Insight: What is the way forward for the propulsion systems of conventional submarines?

AIP has served as a reference point for all countries operating non-nuclear submarines for decades. For example, the Netherlands, which operates older diesel-electric submarines equipped with traditional batteries, has long viewed the future adoption of next-generation AIP submarines as the most logical choice for their procurement needs. Recent developments in LIBs, however, have shuffled the deck in the underwater domain, making AIP-equipped submarines a less obvious procurement option.

Air independent propulsion vs full lithium-ion battery configuration

Whether based on fuel cells or Stirling engines, the AIP system provides energy directly to the electric motor that drives the submarine propeller. The main advantage is that the vessel does not have to recharge its batteries and does not need to snorkel at periscope depth to run the diesel engines. Compared to their conventional diesel-electric counterparts, AIP allows submarines to have a very low “indiscretion rate”, as they can stay underwater for more extended periods (around two weeks).

The benefits of AIP, however, come at the detriment of speed, weight and costs. Indeed, a submarine running on AIP is very slow (generally between 5 and 10 knots). On the contrary, submarines can go much faster when powered by batteries. The system is also very expensive to acquire and adds around 300t of extra weight due to extra systems like Liquid Oxygen (LOX) tanks.

The latest technological advancements in LIBs have made conventional submarines equipped with a full LIB configuration a very attractive alternative to AIP technology.

Compared to standard lead-acid batteries, LIBs provide an advantage in terms of speed, range, and charging times. Thanks to their high power output, LIBs allow the submarine to achieve peak speed regardless of the state of the battery charge, bringing significant benefits during evasive manoeuvres. Lithium-ion cells can also be recharged far more quickly than their lead-acid counterparts, thus lowering the submarine's indiscretion rate.

During an early October press briefing at its submarine building facility in Cherbourg, Naval Group revealed that the abovementioned advantages of LIBs had led the company to abandon the AIP solution in favour of a full LIB configuration for its portfolio of non-nuclear platforms. The French manufacturer sees the new technology as the way forward in the submarine market, as testified by the recent achievements in the Netherlands and Indonesia. It is worth noting that even Japan has adopted a similar approach with the last two vessels of the Soryu-class first and with the Taigei-class later.

Naval Group estimates that submarines equipped with a full LIB configuration can now stay submerged for almost seven-to-eight days, depending on the mission profile. According to the company, the advancements made by the automotive industry have been instrumental in enhancing the performance of the batteries, which are now capable of storing much more energy, increasing the submarine’s submerged autonomy.

As shown in the table above, countries such as Germany, Italy, and South Korea are implementing LIBs with AIP, combining the two technologies.

Chasing market opportunities

During the press briefing in Cherbourg, Shephard understood that Naval Group is following up to 13 future submarine programmes worldwide. Although the shipbuilder did not reveal which navies it is in discussion with, based on Shephard Defence Insight data, Shephard estimates the targeted countries are those highlighted in the map below.

According to Shephard Defence Insight data, the 13 procurement programmes have a combined value of US$69 billion and could see 43 submarines built over the next 15 to 20 years.

From a broader perspective of the conventionally powered submarine market, as shown in the graph below, while almost $28 billion will be invested in already awarded programmes globally, it is forecasted that up to $65 billion worth of unawarded programmes remain to be contracted between 2025 and 2035. Canada, Japan and India are believed to be the countries that will drive most of the spending in the next decade or so.

Naval Group’s approach

As anticipated, the French OEM has shifted its focus on the full LIB configuration for its non-nuclear platforms. The first achievements for the new full LIB arrangement have been reached in Indonesia (Evolved Scorpene) and in the Netherlands (Orka-class/Blacksword Barracuda).

Naval Group has been known to be investing in the full LIB system for years, with the shipbuilder now relying on its self-developed Integrated Platform Management System (IPMS) and on the batteries provided by the French company Saft.

The Paris-based shipbuilder plans to target the market of conventionally powered submarines with two designs: the Evolved Scorpene and Blacksword Barracuda.

The Evolved Scorpene is the entry-level platform that sits at the lower end of Naval Group’s portfolio. Adaptable to customers’ specific requirements, the new platform builds on the experience of the legacy Scorpene, whose 14 platforms have been built worldwide.

The Blacksword Barracuda benefits from the technological advancements introduced onboard the Suffren-class (SSN Barracuda) of the French Navy and arises from the ashes of the Attack-class submarines originally intended for Australia.

Compared to the Scorpene, the Blacksword Barracuda offers a greater range and endurance and is intended for countries aiming to operate in highly contested environments far from home.

What is the way forward?

Developments in LIB technology have undoubtedly widened the range of options available to blue-water navies. Indeed, while a few years ago, the AIP technology was largely perceived as the most viable and obvious solution for all those countries aiming to operate non-nuclear submarines, the introduction of LIBs has opened up new market perspectives.

On the one hand, lithium-ion cells are now offering increased benefits to AIP-equipped platforms, allowing them to store more energy and recharge batteries far more quickly than the previous lead-acid solution. On the other hand, LIBs have contributed to the resurgence of conventional diesel-electric submarines as a competitive option, being now more capable than in the past and considerably cheaper than their AIP counterparts.

Thanks to the latest technological developments, the market now offers a range of innovative solutions that can meet most navies' technical and financial needs.

Related Programmes in Defence Insight

Walrus Replacement Program (WRES) (1-4) [Netherlands]

Submarine Procurement (1-2) [Indonesia]

Canadian Patrol Submarine Project (CPSP) [Canada]

Related Equipment in Defence Insight

U212 NFS (Near Future Submarine)

Type 214

Type 212CD