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Submarines carry out extremely important and diverse roles including intelligence gathering, surveillance, reconnaissance, insertion and extraction of divers or special forces, attacking enemy submarines and their surface warships, mine laying, together with other littoral or choke point operations. Naval Warfare depends a large extent on lethality and survivability of submarines, its principal weapon, therefore Navies have been researching new ways to make them quieter and increasing their underwater endurance. Propulsion system plays an extremely important role in the functioning of a submarine for the completion of its desired operations.
A conventional submarine’s diesel engine generates electricity which can be used to drive the propeller and power its systems. The problem is that such a combustion engine is inherently quite noisy and runs on air—a commodity in limited supply on an underwater vehicle. As battery technology improved, the endurance of these submarines increased proportionally. But it was not enough to last them beyond a week. Traditional diesel-electric submarines need to surface frequently to charge their batteries and have an underwater endurance of only a few days. This is done by snorkeling, which exposes them to detection by enemy radars and makes them an easy target for hostile anti-submarine assets.
Although modern snorkels are coated with radar absorbing paint and have a stealthy shaping, they are still detectable by high resolution radars. There are also sensors called diesel sniffers which can detect the exhaust emissions of the submarines diesel generators while snorkeling. 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. A submarine which needs to surface every day, loses its element of surprise and increases its vulnerability to hostile anti-submarine assets.
The first nuclear-powered submarines were brought into service in the 1950s. Nuclear reactors are quieter, don’t consume air, and produce greater power output, allowing nuclear submarines to remain submerged for months instead of days while traveling at higher speeds under water. Nuclear-powered submarines have traditionally held a decisive edge in endurance, stealth and speed over cheaper diesel submarines.
However, new Air Independent Propulsion (AIP) technology has significantly narrowed the performance gap on a new generation of non-nuclear submarines that cost a fraction of the price of a nuclear-powered boat. 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.
Air-independent propulsion (AIP) is any marine propulsion technology that allows a non-nuclear submarine to operate without access to atmospheric oxygen (by surfacing or using a snorkel) thereby vastly improving their underwater endurance. AIP can augment or replace the diesel-electric propulsion system of non-nuclear vessels. AIP systems permit diesel-electric submarines to recharge their batteries independent of their engines. Also, it helps to reduce the noise without compromising the submarine performance.
Submarine designers and Navy submariners use an indiscretion ratio to indicate the proportion of mission time a submarine is detectable while charging its batteries. For conventional modern submarines the indiscretion ratio ranges typically 7-10% on patrol at 4 knots, and 20-30% in transit at 8-10 knots. This is where Air Independent Propulsion (AIP) comes in. It offers the possibility of increasing underwater endurance by a factor of up to three or four, which reduces the indiscretion ratio significantly.
AIP has a force multiplier effect on lethality of a diesel electric submarine as it enhances the submerged endurance of the boat, several folds. The hydraulics in a nuclear reactor produce noise as they pump coolant liquid, while an AIP’s submarine’s engines are virtually silent. AIP systems have been in high demand due to their increasing advantages in performing stealth underwater operations.
The advantage offered by increased underwater endurance can be used for ‘ambushing’ an approaching fleet. In one such scenario, an AIP equipped submarine can roam near a strait, waiting for its target to approach. The sub will be running at ultra-quiet speeds of 2-4 knots for several weeks and then attack the target when it appears, using its torpedoes. Even though a non-AIP equipped sub can do the same thing, it’s waiting period, which is very essential for an underwater ambush, is significantly lesser.
In another scenario, an AIP equipped sub can roam near enemy territory for far longer compared to a non-AIP sub. Thus in this situation where intelligence is gathered and spy missions are performed, AIP gives these quiet diesel subs an advantage by allowing them to loiter for weeks without the need to surface.
However, speed remains an undisputed strength of nuclear-powered submarines. U.S. attack submarine 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.
AIP also has downsides, Installing AIP increases length and weight of submarines; requires pressurized liquid oxygen (LOX) storage on-board and supply for all three technologies; MESMA and the Stirling engine have some acoustic noise from moving parts; and production costs increase the submarine’s unit cost by around 10%.
