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USS Thresher: The US Navy’s Cautionary Tale

INTRODUCTION

 Today’s Megaproject is a bit of a cautionary tale on the striving for faster, quieter, and more deadly equipment which was a hallmark of the Cold War between the United States of America and the Soviet Union.  

 The USS 593 Thresher was developed as a direct follow on, and improvement of, the Skipjack Class nuclear fast attack submarines that the US had developed to help counter the growing fleet of Soviet Diesel-Electric and Nuclear Ballistic Missile submarines that were becoming a very real threat to global security. The Navy realized that their current fast attack subs, the Skipjack class, though innovative in many ways, was not up to the task of hunting and detecting the newer submarines the USSR was producing, which could dive deeper and travel faster.      

DEVELOPMENT

The Thresher was to be the first real leap forward in many facets of fast attack submarine technology. Initially, she was going to just be an improved version of the Skipjack class submarines. The Skipjacks in their own right represented improvements that changed how submarines were built. They were the first to introduce the teardrop-shaped hulls which provided a more hydrodynamically efficient shape which resulted in high underwater speeds, quieter movement through the water. However, they were not without their flaws.


Photo courtesy of Steve Walsh

The first issue was their sail, not the big white bits of cloth you see on old ships. The sail on a submarine is that blocky protrusion that sticks out on top and contains the periscope, etc. Submarines experience drag, and this reduces efficiency and makes noise, which is very bad for several reasons.      

The idea of fast attack submarines is to detect their prey without being detected. Second, subs have two types of sonar, active which is the one you hear go “ping” and passive which are essentially very large and sensitive microphones that listen for the slightest sounds, allowing them to locate and track targets. It’s the latter of those two that are really hampered by the submarine’s own noise, especially water moving over the aforementioned sail and the noisy forward torpedo room all of which the subs own passive sonar can detect and mask the important noise i.e. the enemy sub. 

DESIGN

Credit: https://www.history.navy.mil/

To address these the Navy had to reduce the size of the sail. They also moved the forward torpedo room to the middle of the sub, which allowed for much longer-range detection as it reduced the noise pollution that plagued the Skipjack class.      

Another improvement was mounting all the major machinery and pump installations on rafts suspended from the sides of the pressure hull, this eliminated their direct contact with the hull and nullified most vibrations from this equipment that made noise, further adding to the boat’s stealth capabilities.  She was also designed with an improved form of HY-80 Steel, which allowed her to dive nearly twice as deep as the Skipjack class submarines giving her an operational depth of 396.24 meters (1300 feet) over the older skipjack’s 210 meters (700 ft). Lastly, they introduced what they called silver brazing to weld the joints of the seawater piping, this was intended to provide superior strength and reduce weight over traditional welding techniques, remember this for later. All these innovations, especially the reduced external drag allowed her to be just as fast as the Skipjacks despite having an additional 900 tons of displacement.  

These innovations meant that the boat was able to dive deeper than anything the Soviets could then field.  At the time the Soviets had diesel-electric submarines, Gulf II Class that could dive 300 meters, and the Nuclear Hotel Class, which could also dive to 300 meters, 90 meters deeper than the Skipjack Nuclear Submarines.  One of the issues early on in her development was the air pressure system for the ballast tanks, the tanks that control the buoyancy of the sub.  The air system could produce pressure up to 4500 psi, but the actual manifolds and piping were off the shelf from earlier subclasses and could only withstand 3000 psi, so they added pressure reduction gear to address this. They did this to further speed up production and reduce costs over having to develop a new version of these systems.  Dockside testing found that these reducers would become clogged with foreign matter from construction which could not be removed.  By adding cone-shaped filters just before the reducers alleviated this problem and the air systems functioned fine with dockside and shallow test diving (400 feet maximum) this was all done before her commissioning and any deep-water testing. 

