It is common trope that active defense systems of all kinds, like these mounted on ships, will not be able to stop all munitions.
What is the most common reasons for this? As far as I know, intercepting long range missiles should be easy as they aren’t very manuverable, having all that fuel optimized for payload and range, especially these against ships or ground targets.
Logically the defender should have advantages, enabling cheap short range but manuverable missiles to shoot down the incoming munitions.
The difficulty of intercepting a munition comes mainly from three factors, I would say.
Detection: Detecting a munition isn’t necessarily easy. Modern guided munitions often have an extremely weak radar trace, or even a stealth one, and have almost no infrared trace. Furthermore, it’s difficult to know/predict where this munition might arrive. Compared to the situation we have in game, it’s relatively simple to intercept a munition because we know where the aircraft spawn and therefore the general trajectory of the munition. In reality, a missile could have been fired several hundred kilometers before its target.
Speed: Depending on the munitions, speed plays an important role. While it’s relatively easy to intercept a missile that’s slowly approaching you once you’ve detected it, a missile that arrives at a higher speed (supersonic or even faster) is complicated to intercept because you have little time before a potential impact. To make an analogy with what we have in the game, intercepting a Gbu-39 is relatively simple, it comes at you because it comes at you at low speed, on the other hand a Kh-38 which is much faster is more difficult because it is faster.
Altitude, most modern munitions have fairly similar flight patterns, either they follow the terrain while remaining as low as possible, which makes their detection and interception complicated, or they fly at very high altitudes, which makes their detection simpler but requires more advanced technology to intercept them, typically a missile that arrives vertically at Mach 4 will be very difficult to retrieve. Taking an example from the game again, a Gbu-39 coming in at low altitude is harder to intercept than a bomb fired at high altitude, even though an AASM or Kh-38 coming at you vertically would be hard to hit due to its trajectory.
Finally I would say that there is a reason why all the munitions cannot be intercepted, saturation, processing a target takes a certain amount of time and if the number of targets is just too large the system simply cannot keep up. (for example, in game it is complicated to stop a complete volley of Gbu-39s, you will hit a few of them for sure, but it is impossible to deal with them all)
But why is speed a issue in itself? Unless used with terrain masking, I don’t think they present much issue? I don’t see computer systems having any problem calculating where to intercept, even if a mutition goes mach 100. It’s just XYZ values that modern chipsets easily handle. (Mach 100 is a extreme example). The only problem is close range camera sensors not having good enough FPS to detect such munitions it seems.
Speed is a problem in itself because the interceptor missile’s guidance system, even if it is very efficient, cannot exactly predict the trajectory of its target. Moreover, when the target in question is supersonic or even hypersonic, the interceptor missile must be able to follow it or match its speed (head-on interceptions are rare and anti-aircraft defense systems are not necessarily exactly next to the intended targets, the interceptor missile must therefore succeed in accelerating to reach its speed while maneuvering to intercept it).
for a very simple reason : speed = distance/time. If you consider two munition launched at the same range, targeting the same point, the faster the munition goes, shorter the time of flight is. Which means that you have a shorter time to : locate the munition, calculate its tragectory, launch a missile, and then for the missile to place itself in an intercepting trajectory. This takes quite some time considering at which speed the munition goes, time that you may not have, which is why intercepting fast munition is hard.