New ways to hunt submarines

Press, 28 May 1983, Page 14

New ways to hunt submarines

From ‘The Economist,’ London

Navies today have split personalities. On the one hand they wish to exploit their unique mobility to play their part in battles ashore. On the other they wish to keep to their traditional role of controlling the sea, or as much of it as possible. Surface gunnery, amphibious assault and air power are the main ingredients of the projection of power, but the ability of modern navies to operate submarines, and their universally poor ability to detect and kill their opponents’ submarines, are central factors of power projection arid sea control. Virtually the only way to detect submarines today is by sound E through sea water. Untold have been spent developing active echo-ranging sonars for surface ships and helicopters, and on active and passive buoys that aircraft can drop and then monitor by radio. Improvements in the performance of submarines, particularly in quietening their own tell-tale noise outputs, have far outstripped improvements in tracking devices over the last two decades.

Submarines have gone almost as far as they can in operating quietly; from now on even small improvements will become increasingly difficult and expensive. And the towed-array detector, which is now coming of age, is beginning to push the hunterhunted pendulum the other way. The towed array is a string of passive hydrophones spaced along a long cable that is towed well astern of a ship, out of the way of its own noise interference. Though much depends on sea conditions — salinity, temperature, background noise and the rest of it — modern

arrays can often detect even quiet submarines at 20 miles, and noisy ones at five or 10 times that distance.

Equipment on board ship measures the time a sound strikes each of the hydrophones on the string. Calculations based on these differences of the arrival times of a signal can produce a bearing line to the submarine. Two arrays, working together, can cross their lines to get a fairly good fix on the submarine. There is considerable controversy as to whether submarines located by towed arrays can be attacked most effectively by shore-based aircraft, surface ships, or helicopters operating from surface ships. The bureaucratic answer is: a combination of all three.

This is not necessarily the correct answer. It does not take an expensive warship to tow an array or to carry a helicopter. The failure of navies to deploy simple towed-array / helicopter-carrier ships as main units of the fleet (although several experimental ones have been tried out) is due largely to obstructionism on the part of “traditional” sailors and airmen.

Another device that has long been a main element in the sub-marine-detection effort is the longrange hydrophone array anchored to the sea bottom. The principal ones of this kind are operated by Russia and by the United States in conjunction with its allies.

The main problem with fixed arrays is that they have had to be fairly close to shore bases: the electrical cabling leading from the hydrophones to the shore is extremely expensive; and the tiny electrical signals they send back are attenuated substantially as they pass through the cables. The loss of signal strength is too great to be tolerated for more than a few dozen miles. As the target submarines got quieter, the only remedy was to increase the sensitivity of the hydrophones. That sensitivity is near its practical limit.

Fibre optics may be able to solve cost and transmission-loss problems. These ultra-thin filaments are cheap (and getting cheaper) and the transmission loss, using lasers to transmit the signals, is extremely low. It may be feasible to “wire up the oceans” by putting hydrophone arrays almost anywhere and monitoring them from shore stations long distances away. Such arrangements could revolutionise antisubmarine warfare.

Regardless of whether surface warships, helicopters or land-based aircraft are used to attack the submarines, the weapon they will almost certainly use is the acoustic torpedo. Torpedo technology is at the beginning of a change, courtesy of microelectronics, and that may be as important to undersea warfare as missile improvements have, been to surface and air warfare:

the torpedo is becoming a true guided missile. It can store information, compare what it sees against this store and make decisions according to programmed logic. It can guide itself not only to the target, but to its most vulnerable part and strike at the proper angle to get the best results from its warhead.

The lightweight, electrically driven Sting Ray torpedo, produced in Britain, is probably the most advanced anti-submarine weapon in the world. It is fired from a surface ship or aircraft or carried in a rocket-propelled stand-off weapon in the direction of a known submarine contact.

It runs towards the target’s position, searching on either side of its launch course and listening passively. If it detects a sound that convinces the logic in its microprocessor brain that it is not coming from fish or from the bottom or some other source of interference, the processor then turns on an active sonar to get the target’s range, shape and direction of motion.

The Sting Ray then homes in on the target, refining its information with each ping, and blasts into the submarine as near to perpendicular to the hull surface as possible in order to give its shaped-charge warhead the best possible penetration.

Sting Ray’s weak point is that it has too little speed — around 45 knots is top — to catch a highspeed nuclear submarine. A better

propulsion plant is needed, but it is by no means certain that an electic one could be made that would deliver the 60 knots or so which is needed.

A thermal plant could easily provide enough usable power, but any exhaust would have to overcome the pressure of the outside water; therefore the deeper it went, the less efficient it would become; With today’s deep-diving submarines — 1000 metres is, fairly common — even thermal plant may not be able to provide enough power for the speed, and to overcome the back pressure, in such a small package.

One exotic scheme is to use a closed-cycle thermal plant consisting of a boiler coil embedded in a block of lithium. When an oxidiser such as sodium hexafluorate is sprayed on the lithium, it heats the block and boils the water into steam, which is expanded through a turbine to drive the torpedo. The steam is then condensed by sea-water cooling and used again. The lithium reaction does not cause any serious gas build-up, so it needs no exhaust.

However, the process is difficult to control, and destroys the engine on each run. Thus exercise torpedoes become prohibitively expensive. Deep-diving submarines remain hard to attack, except by nuclear depth charges, or by heavy wiredguided torpedoes carried in other submarines.