Key Point: The advent of cheap, stealthy and long-enduring diesel submarines is yet another factor placing carriers and other expensive surface warships at greater risk when operating close to defended coastlines.

In 2005, USS Ronald Reagan, a newly constructed $6.2 billion dollar aircraft carrier, sank after being hit by multiple torpedoes.

Fortunately, this did not occur in actual combat, but was simulated as part of a war game pitting a carrier task force including numerous antisubmarine escorts against HSMS Gotland, a small Swedish diesel-powered submarine displacing 1,600 tons. Yet despite making multiple attacks runs on the Reagan, the Gotland was never detected.

This outcome was replicated time and time again over two years of war games, with opposing destroyers and nuclear attack submarines succumbing to the stealthy Swedish sub. Naval analyst Norman Polmar said the Gotland “ran rings” around the American carrier task force. Another source claimed U.S. antisubmarine specialists were “demoralized” by the experience.

How was the Gotland able to evade the Reagan’s elaborate antisubmarine defenses involving multiple ships and aircraft employing a multitude of sensors? And even more importantly, how was a relatively cheap submarine costing around $100 million—roughly the cost of a single F-35 stealth fighter today—able to accomplish that? After all, the U.S. Navy decommissioned its last diesel submarine in 1990.

Diesel submarines in the past were limited by the need to operate noisy, air-consuming engines that meant they could remain underwater for only a few days before needing to surface. Naturally, a submarine is most vulnerable, and can be most easily tracked, when surfaced, even when using a snorkel. Submarines powered by nuclear reactors, on the other hand, do not require large air supplies to operate, and can run much more quietly for months at a time underwater—and they can swim faster while at it.

However, the two-hundred-foot-long Swedish Gotland-class submarines, introduced in 1996, were the first to employ an Air Independent Propulsion (AIP) system—in this case, the Stirling engine. A Stirling engine charges the submarine’s seventy-five-kilowatt battery using liquid oxygen.

With the Stirling, a Gotland-class submarine can remain undersea for up to two weeks sustaining an average speed of six miles per hour—or it can expend its battery power to surge up to twenty-three miles per hour. A conventional diesel engine is used for operation on the surface or while employing the snorkel. The Stirling-powered Gotland runs more quietly than even a nuclear-powered sub, which must employ noise-producing coolant pumps in their reactors.

The Gotland class does possess many other features that make it adept at evading detection. It mounts twenty-seven electromagnets designed to counteract its magnetic signature to Magnetic Anomaly Detectors. Its hull benefits from sonar-resistant coatings, while the tower is made of radar-absorbent materials. Machinery on the interior is coated with rubber acoustic-deadening buffers to minimize detectability by sonar. The Gotland is also exceedingly maneuverable thanks to the combined six maneuvering surfaces on its X-shaped rudder and sail, allowing it to operate close to the sea floor and pull off tight turns.

Because the stealthy boat proved the ultimate challenge to U.S. antisubmarine ships in international exercises, the U.S. Navy leased the Gotland and its crew for two entire years to conduct antisubmarine exercises. The results convinced the U.S. Navy its undersea sensors simply were not up to dealing with the stealthy AIP boats.

However, the Gotland was merely the first of many AIP-powered submarine designs—some with twice the underwater endurance. And Sweden is by no means the only country to be fielding them.

China has two diesel submarine types using Stirling engines. Fifteen of the earlier Type 039A Yuan class have been built in four different variants, with more than twenty more planned or already under construction. Beijing also has a single Type 032 Qing-class vessel that can remain underwater for thirty days. It believed to be the largest operational diesel submarine in the world, and boasts seven Vertical Launch System cells capable of firing off cruise missiles and ballistic missiles.

Russia debuted with the experimental Lada-class Sankt Peterburg, which uses hydrogen fuel cells for power. It is an evolution of its widely produced Kilo-class submarine. However, sea trials found that the cells provided only half of the expected output, and the type was not approved for production. However, in 2013 the Russian Navy announced it would produce two heavily redesigned Ladas, the Kronstadt and Velikiye Luki, expected by the end of the decade.

Other producers of AIP diesel submarines include Spain, France, Japan and Germany. These countries have in turn sold them to navies across the world, including to India, Israel, Pakistan and South Korea. Submarines using AIP systems have evolved into larger, more heavily armed and more expensive types, including the German Dolphin-class and the French Scorpene-class submarines.

The U.S. Navy has no intention to field diesel submarines again, however, preferring to stick to nuclear submarines that cost multiple billions of dollars. It’s tempting to see that as the Pentagon choosing once again a more expensive weapon system over a vastly more cost-efficient alternative. It’s not quite that simple, however.

Diesel submarines are ideal for patrolling close to friendly shores. But U.S. subs off Asia and Europe need to travel thousands of miles just to get there, and then remain deployed for months at a time. A diesel submarine may be able to traverse that distance—but it would then require frequent refueling at sea to complete a long deployment.

Remember the Gotland? It was shipped back to Sweden on a mobile dry dock rather than making the journey on its own power.

Though the new AIP-equipped diesel subs may be able to go weeks without surfacing, that’s still not as good as going months without having to do so. And furthermore, a diesel submarine—with or without AIP—can’t sustain high underwater speeds for very long, unlike a nuclear submarine. A diesel sub will be most effective when ambushing a hostile fleet whose position has already been “cued” by friendly intelligence assets. However, the slow, sustainable underwater speed of AIP-powered diesel submarines make them less than ideal for stalking prey over vast expanses of water.

These limitations don’t pose a problem to diesel subs operating relatively close to friendly bases, defending littoral waters. But while diesel submarines may be great while operating close to home—the U.S. Navy usually doesn’t.

Still, the fact that one could build or acquire three or four diesel submarines costing $500 to $800 million each for the price of a single nuclear submarine gives them undeniable appeal. Proponents argue that the United States could forward deploy diesel subs to bases in allied nations, without facing the political constraints posed by nuclear submarines. Furthermore, advanced diesel submarines might serve as a good counter to an adversary’s stealthy sub fleet.

However, the U.S. Navy is more interested in pursuing the development of unmanned drone submarines. Meanwhile, China is working on long-enduring AIP systems using lithium-ion batteries, and France is developing a new large AIP-equipped diesel submarine version of its Barracuda-class nuclear attack submarine.

The advent of cheap, stealthy and long-enduring diesel submarines is yet another factor placing carriers and other expensive surface warships at greater risk when operating close to defended coastlines. Diesel submarines benefitting from AIP will serve as a deadly and cost-effective means of defending littoral waters, though whether they will can carve out a role for themselves in blue water naval forces operating far from home is less clear.

Sébastien Roblin writes on the technical, historical and political aspects of international security and conflict for publications including The National Interest, NBC News, Forbes.com and War is Boring. He holds a Master’s degree from Georgetown University and served with the Peace Corps in China. You can follow his articles on Twitter. This piece was originally featured in November 2016 and is being republished due to reader's interest.

Image: Reuters