Struggling to contain the submarine threat

Discussion in 'Naval Warfare' started by nandu, Apr 21, 2010.

  1. nandu

    nandu Senior Member Senior Member

    Oct 5, 2009
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    Struggling to contain the submarine threat

    The recent discovery of the remains of the Australian hospital ship Centaur, torpedoed by a Japanese submarine during the Second World War that ignored her clearly displayed markings (markings spotted immediately by the remotely operated vehicle which discovered the wreck), is a poignant reminder of the importance of dominating the underwater battlespace.

    It should never be forgotten that Asia's history in the last century was shaped by the submarine. Between 1943 and 1945 the US submarine force, augmented by British and Dutch boats, slaughtered the Japanese merchant fleet and isolated Tokyo from its ill-gotten territorial gains long before the first atomic bomb descended. The lesson for Asia's navies is that they must be proficient in anti-submarine warfare (ASW) to ensure national survival.

    With the Pacific Rim's economic prosperity depending upon seaborne trade, the threat from the submarine continues and this is partly reflected in the growth of submarine forces within the region. It has been estimated that Asia’s submarine market alone is worth US$32 billion and around 10 countries are acquiring or will acquire some 90 boats, the most recent being Vietnam which is buying six Kilos (Project 636) from Russia.

    Vietnam joins Malaysia as a 'new' submarine operator and, if Bangkok has its way, Thailand will follow suit. India, Indonesia, Pakistan, South Korea, Singapore and Taiwan are all interested in expanding their submarine fleets while the US Navy intends to have the majority of its submarines in the Pacific and is slowly building up its forces. Canberra plans to double its fleet although many regard this is a triumph of hope over experience, given the difficulties of operating six Collins, of which no more than a third appear operational at any one time.

    The value of the submarine is its versatility; it can be used for covert reconnaissance (increasingly using unmanned vehicles) while its offensive weaponry includes the torpedo, surface-to-surface missiles (anti-ship and land-attack) and mines. They are extremely difficult to detect, especially the ever-quieter modern diesel-electric boats as even the mighty United States Navy is discovering. In October 2006 a Chinese Song (Type 039) submarine managed to approach the carrier USS Kitty Hawk and was only spotted by accident by which time she was within torpedo range. The US Navy has also found it a challenge detecting and tracking European and European-designed submarines in its training grounds.

    For these reasons navies are willing to make the massive investments in capital and life-cycle costs of these platforms and in the Pacific the full range is available. Nuclear submarines have the advantage of an endurance dependent only upon that of the crew and very high underwater transit speeds of some 25 knots or more. Diesel-electric submarines are more restricted by the need to recharge their batteries and where once this meant surfacing at night it may now be achieved submerged using a schnorkel. The development of air independent propulsion (AIP) system provides another option for non-nuclear boats to remain quietly submerged for up to several weeks.

    The tactics of ASW have changed radically since the Second World War when the underwater threat was essentially a torpedo-boat which could slip beneath the water but was three times as fast on the surface as it was under the waves. By the end of the war the Germans had developed true submarines with high underwater speeds and almost all of the submarines operating in Asia follow this design philosophy. The tried countermeasure in two world wars was to assemble merchantmen into convoys where they could be more easily shielded and at the same time be an anvil upon which attacking submarines could be hammered. The escorts, augmented by land- and sometimes carrier- based aircraft, could create a tight escort around the merchantmen with dedicated support groups to react to any attack - saturating the area where a submarine was located and tracking the targets with sonar ranges of up to 10 nautical miles (18 kilometres) in ideal conditions.

    This strategy was ruined by the high underwater speed submarine which could strike at will into the convoy while the escorts were over-stretched trying to cover the flanks. The development of longer-ranged torpedoes, typically 15 nautical miles (28 kilometres) but up to 24 nautical miles (45 kilometres) in some cases, meant that the submarines could also strike from a longer range. Moreover they were more accurate thanks not only to more accurate on-board sensors but also to direct links to the submarine weapon control system. The development of underwater-launched anti-ship missiles with the potential for nuclear tips finally doomed the convoy as a means of ASW - indeed the heavyweight torpedo is increasingly used as a weapon against other submarines rather than merchantmen.

