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{{short description|Propulsion system for submarines which operates without access to atmospheric oxygen}}
{{Use dmy dates|date=May 2020}}

'''Air-independent propulsion''' ('''AIP'''), or '''air-independent power''', is any [[marine propulsion]] technology that allows a non-nuclear [[submarine]] to operate without access to [[Earth's atmosphere|atmospheric]] [[oxygen]] (by surfacing or using a [[Submarine snorkel|snorkel]]). AIP can augment or replace the [[Diesel-electric transmission#Submarines|diesel-electric propulsion system]] of non-nuclear vessels.

Modern non-nuclear submarines are potentially stealthier than [[nuclear submarine]]s; although some modern submarine reactors are designed to rely on natural circulation, most naval nuclear reactors use pumps to circulate constantly the reactor coolant, generating some amount of [[Acoustic signature|detectable noise]].<ref>{{cite web |title=S8G (Submarine, 8 for 8th generation and G for General Electric)|url=https://www.globalsecurity.org/military/systems/ship/systems/s8g.htm |website=GlobalSecurity.org |access-date=20 September 2021|location=Alexandria VA}}</ref><ref>{{cite book |last1=Polmar |first1=Norman |title=Cold War submarines : the design and construction of U.S. and Soviet submarines |date=2004 |publisher=Potomac Books |location=Washington, D.C. |isbn=1574885308 |edition=1st |chapter=10: Second Generation Nuclear Submarines}}</ref> 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, speed, range and deep-ocean performance, small, high-tech non-nuclear attack submarines can be highly effective in coastal operations and pose a significant threat to less-stealthy and less-maneuverable nuclear submarines.<ref>{{cite web|title=Tomorrow's Submarines: the Non-Nuclear Option|url=http://argee.net/DefenseWatch/Tomorrows%20Submarine%20Fleet--The%20Non-nuclear%20Option.htm|publisher=DefenseWatch|access-date=2012-07-02|archive-date=7 July 2012|archive-url=https://web.archive.org/web/20120707054338/http://argee.net/DefenseWatch/Tomorrows%20Submarine%20Fleet--The%20Non-nuclear%20Option.htm|url-status=dead}}</ref>

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 [[battery (electricity)|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 [[hull (watercraft)|hulls]] by inserting an additional hull section. AIP does not normally provide the endurance or [[power (physics)|power]] to replace atmospheric dependent propulsion, but allows longer submergence than a conventionally propelled submarine. A typical conventional power plant provides 3 [[megawatt]]s maximum, and an AIP source around 10% of that. A nuclear submarine's propulsion plant is usually much greater than 20 megawatts.

The [[United States Navy]] uses the [[hull classification symbol]] "SSP" to designate boats powered by AIP, while retaining "SSK" for classic diesel-electric [[attack submarine]]s.{{efn|United States Navy Glossary of Naval Ship Terms (GNST). SSI is sometimes used, but SSP has been declared the preferred term by the USN. SSK (ASW Submarine) as a designator for classic diesel-electric submarines was retired by the USN in the 1950s, but continues to be used colloquially by the USN and formally by navies of the British Commonwealth and corporations such as Jane's Information Group.}}

==History==
[[File:Ictineo II.jpg|thumb|right|A replica of ''[[Ictineo II]]'', Monturiol's pioneering submarine, in Barcelona.]]
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.

The earliest attempt at a fuel that would burn anaerobically was in 1867, when Spanish engineer [[Narciso Monturiol]] successfully developed a chemically powered anaerobic or air independent steam engine.<ref>Cargill Hall, R. (1986). ''History of rocketry and astronautics: proceedings of the third through the sixth History Symposia of the International Academy of Astronautics, Volumen 1''. NASA conference publication. American Astronautical Society by Univelt, p. 85. {{ISBN|0-87703-260-2}}</ref><ref>[http://www.lowtechmagazine.com/2008/08/submarines-1.html A steam powered submarine: the Ictíneo] Low-tech Magazine, 24 August 2008</ref>

In 1908 the Imperial Russian Navy launched the [[Russian submarine Pochtovy|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.

==Types==

===Open-cycle systems===
[[File:SS X-1 Midget Submarine.jpg|thumb|X-1 midget submarine on display at the [[Submarine Force Library and Museum]] in the United States]]
During [[World War II]] the German firm [[Hellmuth Walter|Walter]] experimented with submarines that used [[high-test peroxide|high-test (concentrated) hydrogen peroxide]] as their source of oxygen under water. These used [[steam turbine]]s, 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 {{convert|50|nmi|km|abbr=off}}, and the final designs were little better.

