Air-independent propulsion
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.
The United States Navy uses the hull classification symbol "SSP" to designate boats powered by AIP, while retaining "SSK" for classic diesel-electric attack submarines.
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.
[4][5] 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.
[6] 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.
[8] The Soviet Union also experimented with the technology and one experimental boat was built which utilized hydrogen peroxide in a Walter engine.
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.
[11] 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.
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.
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.
[14][15] On the Agosta 90B, the AIP system allows the submarine to operate 16 days under water and gives it a range of 1,400 nautical miles (2,600 km; 1,600 mi).
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.
[17] The Type 212 can remain submerged for 21 days; one such submarine conducted a 1600 nautical mile journey solely on AIP in 2016.
[citation needed] The AIP implemented on the S-80 class of the Spanish Navy is based on a bioethanol-processor (provided by Hynergreen from Abengoa) consisting of a reaction chamber and several intermediate Coprox reactors, that transform the BioEtOH into high purity hydrogen.
[citation needed] 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.
It produces electricity by reacting with hydrogen generated from sodium borohydride and stored oxygen with phosphoric acid acting as an electrolyte.
[citation needed] Air-independent propulsion is a term normally used in the context of improving the performance of conventionally propelled submarines.
For example, a proposal to use a small 200-kilowatt reactor for auxiliary power—styled by Atomic Energy of Canada Limited (AECL) as a "nuclear battery"—could improve the under-ice capability of Canadian submarines.