AIP boats may be considered as something in-between conventional and nuclear submarines, but unlike nuclear ones, AIS boats aren’t so noisy, they’re as silent as diesel electric submarines. However, Diesel electrics, even with AIP, have shorter ranges than nuclear boats and therefore require nearby bases. Although nuclear submarines offer far better endurance and speeds, they are unsuitable for the shallow littoral waters and most navies cannot afford to build and maintain them as they are very expensive. Also diesel submarines possess the advantage of being able to switch off thir engines completely and lie in wait unlike nuclear submarines whose reactors cannot be switched off at will.
Over the past decade, air-independent propulsion (AIP) for submarines has spread rapidly around the world.
AIP configurations found in diesel-electric submarines
The same general concept can be achieved via multiple methods. Japan traditionally has used the Kawasaki Kockums Stirling engine and France the MESMA system, both driving generators and less efficient than Germany’s HDW/Siemens fuel cell plant configurations which produce electricity directly.
The most modern versions range from using Stirling Engines, to the French MESMA (translated as autonomous submarine energy module) closed-cycle steam turbine system, to cutting-edge fuel cells to power the submarine while it is submerged for long periods of time. Each approach has its own advantages and disadvantages, with cost, complexity and technological risk being major factors beyond raw performance.
Closed Cycle Diesel Engines
Closed Cycle Engine is a heat engine in which the working substance is continuously circulated and does not need replenishment. This technology involves storing a supply of oxygen in the submarine in order to run a diesel engine while submerged. Liquid oxygen (LOX) is stored in tanks on board the submarine and sent to the diesel engine for combustion. Since they need to simulate the atmospheric oxygen concentration for the engines to run safely without getting damaged, the oxygen is mixed with an inert gas (usually argon) and then sent to the engine. The exhaust gases are cooled and scrubbed to extract any leftover oxygen and argon from them and the remaining gases are discharged into the sea after being mixed with seawater. The argon which is extracted from the exhaust is again sent into the diesel engine after being mixed with oxygen.
The main challenge with this technology is the storing of liquid oxygen safely on board the submarines. The Soviet subs which used this technology during the 1960s found them to be highly prone to fires and subsequently discontinued their usage. Closed Cycle Diesel AIP is hence not preferred for modern submarines even though it is comparatively cheaper and simplifies logistics by the use of standard diesel fuel.
Closed Cycle Steam Turbines
Steam turbines make use of a source of energy to heat water and convert it into steam in order to to spin a turbine and generate electricity. In nuclear powered submarines, the reactors provide the heat in order to convert water into steam. But in conventional closed cycle steam propulsion, a non-nuclear energy source is used to do the same.
The French MESMA (Module d’Energie Sous-Marine Autonome / Autonomous Submarine Energy Module ) is the only such system available and it makes use of ethanol and oxygen as energy sources. The combustion of ethanol and oxygen under high pressure is used to generate steam. The steam generated is the working fluid and is used to run the turbine. The high pressure combustion allows the exhaust carbon dioxide to be expelled outside into the sea at any depth without making use of a compressor.
The first full-scale undersea application will be in Pakistan’s three new Agosta 90B submarines, which are each be fitted with a 200 kilowatt MESMA system for increasing submerged endurance by a factor of three to five at a speed of 4 knots. Once again, the boat has to lug around ethanol and volatile liquid oxygen as well as complex machinery—which produces noise—to make the system work, but it can produce a lot of power, which is good for high-speed operations.
The advantage of MESMA is it’s higher power output when compared to the alternatives which allows higher underwater speeds but it’s major drawback is it’s lower efficiency. Also the rate of oxygen consumption is said to be very high and these systems are very complex. Cost is a major factor as MESMA is not a cheap technology to acquire or maintain. These drawbacks make several navies opt for sterling cycle and fuel cell alternatives.
Stirling Cycle
A Sterling Engine is a closed cycle engine with a working fluid which is permanently contained in the system. A source of energy is used to heat this working fluid, which in turn moves the pistons and runs the engine. The engine is coupled to a generator, which generates electricity and charges the battery. The source of energy used here is typically LOX as oxidizer and diesel fuel, which is burnt in order to generate heat for the working fluid. The exhaust is then scrubbed and released into the seawater.
