An attack submarine or hunter-killer submarine is asubmarinespecifically designed for the purpose of attacking andsinkingother submarines, surface combatants and merchant vessels.In theSoviet and Russian navies they were and are called"multi-purposesubmarines". They are also used to protect friendlysurfacecombatants and missile submarines. Some attack subs are alsoarmedwith cruise missiles mounted in vertical launch tubes,increasingthe scope of their potential missions to include landtargets.Attack submarines may be either nuclear-powered ordiesel-electric("conventionally") powered. In the United StatesNavy namingsystem, and in the equivalent NATO system (STANAG1166),nuclear-powered attack submarines are known as SSNs andtheirdiesel-electric predecessors were SSKs. In the US Navy, SSNsareunofficially called "fast attacks". In the action of 9February1945, HMS Venturer sank U-864 while both were at periscopedepth.This was the first and so far only intentional sinking ofasubmerged submarine by a submerged submarine. U-864 wassnorkeling,thus producing much noise for Venturers hydrophones (anearly formof passive sonar) to detect, and Venturer was fortunatein havingover 45 minutes to plot the U-boat's zig-zag course byobservingthe snorkel mast. Venturer's commander, James S. "Jimmy"Launders,was astute in assuming the U-boat would execute an"emergency deep"maneuver once it heard the torpedoes in the water,thus the"spread" of four torpedoes immediately available was aimedon thatassumption. One hit, sinking the U-boat. Following World WarII,advanced German submarines, especially the Type XXI U-boat,becameavailable to the Allies, particularly the United States Navyandthe Soviet Navy. Initially, the Type XVII U-boat, with aWalterhydrogen peroxide-fueled gas turbine allowing highsustainedunderwater speed, was thought to be more developed thanwasactually the case, and was viewed as the submarine technologyofthe immediate future. However, the Type XXI, streamlined and withahigh battery capacity for high submerged speed, was fullydevelopedand became the basis for most non-nuclear submarinedesignsworldwide through the 1950s.In the US Navy, the GreaterUnderwaterPropulsion Power Program (GUPPY) was developed tomodernize WorldWar II submarines along the lines of the Type XXI.It was realizedthat the Soviet Union had acquired Type XXI andother advancedU-boats and would soon be putting their ownequivalents intoproduction. In 1948 the US Navy prepared estimatesof the number ofanti-submarine warfare (ASW)-capable submarinesthat would beneeded to counter the hundreds of advanced Sovietsubmarines thatwere expected to be in service by 1960. Twoscenarios wereconsidered: a reasonable scenario assuming theSoviets would buildto their existing force level of about 360submarines, and a"nightmare" scenario projecting that the Sovietscould buildsubmarines as fast as the Germans had built U-boats,with a forcelevel of 2,000 submarines. The projected US SSK forcelevels forthese scenarios were 250 for the former and 970 for thelatter.Additional anti-surface, guided missile, and radarpicketsubmarines would also be needed. By comparison, the totalUSsubmarine force at the end of World War II, excludingobsolescenttraining submarines, was just over 200 boats. Fulltext:https://en.wikipedia.org/wiki/Attack_submarine