Indian Kalvari-class submarine will get Indigenous AIP system in 2025

Indian Navy will get its first AIP equipped submarine in 2025. When the INS Kalvari comes up for retrofitting in 2025, it will be fitted with the indigenous AIP.

In a recent interview with Financial express DRDO Chairman Dr. G Satheesh Reddy said “When the boat comes up for retrofitting in 2025, it will be fitted with the indigenous AIP which has been made in India by DRDO and Indian industry.”

In a major boost to Atma Nirbhar Bharat initiative of the government, the Fuel Cell based Air Independent Propulsion (AIP) System of the Defence Research and Development Organisation will soon be ready to be fitted onboard INS Kalvari (first submarine of Kalvari class commissioned in 2017).

project 75i
Kalvari class submarine

“Work is going on to develop a module which has to be fitted on the submarine when it comes. There are industry partners including Larsen & Toubro and Thermax, and the Naval Materials Research Laboratory (NMRL) of DRDO which has developed this.”

The AIP which is being developed here in India is based on the specifications and requirements shared by the end user – the Indian Navy.


In deep water submarine on board power is given by batteries, but its have a limit for recharging submarine required “snorkeling” but it may be risk of detection by enemy high frequency radars and they can be stabilized by enemy some of snorkels is coated with absorbing paint and made by stealthy material and shape but it can be also detectable by high frequency radars or by the exhaust emission when snorkeling is take place.

So AIP is introduced for continue working in under water submarine,

nuclear submarine has better power and speed than diesel engine submarine but due to noise footprint and as diesel engine nuclear submarine propulsion can’t stop suddenly, and in shallow water nuclear submarine are not able to operate.

WORKING OF AIP – following four type of AIP propulsion

  1. closed cycle diesel engine
  2. closed cycle steam turbine
  3. sterling cycle engine
  4. fuel cell


in this type of propulsion oxygen supplied from on board stored liquid oxygen, as in pure from oxygen is can’t be use in propulsion so the exhaust gases are mixed within them for initially starting the propulsion reactive gases (generally Ar) are mixed with pure oxygen, after when ignition started once exhaust again send in diesel engine with pure oxygen.

but its have some problem too like pure oxygen storing safely is not a simple task.


its also use in Scorpene class submarines, in this type source of energy is hot water which converted into steam and used for run to turbine, its modified version of their nuclear propulsion system with heat generated by ethanol and oxygen. Specifically, a conventional steam turbine power plant is powered by steam generated from the combustion of ethanol and stored oxygen at a pressure of 60 atm. This pressure-firing allows exhaust carbon di oxide to be expelled overboard at any depth without an exhaust compressor.

LIMITATIONS – low efficiency and only use for under water sailing


a closed cycle engine and working fluid is permanently remain inside it, working fluid is heated by a energy source which run the engine and generator which are used for charging batteries but due to extra weight and moving part this class of submarine is heavy and noisy than fuel cell.


fuel cell convert chemical energy to in electrical energy with oxidizer(oxygen) and fuel hydrogen(fuel), fuel cells have two electrode one is positive(cathode negative) and other is (anode positive )

fuel cell is also developed by DRDO india for scorpene class submarine.

AIP is need for combat mission for escaping enemy high frequency radars

in friendly territory its not required, its still on working on efficiency of fuel cell and AIP.

In 2021, in an official statement DRDO has said that the Fuel cell-based AIP has merits in performance as compared to other technologies. temperatures within the ocean and on the surface.

Plans of the Indian Navy

As has been reported in Financial Express Online all the submarines under Project 75 – Kalvari class will be fitted with AIP when they come up for their first upgrades starting 2025.

Earlier the date for the first boat to come up for upgrade was mentioned by top officials as 2023. Six 1615 tonne submarines have been built by Mazagon Dockyards Limited (MDL) Mumbai, and in joint collaboration with the French Naval Group based on Scorpene design.

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