    In response to the new technical threat NATO Cold War ASW strategy was based upon creating 'sanitised' corridors along which merchantmen could travel. The outer defensive layer consisted of submarines as well as fixed-wing maritime patrol aircraft using sonobuoys and magnetic anomaly detectors to detect the presence of submarines and to track them. Maritime patrol aircraft play a major role in maintaining these safe shipping lanes but this article will focus upon the naval element of ASW.

    The inner defensive layer consisted of ASW frigates and destroyers with active sonars (ones which sent out an acoustic pulse to detect targets) on the forward underside of the ship’s hull. Modern hull-mounted sonars are seeking two kinds of noise; narrow- and broad-band. The former is due to rotating surfaces such as propellers or machinery noise while the latter is caused by movement such as that of water of the submarine's hull or fluids moving around internal pipes.

    But sonar performance is handicapped by the physical problems of sound propagation through water. In the surface layer sound travels at a constant velocity because the water is at a constant temperature but warmer waters create a shallower surface layer, and in any event the absolute maximum range is only 10 nautical miles (18 kilometres) with a hull-mounted sonar. Below the surface layer sound velocity tends to vary with depth, water temperature and salinity with signals tending to bend down to a region of minimum sound velocity but in deep waters they refract up to arrive at the surface at successive intervals of 30-35 nautical miles (55-65 kilometres) in what are called convergence zones.

    Convergence zones depend upon water depth and in waters under 180 metres (600 ft), which means in many areas of continental shelf, it is impossible to achieve convergence zone propagation. Moreover hull-mounted sonars in such conditions can suffer from the signal bouncing off the sea floor, 'bottom bounce', which becomes more acute in shallower waters and is also effected by the sea bed's acoustic absorption conditions with rocky sea beds found in most littoral waters causing multiple signal returns ('reverberation').

    Hull-mounted sonars are not only short ranged but also suffer from man-made noise such as that from numerous ship's engines, indeed it might be possible for a submarine to surface and recharge her batteries in waters where there is extensive marine traffic which would conceal the noise of her own diesels. However, as well as being inherently silent through placing noisy machinery on 'rafts' and using propellers which move very quietly, submarines increasingly use anechoic tiles to absorb active sonar pulses. Consequently, hull-mounted sonars are more useful for prosecuting attacks upon submarines especially in shallower waters although these tend to be noisier for environmental reasons and because there are more fresh water layers from rivers to complicate the velocity of acoustic signals.

    From the 1960s helicopters extended sensor ranges detecting targets through active, passive and active/passive sonobuoys or through dipping sonars, the aircraft hovering to lower the sensor, listen and then raise it before moving to a new area.

    The idea of the dipping sonar was exploited by ASW ships with a low-frequency (2 kHz) variable depth sonar being developed initially for operations in the Mediterranean. The system requires a powerful winch to lower the 'fish' beneath the surface layer where it can exploit the reliable acoustic path to listen for submarine activity at longer ranges than hull-mounted sensors, the range depending upon the receive wavelength. Using active and passive sensors such sensor can have a range of up to 15 nautical miles (28 kilometres) in good conditions.

    An even better off-board sensor is the very low frequency towed-array sonar developed for North Atlantic operations reportedly had an optimum range of 100 nautical miles (185 kilometres). However, the sensor system requires an array up to a couple of kilometres long which requires a powerful winch and a large ship. The ship must sail at a relatively low speed to obtain good performance and while the sensor can detect a target it cannot locate it except in general terms, and the ship must usually steer a zig-zag course to obtain a location.

