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MSD · 02-17-2009, 12:27 PM

Lewis Page has a great article in the Register that looks at it from a levelheaded approach. One thing to consider with his point on using active sonar is that it's just like the movies. When you're pinging, you're sending out an audible sound that's easy to detect, and if you ping another sub, everyone on board will hear it.
16th February 2009 Archive ? The Register

Quote:

Updated British and French nuclear missile submarines collided earlier this month beneath the Atlantic, according to reports. Much is being made of the fact that the two subs "failed to see each other", but this is actually quite normal.

The story appears to have first broken with a report in the Sun, stating that HMS Vanguard has been "towed" into her home base at Faslane with "dents and scrapes visible on her hull". It was understood that the nuclear powered, nuclear armed vessel had collided early this month with the French Triomphant, also an ICBM-carrying sub.

The MoD said there had at no time been a break in the British deterrent, but this doesn't mean Vanguard wasn't seriously damaged. The UK now has relaxed rules on maintenance of the deterrent, not requiring a working boat at sea uninterruptedly. Both navies, however, insist there was no damage to anything nuclear, reactors or missiles.

Most of the media have followed the BBC's comment "despite being equipped with sonar, it seems neither vessel spotted the other". But in fact this is not at all unusual.

All submarine captains prefer to refrain from driving about beneath the oceans with their sonar "pinging" sound pulses into the water - this is known as "active" sonar. The pings give a sub's position away, so active sonar is generally only used in special circumstances.

Nuclear-missile subs, whose primary imperative is to remain undetected, are even less likely than normal ones to turn on active sonar. Even where there is no state of war, once a deterrent sub is being tracked by a foreign navy, it is no longer much of a deterrent as it can be attacked and sunk before it will be able to launch its missiles.

Where a nation may have only one deterrent sub at sea - as in the case of Britain and France - the imperative to remain undetected remains even stronger. So neither boat will have been using active sonar, we can be sure.

There remains passive sonar, where one merely listens for the machinery and propulsion noises made by other vessels. But modern Western submarines are deliberately made very quiet in operation - so much so that they are very difficult indeed to detect using passive sonar, and subhunting surface ships are now moving to new forms of active kit.

Vanguard and Triomphant, deterrent subs belonging to competent navies, will be very quiet indeed. If both were patrolling very slowly and listening as hard as possible they might still miss each other entirely. Alternatively, if they were going faster their own speed would tend to blind their passive sonar, and they would still not turn on active equipment and announce their location to the whole ocean.

After all, the whole reason that nations expensively put nuclear missiles on submarines is that it's the only reliable way of making it impossible for anyone to know where the missiles are. Nobody should be surprised at two purposely-designed undetectable launch platforms having remained undetected.

This inability of quiet Western submarines to see each other, especially when moving at speed, has led to the setting-up of a joint traffic-control system by the US and UK, in which submarines are given deconflicted, preplanned moving boxes of sea within which to stay. This avoids British and American boats crashing into each other.

The US and UK trust each other enough to disclose where their subs are, even on occasion their nuclear deterrent boats. Your correspondent isn't aware how closely France participates in/cooperates with this system: on the evidence of the current reports, perhaps not closely enough. ®

Update
At an event held in London this morning, Admiral Sir Jonathan Band (the First Sea Lord, head of the Royal Navy) told reporters including the Reg that both subs were on routine national-deterrent patrols and had hit each other while "moving very slowly". This is a missile boat's normal posture while on deterrent patrol, as it makes the sub as silent and undetectable as possible - evidently quite successfully in this case.

Admiral Band said that damage to Vanguard had been "minor" and the submarine had remained fully able to launch missiles if ordered to. He said she had returned to Faslane "under her own power", contradicting the Sun report. (One should note that it is routine for arriving submarines at Faslane to be accompanied by tugs and assisted into their berths by them, perhaps leading to confusion for the Currant Bun's sources.)