The USS Thresher (SSN-593) disaster was one of the most significant submarine incidents in history, resulting in the largest loss of life aboard a submarine.

On April 10, 1963, the Thresher, an American nuclear-powered submarine, sank during deep-diving tests in the Atlantic Ocean, about 220 miles east of Boston, Massachusetts, claiming the lives of all 129 crew members and civilian technicians on board.

The Thresher had been commissioned in 1961 as the lead boat of her class and represented the most advanced submarine design of her time. She had completed an extensive nine-month overhaul and was in the process of conducting post-overhaul sea trials when the incident occurred.

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Historical context

Understanding the USS Thresher disaster requires a deep dive into the historical and geopolitical context that shaped its conception and construction.

The period was one of increasing tension and competition, mainly between the two world powers, the United States and the Soviet Union. This rivalry permeated into multiple facets of society, including space exploration, military capabilities, and ideological influence – an era now known as the Cold War.

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The late 1950s were marked by a rapid arms race, and the oceans were a significant frontier in this competition.

USS Thresher crest.

The undersea landscape was seen as a critical platform for strategic advantage, particularly for launching nuclear weapons undetected.

This led to a rush to develop advanced submarine technology, capable of remaining submerged for extended periods while also being stealthy and quick. The advent of nuclear-powered submarines revolutionized this field, enabling submarines to remain underwater almost indefinitely, unlike their diesel-electric counterparts.

The USS Thresher was born out of this strategic urgency.

The U.S. Navy commissioned her as part of a new class of submarines intended to incorporate the latest advancements in submarine technology. Launched on July 9, 1960, she was the most sophisticated and silent submarine of her era, designed to detect and destroy Soviet submarines while evading detection herself.

This leap in technology, combined with Thresher’s nuclear propulsion, made her a formidable tool in the U.S.’s naval arsenal.

Beyond the grand strategic design, however, the Thresher was the product of innovative naval engineering and cutting-edge technology of her time.

However, this technological ambition had its drawbacks, as the Thresher was a new breed of submarine. There were unknown risks associated with operating such a complex and advanced machine in the unforgiving deep-sea environment.

The Thresher disaster would later shed a harsh light on these vulnerabilities, but at the time of her conception and launch, she represented a triumphant step forward in undersea warfare technology.

The Thresher’s Final Voyage

On April 9, 1963, the USS Thresher left the Portsmouth Naval Shipyard in Kittery, Maine, embarking on a mission that would become her last.

Accompanied by the submarine rescue ship, the USS Skylark (ASR-20), Thresher set out for a series of deep-diving tests in the Atlantic Ocean, approximately 220 miles east of Boston.

Launch of the Thresher.

These tests were routine for submarines following an overhaul, meant to assess their operational readiness.

In the early morning hours of April 10, the USS Thresher began her descent to her test depth, a classified figure believed to be greater than 1,000 feet.

The descent was monitored by the Skylark, which maintained communication with the submarine throughout the operation.

It was during this period that the Skylark received the first in a series of increasingly puzzling and garbled messages from the Thresher.

At approximately 7:30 a.m., the Thresher reported to the Skylark that she was experiencing minor issues. At 8:45 a.m., communication between the two ships was reestablished, with the Thresher reporting her position and depth but indicating no serious problems.

However, these would be the last relatively normal communications from the Thresher.

In the following minutes, the Thresher transmitted a series of messages that were largely unintelligible due to the poor quality of the transmission. However, the submarine rescue ship did pick up snippets, including phrases such as “experiencing minor difficulties,” “have positive up angle,” and “attempting to blow.”.

These fragments suggested trouble on board the Thresher but did not convey the gravity of the situation.

The final sound from the Thresher came at approximately 9:17 a.m. The Skylark picked up a high-energy, low-frequency noise, which was later identified as the sound of the submarine imploding as she exceeded her crush depth – the depth at which the hull of a submarine will collapse under the pressure of the water above it.

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The implosion happened in less than a second, ending the lives of all 129 crew members and civilian technicians on board. This tragic event marked the single largest loss of life aboard a submarine in history.

