On 10 April 1963, the U. S. Navy suffered the loss of the nuclear submarine Thresher, the nation’s third peacetime submarine loss since World War II and by far the United States’ greatest single submarine disaster in terms of loss of life. The incident made headlines nationwide and led to an extensive inquiry by both the Navy board and Congress. Ultimately, submarine design, construction and operational capabilities had come too far, too fast, revealing flaws in the professional responsibilities of those building, commanding, and administering ships. Though it was a tragic loss of life and a likely preventable misfortune, the sinking of the USS Thresher fundamentally paved the way for impressive technological developments and modern safety procedures. The USS Thresher was the result of the quantum technological advances of the previous decade, meaning she was the most advanced operational attack submarine in the world at that time. When she commissioned on August 3rd, 1961, she was the fastest, deepest-diving, quietest, and best-armed nuclear submarine ever delivered as an operating warship to any fleet. Her test-dive performances at sea fully confirmed the anticipated advances in attack submarine capabilities which the designers had promised, so much so that the Navy asked for and received authority to build 14 of these ships, as well as an additional 11 SSNs with very similar characteristics (Grenfell 2013). The Thresher showed them during her year in the Fleet that she could surpass the best they had hoped for when she was placed in the hands of submarine sailors. She could shoot the most advanced torpedoes, launch the new anti-submarine missile SUBROC, and accelerate to high speeds with very little noise. Her sonar, both active and passive, was extremely impressive, and this vastly improved detection capability was further enhanced by her ability to operate at great depths over a much wider range than ever before. She was shock tested and withstood this depth-charging in a manner far superior to that of any submarine the U.S. Navy had previously tested (“Thresher” 2015). Overall, she was a marvelous machine. After her initial shakedown in the Fleet, the Thresher returned to Portsmouth Naval Shipyard in the summer of 1962 for necessary repairs, corrections, and alterations after having been operated long enough to check the systems thoroughly. She had performed beautifully, and her anticipated repairs were in no way cause for concern. Though she was expected to depart in the early months of 1963, her availability was later extended to permit additional modifications for silencing machinery (“Thresher” 2015). On April 4th, the Thresher was drydocked to permit final work on hull fittings, and a few days later she was undocked and moored to a pier while her crew completed final adjustments and tests with the shipyard personnel in preparation for sea trials. On April 8th, the Thresher, under the command of Lieutenant Commander John W. Harvey, had all of this behind her. Her engineers commenced bringing her nuclear plant critical, and by 0805 on April 9th, after completing all of the checks of communications, instrumentation and ship control equipment, the Thresher was underway and headed for sea for the first time after nearly nine months of overhaul (Grenfell 2013). If the crew managed to accomplish the events on the Sea Trial Agenda, the ship would be well on her way to rejoining the Fleet. The submarine rescue ship USS Skylark (ASR-20) from Submarine Squadron TEN in New London had been ordered to rendezvous with the Thresher and proceed in company to an operating area near Boston where the Thresher was to conduct her initial trim dive and her half-test-depth dive. The Thresher successfully completed her initial submerged tests on in salvageable waters and released the Skylark with instructions to rendezvous the next morning for the conduct of the test-depth dive. The Skylark proceeded to the morning rendezvous point and the Thresher continued to follow her Sea Trial Agenda, conducting various tests en route to the deep-dive position. In the early hours on April 10th, the Thresher joined the Skylark at a position about 220 miles east of Cape Cod. The two ships exchanged calls by radio and underwater telephone and the Thresher reported her range and bearing from the Skylark, as well as her course (Grenfell 2013). She then commenced her dive to test depth around 0900, using the standard practice of stopping at intervals to check carefully the integrity of sea-water systems. Fifteen minutes after reaching her assigned depth, the submarine communicated with Skylark by underwater telephone to apprise the submarine rescue ship of difficulties. Garbled transmissions indicated that far below the surface things were going terribly wrong. At 0917, Skylark received a final transmission with the partially recognizable phrase “exceeding test depth … ” Almost immediately, Skylark personnel heard a noise “like air rushing into an air tank — then, silence.” Efforts to reestablish contact with Thresher failed, and search groups were formed in an attempt to locate the submarine. Photographs taken by bathyscaph Trieste proved that the submarine had imploded, taking all 129 crew members on board to their deaths some 220 miles east of Boston. The USS Thresher was officially declared lost in late April of 1963 (“Thresher” 2015).Immediately after the loss of the Thresher was confirmed, a Judge Advocate General Court of Inquiry was convened to establish what happened on the ship when it was lost, and if possible, why it happened. While the exact cause was never determined, the Navy investigation concluded that the loss of the USS Thresher was in all probability due to an initial flooding casualty in the engine room, likely resulting from a faulty silver-brazed joint, a flexible hose failure, or undiscovered shock damage. This was compounded by the loss of reactor power due to an electrically-induced automatic shutdown from saltwater exposure, inadequate operating procedures with respect to disaster control, and a deficiency in the main ballast-tank blow system (JAG II-15). Consequently, the submarine was unable to overcome the increasing weight of water rushing into the engine room and sank. After extensive investigation of the design, construction, and short operational life, the court concluded that design and construction activities were not