Here’s What You Need to Remember: 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.

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.