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SSN-594 Permit class

Thresher (SSN-593) represented a new generation of multipurpose nuclear submarine and was the prototype for all subsequent attack submarine classes. She was the first quiet and deep-diving submarine and had a new, larger sonar array in the bow in place of the torpedo tubes (which were moved amidships). Thresher was launched in 1960 and commissioned the following year. Tragically, she was lost on 10 April 1963 with 129 crewmembers and civilians during a deep test dive, due to accidental causes still uncertain today. Lessons from the disaster were incorporated into follow-on nuclear submarines. After the loss of the Thresher, the class was renamed for the USS Permit (SSN-594). This was the first new design submarine for which Electric Boat was not the lead yard.

In 1956 Admiral Arleigh Burke, then CNO, requested that the Committee on Undersea Warfare of the National Academy of Sciences to study the effect of advanced technology on submarine warfare. The result of this study, dubbed "Project Nobska" was an increased emphasis on deeper-diving, ultraquiet designs utilizing long-range sonar. The Permit class was based on Project Nobska's recommendations. Hull streamlining, reduction in sail dimensions by approximately 50%, quieting of the propulsion plant and an increase in test depth all led to a dramatic advance in submarine operational capabilities and stealth. In 1957 the Regulus missile program was terminated to free funds for the Polaris project. SSGNs on order were recast as SSN-593 class attack submarines, though existing Regulus submarines continued operations.

The SSN-594 Permit class was the world's first modern, quiet, deep-diving fast attack submarines, integrating such advanced features as a hydrodynamically shaped hull, a large bow mounted sonar array, advanced sound-silencing features, and an integrated control/attack center with the proven S5W reactor plant. The vestigial remains of a wolf-pack concept were found in the original design of the SSN-593 Thresher class, whose operational employment assumed that two such submarines operated together and coordinated through secure underwater communications/data links.

Thresher exploited several technological innovations that had been developed independently in the preceding decade. Taken separately, none of these innovations had transformed the submarine into the dominant ASW platform, and their initial development was not justified for that purpose. Internally, officers expressed concern about why so much money was being spent on her quieting--surely Skipjack-class was quiet enough. Further, some thought at the time that putting torpedo tubes in the middle of the ship instead of in the bow was a dumb idea, that installing such a big sonar array was unnecessary, and that trading any of the Skipjack's speed for Thresher's increased depth capabilities was foolish.

Project KAYO was designed to explore the feasibility of diesel-electric submarines (SSKs) for ASW operations using passive acoustics with low-frequency, bow sonar arrays. As part of KAYO, three small SSKs - Barracuda (SSK-1), Bass (SSK-2), and Bonita (SSK-3) - were built around the large BQR-4 array. Listening with the submarine on battery, these arrays gave then unheard-of convergence zone detection ranges against snorkeling submarines. But the SSKs were hampered by their slow speed and resulting inability to close a target quickly. Thus they were restricted to use in barriers patrols in conjunction with ASW aircraft. Further restricting their utility, the need to snorkel periodically limited their ability to operate in far-forward areas.

Nautilus had proved the feasibility of nuclear power, and Albacore demonstrated the optimum hull form for a fast submarine, and these features were combined in the USS Skipjack (SSN-585). While the SSKs had demonstrated the ASW value of a quiet submarine with a large bow sonar array and the utility of nuclear power for autonomous forward-barrier operations, Nautilus proved to be very loud acoustically. The USS Tullibee (SSN-597) was a smaller and slower SSK(N) with electric drive for quieting. Tullibee featured the first integrated sonar suite that included both a low-frequency passive array for long-range detection and a spherical array for approach and attack.

Tullibee and Thresher were the first submarines designed explicitly for ASW against nuclear submarines. The key was quieting. Tullibee represented an attempt to eliminate the sources of noise in the submarine, while Thresher represented an effort to insulate that noise from the hull and the water beyond. The basic solution adopted in the latter case was rafting, whereby the engineering plant of the submarine, containing all or most of its rotating machinery, was placed on a flexible mount or raft within the submarine. This dramatically reduced the amplitude of the mechanical vibrations coupling to the hull, and therefore the sound that was put into the water. It also drove up the size of the hull and as a consequence, propelled by the same S5W as the Skipjack, Thresher lost a few knots of top end speed. Rafting and other quieting technologies also drove up the cost of the submarine, partly due to the increase in size, but also due to the fiendish engineering problems associated with requirements like circulating water at great pressure from outside the hull into the ship through flexible piping.

