According to reports received today, the Defence Research and Development Organisation (DRDO), in partnership with Larsen & Toubro (L&T), has finalized India’s indigenous Air-Independent Propulsion (AIP) system. In a landmark move, the system is slated for installation on the Kalvari-class submarine INS Khanderi during its upcoming major refit.
The submarine is expected to return to active duty with this new “stealth lung” by the end of 2026. This marks a significant leap in India’s Aatmanirbharta (self-reliance) and fundamentally alters the balance of power in the Indian Ocean Region (IOR).
What is AIP?
To understand the significance of this development, one must first look at how conventional diesel-electric submarines operate. Typically, these vessels use diesel engines to charge their batteries while on the surface or while “snorkeling” (sucking in air through a tube just below the surface).
The problem? Snorkeling makes a submarine vulnerable. It creates a radar signature and thermal trail that modern anti-submarine warfare (ASW) aircraft, like the P-8I Poseidon, can easily detect. A conventional submarine must snorkel every few days.
The AIP Solution: An Air-Independent Propulsion system allows a non-nuclear submarine to operate without access to atmospheric oxygen. This means the submarine can stay submerged for weeks rather than days. India has opted for a Fuel Cell-based AIP, which is technically superior to the Stirling engines used by Sweden or the “MESMA” system used by Pakistan.
The Technical Edge: Fuel Cells vs. Others
India’s DRDO-developed system uses Phosphoric Acid Fuel Cells (PAFC). This system generates electricity by reacting Hydrogen and Oxygen. Unlike engines with moving parts, fuel cells have almost zero vibration.
- Stealth: Because there are no moving parts, the acoustic signature is incredibly low.
- Efficiency: Fuel cells convert chemical energy directly to electricity with higher efficiency than combustion engines.
- Duration: It is expected to extend the submerged endurance of the Kalvari-class from roughly 4-5 days to over two weeks.
The Strategic Impact on Submarine Survivability
The biggest threat to a submarine is detection. In the narrow “choke points” of the Indian Ocean—like the Malacca Strait or the Nine Degree Channel, a submarine that has to surface to breathe is a submarine that is waiting to be hunted.
1. Reduced Indiscretion Ratio: In naval terms, the “indiscretion ratio” is the time a submarine spends snorkeling compared to its total mission time. By installing the AIP on INS Khanderi, the Indian Navy is slashing this ratio to near-zero during critical phases of a mission. A submarine that doesn’t surface is effectively a “black hole” in the water.
2. Tactical Flexibility: With AIP, INS Khanderi can “lie in wait” near an enemy port or a shipping lane for 14+ days without moving or surfacing. This makes it a perfect tool for sea denial, preventing an adversary from using a specific stretch of water.
Meaningful Comparisons: India vs. The Neighborhood
India’s underwater fleet is in a race against both Pakistan and China.
- Pakistan’s Agosta-90B: Pakistan was the first in the region to use AIP, employing the French MESMA system. However, MESMA is a steam-cycle system that is noisier and less efficient than the DRDO’s fuel-cell technology.
- China’s Type 039A/B (Yuan-class): China uses Stirling engine AIP. While effective, Stirling engines have moving pistons and are generally considered noisier than fuel cells.
- India’s Advantage: By jumping straight to Fuel Cell technology, India has bypassed the “noisy” generations of AIP. The Kalvari-class, already known for its extreme “stealthy” hull design derived from the French Scorpene, will now arguably be the quietest conventional submarine in the world.
The Fleet-Wide Roadmap
The installation on INS Khanderi is just the beginning. The Indian Navy plans to retrofit all six Kalvari-class (Scorpene) submarines with this indigenous AIP during their Long Refit and Overhaul (LRO).
Furthermore, the experience gained here will be directly applied to Project-75I, India’s next-generation submarine program. By proving the AIP on the Khanderi, L&T and DRDO have demonstrated that India no longer needs to rely on foreign OEMs (Original Equipment Manufacturers) for the most sensitive part of a submarine’s “brain.”
The Hydrogen Challenge
One technical “deep dive” point is how the DRDO handles the hydrogen. Storing pure hydrogen on a submarine is dangerous. The Indian system is unique because it uses on-board phosphoric acid reforming to generate hydrogen from sodium borohydride as needed. This “plug-and-play” safety feature is what makes the Indian system one of the safest and most compact fuel-cell designs globally.
A Silent Guardian for 2026
The news that INS Khanderi will be ready by end-2026 with an indigenous lung is a massive victory for Indian defense engineering. In the high-stakes game of underwater hide-and-seek, the advantage goes to the one who can stay silent the longest.
With the DRDO-L&T AIP, the Indian Navy isn’t just buying time; it’s buying a strategic edge. When the Khanderi slips back into the water in late 2026, it won’t just be a submarine; it will be a silent sentinel, capable of vanishing into the depths and reappearing only when it chooses.
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