Navy's indigenous AIP system for submarines reaches key milestone

India's home-grown air independent propulsion (AIP) system for the Navy's diesel-electric submarines has reached a key milestone. On Saturday, 24 October, the Setting to Work Phase-2 (STW-2) activity began for the system's energy module at Larsen & Toubro's AM Naik Heavy Engineering Complex at Hazira in Gujarat, the Press Information Bureau and India TV reported.

The energy module was developed by the Naval Materials Research Laboratory (NMRL) of the Defence Research and Development Organisation (DRDO). It is meant to be fitted to the Navy's Kalvari-class submarines, letting them stay underwater far longer without surfacing.

Key points

• Setting to Work Phase-2 of the AIP energy module began on 24 October at L&T Hazira, Gujarat.
• The module was developed by DRDO's Naval Materials Research Laboratory and is based on fuel cell technology using hydrogen.
• Next, the module will be powered up and tested under Navy protocols.
• It is then due to go to Mazagon Dock Shipbuilders for integration on a Kalvari-class submarine in early 2027, India TV reported.
• AIP lets a conventional submarine stay submerged for longer, making it harder to detect.

Who attended

According to India TV and the PIB:

• Rajesh Kumar Singh, Defence Secretary, Secretary of the Department of Defence R&D and DRDO Chairman, joined as chief guest by video.
• Vice Admiral B Sivakumar, the Navy's Chief of Materials, attended at Hazira.
• Dr R Balamuralikrishnan, DRDO's Director General (Naval Systems and Materials), attended at Hazira.

The DRDO chairman said the programme had "culminated into productive reality on the shop floor", and credited the Navy for "testing and validating at every stage of the trials", India TV reported. He said the next phases must meet the Navy's induction timelines while "maintaining the exacting safety culture that hydrogen and fuel cell systems require without compromise".

What AIP does

A conventional diesel-electric submarine runs on batteries when it is underwater. To recharge them, it must come close to the surface and run its diesel engines, which need air. That process, called snorkelling, is when a submarine is most likely to be spotted by aircraft, ships and radar.

An air independent propulsion system lets the submarine generate power underwater without surface air. That means:

• Longer time submerged: a submarine can stay underwater for much longer between snorkelling, often for a week or more instead of a day or two, depending on the system and speed.
• More stealth: less time near the surface means less chance of detection.
• Better patrols: longer, quieter patrols in areas such as the Indian Ocean and the Arabian Sea.

DRDO's system uses fuel cells, which turn hydrogen and oxygen into electricity silently, with water as the by-product. That is one reason the DRDO chairman stressed the safety culture needed for hydrogen and fuel cell systems.

AIP versus nuclear submarines

India operates nuclear-powered submarines too, but conventional submarines remain the backbone of its fleet for coastal and regional operations, and AIP narrows the gap between the two types at a much lower cost.

How the AIP plug is fitted

Fitting AIP to an existing submarine is major surgery. The submarine is cut into two at the shipyard, and a new hull section, often called the AIP plug, containing the energy module, hydrogen and oxygen storage, and power systems, is inserted between the two halves before they are welded back together. This makes the submarine longer and heavier, so its balance, buoyancy and handling all have to be tested again before it returns to service.

That is why the Navy plans to add the system during each submarine's scheduled major refit, when it is already out of service for months, rather than taking boats out of the fleet separately. It is also why each step on land, such as the Setting to Work trials now under way at Hazira, matters so much: problems are far cheaper to fix in a factory than inside a submarine.

Which countries have AIP submarines

AIP technology has been developed by only a few countries, using different methods:

India's DRDO system belongs to the fuel cell family, like Germany's. Several of India's neighbours already operate submarines with imported AIP systems, which is one reason the Navy has pushed for its own.

The Kalvari-class submarines

The Kalvari class is India's version of the French Scorpene design, built by Mazagon Dock Shipbuilders in Mumbai under Project 75. The Navy has six of these submarines. The plan has long been to fit DRDO's AIP to them during their scheduled refits, by cutting each submarine and inserting an AIP "plug" section.

The programme has faced delays. Earlier reports said the AIP plug was not ready in time for the first refits, and later reports expected the first AIP-equipped Kalvari-class submarine by the end of 2026. India TV's report now puts integration on a Kalvari-class submarine in early 2027, after the energy module is tested.

India's wider submarine plans

The AIP upgrade of the Kalvari class is separate from Project 75 India (P-75I), a larger plan to build six new conventional submarines with AIP from the start, in partnership with a foreign designer. The Navy also operates older Russian-origin Kilo-class and German-origin Type 209 submarines, several of which are ageing. Giving the Kalvari class longer underwater endurance helps bridge the gap while new submarines are built.

Why it matters

• Self-reliance: Only a handful of countries have developed their own AIP systems. A working indigenous system would reduce India's dependence on foreign suppliers for a key submarine technology.
• Cost and maintenance: India TV reported that an indigenous system could cut costs, simplify maintenance and open the way for use on more submarines, and possibly exports.
• Ocean security: With more Chinese naval activity in the Indian Ocean, including submarines, longer underwater endurance is a significant boost for the Indian Navy's ability to patrol and deter.
• Industry: The work at L&T Hazira shows Indian private industry's growing role in complex defence manufacturing.

Each successful step also builds the skills of Indian scientists, shipyards and suppliers in a technology that few countries have mastered, which will matter for future submarine programmes.

What happens next

The energy module will now be energised to produce power and put through tests under Navy protocols, India TV reported. If those go well, it will move to Mazagon Dock for integration into a Kalvari-class submarine in early 2027. The main challenge, the report said, is to keep to the induction timeline without compromising safety.

A submarine that does not need to surface is a submarine that is hard to find. India is now a step closer to giving its conventional fleet that edge.