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Air-Independent Propulsion (AIP): Revolutionizing Submarines for Modern Naval Warfare


Submarines are versatile and vital assets in modern naval warfare, serving a multitude of roles that range from intelligence gathering and reconnaissance,  covert insertion and extraction of special forces, attacking enemy submarines and their surface warships, mine laying, to other littoral or choke point operations. Their ability to operate beneath the surface of the ocean, hidden from view, makes them a formidable force.

However, their effectiveness depends largely on their endurance and stealth, two critical factors that directly impact their lethality and survivability. To meet the challenges of the evolving maritime landscape, navies worldwide have been investing in new technologies to make submarines quieter and extend their underwater endurance.

The propulsion system plays an extremely important role in the functioning of a submarine for the completion of its desired operations. To perform stealthy underwater operations and extend their endurance, modern diesel-electric submarines have turned to Air-Independent Propulsion (AIP) technology. In this article, we’ll delve into the concept of AIP and how it creates a force multiplier effect by enhancing a submarine’s endurance and stealth capabilities.


Traditional Challenges of Diesel-Electric Submarines

Historically, diesel-electric submarines have relied on a combination of diesel engines and batteries for propulsion. While diesel engines provide power for surface operations and battery recharge, they are inherently noisy and require atmospheric oxygen for combustion.

While diesel engines provide the necessary power for surface operations and recharging the batteries, they cannot be used while submerged due to the need for oxygen. Submerged operations, therefore, depend solely on battery power, severely limiting the endurance of the submarine. As battery technology improved, the endurance of these submarines increased proportionally. But it was not enough to last them beyond a week.

The Vulnerability Dilemma

Additionally, the need to surface frequently for air intake and battery recharge makes these submarines vulnerable to detection by enemy forces. This is done by snorkeling, which exposes them to detection by enemy radars and makes them an easy target for hostile anti-submarine assets.

Modern surface, airborne and satellite sensors have become so sensitive that they can readily track surface wakes, acoustic and thermal signatures caused by snorkels, diesel engines, and their exhausts. Even with advanced technologies such as radar-absorbing paint and stealthy shaping, submarines remain detectable by high-resolution radars. Diesel sniffers, which detect exhaust emissions while snorkeling, further compound the problem. This increased vulnerability exposes submarines to hostile anti-submarine assets, diminishing their effectiveness.

This limitation on submerged endurance and stealth has long been a challenge for naval strategists.

The Advent of Nuclear-Powered Submarines

Nuclear-powered submarines, introduced in the 1950s, addressed some of the shortcomings of conventional diesel-electric submarines. Nuclear reactors are quieter, do not require atmospheric air, and provide greater power output, enabling these submarines to stay submerged for months instead of days and travel at higher speeds underwater. Nuclear-powered submarines have traditionally excelled in endurance, stealth, and speed, albeit at a significantly higher cost.

Air-Independent Propulsion (AIP): Narrowing the Gap

Air-independent propulsion (AIP) technology has emerged as a game-changing solution, narrowing the performance gap between nuclear-powered and diesel-electric submarines. AIP systems work by providing an alternative source of oxygen for the diesel engine, eliminating the need to surface for air intake.

AIP systems allow non-nuclear submarines to operate without the need for atmospheric oxygen, significantly extending their underwater endurance and enhancing their stealth capabilities. AIP can either augment or replace the traditional diesel-electric propulsion system, enabling submarines to recharge their batteries independently and reducing noise levels without compromising performance. This innovation significantly enhances a submarine’s endurance and stealth capabilities, creating a force multiplier effect.

Key Benefits of AIP:

  1. Extended Endurance: AIP-equipped submarines can remain submerged for weeks or even months, greatly increasing their operational range and versatility. This extended endurance is vital for conducting long-range reconnaissance missions, maintaining a covert presence in critical areas, or waiting for the opportune moment to strike.
  2. Enhanced Stealth: One of the primary advantages of AIP technology is its ability to maintain a near-silent operation.  The hydraulics in a nuclear reactor produce noise as they pump coolant liquid, while an AIP’s submarine’s engines are virtually silent. With reduced noise levels and minimized thermal signatures, AIP-equipped submarines are incredibly difficult for enemy anti-submarine warfare assets to detect. This stealthiness is crucial for covert intelligence gathering and surprise attacks.
  3. Reduced Vulnerability: By eliminating the need for frequent surfacing, AIP systems reduce a submarine’s vulnerability to enemy detection and attacks. This allows submarines to stay hidden beneath the waves, making them challenging targets for adversaries.