This is slightly more efficient, and somewhat less complicated, than the French variant, and is used on Japanese, Swedish and Chinese boats. The advantage of using Sterling engines is the easy availability of diesel fuel and low refueling costs when compared with Fuel Cells. They are also quieter than MESMA and hence preferred by the Japanese for their Soryu class, Sweden for their Gotland and Västergötland class and China for their Yuan class.
The Stirling-cycle engine forms the basis of the first AIP system to enter naval service in recent times. The Swedish builders, Kockums Naval Systems, has fitted three Swedish Gotland-class boats each with two adjunct, 75 kilowatt Stirling-cycle propulsion units that burn liquid oxygen and diesel fuel to generate electricity for either propulsion or charging batteries within a conventional diesel-electric plant. The resulting underwater endurance of the 1,500-ton boats is reported to be up to 14 days at five knots, but significant burst speeds are possible when the batteries are topped up.
They are also bulky when compared to Fuel Cells. Although the technology is well proven and affordable, it also requires the boat to lug around liquid oxygen oxidizer, which can have its own dangers, as well as inert gas to mix with it, writes TYLER ROGOWAY. The Stirling engines and other infrastructure needed to make the system work, much of a relatively small submarine’s bulk gets taken up by the system.
The main drawback is that they are relatively noisy when compared to Fuel Cells due to the presence of a large number of moving parts. Engine can make noise even when a high-degree of soundproofing is designed into the submarine. The operating depth of a submarine using Sterling AIP is limited to 200 m when AIP is engaged.
Fuel Cell
Fuel-cell technology is probably the state of the art in AIP. A Fuel Cell is a device which converts chemical energy into electricity. This is done using a fuel and an oxidizer. A typical fuel cell converts Hydrogen (fuel) and Oxygen (oxidizer) into electricity, with water and heat released as by-products. This is done by an electrolytic cell which consists of two electrodes, one positive (anode) and the other negative (cathode), separated by an electrolytic barrier. The reaction between the cathode and anode produces an electric current, which is used to charge the batteries. A chemical catalyst is used to speed up the reactions.
A fuel cell has almost no moving parts which significantly reduces the acoustic signature of the sub. Fuel Cells can achieve an efficiency of over 80% under certain circumstances. It also is a very efficient system for long endurance missions. They can also be scaled easily into large or small sizes depending on the displacement of the submarine. This is easier than developing different systems for each submarine class. Hydrogen Fuel Cells are also very environment friendly as they generate no exhaust fumes, which in turn eliminates the need to have special exhaust scrubbing and disposal machinery. The only drawback is that they are expensive and complex. They are also not capable of quickly ramping-up its power output like say a MESMA configuration can.
Phosphoric Acid Fuel Cells (PAFC) and Proton Exchange Membrane Fuel Cells (PEMFC) are presently used in submarines. Germany is said to be the world leader in developing and fielding this type of AIP, which is backed by the large number of export orders they have received. France is developing a new generation Fuel Cell AIP as a successor to its MESMA. India is another country which is developing a Fuel Cell AIP to be integrated on their submarines.
There are several alternative configurations, but for submarine propulsion, so-called “Polymer Electrolyte Membrane” (PEM) fuel cells have attracted the most attention because of their low operating temperatures (80 degree Centigrade) and relatively little waste heat. In a PEM device, pressurized hydrogen gas (H2) enters the cell on the anode side, where a platinum catalyst decomposes each pair of molecules into four H+ ions and four free electrons. The electrons depart the anode into the external circuit – the load – as an electric current. Meanwhile, on the cathode side, each oxygen molecule (O2) is catalytically dissociated into separate atoms, using the electrons flowing back from the external circuit to complete their outer electron “shells.”