After the war one Type XVII boat, {{GS|U-1407||2}}, which had been [[Scuttling|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|submarine|6}}, and {{HMS|Excalibur}}. ''Meteorite'' was not popular with its crews, who regarded it as dangerous and volatile; she was officially described as 75% safe.<ref>{{cite book |last1=Paterson |first1=Lawrence |title=Dönitz's last gamble : the inshore U-boat campaign, 1944-45 |date=2008 |publisher=[[Pen & Sword]] |location=Barnsley, UK |isbn=9781844157143}}</ref> The reputations of ''Excalibur'' and ''Explorer'' were little better; the boats were nicknamed Excruciater and Exploder.<ref>{{cite book |last1=Miller |first1=David |title=The Illustrated Directory of Submarines of the World |date=2002 |publisher=MBI Publishing |location=St. Paul, MN |isbn=0760313458 |pages=326-327 |chapter=Explorer - class}}</ref>

The [[Soviet Union]] also experimented with the technology and [[Soviet submarine S-99|one experimental boat was built]] which utilized hydrogen peroxide in a [[Hellmuth Walter Kommanditgesellschaft|Walter engine]].

The United States also received a Type XVII boat, ''U-1406'', and went on to begin two AIP submarine projects. Project [[Ship Characteristics Board|SCB 66]] developed an experimental [[midget submarine]], {{USS|X-1||2}}, which was launched in September 1955. 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.<ref>{{cite web|title=SS X-1 |url=http://www.hnsa.org/ships/x1.htm |publisher=Historic Naval Ships Association |access-date=24 February 2014 |url-status=dead |archive-url=https://web.archive.org/web/20130818031654/http://www.hnsa.org/ships/x1.htm |archive-date=18 August 2013 }}</ref>{{sfnp|Friedman|1994|pages=217-222}}

The second U.S. Navy project was of a full sized AIP submarine under SCB 67 in 1950, later SCB 67A. This submarine, designated SSX, would have one of three propulsion plants under development: a Walther open cycle hydrogen peroxide plant (termed ''Alton''), a liquid oxygen steam plant (''Ellis''), and an AIP gas turbine (''Wolverine''). By late 1951 the Navy realized that while the competing nuclear designs were heavier due to shielding, they were more compact than the three AIP plants: the SSX would be longer than the SSN by nearly 40 feet. The SSN would likely be quieter and less complicated than the AIP technology of this time. By 1952 the [[nuclear reactor]]s were so far along in development it appeared that the SSX submarine would not be needed as a stopgap. The project was cancelled on 26 October 1953.{{sfnp|Friedman|1994|pages=47-48}}

The USSR and the UK, the only other countries known to be experimenting with the technology at that time, also abandoned it when the US developed the nuclear reactor small enough for submarine propulsion. Other nations, including Germany and Sweden, would later recommence AIP development.

It was retained for propelling [[torpedo]]es by the British and the Soviet Union, although hastily abandoned by the former following the {{HMS|Sidon|P259|6}} tragedy. Both this and the loss of the {{ship|Russian submarine|Kursk|K-141|6}} were due to accidents involving hydrogen peroxide propelled torpedoes.

===Closed-cycle diesel engines===
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 would 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 [[Soviet M-class submarine|M-class]] vessels were built using the [[REDO]] system, but none were completed before the German invasion in 1941.

During World War II the German ''[[Kriegsmarine]]'' experimented with such a system as an alternative to the Walter peroxide system, designing variants of their [[Type XVII U-boat]] and their [[Seehund#Type XXVIIB|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 {{sclass2|Quebec|submarine|0}} 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 shaft]]s: a 32D {{convert|900|bhp|kW|abbr=on}} diesel on the centre shaft and two M-50P {{convert|700|bhp|kW|abbr=on}} diesels on the outer shafts. In addition a {{convert|100|hp|abbr=on}} "creep" motor was coupled to the centre shaft. The boat could be run at slow speed using the centreline diesel only.<ref>{{cite book | last = Preston | first = Anthony | title = Submarine Warfare | publisher = Brown Books | year = 1998 | isbn = 1-897884-41-9| page = 100}}</ref>

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, {{ship|Soviet submarine|M-256||2}}, sank following an explosion and fire. They were sometimes nicknamed cigarette lighters.<ref>Cold War Submarines: The Design and Construction of U.S. and Soviet Submarines
By Norman Polmar, Kenneth J. Moore pg 44</ref> The last submarine using this technology was scrapped in the early 1970s.