Submarine designers and naval strategists use the “indiscretion ratio” to assess the proportion of time a submarine is detectable while charging its batteries. Conventional modern submarines typically have indiscretion ratios ranging from typically 7-10% on patrol at 4 knots, and 20-30% in transit at 8-10 knots. AIP technology can increase underwater endurance by a factor of up to three or four, dramatically reducing the indiscretion ratio and enhancing a submarine’s stealth.

AIP-powered submarines have generally cost between $200 and $600 million, meaning a country could easily buy three or four medium-sized AIP submarines instead of one nuclear attack submarine.

Tactical Advantages of AIP

As a result, AIP systems have become highly sought after for stealthy underwater operations. AIP-equipped submarines can employ various tactical advantages. They can lie in wait for extended periods, ambushing approaching fleets with surprise attacks. In intelligence-gathering missions or spy operations, AIP allows submarines to loiter near enemy territory for weeks without the need to surface.

Also, diesel submarines possess the advantage of being able to switch off their engines completely and lie in wait, unlike nuclear submarines whose reactors cannot be switched off at will. While nuclear-powered submarines maintain their supremacy in terms of speed, AIP-equipped submarines offer an ideal balance of endurance and silence, making them well-suited for littoral waters and covert missions.

AIP Challenges and Downsides

While AIP technology offers significant advantages, it is not without challenges. Installing AIP increases a submarine’s length and weight and requires onboard storage of pressurized liquid oxygen (LOX) and supply for all associated technologies. Some AIP systems, like the Stirling engine, produce acoustic noise from moving parts, although they are quieter than traditional diesel engines. Additionally, AIP systems can increase the unit cost of a submarine by approximately 10%.

However, speed remains an undisputed strength of nuclear-powered submarines. U.S. attack submarines may be able to sustain speeds of more than 35 miles per hour while submerged. By comparison, the German Type 214’s maximum submerged speed of 23 miles per hour is typical of AIP submarines. Current AIP technology doesn’t produce enough power for higher speeds, and thus most AIP submarines also come with noisy diesel engines as backup.

Types of AIP Configurations and Technologies

Various AIP technologies are used in submarines worldwide, including Fuel Cell AIP, Stirling Engine AIP, Closed-Cycle Diesel AIP, and Closed-Cycle Steam Turbines.

Closed-Cycle Diesel AIP: Closed-cycle diesel engines are a historical approach to submarine propulsion, relying on the continuous circulation of a working substance.  This system operates similarly to a traditional diesel engine but uses liquid oxygen stored on board for combustion, allowing the submarine to remain submerged for extended periods.

They store liquid oxygen (LOX) on board to enable diesel engines to function without atmospheric oxygen. The stored LOX is mixed with inert gas (usually argon) to simulate the required oxygen concentration for safe engine operation. After combustion, exhaust gases are treated to extract any remaining oxygen and argon before being discharged into the sea. However, the main challenge with this technology is the safe storage of liquid oxygen, which has been prone to fires, leading to its discontinuation in modern submarines. While cost-effective, safety concerns have deterred its use in contemporary submarines.

Stirling Engines: The Stirling Cycle is a closed-cycle engine used in submarines for propulsion and electricity generation.  It employs a permanently contained working fluid that is heated using liquid oxygen (LOX) and diesel fuel, generating motion for the pistons and driving the engine.

In submarines, these engines are typically used to drive generators, which produce electricity. Stirling engines operate by heating and cooling a working fluid, usually helium, which causes it to expand and contract, driving a piston and generating mechanical power. This mechanical power is then converted into electrical power by the generator.

Stirling engines are known for their efficiency, simplicity, and quiet operation compared to other systems like MESMA. They are utilized in Japanese, Swedish, and Chinese submarines due to the easy availability of diesel fuel and lower refueling costs. However, Stirling engines are bulkier than fuel cells and can be noisy, limiting a submarine’s operating depth to 200 meters when the AIP (Air-Independent Propulsion) system is engaged.

