The devastation caused by the German submarines in World War-I led to evolving a standoff anti-submarine warfare (ASW) solution involving use of aircraft in detecting a submarine at sea and attacking it with depth charges, bombs and guns. In 1914, the Royal Navy converted an old ocean liner into a seaplane tender ship and christened it HMS Campania, for which, specially designed seaplanes were ordered. The first of such seaplanes, F.22, inducted in the Royal Navy, had Sunbeam Maori engine of 260 hp, for armament it carried a 7.7mm Lewis machine gun and a bomb load of up to 6x116 lb bombs. Towards the end of World War-I when the US joined the war effort, the US Navy in 1917 asked for a long-range flying boat which could patrol and provide security cover to the supply ships crossing the Atlantic. The Curtiss Aeroplane and Motor Company constructed the Curtiss NC-1 to stringent US Naval standards and the first flight of the same took place in 1918. Despite the fact that Armistice was signed in November 1918, the US Navy went ahead with its order of 10 Curtiss NC aircraft.

During the World War-II, lots of scientific work was being done for detecting the hostile submarines, notable amongst them were the magnetic anomaly detector (MAD) and the use of sonobuoys. Interestingly, since the MAD detects a submarine only after the aircraft has flown over it, a retro rocket system was developed (to fly backwards) to release the depth charge over the submarine. The armament carried by maritime aircraft and helicopters include antiship cruise missiles, lightweight torpedoes, depth charges and bombs.

Anti-submarine and anti-shipping air armament currently in use by major navies

Air-dropped depth charges and bombs: Finland was the first to use air-dropped depth charges from its Tupolev SB aircraft in 1942. Subsequently, the methodology was adopted by RAF Coastal Command. Later depth charges were designed for aerial deployment and have recently once again come into focus because of the antisubmarine warfare (ASW) threat in littorals. These can be very effectively utilised for flushing out the lurking diesel submarines. Two depth charges are worthy of mention; these are the Mk 11 depth charge of the UK and the BDC 204 depth charge of Sweden.

The Mk 11 depth charge was developed by British Aerospace (now BAE Systems) for air delivery from maritime aircraft and helicopters. The Mk 11 depth charge was designed for shallow water operations against submarines on the surface or at periscope depths. It is fully compatible for carriage and release from a wide range of ASW helicopters and fixed-wing maritime patrol aircraft. The Mod 3 version incorporates a 4mm mild steel outer case and nose section, which is designed to withstand entry into the water at high velocities without distortion. It has been cleared for carriage on Lynx, Merlin, NH90, Sea King and Wasp helicopters.

The BDC 204 depth charge was developed by Bofors underwater systems (now Saab Dynamics) for air delivery from maritime aircraft and helicopters of the Swedish Navy. It was designed for use against submarines operating in shallow waters or at periscope depth, and in order to cover a wide range of applications, it was produced in four different weight categories and with different sinking speeds ranging between 5.2 and 6.8 m/s. The depth charge can be deployed in patterns with different depth charges set to detonate at different depths to achieve profound shock and damage to submarines. The BDC 204 family of depth charges is fitted with standard North Atlantic Treaty Organisation (NATO) suspension lugs and their design allows it to be carried as a high drag general purpose bomb or torpedo. These have been cleared for carriage on the Boeing Vertol 107 helicopter and CASA C-212 Aviocar maritime patrol aircraft.

Two air-dropped bomb upgrade kits worth mentioning are joint direct-attack munitions (JDAM) by Boeing Corporation and the dual-mode laser-guided bomb (DMLGB) by Lockheed Martin. The JDAM upgrades unguided bombs to all-weather smart bombs. These bombs then acquire GPS-aided inertial navigation system with a range of up to 24 km. The DMLGB kit upgrades the existing LGBs and is used for precision-bombing against non-hardened targets. It provides LGBs with semi-active laser, GPS/INS guidance.

