Air-independent Propulsion

Air-independent propulsion (AIP) is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing 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 recharging the boat's 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.

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.