A conventional, non-nuclear submarine (SSK) has only two power sources: a battery for submerged,covert operations and, a diesel generator for surface transit (or snorting), and battery charging. The battery on the SSK tends to be bigger than that on the SSN, typically storing 2 to 4 times the energy and so providing reasonable submerged endurance, albeit nowhere near that of SSNs. A submarine is vulnerable to detection when surfaced or snorting, so there is a demand for increasedsubmerged endurance, particularly in the littoral. For SSKs in particular, the development of a stealthy, high performance, air-independent propulsion system is a key requirement.
The A14 Nacken was Sweden's first air-independent propulsion submarine. Stirling engines have been operated by the Swedish Navy in the A14-class submarine Nacken (1000 tons) since 1989. The system produces heat from liquid oxygen in an external combustion chamber that is transferred under a constant and relatively low pressure to the engine via a heat pipe. The external chamber is kept in over pressure to facilitate overboard discharge of exhaust at depths of 300 meters. This indirect heating technology makes it possible to seprarate the engine from an external heating system, which limits risks.
Kockums began testing a prototype Stirling engine in 1983 and carried out tests on a floating platform in 1985. Two Mk 1 engines were then exhaustively trialed aboard the Swedish submarine Näcken. An 8 m long section containing two V4-275 SUB Mk 1 Stirling engines, two large Liquid Oxygen (LOX) tanks, and shock mounts and sound isolation equipment was inserted in the submarine. In the configuration adopted by the Swedish Navy the Stirling AIP plant is intended as an auxiliary low-power, long-endurance system to complement (but not replace) the conventional diesel/electric plant. The main function of the AIP source is to power the boat (with minimum radiated noise) during low- and medium-speed surveillance operations, allowing the main battery to remain fully charged and available for engagements demanding full speed.
In August 1990, the Swedes completed a year long test of the Sterling engine in the 1,030 ton diesel submarine, NACKEN. Submerged patrol time without snorkeling was increased by a factor of 3 to 5. A 1,000 hour running time goal was achieved with the Sterling engine. On the average, NACKEN operated two weeks without snorkeling, compared to the normal five to seven days. During a 20 day patrol, the percentage of time spent snorkeling at periscope depth (50 feet) with snorkel mast raised was reduced from 11% (72 hours) to 0%.
There seems to be a growing acceptance that the use of LOX is necessary for a successful AIP system on manned platforms, and several countries are now considering using it. The Swedish Navy have been assessing its use since 1967. In 1988, a Stirling-based section, which included a small cryogenic tank, was inserted into the submarine Näcken.
This research culminated in the launch of the world's first series-constructed, non-nuclear submarine with an AIP system incorporating LOX at Kockums, in Malmö, in 1995. The new class of submarine, the A 19 Gotland--class (first-of-class laid down 20 November 1992), powered by Stirling engines, was the result of these successful trials.
Sweden leased an air-independent propulsion (AIP) submarine to Denmark. HMS Näcken was delivered in February 2001 and underwent a communications system refit, being undertaken by Swedish shipyard, Kockums. Under the terms of the lease the Danes paid for the submarine in three instalments of DKK28 million (US$3.3m), with a final down payment in 2005 if they decide to retain it. They also had the option of returning the submarine. The arrangement was "a lease to buy or return" that was necessary to maintain capability until Denmark, Sweden and Norway's jointly procured Viking submarines entered service in 2007. The agreement also gave crews a "technological lift" in the operation of more modern systems such as AIP and aid the progress of the Tp 62 torpedo procurement programme. Denmark's existing fleet of five operational submarines dated from the 1960s, and lack the sonar and acoustic capabilities that HMS Näcken can provide.
HMS Näcken returned to the country after spending three years in the service of the Royal Danish Navy (RDN) as HDMS Kronborg. The Danish flag was lowered for the last time on 27 October 2004 after the boat sailed into Karlskrona naval base, southeast Sweden. Denmark had taken over Näcken, partly to boost its small submarine service and partly to gather AIP experience in preparation for the next-generation Viking class, which was to be built in partnership with Sweden. In June 2004, however, the Danish parliament decided to stop operating submarines in Denmark altogether, effectively terminating the country's involvement with the Viking program as well.
The Action Information and Fire-Control System (AI FCS) is a fully integrated system used to acquire, process and display information for tactical evaluation and to form a basis for decisions regarding selected targets and torpedo guidance. The system includes complete hardware and software for controlling wire-guided homing torpedoes. Included also is computer control and monitoring of ship's functions, such as propulsion, steering, depth keeping, trim, storage battery condition and so on.The system has a dual computer configuration, the information being gathered through an extensive data collecting system from the different surveillance and weapon systems as well as from the navigation system. The information is presented to two operators on two separate displays, each of which is equipped with input facilities in the form of special keyboards and trackerballs. After target acquisition the tracking is carried out automatically and relevant fire-control data is calculated continuously. Upon firing, the torpedoes are normally controlled fully automatically with graphic and alphanumeric presentation of all relevant information. The operators at the tactical and fire-control displays can at any time take over or adjust the procedure. By 2005 the Mk 1 was in service with the three 'Näcken' class submarines. The Mk 2 is in service with five 'Sjóormen', four 'Västergötland' and three 'Näcken' class submarines. A Mk 2 has also been installed in a training simulator at Bergas Naval College.
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