    The embarked helicopter has become a vital tool of ASW since the early 1960s. The aircraft provide early warning of the submarine's presence and the helicopters were especially regarded as the prime detection and neutralisation system, some navies (such as the Royal Navy) using them in the autonomous role while others (such as the US Navy) used them as extensions of the shipboard ASW combat system. The value of the helicopter is such that dedicated ASW helicopter carriers were introduced into some NATO navies during the 1960s, and the British Invincible class carriers were originally touted as ASW platforms with the cover name 'through-deck cruisers'. Embarked helicopters tend to be classified as lightweight (about 4.5 tonnes maximum take-off weight) and heavy (9-10 tonnes). The heavyweight aircraft have ranges of 400-600 nautical miles (830-1,100 kilometres) and a tactical radius of up to 150 nautical miles (250 kilometres) compared with 200-300 nautical miles (370-555 kilometres) range and tactical radius of some 50 nautical miles (90 kilometres) for the lightweights. In addition to superior endurance, heavyweights can carry a greater selection of data processing and sensor systems, although both tend to be limited to two lightweight torpedoes

    The lightweight torpedo, a weapon of 32.4-40 centimetres in diameter with a range of 10-13 nautical miles (20-25 kilometres), is the weapon-of-choice for prosecuting an attack on submarines. These weapons are often ‘intelligent’, capable of renewing contact with a target through a search pattern if it is broken and with a speed of up to 50 knots. For quick-reaction to emerging threats such torpedoes may be attached to rocket motors to create Anti-Submarine Rockets (Asroc) launched from dedicated or multi-role launchers such as the Mark 13 in the Oliver Hazard Perry (Adelaide) class frigates. Unguided anti-submarine rockets have been used in the past, and remain a major element of Russian and Chinese ASW ship armouries. They are fired from dedicated multi-barrel launchers and have the advantage of exploding only on contact and thus disturbing the water less than the blast of a depth charge or depth bomb as it is also known.

    The mass of information coming from onboard and offboard sensors as well as the need to integrate operations with other naval and air platforms requires electronic co-ordination, just like anti-air warfare (AAW) operations. Just as in AAW operations the initial solution was the tactical data handling system which provided situational awareness using sonars, radars and datalinks. But these systems need to be integrated with the ASW weapon control systems to produce combat management systems and it is surprising how few of the region’s ASW vessels have this facility.

    Given historical precedent it is not surprising that the Japanese Maritime Self Defence Force possesses what is probably Asia’s most comprehensive ASW force. With the Aegis-equipped Kongou and Atago class anti-air warfare and ballistic missile defence ships complete Tokyo is now focusing upon its ASW forces. Traditionally it has had destroyers with towed array sonars, Asroc missiles, heavyweight helicopters with a and tactical data handling systems as well as frigates which lack almost all of these features and are used mostly for coastal escort missions. Tokyo is emphasising the longer-ranged destroyer and is beginning to replace the Hatsuyuki class destroyers with the 19DD class to shield the Aegis ships. Of even greater interest is the development of the two Hyuga class ‘through-deck destroyers’, officially destroyers but with full-length flight decks for helicopters. While there is much official comment about their value as emergency relief platforms it is clear they are anti-submarine helicopter platforms, and an even larger vessel is to be funded from this year.

    Japanese naval plans envisage four flotillas with two types of division; one with a ‘through-deck destroyer’, an Aegis destroyer and two escort destroyers and the other with an Aegis destroyer and three escort destroyers. The destroyers provide greater capability in terms of electronics and weapons as well as the range to project Tokyo’s maritime shield deep into ‘blue water.’

    South Korea has similar aspirations but not equipment. There are the KDX-1 (Kwanggaeto Daewang class) and KDX-2 (Chungmugong Yi Sun-shin class) multi-role destroyers which feature combat management systems and towed array sonars but only lightweight helicopters, the KDX-2 ships also having Asroc. They provide a long range ASW force while by contrast, the Ulsan class frigates despite receiving new hull-mounted sensors, lack both off-board sonars and helicopters making them largely coastal vessels. The planned FFX frigates, scheduled to join the fleet from 2011, will make up for these omissions and provide a direct escort force.

