Photo: Swedish Gotland-class submarine ‘Halland’ ready for new engine

Photo: Saab

Saab, the Swedish company in charge of the Scandinavian country’s submarine construction and modernization, shared photos of the Gotland-class submarine HMS Halland bisected and ready for the installation of a new Stirling Mk3 engine.

The submarine is being upgraded under a contract signed between Sweden’s Defense Material Administration (FMV) and Saab Kockums on 30 June 2015. The two sides also agreed the construction of two completely new submarines for the Swedish Navy.

Saab refers to the new A26 submarine as “the most modern submarine programme on the planet”.

The new submarines build on the successes of their predecessors, however, as Gotland-class submarines proved their mettle against their nuclear-powered counterparts.

In fact, the U.S. Navy was so impressed by HMS Gotland during wargames that they asked Sweden to borrow the submarine for two years. Sweden acceded and HMS Gotland spent two years (2005-2007) in San Diego, California, participating in training with the US Navy, scoring many successes that included “sinking” many U.S. submarines and approaching aircraft carriers without being detected.

One of the reasons the submarines are so effective is the Stirling engine they are equipped with.

The Stirling engine was invented in 1816 and was named for Robert Stirling, who invented the first closed cycle air engine. The engine is also known as a hot air engine as it is powered by the pressure difference in the working fluid at different temperatures. Heat can be attained from a wide range of sources, and the Stirling engine is used in, among other applications, civil solar energy projects.

In Swedish submarines, heat is derived from low-sulphur diesel which is combusted using liquid oxygen stored in tanks on board. The Stirling engines are integrated in complete modules which also contain oxygen and fuel systems, along with a generator.

The air-independent propulsion system enables Swedish submarines to have a long operating time underwater, without having to surface and reveal their location. Submarines equipped solely with diesel-electric propulsion systems are able to remain submerged for a few days before they are required to surface (or break the surface using a snorkel). The Stirling engine increases underwater duration to several weeks.

The technology is cost-effective in comparison to other air-independent technologies, such as fuel cells and nuclear power. And these are both highly complex and difficult to manage.

All Swedish submarines are equipped with Stirling engines. The first engine was installed on the Näcken submarine in 1988 and, in the years that followed, the system has evolved and much operational experience has been gained.

The system is now in its third generation, Stirling Mk3, which boasts increased efficiency and power density, as well as a new control system and HMI.