The Search For The Thresher

The search for the USS Thresher following the disaster was a considerable and complex operation due to the depth at which the submarine had sunk and the breadth of the area that needed to be searched.

The Thresher’s remains were scattered across the ocean floor, approximately 8,400 feet below the surface, a depth that far exceeded the capabilities of most recovery equipment at the time.

The search began immediately after the Thresher failed to resurface or respond to communication attempts.

The submarine rescue ship USS Skylark (ASR-20), which had been in contact with the Thresher during the dive trials, started the initial search operation.

The U.S. Navy quickly mobilized additional resources, including surface ships and aircraft, to scour the ocean surface, and sonar-equipped vessels to scan beneath.

The breakthrough came when the recovery ship USNS Mizar (T-AGOR-11), equipped with advanced sonar technology, detected underwater acoustic signals, or “pings,” on April 15, 1963.

USS Thresher entering service, 3 August 1961.

Mizar was an oceanographic research vessel that had been rapidly repurposed and equipped for the Thresher search mission. It utilized a precision depth recorder (a type of echo sounder) and other instruments to map the sea floor in the area where the Thresher was believed to have been lost.

The location was confirmed when the Mizar detected large metallic objects on the sea floor. Photographs of the debris field were taken by a specially equipped camera sled, named “Fish,” that was towed by the Mizar at the end of a 10,000-foot cable.

Over the course of the next few months, Mizar made many trips to the debris field, taking thousands of photographs that revealed the broken remnants of the Thresher scattered across the ocean floor.

Finally, in June 1963, the search efforts led to the confirmation of the Thresher’s location and an extensive examination of the wreckage.

Investigation Into The Thresher Disaster

The U.S. Navy’s official report suggests that a failure in a seawater piping system was the initial cause of the Thresher’s demise.

This piping system, which was used to cool the submarine’s reactor, was brazed rather than welded, a construction technique that made it more susceptible to failure. The leak caused by this failure likely led to a short circuit and a subsequent loss of power.

Light debris from the USS Thresher wreckage.

The leak from the seawater piping system likely triggered a short circuit in the electrical bus (the system that distributes electric power to the various components of the submarine). This would have caused a reactor scram, a rapid shutdown of the nuclear reactor.

With the reactor offline, the Thresher lost the propulsion necessary to combat the increasing pressure as it descended deeper into the ocean.

When the submarine lost propulsion, the crew attempted to blow the ballast tanks – a procedure that would fill the tanks with high-pressure air, causing the submarine to become buoyant and ascend.

However, due to design and construction deficiencies, this system failed. The use of “low-pressure” blow, which took much longer to execute, did not provide the necessary buoyancy to counteract the submarine’s descent.

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The Thresher continued to sink, eventually surpassing her crush depth, the maximum depth a submarine can withstand before the hull is compromised by the water pressure.

Rudder from the USS Thresher wreckage.

The enormous pressure at this depth caused the Thresher’s hull to implode, an event that was captured as a distinct acoustic event on the sonar logs of nearby ships.

Aftermath

The Thresher disaster led to a significant reevaluation of submarine safety within the U.S. Navy. In response to the findings of the Court of Inquiry, the Navy instituted the SUBSAFE program, a series of design modifications, and procedural changes aimed at improving submarine safety.

SUBSAFE established rigorous quality control standards for submarine construction and repair, as well as enhanced testing and inspection procedures. The program also emphasized the need for reliable and effective emergency systems, such as the ballast tank blow system.

The Thresher disaster had ramifications beyond the United States.

It led to an increased focus on submarine safety worldwide, with many nations adopting similar safety measures. It also sparked international cooperation in developing submarine rescue strategies, recognising that the deep-sea environment presents universal challenges that transcend national boundaries.

The legacy of the Thresher disaster lives on in these tangible commitments to safety.

The submarine community continues to learn from this tragedy, striving to ensure that such a disaster is never repeated. The Thresher disaster serves as a poignant reminder of the dangers of operating in the world’s deepest and least understood frontiers.

It underscores the importance of meticulous design, rigorous testing, and robust emergency procedures. The lessons learned from the Thresher disaster have made deep-sea exploration safer and have underscored the continual need for vigilance in submarine design and operation.