Evading air-to-air missile
Posted by picard578 on August 17, 2013
Missile evasion is a very important part of modern air combat, yet many people believe it is impossible. But this is false, for several reasons. First reason is that missile cannot turn tighter than the aircraft: in order to pull as tight turn as a fighter aircraft, missile has to pull amount of g that is amount of g’s aircraft can pull multiplied by factor of difference in speed squared. For example, Typhoon can pull 9 g at 360 kts, and IRIS-T can pull 60 g at Mach 3, or 1.984 knots. What this means is that IRIS-T will have 4,5 times as wide turn diameter. If target is pulling 9 g at Mach 0,9, then IRIS-T will still have 1,7 times as wide turn diameter, and if target is pulling sustained 5 g turn at 360 kts, IRIS-T will have 2,5 times as wide turn diameter.
Evasion is made easier in some situations by the fact that missile always attempts to lead the target. Thus if target changes heading, it will be hard pressed to correct. This is the case with BVR missiles, where target fighter aircraft can turn so that missile faces its side and only enter a turn once missile is close. Probability of BVR missile hitting is made worse by the fact that it is both faster and capable of pulling less Gs than WVR missile: AIM-120 can pull 40 g at Mach 4 (2.646 kts), which means that it will have 12,2 times as wide turn diameter as Typhoon in example above. Even if Typhoon turns at 5 g (maximum sustained g in AtG configuration), missile will still have 6,75 times as wide turn diameter. Also, when missile is tracking a maneuvering target, it bleeds off the energy.
Main problem with evading missiles is their speed, which makes timing somewhat difficult. However, even that is far less of a problem than commonly assumed, as missile will be closing at 1.200-1.400 meters per second in the best case. At 20 kilometers, this means 14-20 seconds to reach the target for a BVR missile, or 20-23 seconds for IR missile.
Thus there are several tactics to evade the missile, some of them very simple. First is a barrel roll. As missile is unable to track it, it will fly past and loose a lock in the process. Second is a simple turn, where pilot forces missile to follow it through a turn – this turn however must be well timed, and is very useful as an end move in more complex maneuvers designed to bleed off missile’s energy. It is also very useful in a dogfight, where rear-aspect shots are far more likely than front-aspect shots.
If missile is fired head-on at BVR range, there are several ways to evade it. First is to turn hard to either left or right so as to fly at roughly 90 degrees angle to attacking aircraft. This forces missile to bleed off the energy and to lead the target; as a result, once target aircraft makes a hard turn to reverse a direction, missile – with its far larger turn circle – will be unable to compensate. A variation of this tactic is also useful for head-on WVR launches.
Second tactic, which is also useful at short ranges, is jinking. Aircraft must be positioned so that it is at angle (30-60 degrees is optimum) relative to missile’s flight path; diving is recommended so as to keep energy and take advantage of ground clutter. Once missile gets closer, aircraft will make a hard turn in opposite direction; if missile follows, aircraft will immediately reverse the turn. As there is a lag between aircraft changing the direction and missile following (for several reasons, most important of which is missile’s inertia), this will cause missile to head in wrong direction until it manages to correct, and also to bleed off the energy. Eventually, it will fly past the aircraft and miss. A variation of this maneuver is to continue first turn into a barrel roll; missile will continue to track the aircraft, but in the end its lower turn rate will mean that it won’t be able to keep up.
Third tactic is to turn away and dive for the ground, gaining speed and putting as much distance as possible between aircraft and the missile. This tactic is also useful for rear-aspect BVR shots, but is not useful on its own at shorter ranges, and pilot must evade the missile physically by pulling the aircraft into the turn and forcing missile to overshoot once missile comes sufficiently close. Importance of last part is well displayed in both Gulf wars, where Iraqi failures to time the evasion, or even try to outturn the missile, resulted in unusually high missile Pk. Similar situation, but with WVR missiles, happened in Falklands war.
Fourth tactic, mostly useful at longer ranges, is to climb. Since at long range missile will have burned out its engine, it will rely on inertia to keep it flying, and climbing will mean that it will bleed off energy rapidly. Once missile reaches a close range (maybe around 1,500 meters), dive for the ground, then pull up. This will allow pilot to gain energy and using it to evade the missile.
Fifth tactic is to place the missile at 3 o’clock or 9 o’clock position, then maintain sufficient turn to keep the missile there. This tactic (which is also useful against WVR missiles) forces the missile to execute a continuous turn, bleeding the energy entire time, making it easier to outturn the missile once it comes close.
At very close range, missile will have poor maneuverability as it will not have picked up full speed which means it won’t be able to pull its “official” maximum number of g’s, and that pilot of targeted aircraft will have more time to react than maximum speed of missile might suggest. Missile’s maneuverability peaks at instant before motor burns out, as it will have lowest mass while still possessing motor thrust; this state is what is reflected in “official” statistics, and only under assumption that it did not have to maneuver beforehand.
Physically avoiding the enemy missile is not the only option. If missile is radar-guided, hard turns can cause it to loose lock and miss, regardless of wether it is guided by launch platform or is using its own radar; one of reasons is that most radar-guided missiles track target’s radar centroid, which changes with aspect, and as such target’s maneuvers can result in LOS jitter and return scillintation. It is also possible for a fighter to fly out of seeker’s FoV, regardless of wether missile uses IR or active radar guidance; this has greatest likelyhood of happening if fighter closes with missile at angle. Most modern missiles self-destruct if they have lost the lock. Maneuvers can cause degradation in performance of missile’s fuze, and if aircraft is low-flying, terrain can cause activation of proximity fuze. Jamming can also help evade radar-guided missiles, making acquisition of target difficult for the missile; chaff can also be used to deceive missile’s radar (active-radar missiles are unlikely to use AESA radar due to power and weight requirements) or fuze, forcing either a miss or a premature detonation.
Missile’s effective range is also dependant on altitude, relative speeds. As a rule of thumb, it doubles every 20.000 feet (6.100 meters) above the sea level. Speeds also have a major impact: if target is running away, then every 100 knots in speed advantage for target reduces missile’s range by 5-25%, depending on missile’s own speed. Large target speed advantage can also cause acquisition difficulties for Doppler-radar missiles fired from the rear.