Permit-class submarine

The Permit-class submarine, originally known as the Thresher class, was a class of nuclear-powered fast attack submarines (hull classification symbol SSN) in service with the United States Navy from the 1960s until 1994. They replaced the Skipjack-class submarine class. They were used primarily in the 1960s and 1970s, until replaced by the Sturgeon-class submarine and Los Angeles-class submarine classes.

The Permit class resulted from a study commissioned in 1956 by the Chief of Naval Operations (CNO), Admiral Arleigh Burke. In "Project Nobska," the Committee on Undersea Warfare of the United States National Academy of Sciences considered the lessons learned from various prototypes and experimental platforms.

Design
The new class kept the proven S5W reactor plant from the immediately preceding Skipjack-class submarines, but were a radical change in many other ways. The Threshers had the large bow-mounted sonar and angled, amidships torpedo tubes pioneered by the USS Tullibee (SSN-597). Although it used the same HY-80 as the Skipjacks, the Threshers' pressure hulls were made using an improved process that extended test depth to 1,300 ft. The engineering spaces were also redesigned, with the turbines supported on "rafts" that were suspended from the hull on sound damping isolation mounts. Their hulls were more effectively streamlined and had smaller sails, but the increased displacement over the Skipjacks lead to top speed of around 28kts, five knots slower than the Skipjacks.

Armament
The ships had torpedo launchers moved to the middle of the hull. This made available the required large space in the bow for the BQQ-2, BQQ-5 in modernized boats, sonar system, a new and powerful detection low-frequency sensor. Initially armed with Mark 37 torpedoes, they later carried the improved Mark 48, the UGM-84 Harpoon (replacing four of the Mk-48s) and the UUM-44 SUBROC (replacing six Mk-48s, four after Harpoon was adopted). The maximum weapons load was 23 torpedoes/missiles or, theoretically 46 Mk-57, 60 or 67 mines. Or a mix of mines, torpedoes and missiles.

Construction
The first submarine commissioned in this class was the ill-fated USS Thresher (SSN-593), and so the class was known by her name. When Thresher was lost, the class took the name of the second ship in the class, USS Permit (SSN-594), and the SUBSAFE program began. SUBSAFE includes specific training of SUBSAFE quality assurance inspectors in the engine room crew, and tracks extremely detailed information about every component of a submarine's engine room that contacts seawater. Joints in any equipment carrying seawater must be welded (not brazed), and every hull penetration larger than a specified size can be quickly shut by a remote hydraulic mechanism.

The engine room of USS Jack (SSN-605) was lengthened by ten feet to accommodate an experimental direct-drive propulsion system using concentric counter-rotating propellers. Although counter-rotating propellers produced impressive gains in speed on the experimental USS Albacore (AGSS-569), in Jack the results were disappointing because of the difficulty in sealing the shaft. Jack was also used to test polymer ejection that could reduce flow noises that degraded sonar performance.

USS Flasher (SSN-613), USS Greenling (SSN-614), and USS Gato (SSN-615) were fitted with heavier machinery and a larger sail, to house additional masts, and made ten feet longer than the other units of the class to include more SUBSAFE features, additional reserve buoyancy, more intelligence gathering equipment and improved accommodations.

Boats
The gaps in the hull-number sequence were taken by the unique USS Tullibee (SSN-597), and the George Washington-class submarine, Ethan Allen-class submarine, and Lafayette-class submarine fleet ballistic missile submarine classes.


 * USS Thresher (SSN-593)
 * USS Permit (SSN-594)
 * USS Plunger (SSN-595)
 * USS Barb (SSN-596)
 * USS Pollack (SSN-603)
 * USS Haddo (SSN-604)
 * USS Jack (SSN-605)


 * USS Tinosa (SSN-606)
 * USS Dace (SSN-607)
 * USS Guardfish (SSN-612)
 * USS Flasher (SSN-613)
 * USS Greenling (SSN-614)
 * USS Gato (SSN-615)
 * USS Haddock (SSN-621)