The [[German Navy]]'s former [[Type 205 submarine]] {{ship|German submarine|U-1|1967|2}} (launched 1967) was fitted with an experimental {{convert|3000|hp|kW|abbr=on}} unit.

===Closed-cycle steam turbines===
The French MESMA (Module d'Energie Sous-Marin Autonome) system is offered by French shipyard DCNS. MESMA is available for the [[Agosta-class submarine|''Agosta'' 90B]] and {{sclass2|Scorpène|submarine|1}}s. 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 [[Atmosphere (unit)|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 {{convert|8.3|m|ft|adj=on}}, 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.<ref>{{cite web|url=http://www.dcnsgroup.com/files/pdf/Mesma.pdf|title=DCNS Group|work=DCNS Group|access-date=26 July 2015|url-status=dead|archive-url=https://web.archive.org/web/20081115061409/http://www.dcnsgroup.com/files/pdf/Mesma.pdf|archive-date=15 November 2008}}</ref><ref name=india>{{cite web|url=http://www.defenseindustrydaily.com/india-looks-to-modify-scorpene-subs-with-mesma-aip-propulsion-01954/|title=India Looks to Modify Scorpene Subs With MESMA AIP Propulsion|date=1 March 2006|work=Defense Industry Daily|access-date=26 July 2015}}</ref>

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."<ref name=india/>

===Stirling cycle engines===
[[File:HMS Gotland with USS Ronald Reagan.jpg|thumb|HSwMS ''Gotland'' in San Diego]]

The Swedish shipbuilder [[Kockums]] constructed three {{sclass|Gotland|submarine|1}}s for the [[Swedish Navy]] that are fitted with an auxiliary [[Stirling engines|Stirling engine]] that burns liquid oxygen and diesel fuel to drive 75 kW electrical generators for either propulsion or charging batteries. The endurance of the 1,500-tonne boats is around 14 days at {{convert|5|kn|mph km/h|abbr=on|lk=in}}.

Kockums refurbished and upgraded the Swedish {{sclass|Västergötland|submarine|0}} submarines with a Stirling AIP plugin section. Two (''Södermanland'' and ''Östergötland'') are in service in Sweden as the {{sclass|Södermanland|submarine|4}}, and two others are in service in Singapore as the {{sclass|Archer|submarine|4}} (''Archer'' and ''Swordsman'').{{cn|date=October 2021}}

Kockums also delivered Stirling engines to Japan. Ten Japanese submarines were equipped with Stirling engines. The first submarine in the class, {{sclass|Sōryū|submarine|5}}, was launched on 5 December 2007 and delivered to the navy in March 2009. The eleventh of the class is the first one that is equipped with lithium-ion batteries without a Stirling engine.<ref>{{Cite web|url=https://www.swzmaritime.nl/news/2019/12/31/japanese-submarines-exchange-stirling-engines-for-lithium-ion-batteries/|title=Japanese Submarines Exchange Stirling Engines for Lithium-ion Batteries|website=SWZ|Maritime}}</ref>

The new Swedish {{sclass|Blekinge|submarine|1}} has the Stirling AIP system as its main energy source. The submerged endurance will be more than 18 days at 5 knots using AIP.{{cn|date=October 2021}}

===Fuel cells===
[[File:U Boot 212 HDW 1.jpg|thumb|right|[[Type 212 submarine]] with fuel cell propulsion of the [[German Navy]] in dock]]
[[Siemens AG|Siemens]] has developed a 30–50 kilowatt [[Fuel cells|fuel cell]] unit, a device that converts the [[chemical energy]] from a fuel and oxidiser into electricity. Fuel cells differ from batteries in that they require a continuous source of fuel (such as hydrogen) and oxygen, which are carried in the vessel in pressurized tanks, to sustain the chemical reaction. Nine of these units are incorporated into [[Howaldtswerke-Deutsche Werft|Howaldtswerke Deutsche Werft]] AG's 1,830 [[tonne|t]] submarine {{ship|German submarine|U-31|S181|2}}, lead ship for the [[Type 212 submarine|Type 212A]] of the [[German Navy]]. The other boats of this class and HDW's AIP equipped export submarines ({{sclass|Dolphin|submarine|4}}, [[Type 209 submarine|Type 209 mod]] and [[Type 214 submarine|Type 214]]) use two {{convert|120|kW|hp|abbr=on}} modules, also from Siemens.<ref>{{cite web|url=http://www.naval-technology.com/projects/type_212/|title=U212 / U214 Submarines - Naval Technology|access-date=26 July 2015}}</ref>

