Explained: Submarine tech that India wants

On April 30, France’s Naval Group, one of five shortlisted Original Equipment Manufacturers (OEM) for the Navy’s P-75 India (P-75I) project to build six conventional submarines within the country, announced it would not bid for the project. The reason, Naval Group said, was that the Request for Proposal (RFP) “requires that the fuel cell AIP be sea proven, which is not the case for us yet since the French Navy does not use such a propulsion system.”

AIP refers to Air-Independent Propulsion, a technology for conventional — that is, non-nuclear — submarines.

What is the P-75I project?

In June 1999, the Cabinet Committee on Security approved a 30-year plan for the Navy to indigenously build and induct 24 submarines by 2030. In the first phase, two lines of production were to be established — the first, P-75; the second, P-75I. Each line was to produce six submarines.

The contract for P-75 was signed in 2005 with the Naval Group, then known as DCNS, in partnership with Mazagon Dock Shipbuilders Ltd (MDL). The first Kalvari Class (Scorpene Class) submarine under the project was commissioned in December 2017. Another five submarines have been built since; The final one, Vagsheer, was launched last month, and will be commissioned by late 2023.

While P-75 deliveries were delayed, P-75I has seen long delays even before it has kicked off. The first Request for Information was issued in 2008, then again in 2010, but the RFP was issued only in July 2021.

This will be India’s first project under the Strategic Partnership Model — the government will give the contract to an Indian Strategic Partner (SP), which will partner with a foreign OEM to build AIP-powered submarines in the country. MDL and Larsen and Toubro are the two selected SP; the five selected OEMs are Naval Group (France), ThyssenKrupp Marine Systems (Germany), ROE (Russia), Daewoo Shipbuilding and Marine Engineering (South Korea), and Navantia (Spain).

What is the status of the project?

A Navy representative told the Parliamentary Standing Committee on Defense in 2019-20 that “we are slightly behind the curve” on the P-75I. The final bids — one each by the SP in partnership with an OEM — are yet to be finalised. The project faces choppy waters; the Naval Group has already announced it is pulling out, and sources said the Russian and Spanish companies might also not proceed with their bids.

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Among the concerns, as Naval Group said, is the requirement to demonstrate a sea-proven fuel cell AIP. While some manufacturers may have the technology, it may not have been proven at sea yet. Some analysts believe that while the RFP was clear about these conditions, it is possible that the OEMs were expecting certain concessions in the requirements eventually.

Another problem for the OEMs, sources said, is the transfer of technology, which is built into the process under the SP model. Sources believe that the OEMs are unwilling to share all their expertise, especially the niche technologies that they have built.

As things stand, sources said, only ThyssenKrupp Marine Systems and the South Korean company seem to be in the fray. According to experts, even if all goes smoothly here onward, the earliest the first P-75I submarine can be commissioned is around 2032.

Why does the Navy want AIP subs?

Issuing the RFP last year, the government said it “envisages indigenous construction of six modern conventional submarines (including associated shore support, Engineering Support Package, training and spares package) with contemporary equipment, weapons & sensors including Fuel-Cell based AIP (Air Independent) Propulsion Plant), advanced torpedoes, modern missiles and state of the art countermeasure systems.”

AIP, it said, “has a force multiplier effect on lethality of a diesel electric submarine as it enhances the submerged endurance of the boat several folds. Fuel cell-based AIP has merits in performance compared to other technologies.”

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Simply put, AIP technology allows a conventional submarine to remain submerged for much longer than ordinary diesel-electric submarines. All conventional submarines have to surface to run their generators that recharge the batteries that allow the boat to function under water.

However, the more frequently a submarine surfaces, the higher the chances of it being detected. AIP allows a submarine to remain submerged for more than a fortnight, compared to two to three days for diesel-electric boats.

While the six P-75 submarines are diesel-electric, they can be fitted with AIP technology later in their lives. By the time P-75I is completed under the 30-year project, India is projected to have six diesel-electric, six AIP-powered, and six nuclear attack submarines (yet to be built).

India has been working to develop AIP technology indigenously as well. A tableaux of the Defense Research and Development Organization (DRDO) at this year’s Republic Day parade showcased AIP. In March 2021, the Defense Ministry had said DRDO had achieved an “important milestone in the development of Air Independent Propulsion (AIP) System by proving the land-based prototype”. However, experts have certain doubts about DRDO’s AIP prototype, and many fear it may not be ready even by the time the first Kalvari Class submarine comes for refitting starting 2024.

Around 10 countries have developed or are close to building AIP technology, and almost 20 nations have AIP submarines.

What submarines does India have now?

India has 16 conventional diesel-electric submarines, which are classified as SSKs. After the last two Kalvari Class subs are commissioned under P-75, this number will go up to 18. India also has two nuclear ballistic submarines, classified SSBN.

Of the 16 SSKs, four are of Shishumar Class, which were bought and then built in India in collaboration with the Germans starting 1980s; eight are Kilo Class or Sindhughosh Class submarines bought from Russia (including erstwhile USSR) between 1984 and 2000; and four are Kalvari Class built in India at MDL.


DIESEL-ELECTRIC SUBMARINES must come to the surface batteries or close to it to run their generators to recharge the propel them underwater. AIP is a mechanism that allows the batteries to be charged even while the boat is submerged. However, even with AIP, the submarine needs to surface every three weeks or so. According to a 2015 article on the website of the independent strategic and defense think tank Australian Strategic Policy Institute, AIP “offers the possibility of increasing underwater endurance by a factor of up to 3 or 4, which reduces the indiscretion ratio significantly”.

THERE ARE DIFFERENT TYPES of AIP mechanisms available; what India is looking for under the P-75I project is AIP based on fuel cells. These cells convert chemical energy into electrical energy, recharging the batteries of the submarine.

THERE ARE DOWNSIDES TO AIP. The Australian paper says “installing AIP increases the length and weight of the boats, requires pressurised liquid oxygen (LOX) storage on-board and supply for all three technologies”. Also, “MESMA and the Stirling engine have some acoustic noise from moving parts; and the…submarine’s unit cost [increases] by around 10%.”


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