One cancelled USN interim torpedo defense programs some years ago would have used modified Mk.46 lightweight torpedoes in this role, the Mk.46 Mod 7. And the European MU90 lightweight torpedo has a ‘Hard-Kill’ variant for this role also though its status is unclear.
The current USN system, now being fitted to some carriers, uses much smaller purpose designed anti-torpedo torpedoes, 6.75" diameter v the standard 12.75" for the lightweight torpedoes mentioned.
A different hard kill approach is using salvo’s of unguided rockets flying through the air to impact around the torpedo’s path. The Russian UDAV-1 system, using a version of the ubiquitous Soviet/Russian RBU type anti-submarine rocket launcher, is an example. That system can also launch torpedo decoys along with hard kill rounds, some otherwise similar western systems just launch decoys.
As mentioned, the existence of such systems doesn’t necessarily mean they’d always be effective. Plenty of anti-antishipmissile hardkill systems exist, obviously it doesn’t mean the problem of AShM’s is solved.
The mass of armor necessary to stand up to something with a shaped-sharge warhead is extremely heavy. Considere that there’s no real practical limitation on missile warheads (some Soviet missiles had warheads in the thousands of pounds), and that shaped-charge design isn’t a terribly serious challenge any longer… not even Battleship Turret Face armor is adequate. Then consider that a warship has a LOT of surface area to protect, and that even in the days of ‘all gun’ warships, many areas were left weakly armored, or not armored at all… Well, yeah, Armor’s day as a primary defense on warships is over. As a secondary defense for limited, cruicial systems, it still has a role, but even then those systems mostly rely upon the ‘Not Being Hit’ defense, and the armor is protection against secondary missiles (framentation, essentially).
Kinetic missiles are always a possibility - And growing more so by the day - but getting a kinetic mass up to useful velocities is relatively (heh) energy intensive. Just delivering an intact chemical warhead to close proximity is (currently - That’s changing) the most efficient means of attack.
Picking out a torpedo from the noise does seem the biggest problem, unless it’s a supercavitating torpedo. Those don’t rely on subtlety. Electric propulsion torpedoes must be scary things to know are out there.
Hitting a torpedo with a torpedo might not be as much of a problem as a missile with a missile. Smaller defensive torpedoes are presumably more maneuverable than large torpedoes. Offensive torpedoes will be optimized for stealthy long range (50km+) whereas defensive torpedoes can loudly sprint over shorter distances. Shorter range also means they can use cable for datalink which gives them cheap access to considerable processing resources. Unless I’m mistaken, an offensive torpedo that wants to go beyond 50km range will have to chiefly rely on its own processing resources. Sonar/hydrophone datalinks would seem to pose major issues for the launching/targeting platform if it wants to remain stealthy.
I’m surprised decoys and hard-kill munitions aren’t combined in the same munition. You could use a torpedo which transmits decoy emissions/DRFM (or the sonar equivalent) both to seduce the enemy torpedo and detect it.
Do most torpedoes actively transmit in terminal phase?
Are you talking about something like the ADM-160 MALD or something else?