Advantages: Stirling engines are relatively simple, and their technology is well-proven. They can provide a reasonable amount of power for submarine propulsion.
Disadvantages: Stirling engines are less efficient than some other AIP technologies, and they tend to be noisier due to the presence of moving parts. They also require the storage of an external oxidizer, such as oxygen.

MESMA (Autonomous Submarine Energy Module): The French MESMA (Module d’Energie Sous-Marine Autonome / Autonomous Submarine Energy Module) system is a notable example of closed-cycle steam propulsion. It employs ethanol and oxygen as energy sources. The combustion of ethanol and oxygen under high pressure generates steam, which acts as the working fluid to drive the turbine. The system can expel exhaust carbon dioxide into the sea at any depth without the need for a compressor.

MESMA offers higher power output compared to other alternatives, enabling faster underwater speeds. It eliminates the need for an external oxidizer like oxygen. However, it has drawbacks, including lower efficiency and a high rate of oxygen consumption. Additionally, MESMA systems are complex and expensive to acquire and maintain. As a result, many navies prefer alternative technologies like Sterling cycle and fuel cells for submarine propulsion.

Fuel Cells: Fuel cells represent cutting-edge technology in Air-Independent Propulsion (AIP) for submarines. These devices convert chemical energy into electricity by employing a fuel and an oxidizer, typically hydrogen and oxygen, resulting in electricity generation, with water and heat as by-products. A fuel cell consists of two electrodes, an anode, and a cathode, separated by an electrolytic barrier. The interaction between these components produces an electric current, which is used for battery charging, aided by a chemical catalyst to expedite reactions.

Two common types of fuel cells used in submarines are Phosphoric Acid Fuel Cells (PAFC) and Proton Exchange Membrane Fuel Cells (PEMFC). Germany is a leader in AIP development using fuel cells, with France and India also investing in this technology. PEM fuel cells are particularly favored due to their low operating temperatures and minimal waste heat. They work by catalytically splitting hydrogen molecules into ions and electrons on the anode side, while oxygen molecules are dissociated on the cathode side. The polymer membrane allows ions to migrate to the cathode, where they combine with oxygen atoms to form water. Stacking multiple fuel cells together increases their output, and this technology is being adopted by various countries, including Germany, France, Russia, India, Australia, Israel, and Spain for their submarine propulsion systems.

Germany’s approach involves the use of fuel cells directly to produce electricity for submarine propulsion. Fuel cells convert chemical energy, typically hydrogen and oxygen, into electrical energy through an electrochemical process. In submarines, hydrogen is usually obtained from stored fuel, and oxygen comes from onboard liquid oxygen (LOX) tanks. This process generates electricity and produces water as a byproduct.

Advantages: Fuel cells are highly efficient, quiet, and environmentally friendly, as they produce no exhaust fumes. They offer extended underwater endurance and can be scaled to fit various submarine sizes.  They have minimal moving parts, reducing acoustic signatures, and can achieve efficiency levels exceeding 80%, making them ideal for long-endurance missions. Fuel cells can be easily scaled to accommodate different submarine sizes, simplifying logistics. Furthermore, they are environmentally friendly, producing no exhaust fumes, eliminating the need for exhaust scrubbing equipment.

Disadvantages: Fuel cells can be complex and expensive to develop and maintain. They may not provide the burst speeds that some other systems offer.

Each has its advantages and disadvantages, and their selection depends on factors such as cost, efficiency, and intended operational use.

The Global Spread of AIP Technology

Numerous nations have recognized the strategic importance of AIP technology and have incorporated it into their submarine fleets. Over the past decade, AIP technology has proliferated worldwide, with many countries integrating it into their submarine fleets. Countries like Sweden, Germany, Russia, Japan, and South Korea have developed AIP-equipped submarines.  These submarines employ three types of engines, with approximately 60 currently operational in 15 countries, and another 50 on order or under construction. This technology has revolutionized the capabilities of non-nuclear submarines, making them formidable assets in modern naval warfare.

Stirling engines, such as those used by China’s Yuan-class submarines and Japan’s Soryu-class, are one AIP option. They offer quiet operation but are somewhat less efficient. Germany has developed submarines using electro-catalytic fuel cells, known for their efficiency and quietness. Spain, India, and Russia are also exploring fuel-cell AIP technology. France employs closed-cycle steam turbines called MESMA in their submarines, like the Agosta-90b class used by Pakistan.