Air-launched torpedoes: The prominent air-launched torpedoes are as follows:

Stingray: Stingray is a LWT manufactured by BAE Systems. It has a diameter of 324mm, weight of 267 kg and length of 2.6 metres. Its speed is 45 kts (about 83 kmph) with a range of eight km and its warhead is 45 kg of Torpex. It can dive up to 800 m. Stingray is fed with target data and other associated information prior to its launch. On entering the water, it searches for the target autonomously in active mode and on acquiring the same, attacks it. It is carried by Nimrod aircraft. Stingray Mod1 is reported to have a shaped charge warhead and improved shallow water performance.

Mk46: Designed by Alliant Techsystems Inc, Mk46 Mod5 torpedo is the mainstay of the US Navy’s air-launched lightweight torpedoes. It is manufactured by Alliant Tech systems. It has a diameter of 324mm, length of 2.59 metres and a weight of 231 kg. It runs on Otto fuel, has a range of 11 km with a speed of 40 kts (about 74 kmph) and can dive up to 365 metres. It has a PBXN-103 warhead of 44 kg. It has an advanced digital computer control system with a built in logic and tactics for search and re-attack. It has effectively performed in both deep and shallow waters and can attack both the nuclear as well as the smaller diesel submarine. Over 25,000 MK46 torpedoes have been supplied to customers till date. Interestingly, the Chinese YU-7 torpedo is said to have been developed from the MK46 Mod2.

Mk54: Raytheon’s Mk54 lightweight torpedo is a hybrid of technologies taken from Mk46, Mk48 and Mk50 torpedoes. It is supposed to have homing and warhead of the Mk50 and propulsion package of the Mk46 torpedo. It has incorporated COTS processing technologies for an advanced guidance and control system. It is stated to have sophisticated shallow-water capabilities for littoral threats. It is understood that the Mk54 torpedo has been requested for P-8I aircraft by India.

A244/S: A244/S, developed by WAAS and currently manufactured by the Euro Torp consortium, is a 324mm diameter, 2.8 metres long and 244 kg weight torpedo. It has a cruise/surge speed of 30/39 kts (about 55/72 kmph) with a range of six km and depth of up to 600 metres. Its homing head can function in mixed, active or passive modes. It has special signal processing to distinguish target from decoys.

A244/S Mod.3: This the latest upgrade of the A244/S. It has more powerful propulsion battery with an increased number of cells, which ensures a 50 per cent increase in the endurance of the weapon to 13.5 km. It has an advanced digital signal processor module to counter sophisticated torpedo countermeasures. The homing head has preformed multiple transmission and reception beams and multi-frequency operating capability. It can classify and track several targets simultaneously and discriminate between the target and countermeasures.

MU90: MU90/Impact is in mass production for six major North Atlantic Treaty Organisation (NATO) and allied countries. The MU90/Impact torpedo is 323.7mm ‘NATO standard’ calibre, 2.85mm long, with a weight of 304 kg. It is powered by an aluminium-silver oxide sea water battery using dissolved sodium-dioxide powder as electrolyte with a closed-loop electrolyte re-circulation system. The torpedo is propelled by an electronically controlled high-RPM brush-less motor driving a skewed multi-blade pump jet proposer allowing a continuously variable torpedo speed automatically selected by in built logic of the torpedo. The control and guidance electronics has embedded operational and tactical software including signal processing, data processing and torpedo guidance algorithms, which enable the MU90 to continuously self-adapt its configuration and tactics. The inertial system is based on ‘strap-down’ technology enabling all-attitudes capability including bottom following capability. The warhead consists of V350 explosive, fullyinsensitive shaped charge warhead, with an impact type exploder incorporating two mechanical and six electrical independent safety devices.

A200/A: Low-cost anti-submarine weapon (LCAW) A200/A is a miniature torpedo developed by WASS. LCAW has been developed as an intermediary between air-launched torpedoes and conventional depth charges. It is a low-cost option which provides propulsion and guidance to a depth charge without the costs of a torpedo. The air-dropped version A200/A is deployed from aerial sonar-buoy dispensers. The weapon is primarily designed to engage targets in shallow water, like midget submarines. The A200/A version has a length of 914.4mm, weight of 12 kg and diameter of 123.8mm. The warhead is a 2.5 kg PBX shaped charge and LCAW has an operating depth from 15 to 300 metres. It has a speed of about 18 kts (about 33 kmph) with a range of two km.