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.{{cn|date=October 2021}}

The AIP implemented on the {{sclass2|S-80|submarine|4}} 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 cell]]s from [[Collins Aerospace]] (which also supplied fuel cells for the [[Space Shuttle]]).

The reformer 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 cell]]s.<ref name="armada_cap">{{Cite web|url=https://armada.defensa.gob.es/ArmadaPortal/page/Portal/ArmadaEspannola/buques_unidades/prefLang-en/01_Submarino-S-80--05_capacidades_es|title=Armada Española - Ministerio de Defensa - Gobierno de España|first=Spanish|last=Navy|website=armada.defensa.gob.es}}</ref>

The [[Naval Materials Research Laboratory]] of Indian [[Defence Research and Development Organisation]] in collaboration with [[Larsen & Toubro]] and [[Thermax]] has developed a 270 kilowatt [[phosphoric acid fuel cell]] (PAFC) to power the{{sclass|Kalvari|submarine|1||2015}}s, which are based on the {{sclass2|Scorpène|submarine|5}} design. All six Kalvari class submarines will be retrofitted with AIP during their first upgrade. It produces electricity by reacting with hydrogen generated from [[sodium borohydride]] and stored oxygen with [[phosphoric acid]] acting as an electrolyte.<ref>{{cite news|date=3 November 2014|title=Indian-built Scorpene to carry critical DRDO system|work=The Hindu|url=http://www.thehindu.com/news/national/indianbuilt-scorpene-to-carry-critical-drdo-system/article6558305.ece|access-date=2015-10-22}}</ref><ref>{{Cite news|others=Special Correspondent|date=2021-03-09|title=Key indigenous technology for submarines crosses important milestone: DRDO|language=en-IN|work=The Hindu|url=https://www.thehindu.com/news/national/key-indigenous-technology-for-submarines-crosses-important-milestone-drdo/article34027076.ece|access-date=2021-03-11|issn=0971-751X}}</ref><ref>{{Cite web|last=Dar|first=Younis|date=2021-03-10|title=Success With New Propulsion Technology Will Greatly Benefit Indian Submarines|url=https://eurasiantimes.com/why-is-indias-drdo-going-gaga-over-new-propulsion-system-for-submarines/|access-date=2021-03-11|website=EurAsian Times|language=en-US}}</ref>
[[File:DRDO AIP (Air Independent Propulsion) model for Kalvari-class submarine.jpg|thumb|left|DRDO AIP (air independent propulsion) model for Kalvari-class submarine]]

The Portuguese Navy {{sclass|Tridente|submarine|1}}s are also equipped with fuel cells.

==Nuclear power==
{{main|Nuclear submarine|Nuclear marine propulsion}}

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.<ref name="Ferguson2014">{{cite book|author=Julie H. Ferguson|title=Through a Canadian Periscope: The Story of the Canadian Submarine Service|url=https://books.google.com/books?id=UrgWiHhaHVEC&pg=PA363|date=10 March 2014|publisher=Dundurn|isbn=978-1-4597-1056-6|page=363}}</ref><ref>{{cite web |title=The Nuclear Battery: A Solid-State, Passively Cooled Reactor for the Generation of Electricity and/or High-Grade Steam Heat |first1=K. S. |last1=Kozier |first2=H. E. |last2=Rosinger |publisher=Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Limited |location=Pinawa, Manitoba |year=1988 |url=http://www.nuclearfaq.ca/NB_02.pdf}}</ref>

Nuclear reactors have been used since the 1950s to power submarines. The first such submarine was [[USS Nautilus (SSN-571)|USS ''Nautilus'']] commissioned in 1954. Today, [[China]], [[France]], [[India]], [[Russia]], the [[United Kingdom]] and the [[United States]] are the only countries to have built and operated nuclear-powered submarines successfully.

