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 snort mast 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.

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 3000 kilowatts maximum , and an AIP source around a tenth of that. A nuclear submarine's propulsion plant is around 20 megawatts.

Internal oxygen supply

History

During the Second World War the German firm Walter experimented with submarines that used concentrated hydrogen peroxide as their source of oxygen underwater. These used steam turbines which used 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 savlaged and recomissioned 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 countries 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.

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 they usually dilute the oxygen with recycled exhaust gases. As there are no exhaust gases to use 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.

Currently the German U-1 boat is fitted with a 3000-horsepower experimental unit.

Closed cycle Steam Turbines

The French (Module d'Energie Sous-Marine Autonome) system is the only one which is in current use, and is essentially a modified version of their nuclear propulsion system with heat being generated by ethanol and compressed oxygen.

Stirling cycle engines

The Swedish shipbuilder Kockums Naval Systems has constructed three submarines for the Swedish Navy which are futted with auxilary Stirling-cycle engines 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.

Fuel Cells

Several builders offer fuel-cell auxilary units for submarines. Siemens has developed a 30-50 kilowatt unit, and nine of these are incorporated into Howaldtswerke Deutsche Werft AG's 1,830-ton Type 212 submarine. The enhanced 1700t Type 214 uses two 120kW modules, also from Siemens [1].

Nuclear power

Nuclear reactors have been used for fifty years to power submarines, the first being USS Nautilus. Three countries, the United States, France and Great Britain only operate nuclear powered submarines. 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 auxilary 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 auxilary power source (styled a "nuclear battery") to improve the under-ice capability of Canadian submarines.

References

• Underseas Warfare article
• Seapower article
• Air Independent Propulsion
• Auxilary nuclear reactor for Canadian submarines