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). AIP can augment or replace the diesel-electric propulsion system of non-nuclear vessels.
Modern non-nuclear submarines are potentially stealthier than nuclear submarines; a nuclear ship's reactor must constantly pump coolant, generating some amount of detectable noise (see acoustic signature). Non-nuclear submarines running on battery power or AIP, on the other hand, can be virtually silent. While nuclear-powered designs still dominate in submergence times and deep-ocean performance, small, high-tech non-nuclear attack submarines are highly effective in coastal operations and pose a significant threat to less-stealthy and less-maneuverable nuclear submarines.
AIP is usually implemented as an auxiliary source, with the traditional diesel engine handling surface propulsion. Most such systems generate electricity which in turn drives an electric motor for propulsion or recharges the boat's batteries. The submarine's electrical system is also used for providing "hotel services"—ventilation, lighting, heating etc.—although this consumes a small amount of power compared to that required for propulsion.
AIP can be retrofitted into existing submarine hulls by inserting an additional hull section. AIP does not normally provide the endurance or power to replace atmospheric dependent propulsion, but allows longer submergence than a conventionally propelled submarine. A typical conventional power plant provides 3 megawatts maximum, and an AIP source around 10% of that. A nuclear submarine's propulsion plant is usually much greater than 20 megawatts.
In the development of the submarine, the problem of finding satisfactory forms of propulsion underwater has been persistent. The earliest submarines were man-powered with hand-cranked propellers, which quickly used up the air inside; these vessels had to move for much of the time on the surface with hatches open, or use some form of breathing tube, both inherently dangerous and resulting in a number of early accidents. Later, mechanically driven vessels used compressed air or steam, or electricity, which had to be re-charged from shore or from an on-board aerobic engine.
In 1908 the Imperial Russian Navy launched the submarine Pochtovy which used a gasoline engine fed with compressed air and exhausted under water.
These two approaches, the use of a fuel that provides energy to an open-cycle system, and the provision of oxygen to an aerobic engine in a closed cycle, characterize AIP today.
During World War II the German firm Walter experimented with submarines that used concentrated hydrogen peroxide as their source of oxygen under water. These used steam turbines, employing steam heated by burning diesel fuel in the steam/oxygen atmosphere created by the decomposition of hydrogen peroxide by a potassium permanganate catalyst.
Several experimental boats were produced, though the work did not mature into any viable combat vessels. One drawback was the instability and scarcity of the fuel involved. Another was that while the system produced high underwater speeds, it was extravagant with fuel; the first boat, V-80, required 28 tons of fuel to travel 50 nautical miles, and the final designs were little better.
After the war one Type XVII boat, U-1407, which had been scuttled at the end of World War II, was salvaged and recommissioned into the Royal Navy as HMS Meteorite. The British built two improved models in the late 1950s, HMS Explorer, and HMS Excalibur. Meteorite was not popular with its crews, who regarded it as dangerous and volatile; she was officially described as 75% safe. The reputations of Excalibur and Explorer were little better; the boats were nicknamed Excruciater and Exploder.
The United States also received a Type XVII boat, U-1406, and went on to use hydrogen peroxide in an experimental midget submarine, X-1. It was originally powered by a hydrogen peroxide/diesel engine and battery system until an explosion of her hydrogen peroxide supply on 20 May 1957. X-1 was later converted to a diesel-electric.
The USSR, UK, and US, the only countries known to be experimenting with the technology at that time, abandoned it when the latter developed a nuclear reactor small enough for submarine propulsion. Other nations, including Germany and Sweden, would later recommence AIP development.
It was retained for propelling torpedoes by the British and the Soviet Union, although hastily abandoned by the former following the HMS Sidon tragedy. Both this and the loss of the Russian submarine Kursk were due to accidents involving hydrogen peroxide propelled torpedoes.
This technology uses a submarine diesel engine which can be operated conventionally on the surface, but which can also be provided with oxidant, usually stored as liquid oxygen, when submerged. Since the metal of an engine will burn in pure oxygen, the oxygen is usually diluted with recycled exhaust gas. Argon replaces exhaust gas when the engine is started.
In the late 1930s the Soviet Union experimented with closed-cycle engines, and a number of small M-class vessels were built using the REDO system, but none were completed before the German invasion in 1941.
During World War II the Kriegsmarine experimented with such a system as an alternative to the Walter peroxide system, designing variants of their Type XVII U-boat and their Type XXVIIB Seehund midget submarine, the Type XVIIK and Type XXVIIK respectively, though neither was completed before the war's end.
After the war the USSR developed the small 650-ton Quebec-class submarine of which thirty were built between 1953 and 1956. These had three diesel engines—two were conventional and one was closed cycle using liquid oxygen.
