India’s Defence Research and Development Organisation (DRDO) stands on the cusp of a transformative breakthrough in underwater warfare capabilities.
Reports indicate that DRDO’s indigenously developed Air Independent Propulsion (AIP) plug-in module is ready for integration into INS Khanderi, the second Kalvari-class submarine of the Indian Navy. This upgrade, scheduled during the vessel’s refit in 2026–27, marks a pivotal moment for India’s naval self-reliance.
INS Khanderi, commissioned on 28 September 2019, belongs to the Project-75 Scorpene series. The Indian Navy inducted six such submarines between 2017 and 2024.
Originally, all were slated to receive DRDO’s AIP technology to extend submerged endurance beyond conventional diesel-electric limits. Delays meant the first boat, INS Kalvari, missed out, but Khanderi will now pioneer this indigenous retrofit.
DRDO’s journey with AIP began gaining momentum in June 2023, when it inked a contract with Larsen & Toubro (L&T) for two AIP system modules tailored for Kalvari-class boats.
The Naval Materials Research Laboratory (NMRL), a DRDO entity, transferred the technology to L&T.
Manufacturing and integration occur at L&T’s AM Naik Heavy Engineering Complex in Surat, followed by rigorous factory acceptance trials.
DRDO achieved an important milestone in the development of Air Independent Propulsion (AIP) System by proving the shore-based prototype on 8 March 2021
These AIP modules, termed Energy Modules (EMs), form the core of the plug-in system retrofitted during refits.
The process for INS Khanderi also includes equipping the submarine for DRDO’s heavyweight torpedoes.
By December 2025, DRDO delivered the EM for plug integration. Sources quoted by Times Now confirm the system aced extensive shore-based trials, meeting all benchmarks.
Post-installation, INS Khanderi faces comprehensive trials due to altered dimensions and buoyancy. Sea trials are slated for July–August 2027, with refit completion by early 2028. This timeline underscores DRDO’s precision engineering, transforming a foreign-designed platform into a stealthier asset.
At the heart of DRDO’s AIP lies phosphoric acid fuel cell (PAFC) technology—a testament to the organisation’s innovative prowess. PAFCs employ phosphoric acid (H₃PO₄) as electrolyte, with hydrogen (H₂) at the anode and oxygen (O₂) from air at the cathode. This electrochemical reaction generates electricity directly, bypassing noisy diesel engines for silent, extended submersion.
DRDO’s choice of PAFC outshines alternatives like Alkaline Fuel Cells (AFC), Proton Exchange Membrane Fuel Cells (PEMFC), or Solid Oxide Fuel Cells (SOFC). PAFCs tolerate fuel impurities, operate on reformed hydrocarbons such as methanol or biogas, and function at 150–200°C. This yields steam by-products for onboard heating, boosting efficiency to 70%—a leap in submerged endurance from days to weeks.
Such achievements highlight DRDO’s mastery over complex fuel cell chemistry, once dominated by nations like Germany and Sweden. Developing PAFC indigenously circumvents import dependencies, shielding India from geopolitical supply disruptions. For a submarine-operating nation like India, facing assertive neighbours in the Indian Ocean Region, this stealth edge is non-negotiable.
AIP’s importance to Indian defence preparedness cannot be overstated. Conventional submarines surface frequently for battery recharging, exposing them to detection by advanced anti-submarine warfare assets. DRDO’s AIP enables prolonged underwater loitering, ideal for surveillance, ambush, and precision strikes—critical against China’s expanding submarine fleet and Pakistan’s maturing underwater capabilities.
The retrofit’s success directly fuels Project-76, India’s bid for 12 next-generation diesel-electric attack submarines (SSKs). Envisaged as successor to Project-75 (Scorpene) and Project-75I (likely Type-214), Project-76 demands 90–95% indigenous content, including combat management systems, sonars, and periscopes.
Initiated in late 2023, Project-76 saw the Navy’s Warship Design Bureau receive authorisation for preliminary designs. Early 2024 funding targeted AIP and lithium-ion batteries—DRDO innovations offering higher discharge rates and faster charging over lead-acid predecessors. L&T collaborates with the Navy’s Submarine Design Group, leveraging its track record in Arihant-class hulls and the SOV-400 midget submarine.
As of March 2026, Project-76 advances to detailed design, with Staff Requirements finalised to bridge conventional SSKs and nuclear-powered SSNs under Project-77. L&T officials project design completion by 2026–27. The 3,000-tonne vessels—50% larger than Kalvari—will wield DRDO AIP, lithium-ion batteries, indigenous torpedoes, and tube-launched cruise missiles.
DRDO’s AIP triumph exemplifies ‘Atmanirbhar Bharat’ in defence. By indigenising a technology vital for sea denial and power projection, India bolsters deterrence in contested waters.
Successful Khanderi integration validates the system for fleet-wide adoption and Project-76, ensuring the Navy’s underwater superiority amid rising regional tensions.
This milestone reduces reliance on foreign vendors, cuts costs through local manufacturing, and accelerates production timelines. For India’s strategic autonomy, DRDO’s AIP is more than engineering—it is a cornerstone of maritime security, safeguarding trade routes and national sovereignty.
IDN (With Agency Inputs)