keeping pace with the complexity of newer ship design and materials and newer operational capabilities. Prior to the death of the USS Thresher, seawater connected systems were designed and constructed using silver brazing (silbraze) joint designs that were developed in the 1940s and 1950s. The brazing process joined the metal pipes by heating them to a temperature sufficient to melt the silver ring placed in the middle so that the silver flowed into the space between the closely fitted parts by capillary action. Two standards for silver-brazing pipe joints were used during the construction and overhaul of many ships including the Thresher: induction heating, which provided better joint integrity, was used for easily accessible joints, while hand-held torches were used where accessibility was restricted (Krahn 2002). Malfunctions were a common occurrence on ships in the fleet, and failures included significant flooding casualties, but the condition was commonly accepted in the submarine community as a part of the ship design. Reviewers determined that hand-held torches were used to heat many of the Thresher’s crucial but less accessible pipe joints, and to make matters worse, there was no effective nondestructive testing (NDT) capabilities developed for the design. Therefore, visual inspection and successful system testing were the only criteria used to judge the safety of the ship (JAG I-9). On the USS Thresher, a new technology called ultrasonic testing was tried as an experimental NDT process; of the 3,000 silver-brazed piping joints that were exposed to full submergence pressure, only 145 of those were inspected on a not-to-delay vessel basis and 13.8% of those tested showed sub-standard joint integrity (Krahn 2002). Because this newer NDT technology was not well understood, it was not mandated and the Thresher went on sea trials without further corrective actions.A failure to meet design specifications proved to be the nail in the coffin for the USS Thresher. The submarine was designed and built to meet two sets of standards and because the submarine’s nuclear power plant was the focus of the engineers, the standards used for the nuclear power plant were more stringent than those for the rest of the submarine. As a result of the emphasis placed on nuclear-related aspects of the design, builders assigned less importance to the steam and saltwater systems, even though those systems were crucial to the operation and safety of the vessel. For example, equipment that required immediate access during an emergency was obstructed, in some case by bolted decking, and the submarine’s design did not include the capability to quickly and remotely secure all sea-connected systems during a flooding casualty. Additionally, the Thresher did not have an emergency blow system since specifications for government procurement were not strictly enforced. The Navy later found that the reducing valve components installed in the pressurized air systems used to blow the main ballast tanks of the submarine did not meet design specifications (“Loss” 1963). Because of the magnitude of the pressures anticipated, the valve manufacturer had added a strainer feature upstream of the reducing valves to protect the sensitive valves from particulate matter. When the Navy conducted tests on another Thresher-class (now named Permit-class) vessel, it found that the pressure drop across the component at high flow rates caused entrained moisture to accumulate on the strainers and form enough ice to block the airflow. Venturi cooling, as this phenomenon is called, was thought to be the reason that the Thresher’s attempts to blow its main ballast tanks were ineffective (JAG II-18).Sadly, had there been more strict quality assurance (QA) procedures in place at the time, the USS Thresher may not have ended up at the bottom of the Atlantic. The increase in complexity of nuclear submarines would have resulted in a significant increase in the responsibilities imposed upon the commanding officers during the construction and post shakedown availability periods. Additionally, in the late 1950s and early 1960s, the number of naval officers designated for engineering duties was reduced by almost one third. Therefore, it was highly likely that significant mistakes were slipping through the shipyard unnoticed (Grenfell 2013). This could have easily been counteracted by the creation and implementation of engineering standards that would be strictly observed, especially in the design and construction of complex and technologically sophisticated systems. For example, if the Bureau of Ships had required submarine shipbuilding activities to adhere to specifications, the Thresher probably would have been able to blow its main ballast tanks to help it surface in case of emergency. Additionally, the quality assurance program of the Portsmouth Naval Shipyard would have been greatly improved by the implementation of an organizational chain of command to ensure proper inspection procedures and increase communication between QA divisions and the shipyard commander. For instance, the Chief of Bureau of Ships sent a memo to the Commander of Portsmouth Naval Shipyard to create a test plan to address the silbraze piping joint inadequacies aboard the Thresher, but because the Navy failed to specify the extent of the silbraze testing required, the shipyard discontinued its use as soon as ultrasonic testing proved burdensome and time consuming (“Thresher” 2015). This action was taken despite the fact that 20 of 145 joints passing traditional inspections failed to meet minimum bonding specifications. After sending the memo, the Bureau of Ships did not hear back from the shipyard until after the Thresher sank and they assumed testing went as planned. When asked about why the Bureau did not follow up on the memo during the congressional hearing, Admiral Brockett replied, “I don’t believe it was considered to be a matter of priority” (Krahn 2002). This also proves that the Navy was a bit too hasty when introducing their newest submarine to sea trials. In hindsight, the Navy appears to have acted rashly in sending the Thresher to test depth that fateful day. Following the loss of the Thresher, the Navy was forced to recognize that it moved too fast and too far in areas of offensive and defensive capabilities and that submarine safety did not keep pace. The disaster could have potentially been averted