The polymer membrane that separates anode and cathode is impervious to electrons, but allows the positively-charged H+ ions to migrate through the cell toward the negatively charged cathode, where they combine with the oxygen atoms to form water. Thus, the overall reaction can be represented as 2H2 + O2 => 2H2O, and a major advantage of the fuel-cell approach is that the only “exhaust” product is pure water. Since a single fuel cell generates only about 0.7 volts DC (direct current), groups of cells are “stacked” together in series to produce a larger and more useful output. The stacks can also be arrayed in parallel to increase the amount of current available.
German-built submarines have successfully taken advantage of fuel cell technology, and the French, Russians and Indians are also moving in this direction. It is thought that Australia’s upcoming Shortfin Barracuda submarines, of French origin, will use fuel cell AIP propulsion. These massive submarines will offer as close to nuclear propulsion capabilities as possible. Israel’s latest Dolphin class boats also use fuel cell AIP, which makes sense as they work as Israel’s second-strike nuclear deterrent.
The S-80 is a new generation of submarines in production for the Spanish Navy by the national company Navantia. A total of for S-80 have been ordered by the Spanish Navy. The S-80 submarine will be powered by an air-independent propulsion (AIP) system, based on a bioethanol-processor consisting of a reaction chamber and several intermediate Coprox reactors. Provided by Hynergreen from Abengoa, the system transforms the bioethanol (BioEtOH) into high purity hydrogen. The output feeds a series of fuel cells from UTC Power company. The Reformator is fed with bioethanol as fuel and oxygen (stored as a liquid in a high-pressure cryogenic tank), generating hydrogen and carbon dioxide as sub-products. The produced hydrogen and more oxygen are fed to the fuel cells.
AIP propulsion spread widely around the globe
By the mid-2000s, converging technological developments enabled several major submarine producers around the world to begin to develop practical AIP systems. France, Germany, Japan, Sweden, Russia and China all laid down AIP-capable boats, in some cases exporting those submarines to customers around the world. Germany has four types of SSPs under construction for various navies. Newly constructed Type 209s may also have AIP.
AIP powered-submarines have now proliferated across the world using three different types of engines, with nearly 60 operational today in fifteen countries. Around fifty more are on order or being constructed. China has 15 Stirling-powered Yuan-class Type 039A submarines with 20 more planned, as well as a single large Type 032 missile submarine that can fire ballistic missiles. Japan for her part has eight medium-sized Soryu class submarines that also use Stirling engines, with 15 more planned for or under construction. The Swedes, for their part, have developed four different classes of Stirling-powered submarines.
Germany has also built dozens of AIP powered submarines, most notably the small Type 212 and 214, and has exported them across the globe. The German boats all use electro-catalytic fuel cells, a generally more efficient and quiet technology than the Stirling, though also more complex and expensive. Other countries intending to build fuel-cell powered submarines include Spain (the S-80), India (the Kalvari-class) and Russia (the Lada-class). Finally, France has designed several subs using closed-cycle steam turbine called MESMA. Three upgraded Agosta-90b class subs with MESMA engines serve in the Pakistani Navy.
Sweden has three classes of boats with AIP; the large Japanese Soryus will have AIP, as will the French Scorpenes, French-built Agosta 90Bs (for Pakistan) and Scorpene-inspired Kalvaris (for India). The Soryu-class submarines built by Mitsubishi and Kawasaki are among the most advanced AIP-equipped diesel-electric submarines. The new Spanish S-80s have AIP, as do the two small Portuguese Tridente boats.
The grandfather of AIP submarines, the Swedish Gotland class were the first submarines equipped as a class with the advanced propulsion system. At just 1,600 tons submerged, the Gotlands can do twenty knots submerged and five knots operating off AIP.
Russia’s troubled Lada class has AIP propulsion, and it is expected that the next diesel-electric class (Amur) will also have it. Russia’s latest submarine project, purportedly code named Project Kalina, except that the new sub will be equipped with an air-independent propulsion (AIP) system, a technology that the Russian defense industry has been struggling with for years.
China has been working on getting a working AIP system in one of their Yuan class Type 39B subs for over fifteen years but until now no Chinese AIP equipped boats were seen in action. That changed earlier in 2018 when new Yuan class sub went to sea and operated like an AIP boat (staying underwater for more than seven days at a time). According to the Chinese press releases their AIP sub stayed under for over two weeks at a time, which is typical of what a Stirling AIP system can do. China may now have three of these AIP equipped boats.
