India's second Kalvari-class submarine, INS Khanderi, is set to receive an indigenously developed air-independent propulsion (AIP) system by the end of 2026, marking a pivotal advancement in the Indian Navy's underwater capabilities, according to a report by Asianet News.

Senior defence sources have confirmed that the AIP system, crafted by the Defence Research and Development Organisation's (DRDO) Naval Materials Research Laboratory (NMRL) in Pune, has successfully completed rigorous shore-based trials and achieved all necessary technical standards.

Integration efforts on INS Khanderi are slated to finish before December 2026, while the vessel undergoes a refit at Mazagaon Dock Shipbuilders Limited (MDL) in Mumbai.

A DRDO official revealed that the core "energy module" of the AIP system will be delivered to MDL within the next three to four months, allowing it to be seamlessly embedded into the submarine's hull during the ongoing refit.

Sea trials for the upgraded INS Khanderi are anticipated to commence between July and August 2027, with the entire refit process expected to conclude by early 2028.

This development follows an earlier plan, reported by India Sentinels in January 2023, to fit the AIP system onto INS Kalvari, the lead boat of the six-submarine Kalvari-class fleet under Project 75.

However, due to timing constraints during INS Kalvari's refit schedule, the integration has been pragmatically shifted to INS Khanderi, while INS Kalvari proceeds with a separate refit.

DRDO sources emphasise that this adjustment signifies technological maturity rather than any setback, with the system having advanced through extensive land-based testing phases.

Conventional diesel-electric submarines like those in the Kalvari class rely on periodically raising a snorkel to recharge batteries via diesel engines, a process that exposes them to radar and acoustic detection.

Without AIP, such submarines must surface or snorkel every four to five days, limiting their stealth and endurance. AIP technology transforms this dynamic by enabling electricity generation underwater without combustion, permitting submarines to remain submerged for weeks.

This enhancement significantly boosts stealth, operational range, and endurance, proving invaluable in high-threat maritime domains like the Indian Ocean.

NMRL's AIP employs phosphoric acid fuel cell technology, producing only water as a by-product, which can be safely discharged into the sea.

Upon successful integration and validation on INS Khanderi, India will enter an elite cadre of nations—including Germany, Sweden, Japan, South Korea, Spain, and China—that have independently developed and deployed fuel cell-based AIP for submarines.

France and Russia, by contrast, have adopted alternative AIP methodologies.

India's achievement through indigenous means, rather than imports or licences, aligns squarely with national priorities for self-reliance in defence manufacturing.

The Kalvari-class submarines, constructed at MDL in partnership with France's Naval Group based on the Scorpène design, now comprise a fully commissioned fleet: INS Kalvari, INS Khanderi, INS Karanj, INS Vela, INS Vagir, and INS Vagsheer.

These vessels represent the modern backbone of the Navy's conventional submarine force.

The broader fleet includes 16 conventional submarines, of which 10—comprising Soviet-era Sindhughosh-class (Kilo-class) and German Shishumar-class boats—are over 30 years old and nearing decommissioning.

Replacements under Project 75 India, envisioning six more advanced submarines, remain under governmental review. If the AIP retrofit adheres to its timeline, it will deliver the Navy an interim capability uplift amid delays in fleet expansion.

This milestone underscores DRDO's growing prowess in submarine technologies, potentially paving the way for future upgrades across the Kalvari class and beyond.

Agencies