The Indian Navy plans to install an indigenous fuel-cell-based Air Independent Propulsion (AIP) “plug” on its six French-designed Kalvari-class (Scorpène) diesel-attack submarines to make them much more stealthy vessels.
The Ministry of Defence (MoD) and Mazagon Dock Shipbuilders Ltd (MDL) inked a ₹1,990-crore deal in December 2024 that includes building the hull portion housing the AIP plant and integrating it during each boat’s mid-life repair, starting with INS Kalvari in September 2025.
The program is led by DRDO’s Naval Materials Research Laboratory (NMRL), which in March 2021 successfully operated a land-based prototype in both endurance and maximum-power modes.
In order to power their diesel engines and recharge their batteries, conventional submarines need to snorkel every four to five days. This creates thermal and auditory traces that can be picked up by sonar, radar, or infrared sensors. By electrochemically generating energy from hydrogen and oxygen, a fuel-cell AIP allows the boat to stay submerged for 10 to 14 days at a time, thereby doubling its current endurance. Because hydrogen is produced inside the module as needed, NMRL’s concept is noteworthy because it improves safety by removing the risks associated with storing compressed gas. The system reduces engine noise, shrinks the heat plume, and tightens the submarine’s detectable “acoustic-thermal” footprint in addition to significantly extending the duration of covert patrol.
There is a rigorous “Jumboization” process involved in the work. Before the hull is re-welded and pressure-tested, an 8–10 m, 200-tonne AIP plug is put amidships, the pressure hull is carefully cut open in MDL’s dry-dock during a planned 18-month repair, and power, control, and hotel services are spliced into existing systems. Naval Group—the baseline’s designer
In order to maintain the submarines’ compliance with French safety requirements while maintaining the upgrade’s indigenous character, Scorpène has been retained for supervision, specialized jigs, materials, and certification. An estimated 300,000 skilled man-days will be created by the entire endeavor, contributing to the larger Aatmanirbhar Bharat effort.
Larsen & Toubro, a longtime development partner, is building two production-standard modules at its AM Naik Heavy Engineering Complex in Surat, while NMRL will supply the energy modules (fuel cells, reformer, and control electronics). While Naval Group receives ₹877 crore under a separate contract to integrate the submarines’ fighting system with a new Electronic Heavy-Weight Torpedo (EHWT) also designed by DRDO, MDL handles the hull production and integration. The ₹2,867-crore package modernizes the class’s punch and stealth.
By 2026, Pakistan may begin receiving China’s eight upcoming Hangor-class submarines, all of which are outfitted with AIP. Thus, India’s undersea deterrence in the Indian Ocean region is strengthened and the endurance gap with regional competitors is reduced by adding an indigenous AIP to the Kalvari fleet. When it is finished, India will become one of the few countries that have used domestic fuel-cell AIP on active submarines, including Germany, France, Sweden, and China.
Industry sources warn that several ground-based validation tests are still in progress, even though DRDO and MDL plan to begin installation in late 2025. If trials reveal integration issues, the first refit could still be delayed until 2026. However, the success of the land prototype and the establishment of a new, fully functional testing facility in Surat give assurance that fitting will take place within the updated timeframes.
New Delhi obtains more than ten days of silent endurance and a useful design-production environment for future Project 75(I) submarines by integrating domestic fuel-cell technology into imported hulls. The Indian Navy’s diesel-electric arm will have a powerful combination of increased underwater persistence, decreased detectability, and improved heavy-torpedo firepower after all six Kalvaris have finished their “Jumboization” cycles over the course of the next ten years. This will provide a significant triple boost for sea-denial operations throughout the littoral and blue-water regions.
