Agosta

The Agosta aubmarine was at the time the best conventional diesel-electric submarine. The first member of the conventionally powered Agosta family was designed in the 1970s as a precursor of the Rubis nuclear attack submarine. The first Rubis-class submarine was laid down in 1976 with torpedo and sonar systems inherited from the diesel-electric Agosta class. Besides nuclear-powered submarines, by 1988 the regular French navy included 14 diesel submarines (four "Agosta" class, nine "Daphne" class and one experimental submarine — the "Narval"). It was anticipated that "Daphne" class submarines would be retired into the reserve or dismantled in the next 10 years as nuclear-powered submarines entered the fleet, while "Agosta" class submarines were anticipated to remain in operation until 2005-2010. Besides torpedoes, "Rubis" and "Agosta" submarines are armed with Exocet antiship missiles.

French Agosta class patrol submarine of 1490 tons displacement was launched in 1974. She is powered by two SEMT-Pielstick 16 PA4185VG diesel engines of 3600 hp and one electric motor 4600 hp and one cruising motor of 23 kW providing a top speed of 20 knots dived and a range of 13600 km snorkling. She carries a single crew of 58 personnel, including seven officers. Armaments consist of Aerospatiale SM 39 Exocet anti-ship missiles launched from four 533 mm bow torpedo tubes; ECAN L5 Mod 3 dual purpose and ECAN F17 Mod 2 wire- guided torpedoes; or 36 mines carried in lieu of torpedoes.

The original design featured innovative architectural solutions and state-of-the-art equipment. More importantly, its inherent adaptability made it an excellent candidate for a thorough upgrade, in the early 1990s, resulting in the Agosta 90B. The upgrade - which focused primarily on the combat system - incorporates the latest sensors and computer technologies.

Agosta-class submarines are ideal for missions ranging from anti-shipping and anti-submarine warfare to surveillance, intelligence gathering, mine-laying, special operations and training. French, Spanish and Pakistani Agosta-class submarines have successfully undertaken a wide variety of missions, including long patrols and major multinational exercises, in times of peace or during crises. They have operated worldwide from the Caribbean to the Mediterranean and from the Sea of Oman to the China Sea.

By late 1999 a total of 13 Agosta-class submarines had been ordered by France, Spain and Pakistan. The Agosta 90B, a direct derivative of the Agosta class in service with the French Navy and other navies, features significant improvements in acoustic discretion and detection. Pakistan ordered three Agosta 90B submarines equipped with the MESMA AIP propulsion system and Subtics combat system.

As operations at sea are moving from the "blue water" open ocean to the "brown water" littoral environment, the importance of small conventional submarines is increasing. Finding a quiet, state-of-the-art conventional submarine in a large area is a challenging task and the potential impacts of the threat of such a submarine can delay operations and consume large numbers of military assets. Small, conventionally powered submarines remain suitable for littoral operations because of their low acoustic, magnetic, and thermal signatures.

Increasingly, smaller nations unwilling or unable to accept the high cost of nuclear power to achieve greater underwater endurance and longer range are turning to lower-priced and less ambitious alternatives that still offer significant operational advantages over conventional diesel-electric submarines. The best of the latter boats, such as the German-designed Type 209 or the Russian KILO, can remain submerged on battery at slow speed for periods on the order of three to five days. But now, "Air-Independent Propulsion" (AIP) schemes can increase slow-speed endurance to as much as three weeks or a month. While still dwarfed by the potential of nuclear power, AIP offers diesel submarines a remarkable increase in capability.

Historically, diesel-electric submarine (SSK) operations have been limited in both endurance and range. However, development of air independent propulsion substantially reduced snorkeling requirements and also produced a quieter, more formidable SSK. Due to their smaller size, SSKs offer lower target strengths than their nuclear counterparts; they offer less of a target to ping on and thus produce less return. When operating slowly, they are non-cavitating and provide little Doppler return. The net effect of these improvements is to reduce the detection opportunities for ASW forces.

Currently, system developers are actively pursuing the following generic approaches for achieving "closed cycle" operation: Closed-cycle diesel engines, generally with stored liquid oxygen (LOX); Hydrogen-oxygen fuel cells; Stirling-cycle heat engines with external combustion; and Closed-cycle steam turbines.

The Module Energie Sous-Marin Autonome (MESMA) system is the AIP system that Direction des Construction Navales (DCN) of France developed, mainly for export purposes. The operation of the system is based on a closed Rankine cycle engine. Liquid oxygen stored is pumped into a vaporizer, where it becomes gaseous. It is then led into the combustion chamber, where it mixes with ethanol and produces a thermal output of 700°C, at a pressure of 60 bar, to heat the secondary cycle. The high pressure of the exhaust gases allows for operation of the system at any diving depth without the need for additional equipment. The secondary circuit is a steam-driven Rankine cycle turbine, which drives a high-speed generator.

The MESMA approach is a derivative of French nuclear-propulsion experience using non-nuclear steam generation. Although MESMA can provide higher output power than the other alternatives, its inherent efficiency is the lowest of the four AIP candidates, and its rate of oxygen consumption is correspondingly higher.

The two designs of DCN [now DCNS] that are fitted with the MESMA AIP system are the Scorpene and the Agosta. The propulsion system of Scorpene is different in the two existing variants (SSK Scorpene 2003). The newest variant, the AM-2000, is equipped with a MESMA AIP system. The first of the improved versions of the submarine, the Agosta 90B, was delivered to the Pakistani Navy in 1999, although without the AIP propulsion capability. The PNS Hamza Agosta 90B submarine, fitted with a MESMA AIP system, completed customer acceptance trials in September 2008. The MESMA was retrofitted to their first two submarines. Pakistan's three new Agosta 90B submarines were each fitted with a 200 kilowatt MESMA system for increasing submerged endurance by a factor of three to five at a speed of 4 knots. The Agosta 90B's performance remains the same in all other respects, except that the length increases from 67m to 76m and submerged displacement from 1,760t to 2,050t.