The Indian Navy is set to convert its six French-designed Kalvari-class (Scorpène) diesel- attack submarines into far stealthier platforms by grafting in an indigenous fuel-cell–based Air Independent Propulsion (AIP) “plug.”

The ₹1,990-crore contract signed in December 2024 between the Ministry of Defence (MoD) and Mazagon Dock Shipbuilders Ltd (MDL) covers construction of the hull section that houses the AIP plant and its integration during each boat’s mid-life refit, beginning with INS Kalvari in September 2025.

DRDO’s Naval Materials Research Laboratory (NMRL) leads the programme, having successfully run a land-based prototype in endurance and maximum-power modes back in March 2021.

What The AIP Plug Adds

Conventional submarines must snorkel every four–five days to run their diesel generators and recharge batteries, creating thermal and acoustic signatures that sonar, radar or infrared sensors can detect. A fuel-cell AIP produces electricity electro-chemically from hydrogen and oxygen, freeing the boat from atmospheric air and letting it remain submerged 10–14 days at a stretch—roughly trebling present endurance. NMRL’s design is notable because hydrogen is generated on demand inside the module, eliminating the hazards of storing compressed gas and enhancing safety. Besides vastly prolonging covert patrol time, the system cuts engine noise, shrinks the heat plume and therefore tightens the submarine’s detectable “acoustic-thermal” footprint.

Engineering The Retrofit

The work involves a demanding “Jumboisation” procedure. During a scheduled 18-month refit, the pressure hull is precisely sliced open in MDL’s dry-dock; an 8–10 metre, 200-tonne AIP plug is inserted amidships; and power, control and hotel services are spliced into existing systems before the hull is re-welded and pressure-tested. Naval Group—designer of the baseline Scorpène—has been retained for oversight, specialised jigs, materials and certification to keep the submarines compliant with French safety standards while preserving the indigenous character of the upgrade. The entire activity is projected to create roughly 300,000 skilled man-days, feeding into the wider Aatmanirbhar Bharat drive.

Industrial Consortium And Cost

NMRL will supply the Energy Modules (fuel cells, reformer and control electronics); Larsen & Toubro, long-time development partner, is fabricating two production-standard modules at its AM Naik Heavy Engineering Complex in Surat. MDL undertakes hull fabrication and integration, while Naval Group earns ₹877 crore under a parallel contract to mesh the submarines’ combat system with a new Electronic Heavy-Weight Torpedo (EHWT) also developed by DRDO. The combined ₹2,867-crore package modernises both stealth and punch of the class.

Strategic Overtones

Pakistan’s eight forthcoming Hangor-class submarines from China, all AIP-equipped, could start arriving by 2026. Adding an indigenous AIP to the Kalvari fleet therefore shores up India’s under-sea deterrence in the Indian Ocean region and narrows the endurance gap with regional rivals. On completion, India will join Germany, France, Sweden and China among the small set of nations that have fielded home-grown fuel-cell AIP on operational submarines.

Schedule And Risks

While DRDO and MDL target first installation in late 2025, industry insiders caution that some ground-based validation tests are still underway. A slip to 2026 for the inaugural retrofit remains possible if trials uncover integration wrinkles. Nonetheless, the land prototype’s success and the creation of a new full-scale testing facility at Surat provide confidence that fitment will proceed within revised windows.

Outlook

By weaving indigenous fuel-cell technology into imported hulls, New Delhi gains not only 10-plus days of silent endurance but also a valuable design-production ecosystem for future Project 75(I) submarines. Once all six Kalvaris complete their “Jumboisation” cycles over the next decade, the Indian Navy’s diesel-electric arm will field a formidable blend of extended underwater persistence, reduced detectability and upgraded heavy-torpedo firepower—a decisive triple boost for sea-denial operations across the littoral and blue-water expanse.

IDN (With Agencies Inputs)