Air-independent propulsion

Air-independent propulsion (AIP) is any technology which allows a non-nuclear submarine to operate without the need to access atmospheric oxygen (by surfacing or using a snorkel). AIP can augment or replace the diesel-electric propulsion system of non-nuclear vessels.

The United States Navy uses the hull classification symbol "SSP" to designate boats powered by AIP, while retaining "SS" for classic diesel-electric attack submarines.

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. Non-nuclear submarines running on battery power or AIP, on the other hand, can be virtually silent. While nuclear-powered designs still dominate in terms of submerged endurance and deep-ocean performance, the new breed of small, high-tech non-nuclear attack subs are highly effective in coastal operations and pose a significant threat to less-stealthy and less-maneuverable nuclear subs.

AIP is usually implemented as an auxiliary source, with the traditional diesel engine handling propulsion on the surface. 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"&mdash;ventilation, lighting, heating etc.&mdash;although this consumes a small amount of power compared to that required for propulsion.

A benefit of this approach is it can be retrofitted into existing submarine hulls by inserting an additional hull section. AIP does not normally provide the endurance or power to replace the atmospheric dependent propulsion, but allows it to remain submerged longer than a more conventionally propelled submarine. A typical conventional power plant will provide 3 megawatts maximum, and an AIP source around 10% of that. A nuclear submarine's propulsion plant is usually much greater than 20 megawatts.

History
In 1867 Narcís Monturiol i Estarriol successfully developed a peroxide powered anaerobic or air independent steam engine. In 1908 the Imperial Russian Navy launched the submarine Pochtovy which used a gasoline engine fed with compressed air and exhausted under water.

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, and one, U-1407, which had been scuttled at the end of the war, was salvaged and recommissioned into the Royal Navy as HMS Meteorite. The British built two improved models in the late 1950s, HMS Explorer (submarine), and HMS Excalibur. Meteorite was not popular with its crews, who regarded it as a dangerous and volatile piece of machinery; she was officially described as "75% safe". The reputations of Excalibur and Explorer were little better, the boats were nicknamed Exploder and Excruciator.

The Soviet Union also experimented with the technology and one experimental boat was built. Hydrogen peroxide was eventually abandoned since it is volatile, highly reactive with various metals, and is consumed by submarines at a high rate. The only countries known to be experimenting with it, the Soviets and British, abandoned it when the United States developed a nuclear reactor small enough for submarine propulsion.

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.

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 will burn in pure oxygen, the oxygen is usually diluted with recycled exhaust gas. As there is no exhaust gas upon starting, argon is used.

During World War II the Kriegsmarine experimented with such a system as an alternative to the Walter peroxide system, including a variant of the Type XXVIIB Seehund midget submarine, the "Klein U-boot". It was powered by a 95 hp Diesel engine of a type commonly used by the Kriegsmarine and which was available in large numbers, supplied with oxygen from a tank in the boat's keel holding 1,250 litres at 4 atm. It was thought likely that the boat would have a maximum submerged speed of 12 kn and a range of 70 mi, or 150 mi at 7 kn.

The German work was subsequently expanded upon by the Soviet Union who invested heavily in this technology, developing the small 650 ton Quebec-class submarine of which thirty were built between 1953 and 1956. These had three diesel engines&mdash;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 engines: a 32D 900 bhp diesel on the centre shaft and two M-50P 700 bhp diesels on the outer shafts. In addition a 100 hp "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 for any great length of time these boats could not operate far from a base. It was also a dangerous system; 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 was scrapped in the early 1970s.

The German Navy's former Type 205 submarine U1 was fitted with an experimental 3000 horsepower (2.2 MW) unit.

Closed cycle steam turbines
The French MESMA (Module d'Energie Sous-Marine Autonome) system is being offered by the French shipyard DCNS. MESMA is available for the Agosta 90B and Scorpène-class submarine. It is essentially a modified version of their nuclear propulsion system with heat being generated by ethanol and oxygen. Specifically, a conventional steam turbine power plant is powered by steam generated from the combustion of ethanol (grain alcohol) 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 Scorpène, it requires adding a new 8.3 meter (27 foot), 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."

Stirling cycle engines


The Swedish shipbuilder Kockums has constructed three Gotland-class submarines for the Swedish Navy which are fitted with an auxiliary Stirling engine which uses liquid oxygen and diesel fuel to drive 75 kilowatt generators for either propulsion or charging batteries. The endurance of the 1,500-tonne boats is around 14 days at 5 kn.

