The Buzz

Air-Independent Propulsion Submarines: Stealthy, Cheap and the Future?

Over the past decade, air-independent propulsion (AIP) for submarines has spread rapidly around the world. The technology, which allows conventionally powered submarines to operate without access to outside air, has the potential the shift the balance away from the big nuclear attack submarines (SSNs) that have dominated undersea warfare since the 1950s, and back towards small conventional boats. In global terms, this might again make submarines the great strategic equalizer; small, cheap weapons that can destroy the expensive warships of the world’s most powerful navies. Does this mean that the United States should invest in these kinds of boats? Probably not.


Several navies experimented with AIP during the twentieth century. The earliest work began in World War II in the German and Soviet navies, although none of the experiments produced operationally suitable boats. After the war Britain, the United States and the Soviet Union took advantage of German research to produce their own experimental boats, but nuclear propulsion seemed to offer a more fruitful direction for submarine development.

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In the mid-2000s, converging technological developments enabled several major submarine producers around the world to begin to develop practical AIP systems. France, Germany, Japan, Sweden and China all laid down AIP-capable boats, in some cases exporting those submarines to customers around the world.


AIP systems allow conventional submarines to recharge their batteries without surfacing for air, which enables them to remain underwater for extend periods of time and not expose themselves to detection. Three main types of AIP are found in extant diesel-electric submarines:

Closed Cycle Steam Turbines

Used on French-built submarines, closed cycle steam turbines mimic the energy production process found on nuclear subs (where a nuclear reactor provides heat that turns water into steam) by mixing oxygen and ethanol. This system (dubbed MESMA by the French) is complex, generates a lot of power, but is somewhat less efficient than the alternatives.

Stirling Cycle

A Stirling cycle engine uses diesel to heat a fluid permanently contained in the engine, which in turn drives a piston and generates electricity. The exhaust is then released into the seawater. This is slightly more efficient, and somewhat less complicated, than the French variant, and is used on Japanese, Swedish and Chinese boats.

Fuel Cell

Fuel-cell technology is probably the state of the art in AIP. A fuel cell uses hydrogen and oxygen to generate electricity, and has almost no moving parts. They can generate a lot of energy with minimal waste product, and are very quiet. German-built submarines have successfully taken advantage of fuel cell technology, and the French, Russians and Indians are also moving in this direction.

Procurement Trends

The great thing about AIP is that the technologies involved can be retrofitted into older submarines through the insertion of a hull section. Germany has done this with some boats, including a Type 209, and reports suggest Russia has managed to retrofit a Kilo. Sweden has retrofitted four older boats, and Japan at least one. For navies that want to maximize the lethality of their existing sub flotillas, retrofits can be cost effective.

However, most navies are more interested in new construction. Germany has four types of SSPs under construction for various navies. Newly constructed Type 209s may also have AIP. Sweden has three classes of boats with AIP; the large Japanese Soryus will have AIP, as will the French Scorpenes, French-built Agosta 90Bs (for Pakistan) and Scorpene-inspired Kalvaris (for India). The new Spanish S-80s have AIP, as do the two small Portuguese Tridente boats. Russia’s troubled Lada class has AIP propulsion, and it is expected that the next diesel-electric class (Amur) will also have it. China’s fifteen Type 041 (Yuan) boats have AIP, with another five on the way.

Combat Implications

SSPs can exceed the performance of SSNs under certain conditions. They can take advantage of good endurance and extreme quiet to lay in ambush for approaching enemy vessels, although this requires good intelligence about enemy fleet deployments. They can also conduct short and medium range surveillance of enemy naval forces. In situations that favor small, maneuverable boats (shallow littorals, for example) they can pose a serious combat threat to their larger nuclear cousins.

What This Means for the United States