Key point: 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.

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.  

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.

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:

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.

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.

Diesel and AIP powered submarines have on more than one occasion managed to slip through anti-submarine defenses and sink American aircraft carriers in war games. Of course, such feats have also been performed by nuclear submarines.

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.  While old diesel submarines needed to surface in a matter of hours or a few days at best to recharge batteries, 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.) Of course, surfaced submarines, or even those employing a snorkel, are comparatively easy to detect and attack.

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.  For example, a Gotland class submarine is reduced to just 6 miles per hour if it wishes to remain submerged at maximum endurance—which is simply too slow for long distance transits or traveling with surface ships.  Current AIP technology doesn’t produce enough power for higher speeds, and thus most AIP submarines also come with noisy diesel engines as backup.

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. 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.

Nevertheless, a torpedo or missile from a small submarine can hit just as hard as one fired from a large one, and having three times the number of submarine operating in a given stretch of ocean could increase the likelihood chancing upon an important target, and make it easier to overwhelm anti-submarine defenses.

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 Boring. This first appeared in December 2018.

Image: Reuters.