In the Soviet system, called a "single propulsion system", oxygen was added after the exhaust gases had been filtered through a lime-based chemical absorbent. The submarine could also run its diesel using a snorkel. The Quebec had three drive shafts: a 32D 900 bhp (670 kW) diesel on the centre shaft and two M-50P 700 bhp (520 kW) diesels on the outer shafts. In addition a 100 hp (75 kW) "creep" motor was coupled to the centre shaft. The boat could be run at slow speed using the centreline diesel only.
Because liquid oxygen cannot be stored indefinitely, these boats could not operate far from a base. It was dangerous; at least seven submarines suffered explosions, and one of these, M-256, sank following an explosion and fire. They were sometimes nicknamed cigarette lighters. The last submarine using this technology was scrapped in the early 1970s.
The French MESMA (Module d'Energie Sous-Marine Autonome) system is offered by French shipyard DCNS. MESMA is available for the Agosta 90B and Scorpène-class submarines. It is essentially a 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 atmospheres. This pressure-firing allows exhaust carbon dioxide to be expelled overboard at any depth without an exhaust compressor.
Each MESMA system costs around $50–60 million. As installed on the Scorpènes, it requires adding an 8.3-metre (27 ft), 305-tonne hull section to the submarine, and results in a submarine able to operate for greater than 21 days underwater, depending on variables such as speed.
An article in Undersea Warfare Magazine notes that: "although MESMA can provide higher output power than the other alternatives, its inherent efficiency is the lowest of the four AIP candidates, and its rate of oxygen consumption is correspondingly higher."
The Swedish shipbuilder Kockums constructed three Gotland-class submarines for the Swedish Navy that are fitted with an auxiliary Stirling engine that burns liquid oxygen and diesel fuel to drive 75 kilowatt electrical generators for either propulsion or charging batteries. The endurance of the 1,500-tonne boats is around 14 days at 5 kn (5.8 mph; 9.3 km/h).
Kockums has also refurbished/upgraded the Swedish Västergötland-class submarines with a Stirling AIP plugin section. Two (Södermanland and Östergötland) are in service in Sweden as the Södermanland class, and two others are in service in Singapore as the Archer class (Archer and Swordsman).
Kockums also delivered Stirling engines to Japan. New Japanese submarines will all be equipped with Stirling engines. The first submarine in the class, Sōryū, was launched on 5 December 2007 and were delivered to the navy in March 2009.
The new Swedish A26 submarine has the Stirling AIP system as its main energy source. The submerged endurance will be more than 18 days at 5 knots using AIP.
Siemens has developed a 30-50 kilowatt fuel cell unit, a device that converts the chemical energy from a fuel into electricity. Fuel cells differ from batteries in that they require a continuous source of fuel (such as hydrogen) and oxygen to sustain the chemical reaction, which are carried in the vessel in pressurized tanks. Nine of these units are incorporated into Howaldtswerke Deutsche Werft AG's 1,830 t submarine U-31, lead ship for the Type 212A of the German Navy. The other boats of this class and HDW's AIP equipped export submarines (Dolphin class, Type 209 mod and Type 214) use two 120 kW (160 hp) modules, also from Siemens.
After the success of Howaldtswerke Deutsche Werft AG in its export activities, several builders developed fuel-cell auxiliary units for submarines, but as of 2008 no other shipyard has a contract for a submarine so equipped.
The AIP implemented on the S-80 class of the Spanish Navy is based on a bioethanol-processor (provided by Hynergreen from Abengoa, SA) consisting of a reaction chamber and several intermediate Coprox reactors, that transform the BioEtOH into high purity hydrogen. The output feeds a series of fuel cells from UTC Power company (which also supplied fuel cells for the Space Shuttle).
The reformator is fed with bioethanol as fuel, and oxygen (stored as a liquid in a high pressure cryogenic tank), generating hydrogen as a sub-product. The produced hydrogen and more oxygen is fed to the fuel cells.
Indian Defence Research and Development Organisation has developed an AIP system based on Phosphoric Acid Fuel Cell (PAFC) to power the last two Kalvari-class submarines which are based on the Scorpène design.
The Portuguese Navy Tridente-class submarines are also equipped with fuel cells.
Air-independent propulsion is a term normally used in the context of improving the performance of conventionally propelled submarines. However, as an auxiliary power supply, nuclear power falls into the technical definition of AIP. For example, a proposal to use a small 200 kilowatt reactor for auxiliary power—styled by AECL as a "nuclear battery"—could improve the under-ice capability of Canadian submarines.
