In 2005, the USS Ronald Reagan met its match in the form of a single, diesel-powered Swedish submarine. During war games, the plucky submarine was able to sneak through the passive sonar defenses of the Ronald Reagan and its entire accompanying group to score a few precious torpedo hits, (virtually) sinking the cruiser and getting away without so much as a scratch.
How? As Real Engineering explains, it’s all thanks to a very old-school engine. Instead of using its diesel to power an internal combustion engine (which is quite loud, what with the explosions and all), the Gotland-class sub instead uses highly-optimized Sterling engines, not unlike what you might find as a desk toy, but considerably more high tech. These exceedingly quiet engines are used to charge batteries which in turn can directly run the engines. The result is a sub that’s quieter than any other diesel, and even quieter than its nuclear cousins which require a constant churn of coolant that can give away their position.

While the virtual defeat was obviously concerning, the Navy leased a Gotland-class sub from the Swedes in order to figure out how to counter its unexpected stealth. And, of course, it’s unlikely we’ll be at real war with Sweden any time soon.
Still, the story goes to show how moving forward in engineering doesn’t always mean creating a new solution whole-cloth. Sometimes you can leap forward by taking a careful, considered look back.

source: popularmechanics.com