Damn the Torpedos!

Seen over at Against a Dark Background :

New Scientist : "The US navy wants to protect its warships with a system that will destroy incoming torpedoes by firing massive underwater shock waves at them.

The ships would be equipped with arrays of 360 transducers each 1 metre square - effectively big flat-panel loudspeakers - running along either side of the hull below the waterline. When the ship's sonar detects an incoming torpedo, the transducers simultaneously fire an acoustic shock wave of such intensity that the torpedo either detonates early or is disabled by the pulse's crushing force, according to the Defence Advanced Research Projects Agency (DARPA), which is funding the project."

Active Sonars - very expensive machines that go PING! - can put out significant amounts of energy. One of the major problems of some High Frequency sonars is stopping them from flash-boiling several tonnes of seawater every time they operate. They make one heck of a noise.

But "crushing" a torpedo? Nope. Disabling it by effectively "blinding" it, yes. Because torpedos have got their own small sonars in their guidance system, sometimes passive (listen only), but often active (they go *ping* too). And to avoid having these sensitive devices deafened by nearby explosions - such as when the first of a salvo of torpedos hits its target - there are "cut-out" circuits that prevent damage to the relatively fragile sensors. I say "relatively fragile", because the output energy of thses relatively tiny torpedo sonars often exceeds not just a Jet engine at takeoff, but a Black Sabbath concert.

Make a loud enough noise, often enough, and the cut-out circuits will operate continuously, and the torpedo won't be able to home in. Unless the designer is sneaky, and adds a second homing circuit, one that when the first one fails for predetermined time, homes in on the noise...

One thing such a noisemaker will do though: often a submarine's torpedos are "wire guided", they are steered close to the target by the firing submarine before the torpedo's own homing system takes over, and the wire gets cut. (A submarine guiding such a torpedo has to move slowly, they prefer to cut the weapon loose as soon as they can). Anyway, with a setup like the one above, the target ship can make a very good imitation of a torpedo hitting its target - which will screw up the damage assessment by the firing sub, so they don't re-attack in case they miss. And such a system would make a really loud PING that could be used for detecting submarines in the first place.

But "crushing" a torpedo that's designed to withstand tonnes per square cm pressure (when fired at a deep depth)....I'm sceptical in the extreme. Even if the transducers are fired "in sequence" so that they constructively interefere in a massive pulse at a particular point. Because you would get a greater effect, cheaper and easier, just by putting a dirty great big mortar that fires a depth charge to that place, and get a far more powerful shock than any transucer system could produce.