Shock tactics to destroy torpedoes
06 October 2005
NewScientist.com news service
Paul Marks
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Torpedo ZapperTHE 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.
But these are no ordinary loudspeakers: instead of having a membranous diaphragm that can vibrate in response to a range of audio frequencies, each of the devices has a ram-like cylindrical metal armature at its centre. This is projected outwards by electromagnets at very high speed, producing a shock wave. The array can be fired as many times as needed.
When the six rows of 60 transducers on each side of the ship fire at once, the cumulative action should generate a "destructive pressure pulse capable of disabling an enemy's torpedo", according to DARPA.
Exactly how the system works is shrouded in military secrecy. But by making a speaker several times larger than the wavelength of the sound wave required, a tightly focused beam can be produced in front of the speaker. This is because beam width is partly determined by the aperture of the source - a bigger loudspeaker focuses sound in a smaller area. (New Scientist, 9 September 2000, p 38). And the combined size of the array makes for a very large speaker indeed. This focusing would allow the array to precisely target incoming torpedoes. In addition, the beam can be steered in different directions - probably by slightly altering the phase of the applied signals - a technique that is widely used to steer radio waves using side-by-side antennas. So torpedoes homing in on the ship's wake from many directions can be targeted (see Diagram).
So far, the system's developers, Anteon Technologies of Fairfax, Virginia, and BAE Systems of Farnborough, UK, have only built one transducer. But encouraged by software simulations that show the array should work, they plan to press ahead with a one-quarter-scale test rig.
If it reaches the stage of testing in the open ocean, however, the developers are likely to come into conflict with marine biologists. They have evidence that whales blasted by frequent acoustic signals from submarine or ship sonar appear to develop symptoms of decompression sickness, and die. (New Scientist, 11 October 2003, p 10).
“The torpedo detonates early or is disabled by the pulse's crushing force”But neither DARPA, Anteon, nor BAE Systems was willing to respond to questions about the array's proposed energy levels and any threat to marine mammals they might pose.
