India's Submarine Upgrade Hits Another Wall; DRDO AIP Still Not Ready As Pakistan Races Ahead

India’s efforts to enhance the endurance and stealth of its frontline submarines have suffered another setback.

The first Scorpene-class submarine, INS Kalvari, is undergoing a maintenance refit without the much-anticipated Air Independent Propulsion (AIP) upgrade, as the indigenous system developed by the Defence Research and Development Organisation (DRDO) is still not ready for operational use, Times of India has reported.

AIP allows conventional diesel-electric submarines to remain submerged for extended periods without surfacing or snorkelling. Unlike nuclear submarines, diesel-electric boats must surface every few days to run their diesel engines and recharge batteries, a process known as snorkelling that makes them vulnerable to detection.

AIP systems, such as fuel cells, generate electricity underwater without the need for atmospheric oxygen, enabling submarines to stay submerged for weeks. This significantly enhances stealth and survivability by reducing exposure to radar and aerial surveillance, making AIP-equipped submarines far more lethal in contested waters.

Originally approved in 2014 at a cost of Rs 270 crore and expected to be completed by 2017, DRDO’s fuel cell-based AIP, developed with Larsen & Toubro as the industry partner, has faced repeated delays. Navy sources now hope it will be ready in time for INS Khanderi's scheduled refit in mid-2026.

The upgrade process, known as "jumboisation," involves cutting the submarine's hull to insert an AIP module, extending underwater endurance from a few days to roughly two weeks.

The delay comes as India’s conventional submarine force continues to dwindle, with just six Scorpenes and ageing Russian and German-origin vessels in service.

No Indian submarine is equipped with AIP currently. Meanwhile, Pakistan has raced ahead. It already operates AIP-equipped Agosta-90B submarines of French origin and is poised to induct eight advanced Yuan-class submarines with AIP technology from China.