DRDO's Indigenous AIP Energy Module Enters Phase-2 Setting-to-Work at L&T Hazira, With Kalvari-Class Integration Targeted for Early 2027

India's long effort to give its conventional submarines the ability to stay underwater for much longer has reached the shipyard floor. On 24 October 2026, the Setting to Work Phase-2 (STW-2) activity began for the Energy Module of DRDO's indigenous Air Independent Propulsion (AIP) system. The work is taking place at Larsen & Toubro's AM Naik Heavy Engineering Complex in Hazira, Gujarat.

Defence Secretary Rajesh Kumar Singh, who also chairs DRDO, inaugurated the phase virtually. Vice Admiral B. Sivakumar, the Navy's Chief of Materials, and Dr R. Balamuralikrishnan, DRDO's Director General (Naval Systems and Materials), attended in person at Hazira. The module was designed by DRDO's Naval Materials Research Laboratory (NMRL) in Ambernath.

Why submarines need AIP

A conventional diesel-electric submarine is very quiet when it runs on batteries, but the batteries run down. To recharge them, the boat has to come near the surface and run its diesel generators through a snorkel mast. That is when it is most exposed to radar, aircraft and other submarines. How often a submarine has to snorkel largely decides how detectable it is.

Air Independent Propulsion lets a non-nuclear submarine generate power while fully submerged, without drawing in outside air. This lets it stay down far longer between snorkelling periods. Several navies use AIP based on Stirling engines, closed-cycle steam turbines or fuel cells. DRDO's system uses fuel cells, which turn hydrogen and oxygen into electricity with no combustion and very little noise. Earlier reporting and public references describe the design as a phosphoric acid fuel cell (PAFC) system.

The main technical difficulty with fuel-cell AIP is hydrogen. Storing large amounts of hydrogen inside a submarine hull is a serious safety risk. According to reports of the Defence Ministry's description, NMRL's design generates hydrogen on board, on demand, instead of storing it. This removes the main safety concern, and the only by-product is clean water.

What Setting-to-Work Phase-2 means

"Setting to work" is shipbuilding language for bringing a system to life and proving it works as an integrated unit before it goes into a vessel. In this second phase, the energy module will be energised to produce power. It will then be tested rigorously under the Navy's own test protocols. Only after it passes will it go to Mazagon Dock Shipbuilders Ltd (MDL) in Mumbai, which will integrate it into a Kalvari-class submarine. The integration is targeted for early 2027.

The Hazira work builds on a long development history. DRDO tested a land-based prototype of the AIP system on 8 March 2021, showing that the fuel-cell plant could run in endurance and maximum-power modes on shore. France's Naval Group, which designed the Scorpène submarine on which the Kalvari class is based, was brought in to certify the AIP design for integration into the boats. This step is essential because adding a new hull section changes a submarine's weight, buoyancy and layout.

A timeline that has slipped, but is now concrete

The programme has run late. Earlier plans called for fitting the AIP plug during INS Kalvari's first major refit around 2025. That window passed, and the latest reports put the integration in early 2027. Retrofitting a hydrogen-based fuel-cell plant into an existing submarine is hard engineering. It means cutting the hull, inserting a new section, rerouting systems and meeting strict safety standards for a crew living metres away from the plant.

Still, STW-2 is the kind of milestone that is hard to fake. A full-scale module being powered up in an industrial facility, with Navy testers involved, is a much later stage than laboratory prototypes or land-based demonstrators.

Why it matters for India

The Indian Navy operates six Kalvari-class (Scorpène) submarines built by MDL under Project-75. The lead boat, INS Kalvari, was commissioned in December 2017. Retrofitting AIP would make these boats much harder to detect at a time when the Indian Ocean is becoming more contested. Other navies, including China's, are increasingly active in it, and there is more emphasis on sea denial at the region's chokepoints.

It also matters for industry. Very few countries have developed fuel-cell AIP themselves, and India would join them with a system designed by a DRDO laboratory and built by a private Indian heavy-engineering firm. If the system works at sea, it could be fitted to future Indian conventional submarines and give Indian bids for new submarine programmes a domestic AIP option, instead of depending on a foreign supplier's proprietary plant.

L&T's role is also significant. Its Hazira complex already builds reactor vessels, pressure equipment and defence hardware. Taking an AIP energy module from drawings to a powered unit adds to the private sector's capability in naval propulsion, a field that until recently belonged almost entirely to state-owned shipyards and laboratories.

What to watch next

The next milestones are the energisation and power-production tests at Hazira, the hand-over to MDL, and the cutting-in of the AIP plug during a Kalvari-class refit. After that come harbour and sea trials. The ultimate test is how long a retrofitted boat can actually stay submerged on patrol, and how reliably the hydrogen-generation and fuel-cell plant runs over months at sea. The Navy has not publicly said how many of the six Kalvari-class boats will receive the module, or in what order.

Sources

• https://www.theweek.in/news/defence/2026/10/24/indian-navys-submarines-to-get-major-upgrade-as-indigenous-aip-system-enters-crucial-testing-phase.html
• https://www.indiatvnews.com/news/india/indian-navy-s-submarine-fleet-set-for-major-boost-as-indigenous-aip-system-achieves-key-milestone-2026-10-24-1057828
• https://www.etvbharat.com/en/bharat/navy-underwater-capabilities-set-for-push-after-indigenous-aip-system-achieves-crucial-milestone-enn26102406152
• https://bharatshakti.in/indigenous-aip-system-for-navy-submarines-enters-next-phase-of-testing/
• https://english.khabarpatri.com/2026/10/24/drdos-indigenous-aip-system-for-indian-navy-submarines-reaches-key-milestone-at-hazira/
• https://en.wikipedia.org/wiki/Naval_Materials_Research_Laboratory
• https://euro-sd.com/2023/01/news/29237/fuel-cell-based-aip-to-be-fitted-to-ins-kalvari/