(Published article in SP’s Naval Forces June-July 2012)
Modern surface ships and submarines differ radically as targets with regard to their characteristics of speed, protection, the nature of their sensors and counter measure capabilities. These characteristics impose specific demands upon the design of weapons required to engage them at high seas and even in coastal areas, the submarine continues to dominate by virtue of her ability to remain silent and to strike lethally.
The medium of sea water does not offer itself to comfortable utilisation by the weapon. It offers resistance to its movement at high speeds, imposes crippling constraints on long range target detection capability as it is impervious to electro-magnetic waves and in acoustics, it hinders by limiting propagation of sound waves in unimaginable ways. These factors give rise to various design restrictions of the weapon.
To meet the underwater threat a variety of weapons are available on the shelves of the navies, these include: –
– Depth Charges, Mortars and Rockets.
– Deep sea and Coastal Mines.
– Light Weight Torpedoes.
– Heavy Weight Torpedoes.
– Duel purposes Anti Shipping and Anti Submarine Heavy Weight Torpedoes.
Navy is the only service which operates on the surface, subsurface and in the air. The submarine operates subsurface and is thus the prime platform which uses under water weapons. Traditionally the underwater weapons are torpedoes and mines. This trend has undergone a change since the 1970s when submarines began to be armed with anti ship and later with land-attack missiles, but the torpedo still remains the premier underwater weapon. With advancements in technology, a modern torpedo can destroy target at a range of >40 Km and a speed of about 50 kts. Its destructive power is more than a missile and it can easily differentiate between a target and a decoy. Two incidents, post WW II, demonstrate the accuracy and destructive power of a torpedo. The first was during the Falklands War when General Belgrano, an 8,000 ton cruiser of Argentina, sank within 45 minutes after being hit by three WW II vintage torpedoes. The other example is the sinking of the South Korean corvette Cheonan in March 2010, in which about 46 seamen lost their lives. An investigation concluded that the warship had been sunk by a North Korean torpedo, fired by a midget submarine. For these very potent reasons torpedoes continue to form about 80 per cent of the weaponry of a submarine. The end of cold war has shifted the underwater threat from deep and fast running nuclear submarines to smaller and quieter diesel electric submarines operating in littoral shallow waters. The stealth factor is further aided by the poor shallow water acoustics, which hinders efficient detection, acquisition, tracking and attack on these vessels.
Originally the term “torpedo” was used mainly for weapons which now would be called mines, but since the beginning of the 20th century, torpedo has been used only for an underwater self-propelled missile. The design features of these weapons vary in intricacy from the simple depth charge to the complex dual purpose heavy weight torpedo. Literature is generally silent on the design criterion/ aspects and a student of weapon technology has to perforce interpret these with the help of the technical data available about the weapon.
Some aspects on which propulsion, guidance and lethality of a modern torpedo are designed are outlined below:-
Propulsion. Requirement of high speeds and medium constraints, problems of back pressure, pros and cons of closed systems. Torpedo body and water resistance resulting in penalties on achieving high speeds.
Electric torpedoes are again coming into focus, (as they are much more silent) with vastly improved battery technologies in the market. Sea water activated batteries are providing required power packages for high speeds. Electric motors and pump jets are being used instead of propellers and turbines.
Guidance. The vagaries of the medium towards acoustic propagation and velocity profiles of the sea, Shadow zones, losses and background noise. Target strength of ships and submarines and their radiated noise levels. Design aspects of Homing Head, and detection threshold .Pros and cons of homing, wake and wire guidance systems.
With the advent of integrated circuits and microcontrollers the homing heads have become powerful drivers in guidance of torpedoes, be it passive listening in the initial phase or the active pinging in the attack phase. Cots components with the required ruggedisation enable use of state of the art gyros, accelerometers, and signal processors. Wire guidance using fibre optic cables has enabled enhanced target classification capabilities, so much so that the operator can not only hear what acoustics the torpedo is picking up but also ‘see’ what the homing head is looking at.
Lethality. Phenomenon of underwater explosion, Bubble and Hammer effects. Contact and standoff explosions underwater; torpedo warhead design, and selection basis for underwater explosives.
Design of torpedo proximity fuses is based upon, circular error of probability qualitative requirements and safety aspects of fuses, probable frequencies of operation of acoustic and electromagnetic fuses based on basic physical laws.
Development of safe and long life explosives has led to warhead fillings of ‘insensitive explosives. In the terminal phase the proximity fuses use magnetic, acoustic and laser technologies to determine the optimum distance from the target at which to explode the warhead to inflict maximum damage to the target hull.
A modern torpedo can be divided into two classes namely lightweight and heavyweight, and into two types namely, wire-guided, and the autonomous fire and forget types. They can also be launched from a variety of platforms like ships, submarines, rotary wing and fixed wing aircrafts.
By and large heavy weight torpedoes (HWT) are primarily weapons of choice against ships by submarines and light weight torpedoes (LWT) are anti- submarine weapons launched from both ships and aerial platforms. However some navies like the Russian, PLAN and Indian Navy carry HWTs on ships for deployment against submarines.
One of the LWTs which is claimed to be the most tested torpedo in the world is the US Mk 54 torpedo (Raytheon) and deserves a mention as it also traces its lineage from other Raytheon torpedoes like the MK46, Mk50, Mk 48 and ADCAP. It has proved itself against submarine targets in various exhaustive tests in the US. It will replace Mk 46 and MK 50 torpedoes by 2014.