Sweden, Japan, France, and India have incorporated AIP in their submarines, with advanced models like Mitsubishi’s Soryu-class. Spain’s S-80 submarines utilize a bioethanol-processor-based AIP system. Russia is working on AIP propulsion for their Lada and Amur-class submarines.

China has made strides in AIP technology, with Yuan-class Type 39B submarines demonstrating extended submerged endurance. Thailand is acquiring China’s Yuan Class S26 T boats, equipped with advanced AIP systems.

The Republic of Korea (ROK) Navy operates Son Won II-class submarines, featuring AIP systems built around Siemens polymer electrolytic membrane fuel cells, allowing for extended submerged operations.

Overall, AIP propulsion has become a global standard, enhancing submarine capabilities, extending endurance, and reducing acoustic signatures, with numerous countries adopting these systems in their naval fleets.

Indian AIP Advancements

India’s Defense Research and Development Organisation (DRDO) achieved a significant milestone in AIP technology by demonstrating an AIP system in March 2021, allowing Indian Navy submarines to operate for up to two weeks without surfacing to recharge their batteries. This indigenous AIP technology is a testament to the rapid advancements in AIP capabilities globally.

The AIP system underwent rigorous land-based trials at DRDO’s Naval Materials Research Laboratory (NMRL) in Ambernath, showcasing its 14-day endurance under simulated underwater conditions. NMRL’s AIP solution is based on Phosphoric Acid Fuel Cells (PAFC), which surpasses the Stirling cycle-based AIP used by the People’s Liberation Army Navy. The PAFC-based system offers extended underwater continuous usage capability, comparable to global leaders in AIP technology. Furthermore, NMRL’s modular architecture enhances system survivability, as it can reconfigure operational units in case of module failures.

The DRDO-built AIP is set to be integrated into the Kalvari-class submarines during their refit program, with the first refit for INS Kalvari scheduled for 2023. This technology will also be installed on the six new Scorpene submarines joining the Indian Navy’s fleet in the coming years. The modular and efficient PAFC-powered AIP module technology has been transferred to Thermax Ltd in Pune for production, ensuring its wider implementation in the submarine fleet.

Naval Group, a French defense firm, has invested over ₹100 crore in India for three workshops to maintain critical systems of Scorpene submarines. These workshops are equipped with tools, infrastructure, and spare parts to support important tasks. Naval Group is also working on qualifying the Defense Research and Development Organisation (DRDO)-developed Air Independent Propulsion (AIP) system for installation on Scorpene submarines.

Under Project-75, Mazagon Dock Limited (MDL) is constructing six Scorpene submarines in India, facilitated by technology transfer from Naval Group as part of a $3.75 billion deal signed in October 2005. The project is nearing completion, with several submarines already commissioned: INS Kalvari in December 2017, INS Khanderi in September 2019, INS Karanj in March 2021, and INS Vela in November 2021. The fifth submarine, INS Vagsheer, is currently in the trial phase, and it is expected to be delivered by early 2024. As of now, the Indian Navy operates 16 conventional submarines, including seven Russian Kilo-class submarines, four German HDW submarines, and five Scorpene-class submarines

This development comes as India negotiates with France for three additional Scorpene-class submarines. The new submarines will feature the DRDO-developed AIP system to enhance their endurance, with plans to retrofit AIP modules on all Scorpene submarines during refits. Naval Group is actively supporting the DRDO in this endeavor. In addition to submarines, Naval Group has signed agreements with other Indian companies for various naval projects.

Ultimately, the choice of AIP technology for diesel-electric submarines depends on various factors, including tactical requirements and combat environments. Fuel cell AIP technology offers maximum stealth and is ideal for long-range patrols, while MESMA AIP is suitable for high-speed bursts and endurance. Stirling Engine-based AIP is well-suited for shorter-range littoral combat operations.


Submarine Air Independent Propulsion Market Growth

The submarine air-independent propulsion (AIP) market is expected to grow at a significant rate in the coming years, driven by increasing demand for submarines from navies around the world, growing geopolitical tensions, and the need for more advanced and stealthy submarines.

According to a report by Fortune Business Insights, the global submarine AIP market is expected to reach USD 180.01 billion by 2030, growing at a compound annual growth rate (CAGR) of 5.01% from 2023 to 2030.

