Missiles, at least the ones in question, do not need anti-icing as much as aircraft do.
The typical air-to-air missile has roughly the following mission profile:
- 0 - 2 seconds: Drop from the pylon or internal launcher
- 2 - 10 seconds: Accelerate with the solid fuel rocket motor
- 10 - 120 seconds: Adjust the trajectory with the fins to hit the target
- 120+ seconds: Fall to the ground or self-destruct, model-dependent
The numbers are very approximate, give or take 500%, model-dependent, but this is the general idea. An air-to-air missile is not a fixed-wing aircraft. It's more of a bullet that can turn a little (or a lot) to get on the exact intercept course towards its target.
What happens when a surface gets iced is partial loss of lift production.
Fixed-wing aircraft require L=W (lift=weight) to sustain level flight; if L<W, they will lose altitude. The wing can only produce so much lift. Increasing the angle of attack to add lift reduces the L/D ratio, so the aircraft will lose range - potentially so much that it can't land.
Fuel is the limit. Airliners, for efficiency reasons, try to carry just enough fuel to the destination, plus the mandatory reserve. Combat aircraft have to choose between fuel and payload. All in all, multiple-use aircraft have limited thrust, limited fuel, and have a destination to reach, so the margin for acceptable loss of lift is limited.
An air-to-air missile is never in sustained level flight, the way an airplane is. It may have L>W or L<W, using its kinetic energy from the initial acceleration phase to adjust its trajectory. The maximum range of AAM can be as high as 100 miles and more, but the average range they're fired at is a fraction of that maximum, for many reasons - detection, rules of engagement, effectiveness, repeat launches. In short, range is not very important.
Icing will severely degrade a missile's performance, primarily by reducing the seeker's capabilities. The loss of control surface effectiveness is a secondary, less significant effect. But the control system in any modern missile is fully computerized and relies on feedback from the INS and the sensor to judge the required control surface deflection, not just on assuming a given transfer function. In other words, if the fins are only 30% effective, they will have to be deflected more and for a longer time. The missile will have less range and less maneuverability, but it's still useful.
Half-joke, but "they don't call them hit-iles". Any missile launch being hit or miss is a given. Icing conditions will greatly reduce their effectiveness. But as a design balance, if you expect e.g. 5% of your launches to be in icing conditions, and icing to reduce the Pk by 50%, fitting an anti-icing system could make a difference between an overall Pk of 39% and 40%.
Failing an extra 1% of the time is not decisive for a single-use device such as a missile. On a multiple-use aircraft, on the other hand, that 1% failure rate would be completely prohibitive.
In short, air-to-air missiles don't get deicing because:
- Air-to-air missiles have a much larger margin for loss of lift (as it's only needed for control, the flight itself is ballistic) than fixed-wing aircraft do.
- Missiles are inherently single-use, and not very reliable in the first place, so rare icing-related failures don't make a big difference.
This is for AAM. Some long-range cruise missiles (AGM-109 and foreign counterparts) do have deicing, but only for the engine inlet, as it's too bulky for the wings.