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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 "SSI" to designate boats powered by AIP, while retaining "SSK" for classic diesel-electric hunter-killers.^[1]

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 particular 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 on station underwater for longer than a more conventionally propelled submarine could. A typical conventional power plant will provide 3 megawatts maximum, and an AIP source around a tenth of that. A nuclear submarine's propulsion plant is usually much greater than 20 megawatts.

Contents 1 Internal oxygen supply 1.1 History 1.2 Closed cycle diesel engines 1.3 Closed cycle steam turbines 1.4 Stirling cycle engines 2 Fuel cells 3 Nuclear power 4 Production Non-Nuclear AIP Submarines 5 References 6 External links

During the 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 hydrogen/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 was highly reactive when it came into contact with various metals, was volatile and submarines suffered from 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 succeeded in developing a nuclear reactor small enough to be installed in a submarine.

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.

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 gases. As there are no exhaust gases available when the engine is started, initially argon is used.

The Soviet Union invested heavily in this technology, developing the small 650 ton Quebec-class submarine of which thirty examples were built between 1953 and 1956. These had three diesel engines—two conventional ones and one closed cycle engine which used liquid oxygen. They had a poor safety record, for instance the M-256 was lost following an explosion and fire. They were sometimes sarcastically 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.

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 compressed oxygen.

The Swedish shipbuilder Kockums has constructed three Gotland class submarines for the Swedish Navy which are fitted with auxiliary Stirling engine which burn liquid oxygen and diesel fuel driving 75 kilowatt generators for either propulsion or charging batteries. The AIP endurance of the 1,500 ton 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 the 5th December 2007 and will be delivered to the navy in March 2009.

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.^[2]

After the success of Howaldtswerke Deutsche Werft AG's with regards to its export activities several builders have answered the potential by a development of own fuel-cell auxiliary units for submarines but until today^[clarify] no other shipyard has a contract for a submarine equipped with this technology.

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 happen to 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 has leased nuclear powered submarines from Russia in the past and plans to acquire two used Akula class submarines from Russia to gain experience and build their own class in future. 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 as an auxiliary power source (styled a "nuclear battery") to improve the under-ice capability of Canadian submarines.

See also: Nuclear marine propulsion

As of 2006, three nations offer non-nuclear AIP submarines for export:

• the French-Spanish Scorpène-class submarine (MESMA)
• the German Type 209-1400mod (Fuel cell)
• the German Type 212 submarine (Fuel cell) of the German Navy
• the German Type 214 (Fuel cell)
• the Russian Project 1650 Амур

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

• 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