attack

An attack submarine or hunter-killer submarine is a submarine specifically designed for the purpose of attacking and sinking other submarines, surface combatants, and merchant vessels. In the Soviet and Russian navies they were and are called "multi-purpose submarines". They are also used to protect friendly surface combatants and missile submarines. Some attack subs are also armed with cruise missiles, increasing the scope of their potential missions to include land targets.
Attack submarines may be either nuclear-powered or diesel–electric ("conventionally") powered. In the U.S. Navy naming system, and in the equivalent NATO system (STANAG 1166), nuclear-powered attack submarines are known as SSNs and their anti-submarine (ASW) diesel–electric predecessors are SSKs. In the U.S. Navy, SSNs are unofficially called "fast attacks".

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