Torpedoes challenged: the torpedo is the navy's first guided missile (it was originally called a self-propelled torpedo) and while at first sight there seems to be less interest in underwater warfare, in practice there is a great deal of work to ensure that these weapons remain effective.

The technology of the torpedo transcends the two types: heavyweight (generally 533 mm diameter) and lightweight (generally 324 mm). The heavyweight torpedo is the long lance of the submarine and was originally designed to sink surface ships. During the Cold War the role of the heavyweight weapon was adapted to sink enemy submarines in engagements which were expected to take place in the deep 'blue' waters of the Atlantic and the Pacific at long ranges of between 20 and 50 kilometres (10 to 27 nautical miles). Such long range engagements against a manoeuvring target required tighter control of the weapon, and the old 'free running' weapons were replaced by torpedoes which the submarine could control through copper wires, this allowed the weapon and the platform to exchange data until impact.

The lightweight torpedo has been an anti-submarine weapon since the deployment of the Mk 24 'Mine' or Fido in 1943. The Fido was deployed by air but today lightweight torpedoes are used by both aircraft and by ships for relatively short-range engagements at distances of 10 to 20 kilometres (five to ten nautical miles). During the Cold War the lightweights were largely confined to 'blue water' operations.

In the post-Cold War, post-nine-eleven world, the target remains the submarine although the priority is no longer 'blue water' operations but rather the blue/brown waters nearer the coasts. Here the threat comes from very quiet diesel-electric submarines, with or without air independent propulsion, operating in a very difficult and shallower acoustic environment with a complex mixture of saline and fresh water with different thermal levels. In these circumstances, once a target is acquired it is essential that it is unaware of the approach of danger until the last minute, which means greater reliance on exploiting passive sensors.

Nose Changes

Torpedo design has changed greatly since the first weapons appeared. The original cigar shape was long ago replaced by a more efficient hydrodynamic configuration of a long cylinder with a dome-shaped nose and a tapered rear.

Nose technology has seen some developments with the introduction of electronically steerable planar sonar arrays that give flat noses, which are used in most heavyweight and lightweight weapons. This was especially valuable in Cold War operational scenarios for it provides a more focused field-of-view (120 x 70[degrees] in the case of the Impact) to provide greater accuracy in the terminal phase. But it does have drawbacks; in particular high cavitation effects when the weapon turns, and the prospect of snagging guidance wires during turns because the weapon has to take a snaking course approaching the target. If the target is lost the weapon is usually unable to circle and has to take a pentagonal route, which increases the possibility of snagging.

An alternative solution is to have a conformal array where the transducers are arranged in rows along the nose. This can provide a wider horizontal field-of-view (up to 220[degrees] in the Atlas DM2A4) but a very limited vertical one. This means the weapon has to be rotated to provide a vertical field-of-view but in compensation there is far less cavitation, the weapon can go directly towards the target and can circle to re-acquire it.

Solid-state technology has made a significant improvement to weapon guidance with systems based on active/passive sonar. The passive element is used at low speed to detect, classify and track the target while the active element is used in the terminal phase to provide fire control solutions.

Guidance systems, which incorporate gyros and accelerometers, are increasingly based upon commercial-off-the-shelf hardware and software--both to improve performance and to cut costs. Laser-based gyros, solid-state accelerometers and fibre-optic technology are used to improve life-cycle costs and reliability. Industry-standard cots products are especially valuable in signal processing to execute the demanding software applications required for shallow-water detection and classification.

The software programming language differs from manufacturer to manufacturer and tends to follow the navy's preferences. The Sting Ray and the Spearfish, for example, use Coral 66, but the upgraded Sting Ray Mod 1 weapons have switched to Ada 95. At one time Ada seemed to be the coming language-of-choice for software with defence applications developers, and it is also used in American and Swedish weapons, but Atlas Elektronik is following the growing trend towards using C++.

Software-based products can create pre-formed beams over several frequency bands for improved discrimination and wider coverage. The Thales Underwater Systems' seeker in the Impact features 47 preformed transmission and 33 preformed reception beams for a total acoustic coverage of 120[degrees] horizontal and 70[degrees] vertical. It operates across six different frequency bands (with a bandwidth significantly above ten kHz).

The Advanced Sonar Transmitting and Receiving Architecture (Astra) seeker used in the Black Shark features a multi-beam, steerable planar array with digital pulse compression in any transmission mode, multi-frequency operations in both active (frequency modulation and continuous wave) and passive modes and independent processing of each frequency (active and passive) on each lobe. It operates at 15 kHz (passive medium frequency only) for long-range acquisition and 30 kHz (active and passive high frequency) for high resolution at short range, although it can use both frequencies simultaneously in passive mode to discriminate between signals from a real target and those from a decoy.

The Sting Ray Mod 1, which was trialled during the summer, has an improved sonar with a new active element, a new digital correlator and a solid-state autopilot.

