Air-independent propulsion refers to a technology that can drive a submarine for a long time without obtaining oxygen from the outside air. The self-sustaining power of submarines using this technology is more than twice that of ordinary diesel-electric submarines. That is, the continuous submersible time and submersible distance are longer, but they are still much shorter than nuclear submarines. Its cost is between ordinary diesel-electric submarines and nuclear submarines. Closed cycle diesel (CCD) AIP, Stirling engine (SE) AIP and fuel cell (FC) AIP. These three AIP solutions have been adopted by various naval powers; for example, the United Kingdom and the Netherlands have adopted the CCDAIP solution, Sweden and others have adopted the SEAIP solution, and Germany has adopted the FCAIP solution. At present, only Russia, Germany, Sweden, France, Spain, and China clearly possess this technology. In 1935, S.A. Bazilevskiy, a designer of the 18th Central Design Bureau of the Soviet Union, proposed the use of an air-independent system in submarines and experimented on the M-92 submarine, and obtained a large amount of experimental data. However, the technology was not mature at that time, and the designer proposed air-independent technology only as an experimental subject. During World War II, the German Walter Company attempted to use compressed hydrogen peroxide in submarines as an underwater oxygen source for diesel engines. The principle is that when operating underwater, the thermal energy of the gas turbine heats the hydrogen peroxide with potassium permanganate as the catalyst to produce oxygen for use by the diesel engine. Later, Germany built a few such air-independent propulsion submarines. One of the boats, U-1407, was abandoned at the end of the war. After the war, it was re-entered by the British and renamed HMS Meteorite. Later, the British built another one based on Meteorite. Two identical submarines, HMS Explorer and HMS Excalibur. After the war, the Soviet Union restarted experiments with hydrogen peroxide fuel, but it was finally abandoned due to the extreme instability of hydrogen peroxide when converting into oxygen. The Soviet Union and the United Kingdom both started their own nuclear power research and gave up further research after seeing that the United States already had nuclear reactors that could be used for submarines. But this propulsion system is still used in Soviet and British torpedoes. However, this unstable fuel still caused the tragedy of the British Royal Navy HMS Sidon and the Russian Navy's Kursk nuclear submarine. Japan itself does not claim to possess this technology, but instead imported Stirling engines from Sweden and produced them by Kawasaki Heavy Industries. Japan itself is good at fuel cell technology, but it does not use the current proton exchange membrane fuel cell technology because it cannot meet the needs of Japan's large submarines. Japan is expected to use fuel cells with hydrogen storage metal technology in the next-generation Soryu-type submarine to meet the demand. . In addition, it is said that in response to Japan's related actions, South Korea is also applying to Sweden to introduce the same type of Stirling engine. Because the United States can afford expensive nuclear submarines and has no urgent need for shallow water combat capabilities, it has not developed an AIP propulsion system. On the other hand, nuclear-powered submarines also belong to AIP, but nuclear submarines are generally classified independently and are not included in ordinary diesel-electric AIP submarines. Therefore, countries with nuclear submarines such as China, the United States, Russia, the United Kingdom, and France have no urgent need to develop AIP technology. They are mostly used in periods of financial shortage or for export models. Since Germany and Japan were defeated in World War II, the Allies implemented a ban on the production, introduction and possession of any form of nuclear weapons (including nuclear-powered submarines) against the defeated countries, such as Japan's post-war national policy "Post-War Constitution" and "Three Non-nuclear Principles" ". Compared with Britain, France, the United States, Russia and China, which have nuclear-powered submarines that can conduct offensive operations in the ocean, Germany and Japan are not satisfied with diesel-electric submarines that are only used for offshore defense operations. Therefore, Germany and Japan have further developed nuclear-powered submarines based on traditional diesel-electric submarine power technology. It did not rely on air propulsion engines and became the main underwater combat power of the German and Japanese navies.