A ballistic missile submarine is a submarine equipped to launch ballistic missiles (SLBMs). Ballistic missile submarines are larger than any other type of submarine, in order to accommodate SLBMs such as the Russian R-29 or the American Trident. Although some early models had to surface to launch their missiles, modern vessels typically launch while submerged at keel depths of usually less than 50 meters (164 feet).Ballistic missile submarines differ in purpose from attack submarines and cruise missile submarines; while attack submarines specialise in combat with other naval vessels (including enemy submarines and merchant shipping), and cruise missile submarines are designed to attack large warships and tactical targets on land, the primary mission of the ballistic missile is nuclear deterrence. Accordingly, the mission profile of a ballistic missile submarine concentrates on remaining undetected, rather than aggressively pursuing other vessels. Ballistic missile submarines are designed for stealth, to avoid detection at all costs. They use many sound-reducing design features, such as anechoic tiles on their hull surfaces, carefully designed propulsion systems, and machinery mounted on vibration-damping mounts.
Ballistic missile submarines equipped with nuclear warheads serve as the third leg of the nuclear triad. The invisibility and mobility of submarines offer both a reliable means of deterrence against an attack (by maintaining the threat of a second strike), and a surprise first-strike capability - particularly given the range of the weapons they carry. This card was sent to me by Nancy.