Inertial guidance systems can automatically re-acquire targets and they include sophisticated sub-programmes to discriminate against countermeasures as well as terminal phase programmes to optimise the point of impact.

The Mk 48 now features a cots-based open architecture common torpedo processor based upon the 6U VME64 board. The Mod 6AT (Advanced Technology) introduced an open-architecture guidance-and-control section based upon cots signal- and data-processing hardware and an industry-standard Ethernet databus. This system uses the ubiqitous PowerPC computer and G4DSP signal processors and provides significant growth potential for future processing upgrades including the introduction of a Common Broadband Advanced Sonar System for the Mod 7.

This technology has been incorporated in the Mk 54, which uses the sonar of the Mk 50. The Mk 54 can generate up to 62 independent beams that can be steered in both the horizontal and vertical planes. A fully digital programmable beam former provides the flexibility to select and steer narrow beams for optimum coverage according to the engagement scenario. The transmitter can generate complex signals including frequency-modulated and pure-tone waveforms. A programmable dual-band receiver generates multiple receive beams to enable extensive volume coverage.

One capability being introduced is wake homing. In this the sonar uses high frequency beams pointing upward to detect motion in the water. As it crosses the disturbed water the guidance system turns the weapon until it establishes the wake boundaries and the direction in which they are narrowing. The weapon can then be guided to follow the target until it makes contact. This system has been used extensively by Russian weapons and is now being introduced into Western ones, such as the Atlas DM2A4/Sea Hake 4.

New Wire

Lightweight torpedoes are usually free-running weapons, like those used in the Second World War, but heavyweight weapons (and the Bofors Tp 43) are wire-guided from a submarine. The need to hunt down hostile submarines meant that engagements had to be conducted at longer ranges using low-frequency sonars, and a free-running weapon might not be able to acquire the target with its own sonar.

The solution was to exploit the passive sonars in submarines, with their greater processing capacity. By linking the weapon and the submarine through a wire-based link sensor data could be exchanged and the weapon guided by the submarine close to the target's vicinity and within range of the weapon's sonar. This has meant not only the provision of a guidance-wire spool within the weapon but also a wire dispenser in the submarine to ensure the correct ratio of tension to prevent the wire snapping. This was a major problem in the development of the BAE Systems Tigerfish.

The traditional copper wire has drawbacks, although it is a thoroughly proven technology. The wire is heavy, effecting weapon performance and has a narrow bandwidth, which restricts the amount of data that may be exchanged. In recent years an environmental concern has emerged as it produces a dangerous pollutant of cadmium.

An alternative is to use lighter fibre-optic wire, which is buoyant and has a wider bandwidth (typically 10 Mbit/s), but the performance can begin to deteriorate at distances beyond eight nautical miles (15 kilometres) and it is also vulnerable to both snagging and cutting. However Atlas Elektronik has developed a specific 300-[micro]m wire for the DM2A4, which is also being considered for the Spearfish upgrade.

Propulsion

Many heavyweight torpedoes used a chemical fuel to create a thermal reaction that raises steam to drive a turbine and which also provides high energy levels for sustained periods. One of the most common is Otto II, which is based upon propylene glycol dinitrate augmented by 2-nitrodiphenylamine and dibutyl sebacate. Bofors Underwater Systems' weapons (such as Torpedo 2000) use a combination of 85 per cent high-test peroxide (HTP) and 15 per cent kerosene because of its superior energy density of 6.9 MJ/kg, compared with 3.1 MJ/kg for Otto II--the BAE Systems' Spearfish uses Otto II and hydrogen ammonium perchlorate and can reportedly make up to 60 knots.

This technology retains its popularity with the major naval powers as, for example, Australia recently decided to acquire the Raytheon Mk 48 Adcap weapons, which use Otto II.The problem with thermal propulsion weapons is that they do have an acoustic signature even when the engine is isolated from the main body and other shrouding measures are taken. Concern over HTP technology has grown following the loss of the Russian submarine Kursk to detonation of torpedo fuel although the three Scandinavian users of Bofors heavyweight torpedoes, whose latest versions use seven-cylinder piston engines, have never had any problems in the past 40 years.

Another problem in using thermal fuels is the greatly increased heat signature. By contrast the alternative electrical propulsion is quieter but traditionally has produced lower energy levels, which meant a slower weapon.These power levels have improved by making the motor rotate at high speed for maximum efficiency, but this in turn has often meant introducing a noisy gearbox between the engine and the propulsor while pumps are needed to cool the modern high-performance batteries.

The original electrically driven torpedoes were developed because they left no wake, but they produced low energy levels and the weapons tended to be slow. Battery developments in the past few decades have produced much higher energy levels permitting longer engagement ranges and higher speeds; this by the batteries being activated by exposing them to seawater. This propulsion was developed some 30 years ago and is based on three available technologies; aluminium silver oxide (A1AgO), magnesium silver chloride (MgAgC1) and zinc silver oxide (ZnAgO).