Thailand is buying from China Yuan Class S26 T boats, which were said to have been developed exclusively for Thailand based on China’s Yuan Class Type 039 A submarines. They would be nearly 78 metres long and 9 metres wide, equipped with the latest technology AIP (Air Independent Propulsion) system, that would allow them to dive consecutively up to 21 days without surfacing.
In late 2016 China confirmed that final details have been agreed to on the sale of eight Chinese S20 diesel-electric submarines to Pakistan. These are export versions of the Type 39A that lack many of the advanced features. Four of these will be built in China while at the same time Chinese personnel will assist Pakistan in building another four in Pakistan. Final cost is expected to average somewhere between $500 million and $600 million each and the first one will enter service by 2023.
Another highly advanced submarine class from Japan, the Soryu class is a large, 4,600 ton diesel-electric submarine with AIP propulsion. The Soryu class utilizes the original Swedish Kockums AIP system to stay quietly underwater. The latest submarine in class however, Oryu, was launched in early October 2018 at Kobe, Japan. Oryu is the first submarine equipped with a large bank of lithium ion fuel cells, the same types that power laptop computers and electric cars, to act as a powerful, long-lasting reservoir of energy for long endurance underwater.
The Republic of Korea (ROK) Navy’s seventh Son Won II-class (Type 214) diesel-electric air-independent propulsion (AIP) submarine, christened Hong Beom-do, will be operationally deployed beginning in May 2018, South Korea’s Defense Acquisition Program Administration (DAPA) announced on January 19. Each Son Won II-class sub measures 65 meters (213 feet) in length and seven meters (22 feet) in width. The boat’s top surface speed is around 12 knots and up to 20 knots when submerged powered by its electric motor. “With its air-independent propulsion system, built around Siemens polymer electrolytic membrane fuel cells, the submarine can stay submerged for up to two weeks and can dive up to 400 meters (1,312 feet) deep,” according to Franz-Stefan Gady in Diplomat.
South Korea currently operates a fleet of nine 1,200-ton Chang Bogo-class diesel-electric attack submarines – a variant of the German Type 209 boat. Under the first phase, the ROKN is planning to upgrade all nine Chang Bogo-class submarines with air-independent propulsion and flank-array sonars over the next few years
Indian AIP
The Defence Research and Development Organisation (DRDO) has achieved a crucial milestone when it demonstrated an Air Independent Propulsion (AIP) system in March 2021 which will enable Indian Navy submarines to operate for up to two weeks without having to surface to recharge its batteries. Earlier on 9 March 2021, the Ministry of Defence announced that “The (AIP) plant was operated in endurance mode and maximum power mode as per the user requirements. The system is being developed by the Naval Materials Research Laboratory (NMRL) of DRDO.”
The Defence Research Development Organisation (DRDO) developed Air-independent propulsion (AIP) technology has successfully completed its land-based trials of the AIP Module in its Naval Materials Research Laboratory (NMRL) lab at Ambernath. As per reports, NMRL was able to demonstrate AIP operation for the endurance of 14 days which was monitored by a team deputed from Indian Navy to monitor land-based trials of the AIP Module under simulated underwater condition.
“The DRDO-built AIP would be fitted on the Kalvari class submarines during their refit programme. The first refit of the first boat INS Kalvari is scheduled for the year 2023,” French firm Naval Group’s Senior Executive Vice President Alain Guillou told ANI. Indian Navy sources have confirmed that a decision has been reached on an expensive and time consuming process to install Air Independent Propulsion (AIP) modules on the six new Scorpene submarines to be inducted over the next few years.
NMRL believes that its Phosphoric Acid Fuel Cell (PAFC) based AIP solution is a step ahead of what the People’s Liberation Army Navy has obtained from Sweden in the form of a Stirling cycle based AIP for its submarines. Original Equipment Manufactures of AIP System never usually advertise their underwater continuous usage capability but it is usually around two weeks of usage after which Submarine needs surfacing “.
