nuclear-news

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.

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).

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:…..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. ….

Endurance: Nuclear submarines can operate underwater for three or four months at a time and cross oceans with ease. While some conventional submarines can handle the distance, none have comparable underwater endurance.

AIP submarines have narrowed the gap, however……. new AIP powered vessels only need to surface every two to four weeks depending on type. (Some sources make the unconfirmed claim that the German Type 214 can even last more than 2 months.) ….

Nuclear submarines still have a clear advantage in endurance over AIP boats, particularly on the long-distance patrols.  However, for countries like Japan, Germany and China that mostly operate close to friendly shores, extreme endurance may be a lower priority.

Speed: Speed remains an undisputed strength of nuclear-powered submarines. U.S. attack submarine may be able to sustain speeds of more than 35 miles per hour while submerged. By comparison, the German Type 214’s maximum submerged speed of 23 miles per hour is typical of AIP submarines.

Obviously, high maximum speed grants advantages in both strategic mobility and tactical agility.  However, it should be kept in mind that even nuclear submarines rarely operate at maximum speed because of the additional noise produced.

On the other hand, an AIP submarine is likely to move at especially slow speeds when cruising sustainably using AIP compared to diesel or nuclear submarines. ….

Cost:   Who would have guessed nuclear reactors are incredibly expensive?  The contemporary U.S. Virginia class attack submarine costs $2.6 billion dollars, and the earlier Los Angeles class before it around $2 billion in inflation-adjusted dollars.  Mid-life nuclear refueling costs add millions more.

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. ……

Key point: Air Independent Propulsion technology has significantly narrowed the performance gap on a new generation of submarines

While AIP vessels may not be able to do everything a nuclear submarine can, having a larger fleet of submarines would be very useful in hunting opposing ships and submarines for control of the seas. Nor would it be impossible to deploy larger AIP powered submarines; China has already deployed one, and France is marketing a cheaper AIP-powered version of the Barracuda-class nuclear attack submarine.

It is no surprise that navies that operate largely around coastal waters are turning to cheap AIP submarines. as their disadvantage are not as relevant when friendly ports are close at hand.  The trade off in range and endurance is more problematic for the U.S. Navy, which operates across the breadth of the Atlantic, Pacific and Indian Oceans. This may explain why the U.S. Navy has shown little inclination to return to non-nuclear submarines. However, AIP submarines operating from forward bases would represent a very cost-effective and stealthy means to expand the Navy’s sea-control mission.

Sébastien Roblin holds a Master’s Degree in Conflict Resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is