DRDO AIP Submarine System: Closing India's Undersea Gap

Overview

The Defence Research and Development Organisation's Naval Materials Research Laboratory has developed an indigenous Air Independent Propulsion system. It utilises a phosphoric acid fuel cell to generate hydrogen on board. This extends the submerged endurance of conventional submarines without the explosive hazards of compressed gas storage.

Replacing atmospheric snorkelling with electrochemical power generation cuts indiscretion rates to near zero. This fills a significant undersea operational gap in the Indian Ocean. While Project-75I faces delays and complex retrofits remain, this indigenous system marks a decisive stepping stone toward self-reliance in naval deterrence.

The Defence Research and Development Organisation marked a major milestone on 24 October 2026 by inaugurating advanced integration trials in Hazira, Gujarat. According to the Press Information Bureau, Setting to Work Phase-2 (STW-2) activity for the indigenous Air Independent Propulsion (AIP) Energy Module commenced at Larsen & Toubro's AM Naik Heavy Engineering Complex.

As of October 2026, the Indian Navy is moving to install this technology directly into its operational fleet. The key steps driving this transition include:

• Land-Based Validation: DRDO proved the Naval Materials Research Laboratory (NMRL) prototype in Mumbai across continuous endurance and maximum power modes.
• Industrial Integration Pact: The Ministry of Defence signed a ₹1,990 crore contract with Mazagon Dock Shipbuilders Limited (MDL) on 30 December 2024 to build and install the AIP plug.
• Operational Concealment: Conventional diesel-electric submarines require non-nuclear auxiliary power to remain hidden beneath surface tracks.

Developing a domestic AIP system shields India's undersea deterrent from modern airborne radar and maritime surveillance.

The Indian Navy operates well below the force levels set by national security planners. In July 1999, the Cabinet Committee on Security (CCS) approved a 30-year submarine building plan. It mandated 24 conventional attack submarines by 2030 across Project-75 and Project-75I, followed by the indigenous Project-76.

As of October 2026, the Indian Navy operates just 16 conventional diesel-electric attack submarines, missing the 24-boat target envisioned for 2030. The active conventional fleet comprises:

• Six Kalvari-class (French Scorpène design) submarines;
• Four Shishumar-class (German HDW Type 209) submarines;
• Six Sindhughosh-class (Russian Kilo-class Type 877EKM) submarines.

Project-75 delivered six Kalvari-class boats from Mazagon Dock Shipbuilders Limited, but Project-75I ran into technical and procedural roadblocks. Mandating sea-proven fuel-cell AIP systems narrowed foreign competition and slowed vendor negotiations. This multi-year delay creates a sharp capability gap while regional navies step up patrols across the northern Indian Ocean.

Conventional diesel-electric submarines (SSKs) must surface or snorkel regularly to recharge their batteries using diesel generators. This intake process requires running noisy engines at periscope depth through a raised mast. That operating routine makes the boat vulnerable to detection.

Technical assessments show conventional submarines face an indiscretion rate of 10% to 30%, forcing them to snorkel every 24 to 72 hours. Snorkelling creates thermal plumes, reflects radar off the exposed mast, and generates acoustic noise picked up by maritime patrol aircraft.

NMRL's fuel-cell module reduces the submarine's indiscretion rate to near zero. Generating electricity electrochemically extends submerged patrol runs from 3 to 4 days up to 14 to 21 days at speeds of 4 to 6 knots. Consequently, submarines can loiter undetected in strategic waters without raising masts.

NMRL chose a fuel cell layout that eliminates the hazard of storing compressed gas inside a submarine hull. The power plant is a 270 kW Phosphoric Acid Fuel Cell (PAFC) that uses concentrated phosphoric acid in a silicon carbide (SiC) matrix as its electrolyte. It combines hydrogen with pure liquid oxygen to produce direct-current electricity.

Instead of carrying compressed or cryogenic gas, the system generates hydrogen on demand via the catalytic hydrolysis of Sodium Borohydride ($\text{NaBH}_4$):

$$\text{NaBH}_4 + 2\text{H}_2\text{O} \xrightarrow{\text{catalyst}} \text{NaBO}_2 + 4\text{H}_2 + \text{Heat}$$

This on-demand extraction avoids the explosive hazards of keeping high-pressure hydrogen inside an enclosed pressure hull. However, the chemical design introduces distinct engineering trade-offs:

• Volumetric Deadweight: Carrying aqueous Sodium Borohydride brings a volumetric and weight penalty compared to elemental hydrogen gas.
• Thermal Management: Strong heat from catalytic hydrolysis requires constant cooling loops to prevent thermal runaway.
• Effluent Handling: The reaction leaves Sodium Metaborate ($\text{NaBO}_2$), which cannot be discharged overboard without leaving detectable acoustic or chemical trails.
• Holding Tanks: The spent solution must be retained in dedicated internal holding tanks, requiring trim adjustments as fuel mass converts.

