The Indian Navy is set to take a significant technological leap in underwater warfare capabilities with the integration of the Defence Research and Development Organisation’s (DRDO) indigenously developed Air Independent Propulsion (AIP) system into INS Khanderi, the second submarine of the Kalvari-class fleet.

According to senior defence and security officials, the system will be installed during the submarine’s scheduled refit in July 2026. The AIP system, developed in collaboration with Larsen & Toubro (L&T) and Thermax, had initially been earmarked for INS Kalvari, the lead submarine in the class. However, due to delays in integration and alignment of refit schedules, the Navy has now shifted the focus toward fitting the system onboard INS Khanderi.

The prototype of the AIP is expected to be completed by December this year, after which integration work will commence. The process of installation itself is likely to span nearly 12 months, involving complex engineering to configure the propulsion unit within the submarine’s architecture.

This will be followed by harbour and sea trials aimed at validating the operational efficiency, safety, and endurance of the upgraded vessel. If the schedule holds, initial sea trials are anticipated between March and April 2027, with the complete refit and testing cycle concluding by July 2027.

Technically, the AIP system offers a transformative advantage to India’s conventional diesel-electric submarine fleet. Traditionally, such submarines need to surface every four to five days for battery recharging, thereby increasing vulnerability to detection by adversary forces. With AIP integration, however, endurance levels increase dramatically, enabling continuous submerged operations for up to two weeks at a stretch.

This not only augments stealth and tactical advantage but also substantially enhances operational flexibility in contested waters. Of particular note is DRDO’s adoption of a fuel cell-based AIP system, which produces hydrogen onboard rather than requiring storage of volatile hydrogen on the vessel, thereby reducing major safety risks. Moreover, the system is eco-friendly, generating only clean water as a by-product that can be safely discharged into the ocean.

India’s advancement positions it within an elite group of nations that have successfully developed indigenous fuel cell-based submarine propulsion systems, a realm previously dominated by technologically advanced navies such as those of France, Germany, and Japan. The indigenisation effort also marks a landmark in India’s pursuit of strategic self-reliance in defence technologies under the Atmanirbhar Bharat framework.

The path to AIP integration is, however, exceptionally complex. The process involves a highly intricate engineering exercise referred to as "Jumboisation," wherein the submarine’s pressure hull is precision-cut to allow insertion of the AIP plug. This newly added section, which houses the propulsion system, must be seamlessly connected and structurally reinforced to restore the submarine’s integrity and ensure safety under operational pressures.

To facilitate this, the Naval Group of France and Mumbai-based Mazagon Dock Shipbuilders Limited (MDL) have partnered with DRDO to execute the AIP integration. In December 2024, the Ministry of Defence formalised this collaboration by signing a ₹1,990 crore contract with MDL for the AIP plug’s construction and integration across submarines.

The successful integration of this propulsion technology into India’s Scorpene-class platforms is set to significantly bolster the Navy’s combat endurance, particularly in the Indian Ocean Region (IOR), where prolonged underwater presence is critical to deter adversaries and secure maritime interests. The move not only strengthens India’s indigenous defence ecosystem but also reaffirms its commitment to enhancing underwater dominance in response to growing strategic challenges in the Indo-Pacific.

Agencies