==Non-nuclear AIP submarines==
As of 2017, some 10 nations are building AIP submarines with almost 20 nations operating AIP based submarines:
{|class="wikitable sortable"
|-
! Country || AIP type || Builders || Submarines with AIP || Operators || Numbers with AIP, and notes
|-
| rowspan="5" | {{GER}} || rowspan="5" | Fuel cell || rowspan="5" |[[Siemens]]-[[ThyssenKrupp Marine Systems|ThyssenKrupp]]||{{sclass|Dolphin|submarine|4}}
|{{ISR}}
|5 active / 1 under construction<ref name="defUpdate_6subOrder">{{cite news |url=http://defense-update.com/wp/20110506_enhanced_dolphin.html |title=Israel to Receive a Third Enhanced Dolphin Submarine |first=Tamir |last=Eshel |date=6 May 2011 |access-date=25 July 2011 |publisher=[[Defense Update]]|archive-url=https://web.archive.org/web/20110702071739/http://defense-update.com/wp/20110506_enhanced_dolphin.html|archive-date=2 July 2011}}</ref><ref name="gilad">
{{cite web|url=http://www.israeldefense.com/?CategoryID=426&ArticleID=663 |title=Sixth Submarine: "The Contract Continues" |publisher=israeldefense.com |date=31 October 2011 |access-date=25 December 2014 |url-status=dead |archive-url=https://web.archive.org/web/20131021081137/http://israeldefense.com/?CategoryID=426&ArticleID=663 |archive-date=21 October 2013 }}</ref>
|-
|[[Type 209 submarine|Type 209-1400mod]]
|{{KOR}} 
{{GRE}} 
{{EGY}}
|1 confirmed retrofit with AIP,<ref name="greeceu214submarine">{{cite web|url=http://www.defenseindustrydaily.com/Greece-in-Default-on-U-214-Submarine-Order-05801/|title=The Odyssey: Greece’s U-214 Submarine Order|date=8 October 2014|work=Defense Industry Daily|access-date=19 December 2014}}</ref> up to 9 additional {{sclass|Chang Bogo|submarine|4}} possibly retrofit.<ref>{{cite web|url=http://www.military-today.com/navy/chang_bogo_class.htm|title=Chang Bogo Class Patrol Submarine - Military-Today.com|last=ARG|website=www.military-today.com}}</ref><ref name=janes>{{cite web|url=http://articles.janes.com/articles/Janes-Navy-International-99/SOUTH-KOREAN-SUBMARINE-ARRIVES-AT-PEARL-HARBOR.html|title=Defense & Security Intelligence & Analysis: IHS Jane's - IHS|website=articles.janes.com}}</ref><ref name="nana">{{cite book | last = Kim | first = Duk-Ki | title = Naval Strategy in Northeast Asia: Geo-strategic Goals, Policies and Prospects | publisher = Routledge | year = 2000 | page = 30 | isbn = 0-7146-4966-X }}</ref><ref name="EastAsian">{{cite book | last = Meconis | first = Charles |author2=Wallace | title = East Asian Naval Weapons Acquisitions in the 1990s: Causes, Consequences, and Responses | publisher = Praeger | year = 2000 | page = 229 | isbn = 0-275-96251-2}}</ref>
|-
|[[Type 212 submarine|Type 212]]
|{{GER}} {{ITA}} {{NOR}} (planned)
|10 active / 8 more planned<ref name="Todaro">{{cite web|url=http://www.marina.difesa.it/sommergibili/classetodaro.asp|title=Classe Todaro page at Marina Militare website|access-date = 2010-04-27}}</ref><ref>Holger Naaf: ''[http://vzb.baw.de/publikationen/kolloquien/0/Vortrag_7_Brennstoffzellenantrieb.pdf Die Brennstoffzelle auf U 212 A (PDF, German).]'' Bundesanstalt für Wasserbau, Wehrtechnische Dienststelle für Schiffe und Marinewaffen Eckernförde, 23. September 2008.</ref> 
Norway plans to procure four submarines based on the Type 212 by 2025.<ref name="norway212">{{cite web|url=http://navaltoday.com/2017/02/03/german-tkms-will-build-norways-submarines/|title=German TKMS will build Norway’s submarines|website=navaltoday.com|date=2017-02-03|access-date=2017-05-05}}</ref>
|-
|[[Type 214 submarine|Type 214]]
|{{KOR}} {{GRE}} {{POR}} {{TUR}}
|13 active / 2 under construction / 8 more planned<ref>{{Citation |title=Fuel Cell Propulsion of Submarines |author=Dr. Albert E. Hammerschmidt(Siemens AG, Erlangen) |url=http://www.sea.siemens.com/us/internet-dms/Internet/MarineComm/General/Docs/Siemens%20Presents%20Fuel%20Cell%20at%20the%20Advance%20Naval%20Propulsion%20Symposium.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110716070146/http://www.sea.siemens.com/us/internet-dms/Internet/MarineComm/General/Docs/Siemens%20Presents%20Fuel%20Cell%20at%20the%20Advance%20Naval%20Propulsion%20Symposium.pdf |archive-date=16 July 2011}}</ref><ref>[http://www.ssm.gov.tr/home/projects/naval/Warship/Sayfalar/NewTypeSub(AIP)Pro.aspx New Type Submarine (AIP) Project] {{webarchive|url=https://web.archive.org/web/20110722083530/http://www.ssm.gov.tr/home/projects/naval/Warship/Sayfalar/NewTypeSub(AIP)Pro.aspx |date=22 July 2011 }}, Undersecretariat for Defence Industries of the Republic of Turkey</ref> 