The Thresher design resulted from combining Tullibee, the first quiet nuclear submarine, and Skipjack, the first fast nuclear submarine, to create what became the archetype for all subsequent developments - the quiet, fast-attack SSN, optimized for ASW. These submarines were a major advance over previous submarine designs, and established the pattern of all successive American attack submarine classes, in several extremely important respects:

  • They were the first submarines to have hulls constructed of High Yield-80 (HY-80) steel alloy, which allowed operations at substantially greater depths than previous submarines.
  • They were the first submarines to have raft mountings for turbines, motors and other equipment, resulting in substantially quieter operations.
  • They were the first submarines to have a large bow-mounted sonar requiring the installation of torpedo tubes amidships, aft of the forward crew compartment.

Although they were larger than the previous SSN 585 Skipjack class, and used the same nuclear power plant, their hull design did not compromise their underwater speed. Designed for prolonged periods submerged, they were limited only by the amount of food that they could carry, and were capable of sustained operation at high speed.

While almost all Cold War operations remain classified, recently declassified missions showcase Submarine Force capabilities. USS Guardfish (SSN-612) silently tracked a Soviet cruise missile (SSGN) submarine which was following U.S. aircraft carriers off Vietnam in the 1970's - ready to protect our ships should the SSGN launch her missiles.

The "Permit" class submarines were originally designated the THRESHER class. The USS Thresher was built at the Portsmouth Naval Shipyard and commissioned 30 June 1961. The USS Thresher (SSN 593) was lost 200 miles off the coast of New England on 10 April 1963 while conducting sea trials that followed an overhaul.

In company with Skylark (ASR-20), Thresher put to sea on 10 April 1963 for deep-diving exercises. In addition to her 16 officers and 96 enlisted men, the submarine carried 17 civilian technicians to observe her performance during the deep-diving tests. Fifteen minutes after reaching her assigned test depth, the submarine communicated with Skylark by underwater telephone, apprizing the submarine rescue ship of difficulties. Garbled transmissions indicated that--far below the surface--things were going wrong. Suddenly, listeners in Skylark heard a noise "like air rushing into an air tank". Then just 5 minutes later, the Skylark's sonar picked up the sounds of the submarine breaking apart under the tremendous sea pressure. Efforts to reestablish contact with Thresher failed, and a search group was formed in an attempt to locate the submarine. Rescue ship Recovery (ASR-43) subsequently recovered bits of debris, including gloves and bits of internal insulation. Photographs taken by bathyscaph Trieste proved that the submarine had broken up, taking all hands on board to their deaths in 5,500 of water, some 220 miles east of Boston.

Thresher was officially declared lost in April 1963. The submarine community, the Navy and the nation were stunned. Two days after the disaster President Kennedy issued Executive Order 11104, ordering U.S. Flags to "be flown at half-staff on all buildings, grounds and naval vessels of the Federal Government in the District of Columbia and throughout the United States and its Territories and possessions," from April 12th to 15th. Thresher was the best of the newest. To the Navy, the disaster meant more than the loss of 129 crewmembers and civilians. Thresher had been the most advanced submarine in the world, capable of reaching depths and speeds unimaginable a decade before.

A Navy board of inquiry concluded that the most likely cause of the sinking was a failure in either a pipe, pipe valve, or hull weld which cause flooding near the engine room. According to investigators, a silver-brazed joint in a seawater pipe in the aft engine spaces broke, spraying water into the engine room and shorting one of the main electrical bus boards. The flooding probably short-circuited an electrical system related to the main engine causing the reactor to shut down. The reactor scram (emergency shut down) resulted in a loss of steam power to the primary propulsion system. Submarines rely on power to push them to the surface. Without this power, and taking on water from the leak, the Thresher had negative buoyancy and she began sinking. The aft part of the sub filled up with water and tilted down. Attempts were surely made to stop the leak, but the isolation valve was not nearby and did not have a remote operator. Darkness, a sea mist, and sheer terror may have inhibited the crew from manually actuating the valves.

