The Swedish defense industry has once again demonstrated its prowess with the modernization of the HMS Halland submarine. The Swedish defense contractor Saab completed the upgrade, retaining the submarine’s Stirling engines, which provide it with unique operational capabilities. The official inauguration took place on February 13 at the Naval Shipyard in Karlskrona, Sweden, highlighting the importance of restoring underwater construction sites to Swedish property.

The HMS Halland submarine was built and launched in 1996 and is part of the Gotland submarine class, which includes two other submarines: the Gotland and the Upland, both built in 1995 and launched in 1996. The other two submarines, built earlier, were already modernized, and the Halland was the last of the three. The contract for its modernization was signed in March 2022.

The Halland submarine, like its classmates, is an electric diesel submarine equipped with Stirling engines to benefit from silent propulsion. This technology is particularly advantageous for submarines, as it allows for stealthier operations, crucial for military and intelligence missions. The use of Stirling engines in submarines is a pioneering effort by Saab Kockums, the shipyard responsible for the modernization.

The Stirling engine operates on a simple principle: a cylinder containing gas is heated from any heat source, including electric. Once the cylinder walls are heated, the gas expands and propels a piston connected to a lever that accumulates the final rotary movement. As the piston moves, the gas is evacuated into the circuit, and a new amount of gas enters the cylinder. The process repeats, maintaining the heat within the cylinder without being evacuated. There is no spark or combustion, making the engine silent in operation. Additionally, the engine can theoretically operate in reverse, recovering rotary energy and compressing gas in the cylinder, creating heat by pressure and acting similarly to a heat pump.

“The same gas is constantly recirculated, and the heat is maintained by the cylinder, without being evacuated and used. There is no spark and combustion, and in the absence of these explosions, the engine is silent in its operation.”

Saab Kockums

On the surface, the submarine is propelled by electric motors powered by two MTU diesel engines, which also charge the batteries. Once submerged, it can continue battery-based propulsion or use the two Stirling engines, each producing 75 kW of electric power, to directly supply or charge the batteries. This dual-propulsion system enhances the submarine’s stealth and endurance capabilities.

The maximum submarine speed is 11 knots (20 km/h) on the surface and 20 knots (37 km/h) underwater if the engines are powered by batteries. If directly fed by the energy produced by Stirling engines, the speed is 5 knots (9.3 km/h). This speed reduction is a trade-off for the increased stealth and endurance provided by the Stirling engines.

The HMS Halland’s underwater autonomy can last for several weeks, a feat impossible for typical electric diesel submarines. This Swedish submarine model is considered one of the most silent and hard-to-detect in the world. Its construction allows for exceptional maneuverability, with tightly executed turns, and its magnetic signature is neutralized by 27 electromagnets positioned to screen even the smallest magnetic fields emitted by interior devices to the outside.

The modernization of the Halland submarine was carried out at the Saab Kockums shipyard, which has its main headquarters in Malmo but the largest building halls in Karlskrona. This yard pioneered the use of Stirling engines in submarines, a fact that saved it from closure during critical periods. The recent history of this site is tumultuous, serving as a lesson for many countries, including the United States, about the strategic importance of maintaining domestic defense capabilities.

The shipyard had various owners and was saved in critical periods. In 1999, it was sold to the German company HDW, a ship and submarine builder. In 2005, it became part of Thyssen Krupp. The Swedish authorities insisted on developing a new modern submarine class, code-named A26, but the German owners considered it too risky and costly. The project was stalled for years, leading to a strategic re-evaluation by Thyssen Krupp, which suggested focusing on smaller submarines. The Swedish authorities were dissatisfied, accusing the Germans of diminishing the yard’s capabilities and eliminating competition.

After the annexation of Crimea in 2014, the Swedish authorities decided to repatriate the shipyard to ensure its strategic defense. In July 2014, Saab signed an agreement to buy back the shipyard from Thyssen Krupp. The Swedish army even entered the factory to seize all plans, sketches, and projects for future submarines, convincing the Germans to sell the yard. This decision proved crucial in the current global context.

After the shipyard returned to Swedish property, the Swedish Defense Forces placed two orders for submarines in the A26 class, now called the Blekinge class. These submarines are currently under construction, with the first, Blekingine, expected to be operational in 2027, and the second, Skane, in 2028. The modernization of the Halland includes technologies implemented on the Blekinge-class submarines, comprising over 20 systems, including sensors and control systems.

The modernization of the Halland submarine complements the modernization of current Swedish submarines and significantly strengthens the defense capacity of Sweden. If critical decisions had not been taken in 2014, the Swedish shipyard could have been limited to constructing small, strategically irrelevant submarines. This lesson is particularly relevant for the United States, which faces similar challenges in maintaining domestic defense capabilities.

In the United States, the Navy has been exploring similar advancements in submarine technology. The Virginia-class submarines, for example, have been equipped with advanced propulsion systems and stealth technologies. The modernization of the HMS Halland serves as a reminder of the importance of continuous innovation and strategic investment in defense capabilities. As the global security landscape evolves, countries must prioritize the development and maintenance of cutting-edge technologies to ensure their defense readiness.