3 Turkish orders are being built at [[Gölcük Naval Shipyard]]. 3 more are planned.
|-
|[[Type 218 submarine|Type 218]]
|{{SIN}}
|2 under construction / 2 more planned, with first delivery expected in 2020.<ref name="Navyrecognition">{{cite web|url=http://www.navyrecognition.com/index.php/news/defence-news/year-2013-news/december-2013-navy-naval-forces-maritime-industry-technology-security-global-news/1397--singapore-ministry-of-defence-signs-contract-with-tkms-for-two-new-type-218sg-submarines.html|title=Singapore Ministry of Defence Signs Contract with TKMS for two new Type 218SG submarines}}</ref><ref name="DefenseUpdate">{{cite web|url=http://defense-update.com/20131205_singapore_type_218sg-submarine.html|title=Singapore’s Type-218SG – Forerunner of a new Submarine Class? - Defense Update:|website=defense-update.com}}</ref><ref name="ThomsonReuters">{{cite web|url=http://in.reuters.com/article/thyssenkrupp-submarines-idINL5N0JH37T20131202|title=UPDATE 1-ThyssenKrupp wins submarine order from Singapore|first=Reuters|last=Editorial}}</ref>
|-
| rowspan="4" | {{SWE}} || rowspan="4" | Stirling AIP || rowspan="4" |[[Kockums Naval Solutions|Kockums]]||{{sclass|Gotland|submarine|4}}
|{{SWE}}
|3 active<ref name="kockums">{{cite web | title=The Gotland class submarine - submerged several weeks | work=Kockums | url=http://www.kockums.se/en/products-services/submarines/gotland-class/ | access-date=2008-04-06 | archive-date=25 April 2011 | archive-url=https://web.archive.org/web/20110425220545/http://www.kockums.se/en/products-services/submarines/gotland-class/ | url-status=dead }}</ref>
|-
|{{sclass|Archer|submarine|4}}
|{{SIN}}
|2 active (retrofit of the {{sclass|Västergötland|submarine|4}})<ref name="Ref_">{{cite web |title=Kockums receives Singapore order to two submarines |work=Kockums |url=http://www.kockums.se/news/oldnews/051104order.html |access-date=2005-11-19 |archive-date=6 June 2011 |archive-url=https://web.archive.org/web/20110606154729/http://www.kockums.se/news/oldnews/051104order.html |url-status=dead }}</ref>
|-
|{{sclass|Södermanland|submarine|4}}
|{{SWE}}
| 2 active (retrofit of the {{sclass|Västergötland|submarine|4}})
|-
|{{sclass|Blekinge|submarine|1}}
|{{SWE}}
| 2 planned
|-
| rowspan="2" | {{JAP}} || rowspan="2" | Stirling AIP || rowspan="2" |[[Kawasaki Heavy Industries|Kawasaki]]-[[Kockums Naval Solutions|Kockums]]||{{sclass|Harushio|submarine|4}}
|{{JAP}}
||1 retrofit: ''Asashio.''<ref>{{cite web|last1=Sutton|title=World survey of AIP submarines|url=http://www.hisutton.com/World%20survey%20of%20AIP%20submarines.html|website=HISutton.com|access-date=22 November 2016}}</ref>
|-
|{{sclass|Sōryū|submarine|4}}
|{{JAP}}
|12 active
|-
| rowspan="2" | {{FRA}} || rowspan="2" | MESMA || rowspan="2" | [[Naval Group]]
||[[Agosta-class submarine|''Agosta'' 90B]]
|{{PAK}}
|3 in service
|-
|{{sclass2|Scorpène|submarine|5}}
|{{CHL}} {{BRA}} (planned)
| 6 active (of 7 completed) / 4 under construction / 3 more planned
|-
|{{ESP}}
|Fuel cell
|[[Navantia]]
|{{sclass2|S-80|submarine|4}}
|{{ESP}}
|4 under construction / 4 planned
|-
| {{IND}} || Fuel cell ||[[Defence Research and Development Organisation]]
|{{sclass|Kalvari|submarine|4||2015}}
|{{IND}}
|All six {{sclass|Kalvari|submarine|4||2015}} will be retrofitted with AIP during their first upgrade<ref>{{cite web|url=http://www.thehindu.com/news/national/scorpene-submarine-programme-makes-progress/article22637521.ece|title=Scorpene submarine programme makes progress|website=The Hindu|access-date=2 February 2018}}</ref>
|-
| rowspan="2" | {{RUS}} || rowspan="2" | Fuel cell || rowspan="2" | [[Rubin Design Bureau]] NIISET Krylov ||[[Lada-class submarine|Project 677 ''Лада'' (Lada)]]
|{{RUS}}||Rumoured status: no confirmation that systems are operational on any Russian submarines
|-
|[[Amur-class submarine|Project 1650 ''Амур'' (Amur)]]
|None
|
|-
| rowspan="2" | {{PRC}} || rowspan="2" | Stirling AIP || rowspan="2" |[[China State Shipbuilding Corporation|711 Research Institute-CSHGC]]||[[Type 041 submarine|Type 041 (Yuan class)]]
|{{PRC}}|| 15 completed and 5 under construction
|-
|[[Type 032 Submarine|Type 032 (Qing class)]]
|{{PRC}}
|Experimental submarine
|-
| {{ROK}} || Fuel cell ||[[Hyundai Heavy Industries]] [[Daewoo Shipbuilding & Marine Engineering]]||[[Dosan Ahn Changho-class submarine]]
|{{ROK}}|| 1 completed / 2 under construction / 6 more planned
|-
|}

