Air-independent propulsion

X-1 midget submarine on display at the Submarine Force Library and Museum in the United States

During World War II the German firm Walter experimented with submarines that used high-test (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 (93 kilometres), 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.[4]

The Soviet Union also experimented with the technology and one experimental boat was built which utilized hydrogen peroxide in a Walter engine.

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.[5]

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 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 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 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.[6]

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.[7] The last submarine using this technology was scrapped in the early 1970s.

The German Navy's former Type 205 submarine U-1 (launched 1967) was fitted with an experimental 3,000 hp (2,200 kW) unit.

The French MESMA (Module d'Energie Sous-Marin 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.[8][9]

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."[9]

HSwMS Gotland in San Diego

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 kW 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. Ten Japanese submarines were equipped with Stirling engines. The first submarine in the class, Sōryū, 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.[10]

The new Swedish Blekinge-class 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.

Type 212 submarine with fuel cell propulsion of the German Navy in dock

Siemens has developed a 30–50 kilowatt 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 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.[11]

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 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 cells.[12]

The Naval Materials Research Laboratory of Indian Defence Research and Development Organisation has developed an AIP system based on a Phosphoric Acid Fuel Cell (PAFC) to power the last two Kalvari-class submarines, which are based on the Scorpène design.[13][14]

The Portuguese Navy Tridente-class submarines are also equipped with fuel cells.