    By contrast China, although expanding its naval forces, remains a nation with very limited ASW capability and this is largely confined to operations over the continental shelf - indeed there appear to have been no major exercises outside this area. There are 27 destroyers, most of which have lightweight torpedoes and anti-submarine rocket launchers but their sensors are largely confined to hull-mounted medium-frequency sonars with only the two Luhus (Type 052) having variable depth sensors. They embark lightweight helicopters (Harbin Zhi-9 or Kamov Ka 28 ‘Helix’) as do most of the modern frigates, most of which also lack any long-range sensor. Although information on Chinese warship electronics is limited it appears that many ASW ships lack a combat management system and often they have only a tactical data handling system which provides data on the tactical situation but is not integrated with the weapon systems. It is possible that co-ordinated operations with submarines may not be well practised for Chinese submarines appear to make few long range sea patrols.

    China’s great rival, India, has recently signed a maritime co-operation agreement with Japan and has a different ASW philosophy, much influenced by the Russians. The force of eight destroyers are multi-role vessels with an emphasis upon anti-air warfare. All these ships have hull-mounted sonars but they all possess a longer range sensor capability than the Chinese for they also have variable depth sonars - but with tactical data handling systems rather than fully fledged combat systems. A possible sign of things to come is the fact that the Delhi (Project 15) class INS Mumbai has an Thales ATAS active/passive towed-array sonar. Unusually, the Indian destroyers carry both Russian-designed ASW rocket launchers and heavyweight (53 centimetre diameter) long range torpedoes which include both anti-surface ship and ASW weapons. The ageing heavyweight Sea King helicopter is being phased out and replaced by a variety of light aircraft including the new Dhruv (formerly Advanced Light Helicopter), the Ka 28 or Chetak helicopters which have less than half the older aircraft’s range, restricting the advantage of longer-ranged sensors.

    The most common ASW platforms are the dozen frigates, many of them based upon the British Leander Class. These possess a similar sensor/aircraft fit to the destroyers but most carry lightweight torpedoes, although the Russian-built Talwar (Project 1135.6) class ships carry longer-ranged heavyweight weapons. Interestingly, India is building at least four Project 28 class corvettes which are now described as multi-role vessels but which were originally ASW corvettes. They will feature an active/passive towed array sonar and lightweight helicopter while their armament will consist of the usual mixture of lightweight torpedoes and rocket launchers, but are clearly restricted to coastal operations or the protection of island territories.

    Neighbouring Pakistan’s ASW capabilities are even more restricted and is little more than an offshore or coastal force. None of the seven frigates (including the new Zulfiquar class) possesses any offboard sonars while most feature tactical data handling systems and lightweight helicopters giving only a relatively short-range engagement capability with lightweight torpedoes. However, even this is better than neighbouring Sri Lanka and Myanmar (Burma) which appear to have no ASW capability whatsoever, their navies being primarily coastal patrol and counter-insurgency forces like the Philippines whose sole ASW asset appears to be the 57-year-old frigate PNS Rajah Humabon, the only major surface combatant with a sonar and which is reported to have Hedgehog anti-submarine mortars.

    Within South-East Asia other navies’ ASW capabilities are more varied. Malaysia’s dozen major surface combatants, two Lekiu class frigates and a number of corvettes, have hull-mounted sonars and lightweight torpedoes. They have some limited offshore capability and most feature combat management systems but only the Lekius and the two Katsuris have helicopters, the former having lightweight Lynx while the latter can operate, but not carry, heavyweight aircraft. Singapore’s six new Formidable class frigates feature a variable depth sonar, combat management system and heavyweight helicopter and these are undoubtedly able to provide an adequate AS capability in the waters around the island state.

    Indonesia has a similar capability but with older vessels, the Leander-based former Dutch Ahmad Yani class frigates together with the former East German Kapitan Patimura class corvettes having variable depth sonars, but both lack even tactical data handling systems while the German ships have no embarked helicopter. Surprisingly, the new Sigmas have only hull-mounted sonars and can operate a lightweight helicopter like the Ahmad Yanis, yet they have excellent combat management systems.

