Key Point: Back in the 1960s, Sweden had begun developing a modernized version of the Stirling engine, a closed-cycle heat conversion engine first developed in 1818. This was first used to power a car in the 1970s, then the Swedish ship-builder Kockums successfully retrofitted a Stirling engine to power a Swedish Navy A14 submarine Nacken in 1988. Because the Stirling burns diesel fuel using liquid oxygen stored in cryogenic tanks rather than an air-breathing engine, it can quietly cruise underwater at low speeds for weeks at a time without having to surface.
For decades, submarines came in two discrete flavors: traditional diesel-electric submarines that need to surface every day or two to recharge their noisy, air-breathing diesel engines, and nuclear-powered submarines that could quietly hum along under the sea at relatively high speeds for months at a time thanks to their nuclear reactors.
The downside to the nuclear-powered variety, of course, is that they cost many times the price of a comparable diesel submarines and require nuclear propulsion technology, which may not be worth the trouble for a country only interested in defending its coastal waters. A diesel submarine may also run more quietly than a nuclear submarine by turning off its engines and running on batteries—but only for a very short amount of time. Still, there remains a performance gap in stealth and endurance that many countries would like to bridge at an affordable price.
One such country was Sweden, which happens to be in a busy neighborhood opposite to Russian naval bases on the Baltic Sea. Though Sweden is not a member of NATO, Moscow has made clear it might take measures to ‘eliminate the threat,’ as Putin put it, if Stockholm decides to join or support the alliance. After a Soviet Whiskey-class submarine ran aground just six miles away from a Swedish naval base in 1981, Swedish ships opened fire on suspected Soviet submarines on several occasions throughout the rest of the 1980s. More recently, Russia has run an exercise simulating a nuclear attack on Sweden and likely infiltrated Swedish territorial waters with least one submarine in 2014.