MESMA AIP module which is fitted in Pakistani Navy operated two French-designed Agosta 90B Class Submarines are relatively noisy when compared to Fuel Cell-based AIP module like one DRDO has developed due to the presence of a large number of moving parts. Not only MESMA AIP module is bulky when compared to Fuel Cells based AIP module it also compromises stealthiness of the sub when engaged, risking chances of Submarine detection.
Record set by a Type 212A German Submarine using a Siemens proton exchange membrane (PEM) compressed hydrogen fuel cells based Air-independent propulsion (AIP) system was for 18 days”. he further added ” If DRDO has achieved 14 days endurance with its Phosphoric acid fuel cell (PAFC) based Air-independent propulsion (AIP) system then we are par with Global leaders in AIP System ”
Moreover, NMRL’s modular architecture scores over a composite system, since even if one of the modules fails, the control system for the PAFC stacks can reconfigure the remaining operational units to continue to supply power output, albeit at a reduced quantum. This naturally increases the survivability of the system, which is of utmost importance when being used to propel a submarine stealthily.
DRDO developed PAFC powered AIP module technology already has been transferred to Thermax Ltd in Pune for further production even though first Scorpene submarines to get indigenous AIP System will happen only in 2021-22 when they come for their first refit after which all Six submarines will get AIP module
All sensors and weapons being equal, a Navy has to justify what type of diesel electric submarine to choose based not just on cost but also on what type of tactics they aim to employ and what type of combat environment they are most likely to fight in. For instance, if long-range patrols and ambush tactics are common, along with the need for maximum stealth, fuel cell AIP technology may be best. If bursts of high-speed during attack and evasion maneuvers are needed often, along with high endurance, MESMA may be most appropriate. For shorter-range littoral combat operations, the Stirling Engine-based AIP technology may make the most sense.
Conclusion
The thing to remember about AIP is that just because a submarine is equipped with that technology, it wont necessarily use them on every deployment. During regular patrols or in friendly territory, an AIP equipped submarine will snorkel often to recharge its batteries. Only when it is deployed operationally will it make use of AIP to increase its underwater endurance. This is because most of the fuels, oxidizers and other consumables used in AIP are quite expensive and it would not be economical to replenish them on a monthly basis.
AIP is about two separate choices. The kinds of batteries used in a submarine’s design, and the technology available to generate electricity deep underwater, which directly drives the submarine’s engine and supplies other electrical requirements. Once batteries are chosen for a design, they can’t be swapped for different technologies. Currently, focus is on the promise of Lithium ion Batteries (LiB), which offer significant weight, space and power advantages over classic lead acid accumulators.
Power required to propel a submarine is proportional to the cube of hull speed. To cruise at low speed, LiBs require about half the space of classic accumulators, but at higher speeds they require around 25% of classic accumulator space to provide the same propulsive power. This means more LiBs can be fitted in place of classic accumulators, offering greater underwater endurance. Dependent on their chemistry, if LiBs become overheated or overcharged they can experience thermal runaway, damaged cells and even a fire or explosion. Therefore, the right selection of chemistry—together with stringent control systems—are required to prevent this happening.
The capacity and reliability of batteries is increasing due to extensive research being conducted in that field. The various AIP technologies mentioned will also see large-scale improvement in capabilities. These two technologies combined, will allow AIP equipped submarines of the future to stay underwater for months at a time and make them pseudo-nuclear submarines. This technology has a bright future and we will see more modern navies adopting it for their diesel-electric submarine fleets.
Submarine Air Independent Propulsion Market Growth
Based on the on-going developments in the field of different AIP technologies, the total market size of AIP systems for submarines was valued at $174.1 million in 2016. The Global Submarine Air-Independent Propulsion (AIP) Systems Market is expected to exceed more than US$ 17 Billion by 2023 at a CAGR of 2.5% in the given forecast period.
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.
There are mainly four AIP systems that have been developed namely: 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.
References and Resources also include:
http://idrw.org/drdos-aip-technology-finally-coming-of-age/
https://defencyclopedia.com/2016/07/06/explained-how-air-independent-propulsion-aip-works/
