How submarine letter codes separate missions and propulsion

What do the submarine designators SSK, SSBN, SSN, and SSG describe?

They distinguish conventional hunter-killers, nuclear ballistic-missile boats, nuclear attack submarines, and guided-missile submarines.

SSK is used for a conventional hunter-killer submarine. Ballistic-missile boats fall into the SSB family, with SSBN identifying nuclear propulsion. SSN denotes a nuclear-powered attack submarine, while SSG identifies a guided-missile submarine. The compact explanation shows how submarine letters encode both mission and, in some cases, propulsion.