It is suspected that the Thresher started her emergency propulsion system, but this was not able to push the submarine to the surface from this depth. The submarine apparently managed to point her nose upward in preparation to emergency blow air into her ballast tanks. Some sources say that the Thresher actually began to surface, but the lines to the air pressure-reducing manifold valves iced over and froze shut, an action enhanced by the recent addition of a filter on the air inlet to the manifolds. This frozen pipeline formed an impenetrable barrier between the high-pressure storage air flasks and the manifold valves leading to the Thresher's ballast tanks. So instead of rising to the surface, the Thresher kept sinking. Having a "positive up-angle" with no power or air providing forward momentum, the Thresher slipped backward into the depths of the ocean. Without power, Thresher was not able to surface and the continued flooding caused Thresher to drop below her crush depth where the pressure of the ocean destroyed her. The entire crew of 129 were lost. A ghastly death for an entire crew, and one the US Navy vowed never to allow happen again.

Thresher is in six major sections on the ocean floor, with the majority in a single debris field about 400 yards square. The major sections are the sail, sonar dome, bow section, engineering spaces, operations spaces, and the tail section. Owing to the pressurized-water nuclear reactor in the engine room, deep ocean radiological monitoring operations were conducted in August 1983 and August 1986. The site had been previously monitored in 1965 and 1977 and none of the samples obtained showed any evidence of release of radioactivity from the reactor fuel elements. Fission products were not detected above concentrations typical of worldwide background levels in sediment, water, or marine life samples.

There has been some concern over the Navy's method of performing UnderWater Explosion Shock and Vibration Testing [UNDEX]. Traditionally, explosives are detonated in the vicinity of the ship. Measurements are taken before and after the test to determine any damage to the piping and equipment in the engineering spaces. This test is obviously intended to show the ship's ability to withstand wartime activity. The Thresher was UNDEX tested before her last overhaul. Misaligned equipment continued to be found months later, even as late as April 10, the morning she set sail for her last deep dive.

The ill-fated USS Thresher (SSN-593) and her crew did not suffer in vain. Out of that terror and the lessons learned grew the SubSafe Program. Through this program, every submarine in the US Fleet, every pressure hull integrity-related system aboard those subs, and every pressure-related part within those systems must be certified as being 100% safe for use on a submarine. The goals are to ensure that in case of a casualty, the ship and its crew can be recovered and to ensure that the integrity of the material used on the ship can operate at design test depth. Directly related to the Thresher tragedy, sea-connected joints can no longer be brazed; they must now be welded. The SubSafe program brought other controls, too. Now when an emergency arises aboard a sub, all vital equipment which sailors would need quick access to in the event of an emergency is clearly marked and easily accessible. At all times an operator is one second away from flipping the emergency main ballast tanks to vent, so the sub can rise to the surface.

At the time of the loss of the Thresher, the accepted method of testing the strength of a brazed joint was with a Hydrostatic Test. Basically, one end of the piping system is blocked, then water pressure is applied to the inside of the fitting to about 150% of the rated working pressure. The brazed joints are watched closely for leaks. This Hydrostatic test is applied to the entire shipboard piping system. A new test had recently been developed, Ultra-Sound. At the time, the Navy required a 40% sealing bond between the fitting and the pipe. The Thresher had over 3000 brazed joints and only 145 of them had been tested with this new Ultrasonic Test at the Shipyard prior to getting underway, and then 14% of those joints tested did not meet the minimum 40% bond requirement. Today, every brazed joint is ultrasonically tested - The minimum bond requirement is 80%.

The Navy took other steps to ensure such a tragedy never occur again. After the sinking of the USS Thresher, it was realized that submarines traveled much deeper than any of the potential rescue platforms could possibly reach. Following the recommendations of a special Presidential Deep Submergence Review Group, the Deep Submergence Rescue System was developed. In 1964, the project to build Deep Submergence Rescue Vehicles (DSRVs) was born, and the first DSRV was launched in 1970. The deep submergence rescue vehicles Mystic (DSRV 1) and Avalon (DSRV 2) of the Deep Submergence Unit are the genesis of that program.

The last three units of this class [Flasher, Greenling, Gato] were modified during construction to incorporate lessons learned from the loss of the Thresher. Fitted with heavier machinery and a larger sail, they were ten feet longer than the other units of the class to correct stability problems caused by weight growth.

The SSN 605 Jack was fitted with an experimental direct-drive propulsion system coupled with a pair of counter-rotating propellers. The engine spaces were lengthened by ten feet and the shaft was lengthened by seven feet to accomodate this additional equipment. Although counter-rotating propellers had previously produced impressive gains in speed on the experimental Albacore, in this instance the results were disappointing and led to the abandoment of this approach in subsequent submarine design.

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