With all this development she was rushed into production and corners were cut to get this new “Killer” to sea as soon as possible.  Many of her air ballast and seawater layouts used older pumps and designs of the previous Skipjack class and had a far different layout than previous classes further adding to the crew’s confusion and simulated emergencies. Consideration was not taken regarding the new, much deeper depths that this submarine operated… Which would be a crucial factor in her subsequent loss – foreshadowing alert!      

THE LOSS

Despite her revolutionary design she was lost on April 10, 1963, approximately 220 miles east of Cape Cod, Massachusetts, USA in 8400 feet of water. She went down while conducting a deep-diving test with the loss of 129 crew and civilian shipyard contractors aboard.      

The true details will never be fully known, but sound recording evidence along with the bits of communication from her prior to her loss, point to a leak that started in the area of the engine room or reactor area. It’s believed a failed joint or joints started to leak, and tons of seawater came in at extremely high pressure and caused a short that resulted in the reactor to SCRAM, which is a fancy term for shutting down. 

This removed her primary source of power and propulsion leaving her unable to surface.      

The first indication of trouble broadcast to the outside world was a message on the underwater telephone that they had a “minor problem” (any problem on a deep diving submarine should not be considered minor) and were passing 456 meters (1500 feet) feet so she was approximately 6818 kilos (15,000 lbs.) heavier due to loss of buoyancy at depth. 

She made several attempts to do an emergency “blow” which is pumping high pressure air into the sub’s ballast tanks forcing out all the water and greatly increasing buoyancy. She lost her main source of propulsion when the reactor shut down and 

A post wreck analysis on one of her sister ships, found among all the other factors of the loss that due to higher pressure 4500 psi to lower 3000 psi pressure difference would super cool the compressed air which would condense into ice and essentially freeze and distort the cone-shaped filters, preventing the effective flow of compressed air to remove the water from the ballast tanks. This combined with the loss of her primary driving force from the reactor and the ever-increasing weight of water flooding her rear section kept forcing the submarine deeper. Thresher at approximately 2250 feet (900 feet deeper than her rated test depth) she imploded and was shredded to pieces over the ocean floor. 

After a long search for the wreckage and several boards of inquiry with over 1700 pages of testimony and other documents, experts were able to reach the following conclusions. A contributing factor in addition to the air system filter flaws, remember how we talked about those new joints? Even, though they had been ultrasonically tested and deemed safe when the sub was built, they were never tested again after she put to sea the first time in 1961.  Post analysis of other subs using this form of joining found there was a much higher rate of failure than predicted and the layout of seawater exposed piping was much more extensive than previous designs.

Other non-design factors (or human factors) were that she had been assigned a new captain and executive officer who had no prior experience with this new class of submarine.  Furthermore, her normal complement of 13 officers was reduced to 12 as the chief reactor plant officer, had to remain onshore due to medical issues. Additionally, four of her newly assigned junior officers had never been to sea before, let alone on a submarine about to conduct deep-diving tests and so had no experience with the new layout of systems which were radically different from previous subs.  In fact, the Thresher was so new that no shore base simulators, something that all previous classes had, had been developed to address all of Thresher’s design changes and improvements.      

The bottom line when it comes to the loss of the Thresher was the convergence of many factors of design, rush to production, procedural, administrative and ultimately, everyone involved underestimating the environment in which these subs were intended to operate.  

The loss directly led to the SUBSAFE program which was instituted not only to correct the design flaws and introduce strict manufacturing, construction quality control but also to address procedural items to prevent this from ever happening again. In fact, SUBSAFE was recommended to NASA by the CAIB (Columbia Accident Investigation Board) as the program that NASA should emulate after the shuttle disaster.      

 As for Thresher’s sister subs, they were renamed the Permit Class after the second sub constructed and once all the design flaws were corrected, they eventually numbered 13 additional boats that served from the mid-1960s until early 1990 when they had been eclipsed by the follow-on Sturgeon and Los Angeles class fast attack submarines.  

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