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.

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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.

Kockums went on to build three Gotland-class submarines in the late 1990s, the first operational submarines designed with Air-Independent Propulsion systems. The Gotland became famous for sinking a U.S. aircraft carrier in a 2005 military exercise; its characteristics and operational history are further described in this earlier article. Stirling AIP technology has subsequently been incorporated into numerous Japanese and Chinese submarines, while Germany and France developed more expensive fuel-cell and steam-turbine based AIP submarines instead.

Sweden, meanwhile, converted her four late-80s vintage Västergötland diesel-electric submarines between 2003 and 2005 to use Stirling AIP engines—refits which involved cutting the submarines in two and stretching them out from forty-eight to sixty meters! Two of these submarines were re-designated the Södermanland-class, while the other two were sold to Singapore. The latter Archer-class boats are climatized for operations in warmer waters and boast improved navigation and fire control systems.

Enter the A26: Sweden’s Ghostly Super Sub of the Future—On Paper

Sweden intends to retire its Södermanland boats between 2019 and 2022. Since the 1990s, Kockums had been bouncing around a concept for a next-generation AIP submarine designated the A26 to succeed the Gotland-class, but encountered numerous setbacks. Stockholm canceled A26 procurement in 2014, and at one point there was even a raid by the Swedish government attempting to confiscate blueprints from the German parent firm Thyssen-Krupp which was confronted by company security.

Since then, Kockums has been purchased by the Swedish firm Saab. Finally in June 2015, Swedish defense minister Sten Tolgfors announced Stockholm was finally committing to procure two A26s at a price equivalent to $959 million—less than a fifth the unit cost of a nuclear-powered Virginia class submarine of the U.S. Navy.

The A26 has also been marketed abroad at various times to Australia, India, the Netherlands, Norway, and Poland, but so far without success, due to competition from French and German AIP submarine-makers and an apparent reluctance from smaller European states to commit to submarine purchases at this time.

Kockums claims the A26 will achieve new levels of acoustic stealth thanks to a new ‘GHOST’ (Genuine Holistic Stealth) technology which involves acoustic damping plates, flexible rubber mountings for hardware, a less reflective hull with a lower target strength, and degaussing to lower the submarine’s magnetic signature. Supposedly, the A26’s hull will also be unusually resilient to underwater explosions.

The Swedish firm has unveiled concept art depicting a submarine with a ‘chinned’ sail, X-shaped tail fins for greater maneuverability in rocky Baltic waters, and four 533-millimeter torpedo tubes can fire both heavyweight torpedoes, back up by two 400-millimeter tubes, all of which would use wire-guided torpedoes. The vessel’s four Stirling engines apparently allow allowing for higher sustainable underwater cruising speed of 6 to 10 knots.

Kockums has emphasized the new designs’ modularity, which should lower development costs for specialized variants, such as one configuration accommodating up to eighteen Tomahawk land-attack cruise missiles in a vertical launch system. This is a feature likely meant to appeal to Warsaw, which would like cruise-missile equipped submarines.

Another important features is a special ‘multi-mission’ portal for deploying special forces and underwater vehicles, a much-in demand feature for contemporary submarines. Situated between the torpedo tubes in the nose, the portal can also be used to recover the AUV-6 underwater drone, which can be launched from the torpedo tubes. The A26 would typically belly down on the ocean floor when employing the portal—a maneuver which could also aid it in escaping detection.

Kockums is now marketing three different versions of the A26. The ‘medium’ model intended for Swedish service would measure 63-meters long and displace roughly 2,000 tons surfaced. It would typically have a crew of around twenty-six, and a maximum endurance of forty-five days, including eighteen to thirty days (sources differ) submerged, generally sustaining a speed of 10 knots. This endurance, including a typical range of 6,500 miles, should give it capability for operations in the Atlantic Ocean—in contrast to the Gotlands which are not designed for transoceanic deployments.

There is also a smaller 51-meter ‘Pelagic’ version for short-range patrols, and an Extended Range model stretched to eighty meters long and displacing 4,000 tons that might appeal to operators in the Pacific Ocean due to its 10,000-mile range and 50-day endurance.

Sweden’s two A26s should be completed between 2022 and 2024, at which point it will be possible to gauge whether they can meet their ambitious performance parameters. In general, advancements to AIP submarines are allowing countries across the globe to acquire capable short and medium-range submarines at an affordable price.

Sébastien Roblin holds a Master’s Degree in Conflict Resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.

Image: Swedish Stirling engine-powered attack submarine HMS Gotland transits through San Diego Harbor with the Nimitz-class aircraft carrier USS Ronald Reagan (CVN 76) following close behind during the “Sea and Air Parade” held as part of Fleet Week San Diego 2005. Fleet Week San Diego is a three-week tribute to Southern California-area military members and their families. Wikimedia Commons