Nuclear reactors have been used since the 1950s to power submarines. The first such submarine was USS Nautilus commissioned in 1954. Today, China, France, India, Russia, the United Kingdom and the United States are the only countries to have successfully built and operated nuclear-powered submarines.
As of 2017, some 10 nations are building AIP submarines with almost 20 nations operating AIP based submarines:
|Country||AIP type||Builders||Submarines with AIP||Operators||Numbers with AIP, and notes|
|Germany||Fuel cell||Siemens-ThyssenKrupp||Dolphin class||Israel||3 active / 2 under construction / 1 on order.|
|Type 209-1400mod|| South Korea
||1 confirmed retrofit with AIP, up to 9 additional Chang Bogo class possibly retrofit.|
|Type 212|| Germany
|8 active (of 10 completed) / 14 planned.|
Norway plans to procure four submarines based on the Type 212 by 2025.
|Type 214|| South Korea
|11 active (of 13 completed) / 2 under construction / 17 planned.|
Turkish order is being built at Gölcük Naval Shipyard.
|Type 218||Singapore||2 on order, with first delivery expected in 2020.|
|Sweden||Stirling AIP||Kockums||Gotland class||Sweden||3 active / 3 planned.|
|Västergötland class||Sweden||All 4 retrofit with AIP. Two eventually became the Archer class. and the other two became the Södermanland class.|
|Archer class||Singapore||2 active / 2 planned.|
|Södermanland class||Sweden||2 active / 4 planned|
|Japan||Stirling AIP||Kawasaki-Kockums||Harushio class||Japan||1 retrofit: Asashio.|
|Sōryū class||Japan||8 active (of 9 completed) / 3 under construction / 12 planned.|
||MESMA||Naval||Agosta 90B||Pakistan||3 in service|
|4 in service / 19 planned|
|Spain||Fuel cell||Técnicas Reunidas||S-80 class||Spain||4 under construction / 4 planned|
|India||Fuel cell||DRDO||Kalvari class||India||All 6 Kalvari-class submarines will be retrofitted with AIP during their first upgrade|
|Russia||Fuel cell||Rubin Design Bureau
|Project 677 Лада (Lada)||Russia||Rumoured status: no confirmation that systems are operational on any Russian submarines|
|Project 1650 Амур (Amur)||None|
|People's Republic of China||Stirling AIP||711 Research Institute-CSHGC||Type 041 (Yuan class)||People's Republic of China||15 completed and 5 under construction|
|Type 032 (Qing class)||People's Republic of China||Experimental submarine|
The Amur-class submarine (named for the Amur River), is one of the latest Russian submarine designs. It is advertised as an export version of the Lada-class, a modernised version of the Kilo-class submarine with improved acoustic stealth, new combat systems, and an option for air-independent propulsion (AIP).
The new vessels are the fourth generation of the Kilo submarine family, with two models developed.
As advertised, the Amur-1650 is larger and intended for longer missions. The Amur-950 is armed with a VLS missile system capable of salvo-fire at multiple predesignated targets. Sonar signatures of these submarines are several times lower than the older Kilo-class submarines. Both designs are equipped with electronic warfare armament of the newer generation created on the basis of the latest science and technology. They can be outfitted with AIP fuel cells, considerably improving submerged endurance and range. AIP capability can be added in a hull extension plug either during new build construction, or as a refit to existing boats.
The builder claims these vessels can operate in all areas of the world except areas with solid ice cover, in all weather conditions, and in shallow and deep water.Archer-class submarine
The Archer class submarines are the newest class of diesel-electric submarines in active service with the Republic of Singapore Navy (RSN). Originally launched as the Swedish Navy Västergötland class submarines HMS Hälsingland and HMS Västergötland in 1986 and 1987, the two submarines were sold to Singapore in November 2005 and relaunched in June 2009 and October 2010 respectively after extensive modernisation by Kockums, which included a refit to Södermanland class standards, the insertion of a new hull section with an air independent propulsion system, and additional climatisation for use in tropical waters.German submarine U-1405
U-1405 was a Type XVIIB U-boat of Nazi Germany's Kriegsmarine during the Second World War. She was one of a small number of U-boats fitted with Hellmuth Walter's high test peroxide propulsion system, which offered a combination of air-independent propulsion and high submerged speeds.The U-1405 was laid down on 15 October 1943 at the Blohm & Voss, Hamburg, as yard number 255. She was launched on 1 December 1944 and commissioned under the command of Oberleutnant zur See Wilhelm Rex on 24 April 1944.German submarine U-792
U-792 was a Type XVIIA U-boat of Nazi Germany's Kriegsmarine during the Second World War. She was one of a small number of U-boats fitted with Hellmuth Walter's high test peroxide propulsion system, which offered a combination of air-independent propulsion and high submerged speeds. She spent the war as a trials vessel and was scuttled on 4 May 1945 in the Audorfer See, near Rendsburg.German submarine U-793
U-793 was a Type XVIIA U-boat of Nazi Germany's Kriegsmarine during the Second World War. She was one of a small number of U-boats fitted with Hellmuth Walter's high test peroxide propulsion system, which offered a combination of air-independent propulsion and high submerged speeds. She spent the war as a trials vessel and was scuttled on 4 May 1945 in the Audorfer See, near Rendsburg.German submarine U-794
U-794 was a Type XVIIA U-boat of Nazi Germany's Kriegsmarine during the Second World War. She was one of a small number of U-boats fitted with Hellmuth Walter's high test peroxide propulsion system, which offered a combination of air-independent propulsion and high submerged speeds. She spent the war as a trials vessel and was scuttled on 5 May 1945 in Gelting Bay.
