Air-Independent Propulsion (AIP) is a marine propulsion concept that allows a non-nuclear submarine to operate without needing to access atmospheric oxygen by surfacing or using a snorkel. The origin of AIP technology is rooted in solving the critical vulnerability of conventional diesel-electric submarines, which must surface every few days to run their diesel generators and recharge their batteries, exposing them to enemy detection. AIP systems augment or replace the traditional diesel-electric system, significantly extending the submarine's submerged endurance from a few days to potentially several weeks.
The mechanism of AIP involves various technologies, primarily generating electricity to power the submarine's electric motor or recharge its batteries without combustion that requires external air. The three main types of AIP systems are Stirling engines, Closed-Cycle Diesel Engines, and Fuel Cells. Fuel cell-based AIP, considered the most advanced, uses a chemical reaction between stored hydrogen and oxygen to produce electricity, with the only byproduct being water, which enhances stealth due to low acoustic noise and lack of exhaust fumes.
AIP connects to India's indigenous defense technology development, specifically the Defence Research and Development Organisation (DRDO) and its Naval Materials Research Laboratory (NMRL). The DRDO has developed an indigenous, fuel cell-based AIP system that is unique because it generates hydrogen on demand, addressing the safety concern of storing large quantities of hydrogen onboard. This technology is a key part of the Indian Navy's modernization under Project 75 and Project 75I.
The technology has changed recently in the Indian context, as the indigenous DRDO-developed AIP system is being retrofitted into the Indian Navy's Kalvari-class submarines. The integration involves inserting an additional hull section into the submarine. The first submarine, INS Kalvari, was initially slated for retrofitting during its major refit in September 2024, though other sources suggest the integration on INS Khanderi is likely to begin in mid-2026. This indigenous development replaces the need to import foreign AIP systems, such as the French MESMA (Module d'Energie Sous-Marine Autonome) or German fuel cells, and positions India among an elite group of nations with this capability.