The Stirling engine was designed in 1816, but the design has some inherent benefits and modern iterations of it are shaping the war fought under the world's oceans.
Swedish submarines have proven themselves in exercises against the U.S. One of their subs successfully lodged a kill against the USS Ronald Reagan as the carrier's protectors stood idly by, incapable of detecting the silent and stealthy Swedish boat. Oddly, the Swedish forces succeeded while using an engine based on a 200-year-old design.
The USS Ronald Reagan was sailing with its task force for protection when a single Gotland-class submarine snuck up, simulated killing it, and sailed away without damage.
(U.S. Navy photo by Mass Communication Specialist 1st Class Peter Burghart)
First, a quick background on what engines were available to Sweden when it was looking to upgrade its submarine fleet in the 1980s. They weren't on great terms with the U.S. and they were on worse terms with the Soviets, so getting one of those sweet nuclear submarines that France and England had was unlikely.
Nor was it necessarily the right option for Sweden. Their submarines largely work to protect their home shores. Nuclear boats can operate for weeks or months underwater, but they're noisier than diesel subs running on battery power. Sweden needed to prioritize stealth over range.
But diesel subs, while they can run more quietly under the surface, have a severe range problem. Patrols entirely underwater are measured in days, and surfacing in the modern world was getting riskier by the day as satellites kept popping up in space, potentially allowing the U.S. and Soviet Union to spot diesel subs when they came up for air.
So, the Swedish government took a look at an engine originally patented in 1816 as the "Stirling Hot Air Engine." Stirling engines, as simply as we can put it, rely on the changes in pressure of a fluid as it is heated and cooled to drive engine movement.
That probably sounded like gobbledygook, but the important aspects of a Stirling engine for submarine development are simple enough.
- They can work with any fuel or heat source.
- They generate very little vibration or noise.
- They're very efficient, achieving efficiency rates as high as 50 percent while gas and diesel engines are typically 30-45 percent efficient.
An officer from the HMS Gotland watches the crew of a U.S. patrol plane track his sub during war games near Sweden in 2017.
(U.S. Navy photo by Lt. Brian O'Bannon)
Sweden tested a Stirling engine design in a French research vessel in the 1980s and, when it worked well, they modified an older submarine to work with the new engine design. Successes there led to the construction of three brand-new submarines, all with the Stirling engine.
And it's easy to see why the Swedes chose it once the technology was proven. Their Stirling engines are capable of air-independent propulsion, meaning the engines can run and charge the batteries while the sub is completely submerged. So, the boats have a underwater mission endurance measured in weeks instead of days.
But they're still stealthy, much more quiet than nuclear subs, which must constantly pump coolant over their reactors to prevent meltdowns.
The HMS Gotland sails with other NATO ships during exercise Dynamic Mongoose off the coast of Norway in 2015.
(U.S. Navy photo by Mass Communication Specialist 2nd Class Amanda S. Kitchner)
So much more stealthy, in fact, that when a single Swedish Gotland-class submarine was tasked during war games to attack the USS Ronald Reagan, it was able to slip undetected past the passive sonars of the carriers, simulate firing its torpedoes, and then slip away.
The sub did so well that the U.S. leased it for a year so they could develop tactics and techniques to defeat it. After all, while Sweden may have the only subs with the Stirling engine, that won't last forever. And the thing that makes them so stealthy isn't restricted to the Stirling design; any air-independent propulsion system could get the same stealthy results.
Shortened to AIP, these are any power systems for a submarine that doesn't require outside oxygen while generating power, and navies are testing everything from diesel to fuel cells to make their own stealthy subs. China claims to have AIP subs in the water, and there is speculation that a future Russian upgrade to the Lada-class will introduce the technology (as of August 2017, the Lada-class did not feature AIP).
So, for the U.S., getting a chance to test their mettle against them could save lives in a future war. And, if it saves a carrier, that alone would save thousands of lives and preserve tons of firepower.
For its part, Sweden is ordering two new submarines in their Type A26 program that will also feature Stirling engines, hopefully providing the stealth necessary to catch Russian subs next time their waters are violated. Surprisingly, these advanced subs are also cheap. The bill to develop and build two A26s and provide the midlife upgrades for two Gotland-Class submarines is less than $1 billion USD.
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