U-794 was built by Friedrich Krupp Germaniawerft, Kiel. The keel was laid down on 1 February 1943, the boat was launched on 7 October. She was commissioned on 14 November. The Feldpost Number was M 52 496.
U-794 did not undertake any combat patrols and was instead assigned as a trials boat at first to the 5th U-boat Flotilla, followed by the 8th U-boat Flotilla, before returning to the 5th flotilla for the rest of the war. In late March 1944, Admiral Karl Dönitz and four other admirals took part in a trial of U-794. Although they were enthusiastic, the boat, designed for high underwater speed (over 20 m.p.h.) was difficult to manoeuvre, and the keel to beam ratio was too high.German submarine V-80
The V-80 (German: Versuchs-U-Boot V 80) was a 76-ton experimental submarine and the only representative of the German Type V design produced for Nazi Germany's Kriegsmarine.
The prototype was completed in 1940 in Friedrich Krupp Germaniawerft in Kiel. The four-man vessel was designed to test the Walter hydrogen peroxide-based turbine propulsion system. Its range was 50 nmi (93 km; 58 mi) at 28 knots (52 km/h; 32 mph).
The only earlier attempt to use a chemical reaction based air-independent propulsion system was in the Spanish submarine the Ictineo II.
This midget submarine led to the design of the German Type XVII submarine.Harushio-class submarine
The Harushio class is a diesel-electric submarine class operated by the Japanese Maritime Self-Defense Force (JMSDF). The design is an evolution from the Yūshio class being slightly larger and with better noise reduction. Asashio, has been modified to test air-independent propulsion (AIP), and the remaining vessels were decommissioned and replaced by the Oyashio-class.Lada-class submarine
Project 677 or Lada class (Russian: Лада; NATO Reporting Name: St. Petersburg class, after the name of the lead vessel, Sankt Peterburg) is the new advanced class of diesel-electric attack submarine designed by the Russian Rubin Design Bureau. A program to develop a "fourth generation" diesel-electric submarine, it aimed to produce a highly improved version of the Project 636 Kilo class with much quieter, new combat systems, and possibly air-independent propulsion.List of submarine classes in service
The list of submarine classes in service includes all submarine classes currently in service with navies or other armed forces worldwide. For surface combatants, see the list of naval ship classes in service.Näcken-class submarine
The Näcken-class submarines, also known as the A14 type, were built for the Swedish Navy in the late 1970s. The boats were authorised in 1972 and the programme was completed in 1981. All boats were built by Kockums in Karlskrona. The boats had a teardrop hull and diving depth was 150 metres (490 ft). Between 1987 and 1988 Näcken was cut in half and an 8-metre (26 ft) long hull section containing a prototype Air-independent propulsion (AIP) using a closed cycle Stirling engine was installed between the aft battery/propulsion & power control room and the engine/motor room. This technology increased underwater endurance to 14 days and has been adopted in subsequent Swedish submarines.
By the early 2000s the class was decommissioned from the Swedish navy. HSwMS Näcken was temporary leased to the Royal Danish Navy but was returned in 2005.Project 75I-class submarine
The Project 75I-class submarine is a follow-on of the Project 75 Kalvari-class submarines for the Indian Navy. Under this project, the Indian Navy intends to acquire six diesel-electric submarines, which will also feature advanced air-independent propulsion systems to enable them to stay submerged for longer duration and substantially increase their operational range. All six submarines are expected to be constructed in Indian shipyards.Project Kalina
Project Kalina is a proposed fifth-generation diesel-electric submarine currently being developed by TsKB Rubin and others, for the Russian Navy. It will be fitted with air-independent propulsion technology, and copies may also be sold to China..Scorpène-class submarine
The Scorpène-class submarines are a class of diesel-electric attack submarines jointly developed by the French Direction des Constructions Navales (DCN) and the Spanish company Navantia, and now by Naval Group. It features diesel propulsion and an additional air-independent propulsion (AIP).