A key advantage of NMRL's PAFC is its operating range. Running at 150°C to 200°C, phosphoric acid cells resist trace impurities and carbon monoxide that would foul low-temperature platinum catalysts.

Navies deploy different AIP designs that balance fuel efficiency against underwater radiated noise. These systems rely either on thermal-cycle combustion engines or electrochemical fuel cells.

Sweden's Saab Kockums developed the Stirling engine, a closed-cycle setup burning pure liquid oxygen and low-sulphur diesel fuel. It drives reciprocating pistons to run a generator. The engine is proven, but moving parts generate vibration, and exhaust gases must be forced into the sea.

France built the MESMA (Module d'Energie Sous-Marine Autonome) system, which is a miniature closed-cycle steam turbine burning ethanol and liquid oxygen. MESMA delivers a lower efficiency of 20% to 25% and produces thermal signatures that complicate stealth.

Germany's ThyssenKrupp Marine Systems uses low-temperature Proton Exchange Membrane (PEM) cells. PEM modules run silently at high efficiency, but store hydrogen externally in heavy metal-hydride cylinders.

These trade-offs explain why DRDO chose fuel cells. Eliminating combustion and mechanical pistons gives the platform the acoustic silence needed to elude sub-surface tracking.

Mazagon Dock Shipbuilders Limited faces rigorous fabrication demands in retrofitting Scorpène-class hulls. Fitting an operational submarine with an AIP plug requires a major structural modification called jumboisation. Technicians must slice the hull open midships and insert a modular section measuring roughly 8 to 10 metres in length and weighing 300 tonnes.

Key Stages of the Jumboisation Sequence:

1. Pressure Hull Sectioning: Engineers cut the high-tensile pressure hull midships between accommodation and machinery compartments.
2. Module Insertion: Shipbuilders align and insert the 300-tonne modular AIP plug.
3. Hull Re-integration: Technicians reweld the hull to form a single, lengthened submarine body.

Technical documentation highlights several complex engineering hurdles:

• High-Tensile Metallurgy: Cutting and welding the 80 HLES high-yield stress alloy requires strict thermal controls to prevent micro-fissures under deep-sea pressure.
• Hydrodynamic and Centre-of-Gravity Shifts: A longer hull changes displacement, pitch, and drag, demanding updates to the Submarine Tactical Integrated Combat System.
• Cryogenic Safety: Routing high-pressure liquid oxygen lines across compartments requires vacuum insulation and isolation valves to manage fire hazards.

On 23 January 2023, NMRL and Naval Group France signed an agreement to complete detailed design and certify structural safety. The first installation will occur during the Major Refit and Life Certification (MRLC) of the lead Kalvari submarine in early 2027. Completing this work on schedule will test domestic shipyard throughput.

The Indian Maritime Security Strategy prioritises layered undersea deterrence to protect commercial sea lanes across the Indian Ocean. Operational doctrine divides missions by submarine type. Silent, AIP-fitted conventional submarines (SSKs) excel at choke-point ambushes across the Malacca, Sunda, and Lombok straits. Meanwhile, nuclear-powered attack submarines (SSNs) handle sustained, high-speed open-ocean patrols.

To sustain fleet numbers while indigenous AIP matures, the Defence Acquisition Council approved Acceptance of Necessity on 13 July 2023 for three additional Scorpène submarines built with factory-fitted AIP plugs. Concurrently, the Cabinet Committee on Security approved the construction of two nuclear attack submarines under Project 77 on 9 October 2024 at an estimated cost of ₹40,000 crore at the Ship Building Centre in Visakhapatnam.

Indian naval planners must focus on three operational priorities:

• Accelerate Industrial Production: MDL and domestic partners must streamline plug fabrication so MRLC refits stay within scheduled dockyard cycles.
• Secure Chemical Supply Chains: Naval commands must guarantee steady supplies of high-purity Sodium Borohydride and liquid oxygen at operating bases.
• Scale Project-76 Design Work: Technical insights from the 270 kW PAFC system should guide higher-output fuel cells for the future, fully indigenous Project-76 line.

Deploying the NMRL fuel cell strengthens domestic naval design. While refits pose manufacturing challenges, domestic AIP technology ensures India's conventional fleet can hold regional choke points silently and reliably.

• The Naval Materials Research Laboratory developed a 270 kW Phosphoric Acid Fuel Cell AIP system, generating hydrogen on demand to eliminate hazardous gas storage.
• Hydrogen is produced by the catalytic hydrolysis of Sodium Borohydride ($\text{NaBH}_4$), with spent sodium metaborate ($\text{NaBO}_2$) stored internally.
• Adding the AIP module cuts a submarine's indiscretion rate to near zero, expanding submerged endurance from 3 to 4 days up to 14 to 21 days.
• As of October 2026, the Indian Navy operates 16 conventional submarines, remaining short of the 24-submarine target set for 2030 by the 1999 CCS plan.
• Fleet expansion relies on a twin approach: retrofitting Kalvari-class hulls during early 2027 refits while initiating nuclear attack submarine builds under Project 77.