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A cruise missile submarine is a submarine that carries and launches cruise missiles (SLCMs consisting of land-attack cruise missiles and anti-ship missiles) as its primary armament. Missiles greatly enhance a warship's ability to attack surface combatants and strike land targets; although torpedoes are a more discreet option for submerged submarines, missiles give a much longer stand-off range, shorter time to impact the target, as well as the ability to engage multiple targets on different headings at the same time. Many cruise missile submarines retain the capability to deploy nuclear warheads on their missiles, but they are considered distinct from ballistic missile submarines due to the substantial differences between the two weapons systems' flight characteristics; cruise missiles fly aerodynamically using flight surfaces like wings or fins, while a ballistic missile uses its engine power alone as it may exit the atmosphere.
The United States Navy's hull classification symbols for cruise missile submarines are SSG and SSGN – the SS denotes submarine, the G denotes guided missile, and the N denotes that the submarine is nuclear-powered.
The earliest designs of cruise missile submarines had to surface to launch their missiles, while later designs could do so underwater via dedicated vertical launching system (VLS) tubes. Many modern attack submarines can launch cruise missiles (and dedicated anti-ship missiles) from their torpedo tubes while some designs also incorporate a small number of VLS canisters, giving an overlap between cruise missile submarines and traditional attack submarines. Nonetheless, vessels classified as attack submarines are designed to use torpedoes as their main armament and have a more multi-role mission profile due to their greater speed and maneuverability. This is in contrast to cruise missile submarines which are typically larger, slower boats carrying a larger number of missiles and often possess a special compartment dedicated solely to the cruise missile tubes.