Modern surface ships and submarines differ radically as targets with regard to their characteristics of speed, protection, the nature of their sensors and countermeasure 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 its ability to remain silent and 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 antisubmarine 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 underwater 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 a 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-World War II, demonstrate the accuracy and destructive power of a torpedo. The first was during the falklands War when General Belgrano, an 8,000 tonne cruiser of argentina, sank within 45 minutes after being hit by three second World War 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.

Torpedo

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 aspect on which propulsion, guidance and lethality of a modern torpedo are designed, are outlined below:

Propulsion: The requirement of high speed and medium (sea water) constraints, problems of back pressure, and pros and cons of closed systems are the challenges. Torpedo body and water resistance results 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 constraints are 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 and 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 stateof-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: The challenges are the phenomenon of underwater explosion; bubble and hammer effect; 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.

Class of torpedoes

A modern torpedo can be divided into two classes namely lightweight and heavyweight, and into two types namely, wireguided, 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 aircraft.

By and large heavy weight torpedoes (HWT) are primarily weapons of choice against ships by submarines, and light weight torpedoes (LWT) are antisubmarine 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.

US LWT

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 MK 46, Mk 50, 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 271 cm to 287 cm, weighs between 276 kg and 293 kg 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 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 puretone 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 control synergises established MK 48 and MK 50 torpedo technologies with the latest 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 us 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.