The indigenous AIP system for the Indian Navy's diesel-electric submarines has entered its second setting-to-work phase, with an inauguration ceremony on Saturday at Larsen and Toubro's AM Naik Heavy Engineering Complex in Hazira, Gujarat.

The Air Independent Propulsion (AIP) Energy Module has been developed in-house by DRDO's Naval Materials Research Laboratory (NMRL). Next, it will be switched on to produce power and tested to the Navy's protocols. It then goes to Mazagon Dock Shipbuilders Limited (MDL) for fitment on a Kalvari-class submarine in early 2027, according to the Ministry of Defence.

Indigenous AIP system begins Phase-2 at L&T Hazira

Defence Secretary Rajesh Kumar Singh, who is also Secretary, Department of Defence R&D and Chairman DRDO, joined the ceremony virtually as Chief Guest. Vice Admiral B Sivakumar, Chief of Materiel, Indian Navy, and Dr R Balamuralikrishnan, Director General (Naval Systems and Materials), DRDO, were present at Hazira.

The Defence Secretary congratulated DRDO and its industry partners. He credited the scientific perseverance and industrial cooperation behind the programme, which he said had now turned into real output on the shop floor.

He also thanked the Navy for backing DRDO throughout, including testing and validation at every stage of the trials.

Hydrogen safety stays non-negotiable

The Defence Secretary asked the teams to move on to the next phases and meet the Navy's induction timelines for the system. That work, he said, has to keep the exacting safety culture that hydrogen and fuel cell systems require, without compromise.

AIP cuts a submarine's need to come up for air

Underwater, a diesel-electric submarine runs on batteries. Recharging them means coming near the surface to run diesel engines, which need air. That is when a boat is easiest to find.

AIP reduces this dependence on atmospheric oxygen and boosts the submarine's underwater endurance, the ministry said.

DRDO's design is fuel cell based. Its land-based prototype was proven in endurance and maximum power modes in March 2021, with L&T and Thermax as industry partners, and DRDO said at the time that the hydrogen is generated on board.

Private shipyard builds the Energy Module

L&T signed a contract with DRDO in June 2023 to manufacture two AIP system modules, according to reports at the time. Hazira has carried the Navy's submarine work before. The complex was at the centre of attention for Project-76, the Navy's first conventional submarine to be designed fully in India.

Nibe Group and France's Naval Group signed a memorandum of understanding in September covering underwater drones, mine countermeasure vessels and submarine systems.

Kalvari-class submarine to carry the Indigenous AIP system from 2027

The Indian Navy operates six Kalvari-class submarines, built by MDL in Mumbai to the French Scorpene design under Project-75. INS Kalvari was commissioned in December 2017 and the sixth boat, INS Vagsheer, in January 2025.

The Indigenous AIP system goes into a submarine as a new hull section, known as a plug. The Ministry of Defence signed a Rs 1,990 crore contract with MDL on 30 December 2024 to build this plug for the DRDO-AIP system and integrate it on board.

Earlier this month, the Defence Secretary had listed the AIP retrofit among several underwater programmes in motion, alongside the P75I submarine deal and a DRDO combat system for the fleet.