The Sturgeon class (known colloquially in naval circles as the 637 class''') was a class of nuclear-powered fast attack submarines (SSN) in service with the United States Navy from the 1960s until 2004. They were the "workhorses" of the Navy's attack submarine fleet throughout much of the Cold War. The boats were phased out in the 1990s and early 21st century, as their successors, the , followed by the and -class boats, entered service. Design The Sturgeons were essentially lengthened and improved variants of the Thresher/Permit class that directly preceded them. The five-compartment arrangement of the Permits was retained, including the bow compartment, operations compartment, reactor compartment, auxiliary machinery room no. 2, and the engine room. The extra length was in the operations compartment, including longer torpedo racks to accommodate additional Mark 37 torpedoes, the most advanced in service at the time of the class's design in the late 1950s. The class was redesigned to SUBSAFE requirements concurrently with the construction of the first units, with seawater, main ballast, and other systems modified for improved safety. The biggest difference was the much larger sail, which permitted a second periscope and additional intelligence-gathering masts, and which reduced the risk of the submarine broaching the surface in heavy seas. The fairwater planes mounted on the sail could rotate 90 degrees, allowing the submarine to surface through thin ice. Because the S5W reactor was used (the same as in the Skipjacks and Thresher/Permits), the sail was enlarged (increasing drag), and the displacement was increased, the Sturgeons' top speed was , 2 knots slower than the Thresher/Permits. The last nine Sturgeons were lengthened to provide more space for electronic equipment and habitability. The extra space also helped facilitate the use of dry deck shelters first deployed in 1982. The class received mid-life upgrades in the 1980s, including the BQQ-5 sonar suite with a retractable towed array, Mk 117 torpedo fire control equipment, and other electronics upgrades. Armament The Sturgeon-class boats were equipped to carry the Harpoon missile, the Tomahawk cruise missile, the UUM-44 SUBROC, the Mark 67 SLMM and Mark 60 CAPTOR mines, and the MK-48 and ADCAP torpedoes. Torpedo tubes were located amidships to accommodate the bow-mounted sonar. The bow covering the sonar sphere was made from steel or glass reinforced plastic (GRP), both varieties having been produced both booted and not booted. Booted domes are covered with a half-inch layer of rubber.