The Chilean Navy ordered two Scorpène-class boats, which replaced two Oberon-class submarines retired by the Chilean Navy. In 2005, the Indian Navy ordered six Scorpène-class; all the Indian boats will be built in India, at Mazagon Dock and elsewhere, and the last two are to be fitted with an Indian Fuel cell AIP module. For the follow-on requirement of six submarines, DCNS plans to offer a larger version of the submarine to the Indian Navy. In 2008, the Brazilian Navy ordered four Scorpènes.
The Chilean Scorpène-class O'Higgins and Carrera were completed in 2005 and 2006, respectively. In 2009, the Royal Malaysian Navy commissioned Tunku Abdul Rahman and Tun Abdul Razak. The Indian Navy commissioned Kalvari in 2017.Södermanland-class submarine
The Swedish Södermanland class of diesel-electric submarines consists of HSwMS Södermanland and HSwMS Östergötland. These two submarines were originally launched as Västergötland-class submarines in 1987 and 1990, and have been relaunched as a new class after extensive modernization in 2003 and 2004 by Kockums AB. The pressure hull had been cut in two after the sail and a 12 m (39 ft 4 in) long new section with an air-independent propulsion system was inserted. It contains two Stirling engines which are coupled to electric generators and heated by burning diesel fuel with liquid oxygen stored in cryogenic tanks. The AIP system can provide electric energy to extend the submarine's submerged time from days to weeks.
The class is planned to remain in service until 2019-20 when it will be replaced by the future A26 submarine.Two submarines of similar modifications, known as the Archer class are in service with the Republic of Singapore Navy.Sōryū-class submarine
The Sōryū-class submarines (16SS) are diesel-electric attack submarines. The first boat in the class entered service with the Japan Maritime Self-Defense Force in 2009. The design is an evolution of the Oyashio-class submarine, from which it can most easily be distinguished by its X-shaped stern combination diving planes and rudders. The Sōryūs have the largest displacement of any submarine used by post-war Japan.It is Japan's first air-independent propulsion submarine. The boats in the class are fitted with Kockums Naval Solutions Stirling engines license-built by Kawasaki Heavy Industries, allowing them to stay submerged for longer periods of time.
The cost of the sixth submarine (Kokuryū) was estimated at 540 million USD.Type 212 submarine
The German Type 212 class, also Italian Todaro class, is a highly advanced design of non-nuclear submarine developed by Howaldtswerke-Deutsche Werft AG (HDW) for the German and Italian navies. It features diesel propulsion and an additional air-independent propulsion (AIP) system using Siemens proton exchange membrane (PEM) compressed hydrogen fuel cells. The submarines can operate at high speed on diesel power or switch to the AIP system for silent slow cruising, staying submerged for up to three weeks without surfacing and with little exhaust heat. The system is also said to be vibration-free, extremely quiet and virtually undetectable.
Type 212 is the first fuel cell propulsion system equipped submarine series.Type 214 submarine
The Type 214 is a diesel-electric submarine developed by Howaldtswerke-Deutsche Werft GmbH (HDW). It features diesel propulsion with an air-independent propulsion (AIP) system using Siemens polymer electrolyte membrane (PEM) hydrogen fuel cells. The Type 214 submarine is derived from the Type 212, but as an export variant it lacks some of the classified technologies of its smaller predecessor, the most important of which is probably the non-magnetic steel hull, which makes the Type 212 submarine difficult to detect using a magnetic anomaly detector.A contract to build three boats for the Hellenic Navy was signed 15 February 2000 and a fourth unit was ordered in June 2002. The first boat was built at HDW in Kiel, Germany and the rest at the Hellenic Shipyards Co. in Skaramangas, Greece. The Hellenic Navy named them the Papanikolis-class.
The Republic of Korea Navy has ordered nine Type 214 submarines, designated as Son Won-Il-class, to be built in Korea by Hyundai Heavy Industries and Daewoo Shipbuilding & Marine Engineering; three first batch models had entered service since 2007, and six second batch models will enter service from 2012.
Due to improvements in the pressure hull materials, the Type 214 can dive nearly 400 meters. It can also carry food, fresh water and fuel for 84 days of operation.Type XVII submarine
The Type XVII U-boats were small coastal submarines that used a high-test peroxide propulsion system, which offered a combination of air-independent propulsion and high submerged speeds.