Air Independent Propulsion (AIP) is the latest technology in submarine development. Many countries are working on this including India (DRDO is developing an indigenous AIP system for its Kalvari class subs).

Source: Internet, Courtesy: Wikipedia

The submarines can go up to a speed of 20 knots under water and can go deep inside the water up to 800 ft which can be used for spooking and oceanographic research and many more.


Basically subs are of two types – Diesel subs (Conventional subs) and Nuclear powered subs


Diesel subs are the oldest and cheapest method of propulsion used not only in subs but also in merchant and naval warships. This consists in first converting the work of the combustion engine into electric energy via a dedicated generator.

These subs have limitations, to inhale fresh air, these giant have to surface themselves but when they are under water, and all this is done by combination of batteries which run the whole propulsion along with electrical equipments onboard. Once the batteries are drained up, subs come up, charge the batteries and submerged again but this leads to the chances of detection by enemy.


Nuclear-powered submarines have a relatively small battery and diesel engine/generator power plant for emergency use if the reactors must be shut down. Nuclear power is now used in all large submarines, but due to the high cost and large size of nuclear reactors, smaller submarines still use diesel-electric propulsion. 

These subs can’t work in shallow waters and need to be large in size for the nuclear reactor to be inside it. Major problem in this is, these have many rotating components which are a source of noise and thus not good for its stealth.


Air-independent propulsion (AIP) represents a technological break-through for conventional submarines. In addition to the diesel-run energy generator that requires exterior air for combustion of the conventional submarine; those with an AIP system also use energy sources that do not require surface air, such as fuel cells.


  • Closed cycle diesel engine
  • Closed cycle steam cycle
  • Stirling cycle engine (Used by SAAB Defence & Security)
  • Fuel cell (DRDO is working on this)


The fuel cell technology is the cheapest of all the above methods. In this we use fuel and oxidizer to convert chemical energy into electrical energy (same as electrolytic cells).In this hydrogen is the fuel and oxygen is the oxidizer which with some byproducts gives electrical energy.

If we compare this with nuclear method, no doubt the nuclear is much more sophisticated and efficient but due to cost and maintenance constraints, AIP is better. The diesel subs can be easily converted to AIP by some changes in its hull only.


  • No rotating components in it so less noise production
  • Can be used in medium size subs.
  • Easy to operate
  • Cheapest AIP
  • With some changes can be used in diesel subs

Authored By:- Cdt. Suraj Mohanty, TMI

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