USN:
On Eternal Patrol - USS Thresher (SSN-593). 60 years ago USS Thresher (SSN-593), the lead boat of her class of nuclear-powered attack submarines, was lost with all hands during deep diving tests beyond the continental shelf east of Cape Cod, Massachusetts.
Launched on July 9, 1960, the USS Thresher represented a massive leap forward in submarine technology. Built as a replacement to the Skipjack-Class subs, she was the first in a new generation of nuclear attack submarine developed by the U.S. Navy. Even by the standard of the early 60’s, the Thresher was a remarkable submarine; at least on paper. Capable of diving to depths of up to 1,300 ft and equipped with next generation sonar arrays, fire-control systems, and a new generation of nuclear reactor, she was a formidable foe to any enemy combatants. Her sleek hull and narrow sail (the smallest ever fitted to a American SSN) gave her an impressive top speed of 33 knots.
On 9 April 1963, Thresher, commanded by Lieutenant Commander John Wesley Harvey, left from Kittery, Maine, at 8:00 a.m. and met with the submarine rescue ship Skylark at 11:00 a.m. to begin her initial post-overhaul dive trials, in an area some 220 mi; 350 km (190 nmi) east of Cape Cod, Massachusetts. That afternoon, Thresher conducted an initial trim-dive test, surfaced, and then performed a second dive to half of her 1,300-foot test depth. She remained submerged overnight and re-established underwater communications with Skylark at 6:30 a.m. on 10 April to commence deep-dive trials. Following standard practice, Thresher slowly dove deeper as she traveled in circles under Skylark – to remain within communications distance – pausing every 30 m (100 ft) of depth to check the integrity of all systems. As Thresher neared her test depth, Skylark received garbled communications over underwater telephone indicating " ... minor difficulties, have positive up-angle, attempting to blow," and then a final, even more garbled message that included the number "900". When Skylark received no further communication, surface observers gradually realized Thresher had sunk.
Deep-sea photography, recovered artifacts, and an evaluation of Thresher's design and operational history permitted a court of inquiry to conclude that the submarine had probably suffered the failure of a salt-water piping system joint that relied heavily on silver brazing instead of welding. Earlier tests using ultrasound equipment found potential problems with about 14% of the tested brazed joints, most of which were determined not to pose a risk significant enough to require repair. But on 30 November 1960, nearly three years prior to the accident, USS Barbel suffered such a silver-braze joint failure near test depth while on an exercise, flooding the engine room with an estimated 18 tons of water in the 3 minutes it took to surface under power and with blown tanks. This incident was followed months later by more silver-braze failures aboard the ballistic missile submarine USS Abraham Lincoln during trials. High-pressure water spraying from a broken pipe joint may have shorted out one of the many electrical panels, causing a shutdown ("scram") of the reactor, which in turn caused loss of propulsion.
The inability to blow the ballast tanks was later attributed to excessive moisture in the submarine's high-pressure air flasks, moisture that froze and plugged the flasks' flowpaths while passing through the valves. This was later simulated in dockside tests on Thresher's sister sub, Tinosa. During a test to simulate blowing ballast at or near test depth, ice formed on strainers installed in valves; the flow of air lasted only a few seconds. Air dryers were later retrofitted to the high-pressure air compressors, beginning with Tinosa, to permit the emergency blow system to operate properly. Submarines typically rely on speed and deck angle (angle of attack) rather than deballasting to surface; they are propelled at an angle toward the surface. Ballast tanks were almost never blown at depth, as doing so could cause the submarine to rocket to the surface out of control. Normal procedure was to drive the submarine to periscope depth, raise the periscope to verify that the area was clear, and then blow the tanks and surface the submarine.
Subsequent study of SOSUS (sound surveillance system) data from the time of the incident has given rise to doubts as to whether flooding preceded the reactor scram, as no impact sounds of the high pressure water in the compartments of the submarine could be detected on instrument recordings from SOSUS at the time. Such flooding would have caused a significant sonic event, and no evidence of such may be found in the recorded data.
At the time, reactor-plant operating procedures did not allow for a rapid reactor restart following a scram, or even the ability to use steam remaining in the secondary system to propel the submarine to the surface. After a scram, standard procedure was to isolate the main steam system, cutting off the flow of steam to the turbines providing propulsion and electricity. This was done to prevent an overly rapid cool-down of the reactor. Thresher's reactor control officer, Lieutenant Raymond McCoole, was not on the boat during the fatal dive; his trainee, Jim Henry, fresh from nuclear power school, probably followed standard operating procedures and gave the order to isolate the steam system after the scram, even though Thresher was at or slightly below its maximum depth. Once closed, the large steam system isolation valves could not be reopened quickly. Reflecting on the situation in later life, McCoole was sure that he would have delayed shutting the valves, thus allowing the boat to "answer bells" and drive itself to the surface, despite the flooding in the engineering spaces. Admiral Rickover noted that the procedures were for normal operating conditions, and not intended to restrict necessary actions in an emergency involving the safety of the ship. After the accident, Rickover further reduced plant restart times, which had already been gradually improving with new technology and operating experience, in addition to limiting factors that could cause a shut down. The Fast Recovery Startup involves an immediate reactor restart and allows steam to be withdrawn from the secondary system in limited quantities for several minutes following a scram.
In a dockside simulation of flooding in the engine room, held before Thresher sailed, the watch in charge took 20 minutes to isolate a simulated leak in the auxiliary seawater system. At test depth with the reactor shut down, Thresher would not have had 20 minutes to recover. Even after isolating a short circuit in the reactor controls, it would have taken nearly 10 minutes to restart the plant.
It was believed at the time that Thresher likely imploded at a depth of 400–610 m (1,300–2,000 ft), though 2013 acoustic analysis concluded implosion occurred at 730 meters.
The U.S. Navy has periodically monitored the environmental conditions of the site since the sinking and has reported the results in an annual public report on environmental monitoring for U.S. naval nuclear-powered craft. These reports provide results of the environmental sampling of sediment, water, and marine life, which is performed to ascertain whether Thresher's nuclear reactor has had a significant effect on the deep-ocean environment. The reports also explain the methodology for conducting deep-sea monitoring from both surface vessels and submersibles. The monitoring data confirm that there has been no significant effect on the environment. Nuclear fuel in the submarine remains intact.
Following the sinking of Thresher, the class that she was supposed to represent was changed to the Permit-Class, after the second boat commissioned. Due to the Thresher disaster, almost everything onboard the Permit and her sisters were overhauled and redesigned, to the point that the Permit-Class boats barely resembled the Thresher in almost every respect. The navy created the SUBSAFE program to ensure that future submarine hulls would stay watertight and that they can recover from unanticipated flooding.
While other submarines far larger have sunk, such as the Kursk in 2001, the Thresher still holds the unwanted record for the highest casualty number of any submarine disaster in history; with 129 souls aboard at the time of sinking.
Having been lost at sea, and in the tradition of the U.S. Navy Thresher has never been decommissioned and remains on "Eternal Patrol".