The growing demand for submarines is one of the key factors driving the growth of the submarine AIP market. Submarines are becoming increasingly important for navies around the world, as they offer a number of advantages, including stealth, maneuverability, and long-range capabilities.

Geopolitical tensions are also driving the demand for submarines. As tensions rise between countries, navies are looking to upgrade their fleets with more advanced and capable submarines.

The market of AIP systems is expected to show robust growth due to increasing need for safe and secure underwater military operations and demand for submarine modernization plans by the naval forces. Saab, DCNS, ThyssenKrupp Marine Systems, Howaldtswerke-Deutsche Werft (HDW), Siemens and United Technologies Corporation, among others, are some of the major players of the AIP systems market.

Four AIP systems have been developed: closed cycle diesel engine (CCD), autonomous submarine energy module (MESMA), stirling engine and fuel cells. Out of all the AIP systems, stirling engine and fuel cell AIP modules are the most prominent systems that have been used in 2016 and is estimated to witness the higher demand during the forecast period 2017-2026. The fuel cell module market for AIP systems is estimated to generate the highest revenue during the forecast period.

The AIP systems can be installed in submarines by two ways namely, line fit and retro fit. Retro fitting an AIP system into an old conventional submarine is a complex task as compared to equipping AIP systems into the submarine during its construction. Therefore, line fit AIP systems into submarines is expected to have the highest demand as compared to retro fit during the forecast period 2017-2026.

Asia-Pacific is expected to have the highest market during the forecast period (2017-2026), followed by Europe and Middle-East. The increase in the demand for AIP systems in Asia-Pacific is due to the adoption of military modernization by various naval forces and the need for underwater security. Japan, China, India, Australia, Thailand, Singapore and South Korea are some of the prominent nations for the development of AIP systems. Moreover, China holds the largest fleet of AIP equipped submarines, globally.

Future Prospects

As battery technology improves, AIP systems continue to evolve, enabling submarines to stay submerged for extended periods and significantly enhancing their stealth capabilities. The combination of advanced batteries and AIP technology promises a future where submarines can operate with the endurance and silence of nuclear-powered boats but at a fraction of the cost. The choice of AIP technology will depend on the specific operational requirements of each navy, ensuring a diverse and flexible approach to underwater warfare.

In conclusion, it’s essential to understand that submarines equipped with Air Independent Propulsion (AIP) technology won’t use it during every deployment. During routine patrols or in friendly waters, AIP-equipped submarines will frequently snorkel to recharge their batteries. AIP is reserved for operational deployments due to the relatively high cost of the fuels, oxidizers, and consumables used in the system, which makes monthly replenishment uneconomical.

AIP technology involves two distinct choices: the type of batteries used in the submarine’s design and the technology for generating electricity deep underwater, which powers the submarine’s engine and other electrical systems. Once the batteries are selected, they cannot be easily replaced with different technologies. Currently, there is a focus on Lithium-ion Batteries (LiB), which offer significant advantages in terms of weight, space, and power over traditional lead-acid accumulators.

The power required to propel a submarine is related to the cube of its hull speed. LiBs require less space than classic accumulators for low-speed cruising but even less space at higher speeds to provide the same propulsive power. However, careful selection of the battery chemistry and robust control systems are necessary to prevent overheating, overcharging, and potential safety issues.

Batteries’ capacity and reliability are continually improving due to extensive research, and AIP technologies are also undergoing significant enhancements. These advancements in batteries and AIP systems will enable future AIP-equipped submarines to stay submerged for extended periods, akin to pseudo-nuclear submarines. The technology holds great promise, and we can expect more modern navies to adopt it for their diesel-electric submarine fleets in the future.


Air-independent propulsion (AIP) has ushered in a new era for diesel-electric submarines, making them more lethal, stealthier, and capable of enduring extended underwater deployments.

With the global proliferation of AIP technology and ongoing advancements, submarines equipped with AIP systems are poised to play a crucial role in modern naval warfare. AIP-equipped submarines can remain submerged for extended periods, conduct covert operations, and strike with precision when necessary.

As naval technology continues to evolve, AIP will play an increasingly vital role in ensuring the effectiveness of submarines in modern warfare. The force multiplier effect created by AIP technology strengthens a nation’s maritime defense and power projection capabilities, making it an indispensable asset in the naval arsenal.



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