by postponing sea trials in order to run more tests and conduct more thorough inspections, given that it was a brand new type of submarine. Instead, all post shakedown maintenance was performed on a not-to-delay vessel basis and the ship was instead put through a gauntlet of depth charges causing structural damage that was then measured and evaluated (Sullivan 2003). Ultimately, the Thresher’s management had a “mission above safety” attitude, and they thought that the safety standards at the time were adequate, even though the Thresher was to undergo more stress than any submarine ever built. Had the testing occurred in shallower water (perhaps with the ocean bottom just slightly below test depth), in which the USS Skylark could have potentially come to their aid, the crew might have been saved, if not the USS Thresher itself. Instead the risky tests took place in 8,400 feet of water with only one rescue ship nearby, giving the ship and the crew low chances of survival and impossibility of rescue if an incident occurred (Jabaley 2015). The nuclear submarine was built using tried and true methods and procedures that had worked for decades, but with the adoption of new materials, equipment, and operating conditions, the old ways became suddenly, tragically inadequate.The loss of the USS Thresher was a watershed event in the submarine community and generated an immediate response. In June of 1963 – less than two months after the tragedy – the Submarine Safety (SUBSAFE) Program was created which established and implemented clear requirements for the design, construction and maintenance of all Navy submarines. The SUBSAFE Certification Criterion was issued December 20th of 1963, formally implementing the program and providing the basic foundation and structure of the program that is still in place today (Jabaley 2015). Its purpose is to provide maximum reasonable assurance of watertight integrity and recovery capability, which is achieved by certifying that each submarine meets safety requirements upon delivery to the Navy and by maintaining that certification throughout the life of the submarine. The heart of the SUBSAFE program and its certification processes is a combination of three concepts: work discipline, material control, and documentation. Work discipline demands knowledge of the requirements and compliance with those requirements for everyone who performs any kind of work associated with submarines. Material control is everything involved in ensuring that correct material is installed correctly, beginning with contracts that purchase material all the way through receipt inspection, storage, handling, and finally installation in the submarine. Documents include specific design products that are created when the submarine is designed, such as system diagrams or ship systems manuals, which must be maintained throughout the life of the submarine to enable maintenance of the SUBSAFE certification. It also includes work records, which document the work performed and the worker’s signature that certifies it was done per the requirements (Sullivan 2003).SUBSAFE requirements are invoked in the design and construction contracts for new submarines, in the work package for submarines undergoing depot maintenance periods, and in the Joint Fleet Maintenance. Audits are conducted on a regular basis to ensure that the material condition of that submarine is satisfactory for continued sea trials and unrestricted operations, as well as to periodically review the policies, procedures, and practices used by each organization that performs SUBSAFE work (Sullivan 2003). Overall, the SUBSAFE Program has been very successful. Between 1915 and 1963, sixteen submarines were lost due to non-combat causes — an average of one every three years. Since the inception of the SUBSAFE Program in 1963, only one submarine has been lost. The USS Scorpion (SSN 589) was lost in May of 1968 with 99 officers and men aboard, but she was not a SUBSAFE certified submarine and the evidence indicates that she was lost for reasons that would not have been mitigated by the SUBSAFE Program. Fortunately, a SUBSAFE certified submarine has never been lost at sea (Jabaley 2015).During the search for debris and clues on the deep ocean floor following the loss of the USS Thresher, the Navy recognized the need for better deep submersibles. This led them to pioneer the concept of the Deep Submergence Recovery Vehicle (DSRV) in 1970. The DSRV is designed to rescue 24 people at a time at depths of up to 2,000 feet, and its maximum operating depth is 5,000 feet. Power is provided by 2 large batteries — one fore and one aft — that power the electrical, hydraulic and life support systems. The vehicle uses mercury in a completely sealed system to allow itself to match any angle (up to 45°) in both pitch and roll so as to attach to a downed submarine that may be at an angle on the seafloor. The DSRV is capable of being transported by Air Force C-5 to anywhere in the world within 24 hours. It is then loaded onto a “Mother Submarine” (MOSUB) which carries the DSRV to the rescue site where several trips are made to rescue all personnel. Rescue is usually accomplished by ferrying submariners from the stranded sub to the MOSUB; however, they can also be taken to a properly equipped surface support ship (Hooton 2006). The legacy of the USS Thresher inspired the remarkable new designs that can now explore some of the deepest trenches of the sea, that helped discover the remains of the Titanic, and that can even rescue the occasional stranded sailor. No system with extensive vulnerability to human error can be executed flawlessly, especially when it involves the design and production of complicated technology. However, the application of reliable engineering practices and SUBSAFE procedures help reduce the margin of error to prevent deadly situations. Vast improvements to safety have since been made in submarine programs and throughout the Navy since 1963, but it took a disaster with expensive technology and many lives lost to provoke the Navy to change. The loss of loved ones is always difficult to bear, especially in the case of the USS Thresher since the submarine sank during what should have been a routine test. But even now, more than 50 years later, families of those lost in the Thresher disaster can take comfort in knowing that the incident provoked major overhauls in naval safety.