Submarines: Torpedo Defenses

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March 4, 2007: With the proliferation of anti-missile systems like the American Phalanx , the Rolling Airframe Missile, the Dutch Goalkeeper, and the Russian CADS-N-1, a natural question has arisen: Why can't similar systems for use against torpedoes be designed? The answer is really quite simple - and much of it has to do with the relative amount of time to and distance deal with each type of attack.

Anti-ship missiles are often launched at a much-longer range than torpedoes (often as much as 500 kilometers for some of the Russian systems). Not only does this distance give you plenty of time (and multiple chances) to shoot down the anti-ship missiles, it also means the missile can be destroyed with relatively little harm to the target vessel. Note, the word is relatively little. Even a near-miss by a missile can cause damage - look at the case with Israeli corvettes close encounter with a Hizbollah C-802 missile off Lebanon last year. The same is true from World War II combat experiences - a near-miss often was as bad as actually being hit.

How so? Because when the warhead detonates, it sends out tons of fragments. These fragments will fly all over the place. They might not sink a ship, but they will knock out weapon mounts and sensors, kill crewmen, and possibly even start fires. This could be worse, because it now renders the ship helpless against a second wave of attacks. This is why the push has gone more to missiles like the SA-N-11 and the Rolling Airframe Missile, rather than for guns like the AK-630 and the Phalanx. The further away the missile is when it detonates, the fewer fragments that will be likely to hit the ship. Detonate it far enough away, and no damage occurs at all.

Torpedoes are different - not only via their damage mechanism (they usually detonate via impact of from a magnetic detonator, but some detonate on impact), but because of the attack profile. In essence, a torpedo attack from a submarine hasn't changed much from World War II. You get in reasonably close (often within three kilometers), and you fire the weapons . This cuts down the time one has to deal with the incoming threat.

The other problem is that the underwater sensors have a much shorter range. In order to reliably take out an incoming torpedo, one needs to see it. The "eyes" underwater usually consist of passive sonar - in essence, underwater microphones, with operators who try to pick out the torpedo in the whole underwater cacophony (whale songs, a ship's own engines, and other sounds of the sea). Active sonars can also be used, but, like radar, they also tell everyone where the ship using them is. Sonars have shorter ranges than radars, especially as submarines get quieter (submariners often consider themselves to be the original stealth weapons).

There are defenses for modern torpedoes, most of which are guided by sonar systems. Often these are towed decoys, which will be single-use (as they induce the torpedo to hit them instead of the ship towing them). The first of these was the Fanfare, which was later supplanted by the Nixie. These decoys gave off acoustic signatures that mimicked those of the ships towing them or those of larger vessels. The hope is that the torpedo will go after the decoy and not the real ship. These are known as "soft kill" systems. The problem is that not all torpedoes home in on acoustic signatures. Some home in on a surface ship's wake. This neutralizes decoys like the Fanfare and Nixie.

The Russians, who often faced superior Western sensors, began developing hard-kill systems. These were initially variants of the RBU anti-submarine rockets (an ahead-thrown weapon similar to the Hedgehogs of World War II). The Americans and British have gone with a new twist on this concept, devising a system that will fire an anti-torpedo torpedo. A wake-homing torpedo might not be decoyed, but it also is unable to take evasive maneuvers, and can be induced to take a predictable path. This is called the "hard kill" concept, and it is ultimately the best way to deal with anything incoming, particularly if the incoming object has a big warhead.

Still, a close detonation can cause as much trouble for a ship. Near-misses from bombs often could keep a ship in drydock (and unable to fight) for months. Like many other areas of warfare (such as tanks vs. ATGMs, and SAMs vs. aircraft), the competition between the submarine and the surface ship as to who will have the edge will go on for a long time, as innovations shift the balance one way, then another. - Harold C. Hutchison (haroldc.hutchison@gmail.com)


 


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