    Thailand, with 15 frigates and corvettes, is also largely an offshore ASW force. These ships feature hull-mounted sonars with only two Knox class frigate having towed array sensors, only lightweight helicopters can be embarked while only a few ships have even a tactical data handling system.

    Vietnam has had an even poorer ASW capability in the past with five Petya class frigates and about 10 Tarantul class corvettes all lacking offboard sonars and embarked helicopters. But Hanoi is in the process of acquiring two Russian-built Gepard class frigates which are likely to feature Ox Tail variable depth sonars improving detection ranges and while these ships will not embark helicopters they will have similar heavyweight torpedo systems to the Indian destroyers as well as the ubiquitous rocket launchers. Two ships will not be sufficient to provide much ‘blue water’ ASW capability but Hanoi is more interested in protecting its offshore interests and these two vessels, which should have joined the fleet by next year, will be a step in the right direction

    A navy which requires a ‘blue’ as well as a ‘brown’ water ASW capability is that of the Republic of China, more usually called Taiwan. The island state is under notice from Beijing that it will try to bring it back under the mainland’s control and it needs to secure its maritime lines of communications. The Keeling (formerly Kidd) class destroyers provide a useful keel for such a force with a towed array sonar, a combat management system and heavyweight helicopters. The Oliver Hazard Perry (Cheng Kung), La Fayette (Kang Ding) and Knox class frigates augment this force and all have towed array sonars which are well suited to ‘blue water’ missions, all but the Knox having active sensors. The La Fayettes have a full combat management system but the others benefit from tactical data handling systems, although the Knoxes can embark only lightweight helicopters. These are scheduled for replacement - although Taipei has yet to decide how and with what.

    Australia and New Zealand also face threats to their maritime lines of communications but financial limitations have forced both countries to adopt multi-role ships as the major surface combatants. Australian Anzac class ships are being upgraded with a new combat management system and the MU 90 lightweight torpedo while the Seahawk heavyweight helicopters will enhance ASW performance. However, even after some 14 years there must be question marks over the ability of these relatively diminutive ships to conduct ‘blue water’ ASW operations. Like the upgraded Adelaides they lack any long range sensor and even a helicopter as good as a Seahawk cannot compensate for such a key element of ASW. The Government has recently announced that the ageing Seahawks will be replaced by up to 24 state-of-the-art maritime helicopters, which will either be NHI’s NF90 or Sikorsky’s MH-R60.

    The overall strategy of the Royal Australian Navy and the Royal New Zealand Navy for the past six decades has been based upon operations with the US Navy and it appears that where ASW is concerned both will exploit the technological superiority of American ships such as the Arleigh Burke (DDG 51) class destroyers which do possess the complete range of combat management systems and sensors with which to meet the submarine threat. It is interesting to note that Spain's Alvaro de Bazán class destroyers, upon which the Hobarts are based, are similar in capability to the Arleigh Burkes and it appears the Air War Destroyer will also have a substantial ASW capability through its Ultra integrated sonar suite which is likely to include a towed array sensor. It is interesting to speculate, however, that one of the Canberras might occasionally be press-ganged into acting as an anti-submarine helicopter platform on an ad hoc basis, although this is not currently part of Australian Defence Force thinking.

    Regional ASW capability is, inevitably, extremely localised, essentially into littoral waters and their immediate vicinity. Only Japan, Korea and perhaps India currently possess the resources for prosecuting 'blue water' ASW operations relying upon their own resources, including long-range maritime patrol aircraft, and even here the need to use ASW forces to protect naval task groups will reduce the numbers of hulls available to protect from submarine attack the merchant fleets upon which the region depends. Those who fail to learn the lessons of history are doomed to repeat the mistakes and unrestricted submarine warfare brought Japan to its knees. In the face of the region’s growing force of underwater sharks this prospect becomes ever more likely and while much is made about the potential of autonomous, or remotely-controlled, platforms in ASW they will still require both a manned base ship and large numbers to meet the threat. The region will, therefore, have to re-evaluate both the threat from submarines and the response.

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