Thyssenkruppe partners MDL for potential Indian Navy submarine order

6 submarines, powered by air-independent propulsion, are planned to be built under Rs 45,000 cr programme

Thyssenkruppe Marine Systems (TKMS), the German shipbuilder, signed an agreement on Wednesday with India’s premier defence shipyard, Mazagon Dock Limited (MDL), to cooperate in building six state-of-the-art submarines for the Indian Navy.

The submarines, powered by air-independent propulsion (AIP), are planned to be built under a Rs 45,000 crore programme called Project 75-I under the “strategic partner” (SP) acquisition category. It requires qualified Indian firms to build a weapons platform in partnership with a foreign original equipment manufacturer (OEM) chosen by the defence ministry.

While conventional diesel-electric submarines can operate underwater silently for up to 48 hours, AIP-driven submarines remain underwater for up to two weeks before they must surface for battery charging.

That gives AIP-driven submarines significant operational advantages over legacy diesel-electric submarines that are easily detected by enemy radar for they have masts or snorkels protruding out of the water while they are surfaced.

None of today's AIP power plants are ideal; each has its merits and drawbacks. Besides, each navy operates in different geographical zones and hydrological conditions and has varied levels of crew training and conditions at naval bases.

Experience has shown, however, that there are only two types of practicable AIP systems: “fuel cell AIP” and “Stirling AIP”. Another fuel cell AIP—the closed cycle steam turbine “Module d'Energie Sous-Marine Autonome (MESMA)—has shown its practicality but has remained a niche product.

Fuel cell AIP

Fuel cell-based AIP generates power through the reverse electrolysis of oxygen and hydrogen. In this process, the two elements chemically combine, generating electricity to charge the submarine’s batteries.

This process does not need air, but requires storage of highly inflammable hydrogen on board.

Fuel cell AIP systems are associated with German submarines of Class 212A and Class 214, and will power the Project 75-I submarines too.

Other fuel cell AIPs are being developed in other countries, such as Russia (alkaline fuel cells) and India (phosphoric-acid fuel cells).

German designers developed an AIP system with low noise, low temperature, and ordinary water at the process output. The system is complex, expensive and bulky. Class 212A submarines are three times larger than previous German Navy Class 206 submarines.

In addition, fuel cell-based AIP systems require meticulous crew training and dedicated infrastructure.

The development of this AIP system took 25 years, but it was supported by concerted European efforts to bring hydrogen energy into every field, notably in the automotive industry.

A key problem in fuel cell development was to master hydrogen storage. Hydrogen metal hydride storage used in Class 212A and 214 submarines achieves high safety, but also to a heavy storage system and, thence, to an unacceptably large submarine.

This was acceptable to the German Navy, which designed its submarines for operations in the Baltic and Northern Seas. The submerged endurance of Class 212А submarine is sufficient for these theatres and remained the same when the Italian Navy joined the 212A programme since the German and Italian submarines operate in the areas controlled by allied surface and air forces.

The navies that have bought Class 214 submarines—Portugal, Greece and Turkey—rely on an already engineered hydrogen infrastructure, which they further expand having joined the users of the Class 214 submarine.

In South Asia, the situation is quite different. The need is for a submarine with a submerged endurance of over two weeks, while the deployment of expensive hydrogen infrastructure remains and might take dozens of years.

Stirling

The second type of AIP, based on the Stirling engine, is the first modern combat-ready system. It is a simple system that uses diesel fuel (typical for the submarine) and liquid oxygen, discharging the plant overboard at small and medium depths.

Low-power Stirling engines are much quieter than main diesel generators of submarines, providing considerable tactical gains. It took less than 15 years for the creation of this system from concept to implementation.

Although this system lacks stealth, it has proved relatively good in a small area, shallow depths, complex hydrology and heavy traffic, which require a submarine’s acoustic signature to be minimised. The Stirling engine is compact enough for small submarines, which renders it ideal for countries like Sweden and Singapore since conditions around the Strait of Malacca are similar to the Baltic Sea.

Meanwhile Japanese designers have extensively enhanced AIP system capabilities. Japanese submarines have four-engine AIP systems, rather than two, as is the case with more compact Swedish submarines. Despite Japan’s developed defence industry, the Japan Maritime Self Defence Force preferred licensing the existing system, developed by Sweden for different conditions instead of developing the indigenous one.

India’s AIP system

The Defence R&D Organisation’s (DRDO’s) AIP system relies on the innovative Phosphoric Acid Fuel Cell (PAFC) technology.

This PAFC process is more rugged, tolerant of fuel impurities, offers longer life and efficiency, and is much safer, since it generates hydrogen on board, rather than requiring the storage of hazardous hydrogen.

Having tested its AIP on a land-based prototype in March 2021, the DRDO is now required to evolve this into a “marinised” AIP system, which can fit into an actual submarine and operate underwater in live conditions.