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Tuesday, January 07, 2014

USS Thresher (SSN-593)

The USS Thresher was the lead ship in a new, advanced class of US Submarines.  Her keel was laid in May of 1958 and she was launched in July 1960.

At the time of Thresher's launch, she was the deepest diving attack submarine in the world.  Until the arrival of Thresher, all submarines had bow-mounted (and most also had stern-mounted) torpedo tubes.  USS Thresher was the first to use angled, midship-mounted torpedo tubes.  This arrangement freed the bow for installation of an extremely high-sensitivity sonar array. 

The bow-mounted sonar consisted of a sphere covered with hydrophones, placed as far away as possible from the propulsion plant.  This new arrangement offered a huge improvement in sonar sensitivity, and Thresher had the ability to detect other submarines at far greater range than they could detect her.  The importance of detecting an enemy ship first cannot be overstated!!! 

In addition to the improvements in the sonar/torpedo tube arrangement, huge improvements had been made to silence the ship.  This was also the quietest nuclear submarine in the world.  For the first time ever, all rotating equipment in the ship was sound-isolated from the ship's hull.  The heavier equipment (propulsion turbines, turbine generators, primary coolant pumps, and reduction gears) were mounted to a subfloor that was suspended and accoustically isolated from the hull.  Smaller machinery and steam piping was mounted using sound-isolating snubbers. 

Thresher was also the first ship to carry the SUBROC missile. The SUBROC was a Stand-off weapon system for attacking enemy ships at greater distances than a torpedo could reach.  The SUBROC was a SUBmarine ROCket that carried a W55 nuclear warhead with a yield of 1-5 kilotons TNT. It was propelled by a solid fuel rocket motor that would ignite following ejection from a torpedo tube. The SUBROC would then angle up, clear the ocean surface, fly a certain distance, then release the warhead. The warhead would enter the water and detonate at a depth that was programmed prior to launch.

In short, the ship was revolutionary.  She was quiet, fast, deep-diving, had the keenest hearing on the planet, and could shoot a nuclear-tipped rocket out her torpedo tubes.  

Then on April 9, 1963 she sank with all hands.  She took 129 men along with her.  Seven days after being lost, Thresher was stricken from the ship's registry. 

As the lead ship of a new class of submarines, this new class of ship had been called the Thresher (593 class).  After striking Thresher from the registry, the class was renamed after the second ship in the class, the USS Permit class (594 class).

What happened:
Thresher had performed operational testing of all her systems, particularly the weapons and sonar systems for two years. In July 1962 she entered shipyard to inspect equipment conditinos, make repairs and correct deficiencies noted during the operational testing.   She un-docked on April 8.

On April 9, Thresher got underway from Portsmouth Naval Shipyard in Maine, for the purpose of conducting a dive to test depth.  Diving to test depth is always done immediately following any major overhaul or repair, so that the sea-worthiness of the ship can be tested.

Thresher rendezvoused with a submarine rescue vessel named Skylark, and then conducted a slow descent in 100ft increments, while circling Skylark in order to maintain communications.  As Thresher neared test depth, Skylark received a message over the underwater telephone stating "...minor difficulties, have positive up-angle, attempting to blow".  Lastly there was a final garbled message that included the number "900".  When Skylark received no further communication, surface observers gradually realized Thresher had sunk.

Thresher had gone down in 8400ft of water, and it was difficult to gather evidence or get photographs to determine what had caused the loss of the ship.  A court of inquiry was convened, and a picture of events began to unfold.

In this class of ship, the major heat loads were cooled by seawater-supplied heat exchangers.  A/C Units, Generators, oil coolers - everything was supplied by high-pressure seawater piping.  At the time, most of the joints in these seawater pipes were silver-soldered, rather than welded.  Many of the silver-soldered joints were faulty, and there was no process in place for quality control.

It is conjectured that one of the larger silver-soldered joints sheared off near test depth, filling the ship from *both* ends of the damaged seawater pipe.  This in itself would not have sunk the ship, but several other things likely compounded the problem.  The seawater would have blasted in and created a mist as it struck other surfaces.  This saltwater mist probably created enough electrical grounds and disturbances to trip (or scram) the reactor. 

A shutdown reactor also should not have caused the ship to be lost.  Enough heat remains in the primary coolant to generate propulsion steam for several minutes.  However, the procedures in place at that time called for the main steam stop valves to be shut whenever the reactor scrammed (to conserve heat in the primary coolant).  This action eliminated the option of driving the ship toward the surface using the propulsion turbines, just as Thresher was taking on additional weight due to flooding.

The captain ordered an emergency main ballast tank blow.  An emergency blow releases extremely high pressure air from large air tanks into the main ballast tanks.  The air displaces water out the bottom of the ballast tanks and makes the ship more bouyant.   The emergency blow also should have saved the ship, but it did not. 

Previous classes of submarines did not dive this deep, and therefore did not require air at such high pressure to blow their main ballast tanks while at maximum depth.  No previous submarine had needed to dehydrate compressed air, and this class of ship did not have compressed air dryers either.

The air in the high pressure air banks was moist air.  It is believed that during the emergency main ballast tank blow, as the air pressure dropped, ice formed in the piping and blocked further air flow, preventing a full blow of the ballast tanks.

The Thresher is believed to have made some progress toward reaching the surface, before flooding overcame the bouyancy created by the partial ballast tank blow.

Loss of the Thresher came as a huge shock to the Navy, and to the Submarine Service.  As a result of the investigation, the SUBSAFE program was initiated.  This was a QA/QC program to ensure all parts of the ship subject to seawater pressure were of the highest quality, and pedigreed with a paper trail, from smelting to installation.

Other improvements included remotely-operated hydraulic shutoff valves for all seawater systems that didn't rely on electrical power, air dryers and larger diameter piping for the compressed air systems, and procedural changes that allowed crews to use residual primary coolant heat for emergency steam propulsion.  Not much point protecting the reactor, if the ship will be lost in the process...

Later classes of submarines (engineered from the ground up with SUBSAFE) in would use seawater-freshwater heat exchangers for everything except the main propulsion condensers.  This other cooling system circulated low pressure fresh water to machinery that required cooling, and so minimized the amount of high-pressure seawater piping within the ship.

Those are the engineering improvements that we are able to take away from this tragedy. 

Thresher was lost at sea and never decommissioned.  Both she and her crew, as well as a large number of civilian shipyard workers, remain on Eternal Patrol.

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