India is advancing towards true underwater endurance autonomy with the Defence Research and Development Organisation (DRDO) finalising a scalable phosphoric acid fuel‑cell Air‑Independent Propulsion (AIP) system for its upcoming Project‑76 conventional submarines.

Conceived as a high‑efficiency, modular upgrade to the existing AIP intended for Kalvari‑class boats, the system represents a decisive step in India’s quest for long‑duration stealth operations and indigenous clean‑energy maritime technology.

Developed by DRDO’s Naval Materials Research Laboratory (NMRL) under the Ministry of Defence, the phosphoric acid fuel‑cell technology utilises hydrogen reformed onboard from sodium borohydride and stored liquid oxygen to generate electrical power without combustion.

This allows the submarine to operate submerged for over two weeks without surfacing to charge its batteries, reducing acoustic and infrared signatures and increasing tactical survivability in contested littoral environments.

While the first installation will take place as a retrofit during the mid‑life upgrade of the INS Kalvari, the system’s design philosophy is deliberately modular.

The scalable AIP architecture allows power cell clusters to be added or replaced depending on submarine displacement and mission profile, ensuring compatibility not only with the 1,800‑ton Scorpène class but also with larger, 3,000 to 4,000‑ton Project‑76 boats that will follow.

The AIP will work in hybrid tandem with advanced lithium‑ion (Li‑ion) battery banks being developed in parallel by the Indian Navy and industry partners.

Li‑ion cells, providing rapid charge‑discharge cycles and higher energy density than lead‑acid units, will handle peak power loads such as high‑speed manoeuvres. In contrast, the fuel‑cell AIP will handle steady‑state cruising, maintaining a low acoustic footprint essential for modern anti‑submarine evasion.

Future Project‑76 submarines—expected to be built indigenously by Mazagon Dock Shipbuilders Ltd (MDL) and Larsen & Toubro (L&T) under a strategic partnership model—will integrate this hybrid endurance system from inception.

The design aims to match or surpass the submerged endurance of advanced regional platforms such as Japan’s Taigei‑class and South Korea’s KSS‑III, both equipped with fuel‑cell or Li‑ion solutions.

DRDO’s scalable AIP effort aligns closely with national initiatives promoting self‑reliance in strategic technologies under the “Aatmanirbhar Bharat” framework. The program also widens India’s future technological base for clean‑energy propulsion, providing a platform for eventual transfer of know‑how to civil maritime or underwater autonomous vehicle sectors.

By achieving a field‑deployable, modular AIP proven on operational submarines, India will possess one of the few flexible submarine endurance platforms outside Europe and East Asia.

This capability will allow Indian Navy attack submarines to patrol critical chokepoints and extended Indian Ocean sectors for weeks—silent, self‑sustaining, and strategically unpredictable.

Technical specifications and lifecycle integration details for the proposed Project‑76-class Submarines

FeatureDetails
Project NameProject-76 (P-76)
Number of Submarines12 (planned)
Submarine TypeConventional diesel-electric attack submarines
Submerged DisplacementAround 3,000 to 4,000 tonnes
Propulsion SystemIndigenous phosphoric acid fuel-cell Air-Independent Propulsion (AIP) combined with lithium-ion batteries
AIP DetailsModular design scalable for different submarine sizes; uses hydrogen reformed onboard from sodium borohydride and liquid oxygen
EnduranceWeeks-long submerged endurance without surfacing
Stealth FeaturesLow acoustic and infrared signatures due to AIP and hybrid battery system
Primary BuildersMazagon Dock Shipbuilders Ltd (MDL) and Larsen & Toubro (L&T)
Indigenous Content TargetApproximately 80% indigenous materials
Design TimelineDesign finalisation expected by 2026-2028
Prototype LaunchExpected by 2030
Combat SystemsIndigenous weapon control system; armed with torpedo tube-launched cruise missiles
Strategic RoleMaintain robust conventional underwater fleet with enhanced endurance and stealth for Indian Ocean Region dominance
Technological InfluenceIncorporates lessons from Project-75 (Kalvari-class) and nuclear submarine Project-77 programs
Collaborative DesignDesign led by Indian Navy’s Warship Design Bureau with private sector involvement (L&T)
Future CapabilityEnhanced underwater endurance and stealth, making Indian Navy one of the few with flexible modular AIP conventional submarines


IDN (With Agency Inputs)