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 early 1960s until 1996. They were a significant improvement on the class, with greatly improved sonar, diving depth, and silencing.〔 They were the forerunners of all subsequent US Navy SSN designs. They served in the 1960s through middle 1980s, until retired due to age.〔Friedman, pp. 235-236〕 They were followed by the and classes.
The ''Thresher'' class resulted from a study commissioned in 1956 by Chief of Naval Operations (CNO) Admiral Arleigh Burke. In "Project Nobska", the Committee on Undersea Warfare of the United States National Academy of Sciences, collaborating with numerous other agencies, considered the lessons of submarine warfare and anti-submarine warfare learned from various prototypes and experimental platforms.
The new class kept the proven S5W reactor plant from the immediately preceding s, but were a radical change in many other ways. The ''Thresher''s had the large bow-mounted sonar sphere and angled, amidships torpedo tubes used in the concurrently-built . This placed the sonar sphere in the optimum position for detection of targets at long range. ''Tullibee'' was an alternate design optimized for anti-submarine warfare, much smaller and slower than the ''Thresher''s and with a quiet turbo-electric propulsion system.〔Friedman, pp. 136-142, 243〕 Although they used the same HY-80 steel (yield strength ) as the ''Skipjack''s, the ''Threshers pressure hulls were made using an improved design 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 isolation mounts for acoustic quieting. Drag was reduced, with external fittings kept to a minimum and the sail greatly reduced in size.〔
The small sail of ''Thresher'' (the smallest fitted to an American SSN) compensated for the increased drag of the longer hull, giving ''Thresher'' a top speed of 33 knots, the same as the ''Skipjacks'', according to one recollection.〔Polmar, Norman; Moore, K. J. (2004) ''Cold War Submarines: The Design and Construction of U.S. and Soviet Submarines'', Potomac Books, p. 363〕 However, the small sail had disadvantages as well, including room for only one periscope and a reduced number of electronics masts, less convenient surfaced operation in rough seas, and an increased possibility of "broaching" (inadvertent surfacing) at periscope depth in rough seas.〔Friedman, pp. 143-146〕
Only ''Thresher'' was fitted with a five-bladed symmetric screw, very similar to the ones originally fitted to the "Skipjacks", which allowed her to reach this speed. During trials of the ''Skipjack'' class, it was found that the propeller produced noise below cavitation depth. It was determined that the source of this noise, called blade-rate, was the blades of the screw vibrating when they hit the wake of the sail and control surfaces.〔 This produced a noise that could carry for many miles and could be used by an enemy submarine to set up a firing solution because the frequency of blade-rate was directly related to the speed of the submarine (the RPM of the screw). The solution was to either make the screw smaller so it did not hit the wakes of the sail and control surfaces, which would cavitate more easily because of its increased speed, or have a large screw that gently interacted with these areas of disturbed water. The latter solution was chosen for all subsequent American SSNs. ''Permit'' and later submarines of this class had seven-bladed skewback screws, which reduced the problem of blade-rate, but reduced the submarines' top speed to 29-28 knots. ''Jack'' was designed with counter-rotating screws, each of which were smaller than the standard seven-bladed screw, as an alternative solution to the blade-rate problem.〔
The class received mid-life upgrades in the late 1970s and 1980s, including the BQQ-5 sonar suite with a retractable towed array, Mk 117 torpedo fire control equipment, and other electronics upgrades.
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