1,7 F-22 and ambient air will be around 130-145 * C, while temperature difference between airframe and ambient air will be cca 111 * C at Mach 1,6 and cca 172 * C at Mach 2,0.
While fighter’s IR signature can be reduced by reducing speed, such course of action also has the effect of reducing one’s own weapons range, as well as making a rear-quarter surprise more likely. In either case, fighter will get detected by modern QWIP IRST before it reaches missile effective range (
10-40 km for AIM-120D at most, and can be as low as 2 km).
It is possible to apply IR absorbent paints to a fighter in order to reduce IR emissions from systems inside it. This, at best, does not have any impact on aerodynamic heating. Some IR absorbent paints cause more friction than would otherwise be the case, increasing aerodynamic heating. RAM coatings also can increase friction. While it is not a significant factor in MWIR band, LWIR detectors can detect aircraft by detecting sunshine reflections from its surfaces, such as canopy.
Modern IRST systems can even detect missile launch from its nose cone heating – this is in fact a significant advantage for IR MAWS, as UV MAWS cannot detect missiles that have spent fuel. They are also sensitive enough for planets, birds, and (in air-to-ground) barbecue grills to be sources of clutter.
Note that even if an object is at the exact same temperature as its environment, it still emits blackbody radiation, most of it at longer wavelengths.
Tactical impact
Unlike radar, IRST is primarily a passive system. This allows a fighter aircraft, or a fighter group, to detect and track the enemy without latter being aware of their presence, thus gaining a significant initial advantage in the OODA loop. Even when the enemy is aware of the fighter’s presence, he has no way of knowing wether he has been detected, or is being targeted, until a significant shift in fighters’ posture (such as painting target with a rangefinder or shifting flight path or formation). For comparison, just turning on the radar warns the aircraft in very large area of scanning fighter’s presence – and said area is far larger than one covered by the radar. Not only does it give away fighter’s presence, but if the enemy has good enough listening equipment, it is possible to triangulate location and even identify the target through its unique radar signals. Even radio communications and datalinks can serve the same purpose.
If the enemy is using radar, it is possible to use data from radar warner to generate a bearing, after which IRST can be used in a “stare” mode – continuous track, during which photon impacts are combined over prolonged timeframe to detect a target at greater distances than would normally be possible. This mode is also present in radar systems, and like IRST, radar also has to be cued by other sensors to make use of it. But while using radar in such a manner basically guarantees than the enemy with a competent RWR will detect radar transmissions, IRST is undetectable. Even a short radar burst can allow the passive fighter to generate such bearing, albeit it will somewhat limit the precision.
If radars are jammed, or more likely turned off for fear of detection, first indication of IRST-equipped fighter’s presence that the enemy aircraft will get may be alarm from a missile warning system (or radar warning system if missile is using an active seeker), thus allowing only a short time for defensive reaction. (Simulated trials of ECR-90 have shown that its airborne detection range could be cut to less than 9 kilometers by jamming). If both sides have IRST, it comes down to sensor quality and IR signature differences.
Aircraft equipped with IRST, and using IR MAWS, can remain completely silent during the mission. If the enemy has no IRST, then he will have to turn on his own radar(s), allowing the passive aircraft excellent situational awareness, well beyond what using radar in addition to IRST would allow. Further, active usage of radar will allow geolocation of radar emitters, allowing the passive fighter to use IRST to engage such targets with high precision – thus gaining a “see first, strike first” capability. IRST-equipped aircraft is also not vulnerable to anti-radiation missiles. (Note that such missiles are not very hard to make, with basically all air-to-air engagement radars being in X band).
IRSTs shortcomings can be compensated for by using datalinks to network the fighter with other assets, such as other IRST-equipped fighters and radar-equipped AWACS. As a result, radar is not the primary onboard sensor any more, and is not actually even required.
Using datalink from AWACS (though AWACS is unlikely to survive for long in a shooting war) or ground radars, fighter can then approach the enemy from side or rear, in order to prevent detection by enemy’s own radar and maximize IRSTs detection range. Once target is acquired on IRST, fighter can pursue engagement completely independently. Of course, if enemy fighter uses its own radar, no AWACS is required. It should be noted that most, possibly all, fighter aircraft today lack the datalink capable of transferring amount of data necessary for a firing solution. Even if such datalink is deployed, it will be easy to jam. As a result, fighters have to rely on onboard sensors to create a firing solution (when Rafale
shot down a target at 6 o’clock, shot was done with onboard sensors and within visual range; F-35 may have a similar capability).
Large radar-based fighters – such as the F-15, F-22, Flanker variants – can act as AWACS of sorts, providing radar image to smaller IRST-only fighters, which can then use such image to achieve optimal position for a surprise attack. This in turn will allow IRST-equipped fighters to focus the IRST and achieve detection ranges larger than could normally be achieved. Even if radars are jammed, radar-based fighters should be able to roughly tell positions of enemy fighters, unless DRFM, active cancellation or standoff jamming is used. Using IRST to generate a firing solution, and then launching an IR BVRAAM or ramjet RF BVRAAM (or, ideally, a ramjet IR BVRAAM, though such missile does not exist in Western inventory) at a surprised opponent will allow far higher kill probabilities than using an obvious radar for firing solution.
Still, using an AWACS with a huge IRST plus extensive ESM arrays might allow the same tactics without a drawback of warning the enemy that he has been detected, and without suffering vulnerability to decoys and jamming that radar has. Additional advantage of such system is that its effectiveness will not be significantly degraded even against VLO targets. On the other hand, while bad weather degrades IRSTs performance, it also degrades performance of stealth coatings (assuming that stealth fighters can safely enter storm clouds), thus combining radar AWACS with IRST-equipped fighters does make some sense, as does using both types of AWACS.
IRST is the best solution for engaging stealthy aircraft and cruise missiles. As it can be seen from the previous section, is impossible to significantly reduce IR signature of a high-speed, highly maneuverable aircraft, and even low-performance aircraft that do have very extensive IR signature reduction measures are still detectable at large distances by new QWIP imaging IRSTs. Even against “legacy” aircraft its is a better choice than radar, as radar cannot separate valid contacts from decoys except at very short range – especially if it is being jammed. As a result, only IRST-equipped fighters can effectively engage modern fighters at beyond visual range.
IRST can be used as a relatively cheap way of turning an old, possibly even WVR-only, platform into one capable of BVR combat. With PIRATE + MICA IR combination, even an old F-86 would gain a capability to shoot down enemy fighters from beyond visual range (that being said, issues of low cruise speed, deficient acceleration by today’s standards and no defense suite at all would remain, and would mean that even against the F-35, F-86 would not achieve positive kill/loss ratio).
Analytic simulations indicate that an IRST-equipped aircraft will have 230% better exchange ratio than a non-IRST equipped aircraft against a “legacy” target, and 370% better against a LO target.