== References ==
{{reflist}}

==Notes==
{{notelist}}

==Sources==
* {{cite book
|title=U.S. Submarines Since 1945: An Illustrated Design History
|last=Friedman|first=Norman |author-link=
|publisher=United States Naval Institute
|year=1994
|location=Annapolis, Maryland |url= |doi=
|isbn=1-55750-260-9
}}

==Further reading==
* {{cite thesis |degree=M.S. Naval Architecture and Marine Engineering and M.S. Mechanical Engineering |first=LCDR Grant B.|last=Thornton |title=A Design Tool for the Evaluation of Atmosphere Independent Propulsion in Submarines |publisher=Massachusetts Institute of Technology |date=1994| url=http://www.hydrogen-peroxide.us/uses-oxygen-generation/US-Naval-Acad-Design-Tool-Air-Independent-Submarine-Propulsion-1979.pdf }}
* {{citation|title=Air-Independent Propulsion: AIP Technology Creates a New Undersea Threat |first=Edward C. |last=Whitman |work=Undersea Warfare |issue=13 |date=Fall 2001 |publisher=United States Navy |url=http://www.navy.mil/navydata/cno/n87/usw/issue_13/propulsion.htm |url-status=dead |archive-url=https://web.archive.org/web/20071208164906/http://www.navy.mil/navydata/cno/n87/usw/issue_13/propulsion.htm |archive-date=8 December 2007 }}
* {{Cite news|url = http://foxtrotalpha.jalopnik.com/sweden-has-a-sub-thats-so-deadly-the-us-navy-hired-it-t-1649695984|title = Sweden Has A Sub That's So Deadly The US Navy Hired It To Play Bad Guy|last = Rogoway|first = Tyler|date = 23 October 2014|work = Foxtrot Alpha|access-date = 17 January 2017}}

{{DEFAULTSORT:Air-Independent Propulsion}}
[[Category:Submarine design]]
[[Category:Marine propulsion]]
[[Category:Spanish inventions]]

Air-independent propulsion - Revision history

>Dough34 at 17:53, 27 December 2021