Air-independent propulsion

Air-independent propulsion

Air-independent propulsion (AIP) is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing the diesel-electric propulsion system of non-nuclear vessels. The United States Navy uses the hull classification symbol "SSP" to designate boats powered by AIP, while retaining "SS" for classic diesel-electric attack submarines. [United States Navy Glossary of Naval Ship Terms (GNST). SSI is sometimes used, but SSP has been declared the preferred term by the USN. SSK (ASW Submarine) as a designator for classic diesel-electric submarines was retired by the USN in the 1950s, but continues to be used colloquially by the USN and formally by navies of the British Commonwealth and corporations such as Jane's Information Group.]

AIP is usually implemented as an auxiliary source. Most such systems generate electricity which in turn drives an electric motor for propulsion or recharging the boat's batteries. The submarine's electrical system is also used to provide "hotel services"—ventilation, lighting, heating etc—although this consumes a small amount of power compared to that required for propulsion.

A benefit of this approach is that it can be retrofitted into existing submarine hulls by inserting an additional hull section. AIP does not normally provide the endurance or power to replace the atmospheric dependent propulsion, but allows it to remain submerged longer than a more conventionally propelled submarine. A typical conventional power plant will provide 3 megawatts maximum, and an AIP source around 10% of that. A nuclear submarine's propulsion plant is usually much greater than 20 megawatts.

Internal oxygen supply


In 1867 Narcís Monturiol i Estarriol successfully developed an early form of anaerobic air independent propulsion. In 1908 the Imperial Russian Navy launched the Russian submarine Pochtovy which used a gasoline engine fed with compressed air and exhausted under water.

During World War II the German firm Walter experimented with submarines that used concentrated hydrogen peroxide as their source of oxygen underwater. These used steam turbines, employing steam heated by burning diesel fuel in the steam/oxygen atmosphere created by the decomposition of hydrogen peroxide by a potassium permanganate catalyst.

Several experimental boats were produced, and one, U-1407, which had been scuttled at the end of the war, was salvaged and recommissioned into the Royal Navy as HMS "Meteorite". The British built two improved models in the late 1950s, HMS "Explorer", and HMS "Excalibur".

The Soviet Union also experimented with the technology. Hydrogen peroxide was eventually abandoned since it is highly reactive when in contact with various metals, is volatile, and submarines had a high rate of consumption. Both the British and the Soviets, the only countries known to be experimenting with it, abandoned it when the United States developed a nuclear reactor small enough for submarine propulsion.

It was retained for propelling torpedoes by the British and the Soviet Union, although hastily abandoned by the former following the HMS "Sidon" tragedy. Both this and the loss of the Russian Submarine "Kursk" were due to accidents involving hydrogen peroxide propelled torpedoes.

Closed cycle diesel engines

This technology uses a submarine diesel engine which can be operated conventionally on the surface, but which can also be provided with oxidant, usually stored as liquid oxygen, when submerged. Since the metal of an engine will burn in pure oxygen, the oxygen is usually diluted with recycled exhaust gas. As there is no exhaust gas upon starting, argon is used.

The Soviet Union invested heavily in this technology, developing the small 650 ton "Quebec"-class submarine of which thirty were built between 1953 and 1956. These had three diesel engines—two were conventional and one was closed cycle using liquid oxygen. They had a poor safety record; the "M-256" was lost following an explosion and fire. They were sometimes nicknamed "cigarette lighters". The last was scrapped in the early 1970s.

The German Navy's former Type 205 submarine U1 was fitted with an experimental 3000 horsepower (2.2 MW) unit.

Closed cycle steam turbines

The French MESMA (Module d'Energie Sous-Marine Autonome) system is being offered by the French shipyard DCN. Currently a MESMA Section is retrofitted to an Agosta 90B of the Pakistan Navy. It is essentially a modified version of their nuclear propulsion system with heat being generated by ethanol and oxygen.

tirling cycle engines

The Swedish shipbuilder Kockums has constructed three Gotland class submarines for the Swedish Navy which are fitted with an auxiliary Stirling engine which uses liquid oxygen and diesel fuel to drive 75 kilowatt generators for either propulsion or charging batteries. The AIP endurance of the 1,500 tonne boats is around 14 days at five knots (9 km/h).

