|I know this sounds very odd, however it just occured to me that all other platforms have capability against all others as well as their opposite numbers, ie. ships can engage not only other ships but submarines and aircraft, and aircraft can engage not only other aircraft but ships and submarines.
However submarines can only engage other submarines and ships, which gives them no capability against ASW helicopters or maritime patrol aircraft, except presumably to hide. Now as we have weapons systems capable of being launched from a submarine and breaking the surface - I am talking of Sub Harpoon, not just the expensive cases of Trident and Tomahawk, why not a submarine launched air defence missile, something along the lines of a Sub Seawolf, to give the boats something to counter air threats?
I read an account by a USN exchange officer during a "Perisher" course, I think it is linked to from dutchsubmarines.com, of being searched for by a Sea King for hours, and seeing the Sea King through the periscope, and thought, how useful would an antiair missile be to them at that moment?