India’s DRDO (Defence Research & Development Organization) is reportedly poised to deliver its indigenously developed Air Independent Propulsion (AIP) plug-in time for integration with INS Khanderi during its scheduled refit in 2026–27.
INS Khanderi is the second Kalvari-class (Project-75, Scorpene) submarine inducted into the Indian Navy (IN). It was commissioned on September 28, 2019.
The Indian Navy inducted six Kalvari-class Scorpene submarines under Project-75 between 2017 and 2024. Originally, it was planned to integrate DRDO-developed AIP plugs—designed to facilitate extended submerged endurance—into all Kalvari-class boats. However, the DRDO plug was not ready in time for installation on the first boat, INS Kalvari.
AIP Development Landmarks and Timeline
In June 2023, DRDO signed a contract with L&T to provide two AIP system modules for Kalvari-class submarines.
Under the contract, the Naval Materials Research Laboratory (NMRL), a DRDO laboratory, transferred AIP technology to L&T. The AIP modules are manufactured, integrated, and undergo factory acceptance trials at L&T’s AM Naik Heavy Engineering Complex in Surat.
The AIP modules, also called Energy Modules (EMs), are integrated into the AIP plug, which is then retrofitted into the submarine during refit. The refit will also include equipping the boat to launch DRDO-developed heavyweight torpedoes.
In December 2025, DRDO was expected to deliver the EM for integration with the plug that will be inserted into INS Khanderi during its refit.
“The system has undergone extensive shore-based trials and has met the required benchmarks. Integration work on the second submarine is expected to be completed before December 2026,” Times Now reported, quoting sources.
The submarine will need to undergo extensive trials following the installation of the AIP plug, since the boat’s dimensions and buoyancy characteristics will change.
Sea trials are reportedly expected to commence between July and August 2027, with the full refit process expected to conclude by early 2028, the sources added.
Characteristics of the DRDO-Developed AIP
The DRDO-developed AIP uses phosphoric acid fuel cell (PAFC) technology.
A fuel cell converts chemical energy from a fuel into electricity through a chemical reaction between positively charged hydrogen ions and oxygen (or another oxidizing agent).
Several fuel cell types exist, including:
- Alkaline Fuel Cell (AFC)
- Proton Exchange Membrane Fuel Cell (PEMFC)
- Direct Methanol Fuel Cell (DMFC)
- Molten Carbonate Fuel Cell (MCFC)
- Phosphoric Acid Fuel Cell (PAFC)
- Solid Oxide Fuel Cell (SOFC)
A PAFC uses phosphoric acid (H₃PO₄) as the electrolyte. Hydrogen gas (H₂) is used as the fuel at the anode, while oxygen (O₂) from air is supplied at the cathode.
PAFC fuel cells offer several advantages over other fuel cell types. They provide greater fuel flexibility and are more tolerant of fuel impurities. They can operate using reformed hydrocarbon fuels such as methanol or even biogas.
PAFC operating temperatures (150–200°C) are relatively high. As a result, they generate steam as a byproduct in addition to electrical power for propulsion. The steam can be utilized for other onboard heating requirements, raising overall operating efficiency to as high as 70%.
DRDO AIP Critical for Project-76 Submarines
Successful integration of the DRDO AIP plug will not only augment the capabilities of Project-75 submarines but will also facilitate finalization of the design of Project-76 submarines.
Under Project-76, India has embarked on an ambitious design, development, and manufacturing program to deliver 12 next-generation diesel-electric attack submarines (SSKs) to the Indian Navy.
Project-76 is envisaged as the logical successor to the foreign-designed Project-75 (French Scorpène) and the upcoming Project-75I (Most likely German Type-214) programs.
Project-76 was initiated in late 2023 when the Indian Navy’s Warship Design Bureau (WDB) received formal authorization to begin the preliminary design phase.
In early 2024, the MoD allocated initial funding for the indigenous development of two pivotal enabling technologies for modern submarines: AIP systems and advanced lithium-ion batteries. As noted earlier, AIP offers increased submerged endurance, while lithium-ion batteries provide higher discharge rates and faster charging compared with traditional lead-acid batteries.
Project-76 aims for an unprecedented 90–95% indigenous content, including the Combat Management System (CMS), sonar suites, and periscopes.
Development Timeline
L&T is confirmed to be part of the design process along with the Navy’s Directorate of Naval Design (Submarine Design Group) (DND-SDG).
L&T’s credentials in submarine construction are impressive and extend beyond the development of the AIP plug for Project-75 submarines. The company previously carried out detailed engineering and hull construction for the Arihant-class SSBNs. It also designed and developed the SOV-400 Midget Submarine, a 400-tonne special operations vessel for commandos.
Additionally, L&T designed and built the Submarine Escape Training Tower (SETT) facility in Visakhapatnam, which is used to train naval crews in emergency escape procedures.
P-76 Progress
In September 2025, a senior L&T official reportedly stated that the design phase of the submarine could be completed by 2026–27.
As of March 2026, the project has moved into the detailed design phase. The Indian Navy is currently finalizing the Staff Requirements to ensure the P-76 can act as a bridge between conventional SSKs and nuclear-powered attack submarines (SSNs) being built under Project-77.
Initially, six submarines are proposed for construction. The first submarine could be delivered in six to seven years, with all six delivered within ten years.
As mentioned earlier, follow-on orders are likely to meet the Indian Navy’s expanding requirements.
The P-76 submarine is envisioned as a 3,000-tonne class vessel, roughly 50% larger than the current Kalvari class.
It will feature a fully indigenous AIP system and incorporate advanced lithium-ion batteries, both designed and developed by DRDO in collaboration with L&T.
The submarine will be armed with indigenously developed torpedoes as well as torpedo-tube-launched anti-ship and land-attack cruise missiles.
- Vijainder K Thakur is a retired IAF Jaguar pilot, author, software architect, entrepreneur, and military analyst.
- Follow the author @vkthakur