Saft produces both magnesium silver chloride batteries for the Stingray Mod 1 and aluminium silver-oxide ones for the rival MU90 Impact. The company opened a new $ six million ([euro] five million) factory in Poitiers last year to meet growing demands for batteries. Since the development of improved batteries there have been few significant technical developments although lithium-ion technology is being examined.

The zinc silver oxide technology is also exploited in the DM2A4 with batteries produced by Atlas. Unusually these are in three-, two- and single-battery configurations with each energy pack consisting of 86 cells and generating more than 150 V.

Thrust

Propulsion technology has also changed rapidly, with turbines and propellers increasingly replaced by electric motors and propulsors. A propulsor consists of two contra-rotating sets of propellers housed in a duct, these create a pump-jet effect that is quieter than conventional propellers. The Torpedo 2000 uses a propulsor based upon the one used in the Spearfish. The latter used a Sundstrand 21TP01 gas turbine but the Torpedo 2000 uses a seven-cylinder piston engine. The Mk 48 Adacap uses a six-cylinder hot-gas piston engine, which has a distinct audio signature. To reduce this signature in the latest Mod 6 version, sound isolation techniques and mechanisms have been introduced. This includes isolating the engine, exhaust muffling, bringing in flexible fluid hoses and isolating the drive shaft bearing as well as 'damping' the torpedo body.

Electrically driven weapons tend to use skewed contra-rotating propellers made of carbon fibre. The DM2A3 has a DC motor with contra-rotating drive shafts while the DM2A4 features a high frequency, permanent-magnet AC motor operating at 93 per cent efficiency to deliver 300 kW. The manufacturers claim this provides a 50 per cent increase in maximum speed and a 400 per cent increase in energy content compared with the DM2A3 while the weapon is 50 per cent quieter. The Black Shark uses a brushless axial flux, slow rotating, permanent-propulsion motor. The advantage of this motor is that there is no need for reduction gearing which therefore improves efficiency and reduces noise.

Most lightweight torpedoes use electric motors that are quieter while retraining enough reserve power to chase the target. One of the latest used in Eurotorp's MU 90 Impact is an Atlas Elektronik stepless, variable speed, 120kW electric motor. In this weapon and the BAE Systems Sting Ray, which has a 63kW motor, a propulsor is used. Some weapons of slightly earlier design, such as the Whitehead Alenia Sistemi Subacquei (Wass) A244/S (which uses a DC counter-rotating motor) and the Bofors Underwater Systems Tp43/Tp 45, continue to use conventional propellers.

American lightweight weapons not only use propellers but also have propulsion systems based upon high-energy chemical reactions, a hangover from the Cold War. The Raytheon Mk 46 uses Otto II fuel and a five-cylinder motor. Its longtime heir apparent, the Mk 50, features a closed-cycle steam turbine engine fuelled by solid lithium. This is ignited to produce an oxidant that is injected into a boiler and generates superheated steam that drives the turbine and is recycled through the boiler. Developed by AlliedSignal's Garrett Division it can develop full power at all depths but is extremely complex.

Of significance is that the Raytheon Mk 54, which combines the best features of its two predecessors, has reverted to the Mk 46 propulsion system. However, the US Naval Underwater Warfare Center is developing new technologies for lightweight torpedoes and is investigating both an integrated motor propulsor (IMP) being developed by SatCon Technology as well as a rechargeable, low cost energy source developed by Saft America's Advanced Battery Systems Division. The IMP has the motor stator in the fixed shroud assembly with permanent magnets on the propulsor rotor tips, thereby reducing volume, weight and acoustic signature while generating some 112 kW. It is to be powered either by lithium-ion or aluminium-silver oxide batteries.

Warheads

Warhead technology has not changed very much, with warheads still featuring a TNT-based explosive such as PBX and with both direct and influence fuzing. The latter can be both acoustic and magnetic and, when used against surface targets, is designed to detonate beneath the keel to produce a gas bubble that will break the ship's back. Heavyweight weapons have warheads of 240 to 300 kg, but lightweight weapons have smaller payloads ranging from 35 to 50 kg, which are usually hollow-charge ordnance to provide a focused burst of energy to penetrate thick pressure hulls.

While there is no difference in the types of targets there is a change developing in the explosive fill. Demands for explosive which will not be detonated by shock or fire have led to the development of insensitive munitions, which are gradually being considered for all services. Such fills are already being introduced into torpedoes, notably the Sting Ray, and will certainly be required for both new weapons and upgrades.

Despite the changing operational scenarios the torpedo remains one of the most lethal weapon systems available to modern navies. Indeed, it might be argued that it is more effective against ships than surface-to-surface missiles and this situation is unlikely to change within the foreseeable future.