Kockums has also refurbished/upgraded the Swedish Västergötland class submarines with a Stirling AIP plugin section. Two of these submarines (Södermanland and Östergötland) are in service in Sweden as the Södermanland class, and two of them are in service in Singapore as the Archer class (Archer and Swordsman).

Kockums has also delivered Stirling engines to Japan. The 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 will have the Stirling AIP system as its main energy source. The submerged endurance will be more than 18 days at 5 knots using AIP.

Fuel cells
Siemens has developed a 30-50 kilowatt fuel cell unit. Nine of these units are incorporated into Howaldtswerke Deutsche Werft AG's 1,830t submarine U31, lead ship for the Type 212A class of the German Navy. The other boats of this class and HDW's AIP equipped export submarines (Dolphin class submarine, Type 209 mod and Type 214) use two 120 kW modules, also from Siemens.

After the success of Howaldtswerke Deutsche Werft AG's in its export activities, several builders have developed their own 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 will transform the BioEtOH in 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.

Nuclear power
Nuclear reactors have been used for 50 years to power submarines, the first being USS Nautilus. The United States, France, the United Kingdom, Russia, India and the People's Republic of China are the only countries currently operating nuclear-powered submarines. India, having successfully developed a miniaturised reactor for submarine application, is developing Arihant-class nuclear submarines, the first of which is undergoing sea trials and induction is expected during mid-2012. India in the past has leased a Charlie-class nuclear-powered submarine from Russia and plans to acquire two used Akula-class submarines which would be used for training purposes. Many other developing countries have also attempted to research nuclear propulsion for submarine use in the past, but with disappointing results. However, air independent propulsion is a term normally used in the context of improving the performance of conventionally propelled submarines.

There have nevertheless been suggestions for a reactor as an auxiliary power supply, which does fall into the normal definition of AIP. For example, there has been a proposal to use a small 200 kilowatt reactor for auxiliary power (styled a "nuclear battery") to improve the under-ice capability of Canadian submarines.

Production non-nuclear AIP submarines
As of 2013, some 10 nations have non-nuclear AIP submarines:
 * 🇮🇱-Germany Dolphin class submarine (2,300 tonnes) (fuel cell) 1 operational, 2 building, of the Israeli navy
 * 🇫🇷-Spain Scorpène-class submarine (1,700 tonnes) (MESMA)
 * S-80 class (2,400 tonnes) of the Spanish Navy. None built as of 2012.
 * 🇩🇪 Type 209-1400mod (1,810 tonnes) (Fuel cell)
 * 🇩🇪 Type 212 submarine (1,830 tonnes) (fuel cell) of the German Navy and Italian Navy
 * 🇩🇪 Type 214 (1,980 tonnes) (Fuel cell)
 * 🇷🇺 Project 677 Лада (Lada)
 * 🇷🇺 Project 1650 Амур (Amur)
 * Asashio (2,750 tonnes) (Stirling AIP) of the Japan Maritime Self-Defense Force
 * Sōryū-class submarine (4,200 tonnes) (Stirling AIP) of the Japan Maritime Self-Defense Force
 * 🇸🇪 Gotland-class submarine (1,450 tonnes) (Stirling AIP) of the Swedish navy
 * 🇸🇪 Södermanland-class submarine (1,500 tonnes) (Stirling AIP) of the Swedish navy
 * Archer-class submarine. Its two submarines are originally Swedish Västergötland-class submarines. They are upgraded to Södermanland-class submarine standards.
 * Type 041 (Yuan class) submarine (Stirling AIP) of the PLAN
 * Type 032 (Qing class) submarine (Stirling AIP) of the PLAN
 * 🇰🇷 Chang Bogo-class submarine, which was modified out of the German Type 209 submarine, is an AIP submarine of the Republic of Korea Navy.

Also several shipbuilders offer AIP upgrades for existing submarines:
 * 🇩🇪 Nordseewerke (Closed-cycle diesel)
 * 🇸🇪 Kockums (Stirling), owned by German company ThyssenKrupp
 * 🇵🇰 Agosta 90B class submarine made in cooperation with France
 * 🇫🇷 Scorpène made by French Company DCNS