Effects of clipping the fins on the AMRAAM?

[RadicalDisconnect]

I'm wondering what effect clipping the fins on the AMRAAM C/D has on maneuverability. Doing some brief reading on aerodynamics, it would seem like clipping the fins decrease the aspect ratio, so for the fins to generate the same mount of lift force compared to unclipped ones, there would be more induced drag. At the same time the clipping also reduces the wing area, so would the maximum force be less compared to unclipped fins? Given this, would the AIM-120C/D have overall inferior turning envelope compared to A/B?

 

[totoro]

i would think yes, if all other variables remain the same.

that being said, missiles tend to achieve their kill rates more by virtue of their guidance/seekers not being jammable/decoyed, by speed and by not being detected properly - more so than the ability to snake and loop their way to a aware, perfectly timed dodging enemy. It's mostly about the element of surprise and reliability in all circumstances methinks.

 

[overscan (PaulMM)]

More powerful actuators and better control systems may compensate for reduced area.

 

[pathology_doc]

You pay for the wing clipping in many ways:


1) Reduced wing area leading to less lift (and hence turning force) for a given control deflection


2) Higher angle of attack required for the same lift/manoeuvre force, leading to increased induced drag for the same lift.


3) Lower aspect ratios cause stronger tip vortices anyway, but OTOH a lower aspect ratio wing has a smoother and more gradual "stall" (probably not relevant to AAMs, as their AR is quite low to start with and their wings/fins are frequently deltas, which inherently enjoy a gentler stall).


The missile designer has to adapt by altering the missile's control-loop functions appropriate to the aerodynamics. Otherwise you get Sidewinder-like behaviour with the weapon oscillating from side to side as the guidance loop wrestles with the suboptimal control system (it was worth it in Sidewinder to keep things simple).


Ultimately it's a question of ENERGY - if the missile is still in boost phase, the motor is still supplying the energy and the drag caused by the loss of wing area etc. might slow the missile down a bit but usually the speed differential is so high in the first place that this doesn't matter. If it's in coast phase, that's a different matter - a target which becomes aware of the missile might evade by turning away and taking evasive action in a full-thrust climb, to force the missile to "stretch its legs" and waste kinetic energy trying to follow. At extreme range, this can lead to the missile falling away or failing to catch up.


That being said, there are too many variables. If it's a snap-up or co-altitude shot with the missile in coast phase, the evading target has an energy advantage in the climb (possibly its only advantage). If the missile was following an up-and-over trajectory, it still has height to trade for speed and manoeuvre energy and the evading pilot might as well reach for the eject handle.

 

[Trident]

More powerful actuators and better control systems may compensate for reduced area.

More powerful actuators may restore most of the maximum turning ability, they will not get rid of the turning drag penalty however (how big that is, going from fins that are already very low aspect ratio to a planform that is merely slightly more so, is open to debate). So all else equal, a late-model AMRAAM might have less turns/turning-time available to hit the target when it enters the terminal phase, meaning an alerted opponent may find it easier to run the missile out of energy by forcing it into a long, sustained turn or multiple hard reversals of direction.

Trouble is, all else is not equal because the clipped fins likely give a slight benefit in mid-course drag and later models also have bigger rocket motors tailored for improved end-game energy levels - recent variants should therefore enter the terminal phase with more energy on tap. Perhaps the best way of putting it would be to say the clipped fins erode the range advantage of the new rocket motors somewhat because more energy needs to be reserved to maintain the same maneuvering ability?

 

[pathology_doc]

More powerful actuators and better control systems may compensate for reduced area.

later models also have bigger rocket motors tailored for improved end-game energy levels - recent variants should therefore enter the terminal phase with more energy on tap.

Another aspect of this (which is possibly classified) is that the propellant "stick" may not be homogenous in its burning rate all the way through - so you may actually get your highest performance vis a vis raw thrust at the END of the motor burn, rather than simply more energy overall. How the burn rate is distributed throughout the charge (if this is indeed the case) is, of course, probably even more of a state secret.