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By comparison, AIP powered submarines have generally cost between $200 and $600 million, meaning a country could easily buy three or four medium-sized AIP submarines instead of one nuclear attack submarine. Bear in mind, however, that the AIP submarines are mostly small or medium sized vessels with crews of around 30 and 60 respectively, while nuclear submarines are often larger with crews of 100 or more.  They may also have heavier armament, such as Vertical Launch Systems, when compared to most AIP powered vessels.

Nuclear-powered submarines have traditionally held a decisive edge in endurance, stealth and speed over cheaper diesel submarines. However, new Air Independent Propulsion (AIP) technology has significantly narrowed the performance gap on a new generation of submarines that cost a fraction of the price of a nuclear-powered boat.

A conventional submarine’s diesel engine generates electricity which can be used to drive the propeller and power its systems. The problem is that such a combustion engine is inherently quite noisy and runs on air—a commodity in limited supply on an underwater vehicle. Thus, diesel-powered submarines must surface frequently to recharge their batteries.

The first nuclear-powered submarines were brought into service in the 1950s. Nuclear reactors are quieter, don’t consume air, and produce greater power output, allowing nuclear submarines to remain submerged for months instead of days while traveling at higher speeds under water.  

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These advantages led the U.S. Navy to phase out its diesel boats in favor of an all-nuclear powered submarine fleet.  However, most other navies have retained at least some diesel submarines because of their much lower cost and complexity.

In the 1990s, submarines powered by Air Independent Propulsion (AIP) technology entered operational use. Though the concept dated back to the 19th century and had been tested in a few prototype vessels, it was left to Sweden to deploy the first operational AIP-powered submarine, the Gotland-class, which proved to be stealthy and relatively long enduring. The 60-meter long Gotlands are powered by a Stirling-cycle engine, a heat engine consuming a combination of liquid oxygen and diesel fuel.

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Since then, AIP powered-submarines have proliferated across the world using three different types of engines, with nearly 60 operational today in fifteen countries. Around fifty more are on order or being constructed.

China has 15 Stirling-powered Yuan-class Type 039A submarines with 20 more planned, as well as a single large Type 032 missile submarine that can fire ballistic missiles. Japan for her part has eight medium-sized Soryu class submarines that also use Stirling engines, with 15 more planned for or under construction. The Swedes, for their part, have developed  four different classes of Stirling-powered submarines.

Germany has also built dozens of AIP powered submarines, most notably the small Type 212 and 214, and has exported them across the globe. The German boats all use electro-catalytic fuel cells, a generally more efficient and quiet technology than the Stirling, though also more complex and expensive. Other countries intending to build fuel-cell powered submarines include Spain (the S-80), India (the Kalvari-class) and Russia (the Lada-class).

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Finally, France has designed several subs using closed-cycle steam turbine called MESMA.   Three upgraded Agosta-90b class subs with MESMA engines serve in the Pakistani Navy.

Nuclear vs. AIP: Who Wins?:

Broadly speaking, how do AIP vessels compare in performance to nuclear submarines?  Let’s consider the costs and benefits in terms of stealth, endurance, speed and cost.

Stealth:

Nuclear powered submarines have become very quiet—at least an order of magnitude quieter than a diesel submarine with its engine running.  In fact, nuclear-powered submarines may be unable to detect each other using passive sonar, as evidenced by the 2009 collision of a British and French nuclear ballistic missile submarines, both oblivious to the presence of the other.

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However, there’s reason to believe that AIP submarines can, if properly designed, swim underwater even more quietly. The hydraulics in a nuclear reactor produce noise as they pump coolant liquid, while an AIP’s submarine’s engines are virtually silent. Diesel-powered submarines can also approach this level of quietness while running on battery power, but can only do so for a few hours whereas an AIP submarine can keep it up for days.

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