The Mk 54 torpedo is 271cm to 287cm, weighs between 276 Kgs to 293 Kgs and has a diameter of 32 cm. As per Raytheon, Mk 54 Sonar has evolved from Mk 50 and 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. The sonar nose construction provides low array self-noise; the acoustic array and digital beam former provide superior performance in littoral areas; and the transmitter flexibility and dual-band receiver provide extensive and effective volume coverage. The Guidance and Controlsynergises established MK 48 and MK 50 torpedo technologies with the latest commercial-off-the-shelf (COTS) processing power (14 gigaflops) in an expandable open architecture. This processing power, coupled with the MK 48 ADCAP and MK 50 detection, editing and tracking algorithms, provides an effective weapon in both littoral and open-ocean waters. The Warhead comprises of the MK 103 Mod 1 Warhead and the MK 20 Mod 0 Exploder. The Warhead System has been deployed on U.S. Navy ships, aircraft and helicopters for over two decades with no safety incidents. The propulsion provides a reliable, variable-speed weapon with full performance over the depth and speed ranges of interest. It owes its origins to Mk 46, Mk48 ADCAP and Mk 50 Torpedoes.
The European torpedo of interest is the MU90/IMPACT(A 244/S Mod 3) Advanced Lightweight Torpedo produced by EUROTORP and jointly developed by WHITEHEAD ALENIA Sistemi Subacquei (Italy, a FINMECCANICA company), DCNS International and THALES (France). As per the manufacturers, The MU90 is a NATO-standard-calibre (323,7mm) fire-and-forget LWT of 304 Kg and 285 cm length, designed to counter any type of nuclear or conventional submarine, acoustically coated, fast-evasive, deploying active or passive anti-torpedo effectors. The torpedo can be deployed from surface vessels, fixed/rotary wing aircraft or missile. Pre-arrangements to cope with submarine-launched SLAAM threats have been incorporated in the weapon as well as Hard-Kill (anti-torpedo torpedo), continental shelf mine and submarine launching capability. It is currently in mass production and it has entered into service with the French, Italian, German, Danish and Polish Navies and accepted by the Australian Navy. It is powered by an Aluminium-Silver Oxide sea water battery, using dissolved sodium-dioxide powder as electrolyte and incorporating an advanced closed-loop electrolyte re-circulation system, the torpedo is propelled by an electronically controlled high-RPM brushless motor driving a skewed multi-blades pump-jet propulsor allowing a linearly variable torpedo speed. The MU90 operates without any degradation in water depths in excess of 1000 m and as shallow as 25 m and retaining navigation capability up to 3m.The advanced acoustic seeker features 47 transmission and 33 reception pre-formed beams. Its multi-frequency, parallel processing and simultaneous acoustic modes operation, allow multi-target tracking capability, high engagement distance, high performance in very shallow water providing the weapon with high immunity to the most advanced anti-torpedo countermeasures. The control and guidance electronic accommodates the operational and tactical software including the signal processing, the data processing and the torpedo guidance laws. The torpedo guidance is achieved through an inertial system based on ‘strap-down’ technology.
The very low radiated noise, achieved through the use of the modern pump-jet technologies combined to an extended selection of torpedo preset parameters, allow a silent approach to the target, minimising the alert range of the submarine detection systems and resulting in high killing probability. The Insensitive Munitions, direct energy charge warhead is proven to kill double hull submarines; it is ignited through an impact type exploder incorporating independent safety devices. The warhead fully meets any STANAG safety requirement.
The exercise section, interchangeable with the warhead section, allows live exercise firings. Composed of a pneumatic recovery system based on inflatable collar technology, it incorporates underwater tracking capability and solid-state memories data acquisition system. The MU 90 operates at speeds between 29Kts to >50 Kts and has a range between 11000m and 23000m.
South Korea’s Defense Acquisition Program Administration (DAPA) has spear headed the development of the Cheong Sangeo (Blue Shark) torpedo, on the lines of Mk 46 Mod 5, indigenously, through The Naval Center and LG Innotek .
The Japanese torpedo GRX-4 (Type 73) is a lightweight anti submarine weapon, for tracking and attacking the high-speed nuclear submarines. This torpedo has higher speed, better operating depth and target detection capabilities than other Japanese torpedoes.
The Chinese YU-7/ET-52 torpedo is probably a reverse engineered A 244-S Mod 2 version with some technologies from the US Mk 46 Mod.2. 507 Institute and 705th Institute have undertaken the reverse engineering and development.
India’s National Science and Technology Laboratory (NSTL), Visakhapatnam has developed TAL a lightweight torpedo, 2.75 meters in length and weight of 220kg.It is produced and assembled by Bharat Dynamics Limited (BDL). TAL is an anti-submarine torpedo which can be launched from both, surface ships and helicopters. It can attack and cripple submarines at speeds of up to 33 knots and up to a depth of 540 meters. It uses sea water activated batteries and can travel up to a maximum distance of seven km; it has an explosive weight of 50Kg. The Indian Navy has placed an order for 25 TALs.
The fact remains that for the elusive underwater lethal threat the submarine, the torpedo is the most potent weapon in the inventory of the Navies. The HWTs on ships to attack enemy submarines, still suffer from the ship – submarine detection range disadvantage vis-a-vis the submarine – ship detection and anti ship torpedo launch capability. The helicopter/aircraft launched LWTs coupled with advanced detection and tracking systems provide a very safe and secure way of dealing with hostile submarine threats.