Kockums has also delivered Stirling engines to Japan. The new Japanese submarines will all be equipped with Stirling engines. The first submarine, "Sōryū", in the class was launched on 5 December 2007 and will be delivered to the navy in March 2009.

Fuel cells

Siemens has developed a 30-50 kilowatt fuel cell unit. Nine of these units are incorporated into Howaldtswerke Deutsche Werft AG's 1,830t submarine "U31", lead ship for the Type 212A class of the German Navy. The other boats of this class and HDW's AIP equipped export submarines (Type 209 mod and Type 214) use two 120 kW modules, also from Siemens. [ [ Naval Technology - U212/U214 - Attack Submarine ] ]

After the success of Howaldtswerke Deutsche Werft AG's in its export activities, several builders have developed their own fuel-cell auxiliary units for submarines but until todayClarifyme|date=March 2008 no other shipyard has a contract for a submarine so equipped.

Nuclear power

Nuclear reactors have been used for 50 years to power submarines, the first being USS "Nautilus". The United States, France, the United Kingdom, Russia and the Peoples Republic of China are the only countries known to operate nuclear powered submarines. These five countries also have permanent seats on the United Nations Security Council and are the only countries allowed to possess nuclear weapons according to the Nuclear Non-Proliferation Treaty. India is currently constructing a nuclear powered submarine codenamed ATV. India in the past has leased a Charlie class nuclear powered submarine from Russia and plans to acquire two used Akula class submarines which would be used for research purposes. Brazil is also known to research nuclear propulsion for submarine use. However, Air Independent Propulsion is a term normally used in the context of improving the performance of conventionally propelled submarines.

There have nevertheless been suggestions for a reactor as an auxiliary power supply, which does fall into the normal definition of AIP. For example, there has been a proposal to use a small 200 kilowatt reactor for auxiliary power (styled a "nuclear battery") to improve the under-ice capability of Canadian submarines.

Production Non-Nuclear AIP Submarines

As of 2008, some nations have non-nuclear AIP submarines:
* the Chinese Type 041 submarine "Yuan" (Stirling AIP) of the People's Liberation Army Navy
* the French-Spanish "Scorpène"-class submarine (1,700 tonnes) (MESMA)
* the German Type 209-1400mod (1,810 tonnes) (Fuel cell)
* the Italo-German Type 212 submarine (1,830 tonnes) (Fuel cell) of the German Navy and Italian Navy
* the German Type 214 (1,980 tonnes) (Fuel cell)
* the Russian Project 1650 "Амур"
* the Japanese Asashio (2,750 tonnes) (Stirling AIP) of the Japan Maritime Self-Defense Force
* the Japanese Sōryū class submarine (4,200 tonnes) (Stirling AIP) of the Japan Maritime Self-Defense Force
* the Swedish Gotland class submarine (1,450 tonnes) (Stirling AIP) of the Swedish navy
* the Swedish Södermanland class submarine (1,500 tonnes) (Stirling AIP) of the Swedish navy
* The Turkish/German Type 214TN to be co-produced in Turkey, with 80% Turkish systems.Sweden is going to sell its remaining two Västergötland class submarines to the Republic of Singapore Navy after they have been refitted with Stirling AIP systems like the Södermanland class submarines.

Also several shipbuilders offer AIP upgrades for existing submarines:
* German Nordseewerke (Closed-cycle diesel)
* Sweden Kockums (Stirling), owned by German company ThyssenKrupp
* Pakistan Agosta 90 B submarine Hamza Made with cooperation with France


External links

* [ Underseas Warfare article on AIP]
* [ Seapower article]
* [ Auxiliary nuclear reactor for Canadian submarines .PDF]
* [ Siemens fuel cells for submarines .PDF]
* [ Research paper describing Siemens submarine fuel cells .PDF]

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