April 10 in History- the 49th Anniversary of the Sinking of the USS Thresher
Yesterday, April 10th marked the 49th anniversary of the loss of the USS Thresher (SSN-593), the worst submarine disaster in the history of the United States Navy.
USS Thresher conducting pre-commissioning trials
The Thresher, the most advanced submarine yet built by the United States, was to be the lead unit of the next class of attack submarines, as well as the engineering prototype for the entire next generation of submarines. Her design combined two of the prior strands of nuclear submarine development, that of the high-speed attack submarine such as the Skipjack-class and the quiet anti-submarine hunter-killer like the Tullibee (SSN-597). Among the innovations in Thresher's design were the use of modified fittings, which combined with the use of the recently developed HY-80 steel alloy in the construction of the hull, allowed a diving depth of 1,300 feet, almost twice that of earlier nuclear submarines, utilized recent advancements in hydrodynamics to streamline the hull to the point where it was only 1 kt slower than Skipjack despite being 15% heavier and over 25' longer while still using the same powerplant, further refined advances in sonar technology such as the spherical bow sonar array first used in Tullibee so she was better able to detect ships and submarines while submerged than any other submarine at the time, and the latest silencing techniques, making her the quietest submarine yet designed.
Authorized in the defense appropriations for 1958, Thresher was built by the Portsmouth Naval Shipyard in Kittery, Maine, being laid down on May 28, 1958, launched on July 9, 1960, and commissioned on August 3, 1961. After completing her trials, shakedown cruise, and crew training, Thresher spent over a year participating in a series of exercises designed to fully explore the capabilities of the new submarine, as well as possible ways to counter then-notional Soviet submarines of similar capability. Following the excercises, Thresher underwent an overhaul, that in addition to taking care of accumulated maintenance needs and correcting defects, would carry out a number of systems upgrades. After completing the overhaul in the spring of 1963, Thresher began conducting a series of post-overhaul trials. On April 10, 1963, Thresher began a deep-submergance test some 220 miles east of Cape Cod, with the submarine-rescue vessel Skylark (ASR-20) standing by.
As can best be reconstructed from a series on unclear underwater telephone transmissions between Thresher and Skylark, as well as the subsequent investigation, shortly after 9:10 AM local time, while Thresher was operating at a depth of approximately 1,000 feet, a seawater pipe joint in the engine room failed, causing the submarine to take on water, and her captain attempted to surface the boat.
However, the flooding likely caused a short circuit which caused the reactor to enter emergency shutdown, and the emergency battery could not provide enough power to drive the submarine to the surface. Furthermore, the standard operating procedures of the era would have prevented the crew from using the waste heat & remaining steam to keep the main engines running or speed up the restart of the reactor, which going by the book, would have taken over 7 minutes. (Thresher's chief engineer had been granted compassionate leave just before her sailing to care for his sick wife, leaving his inexperienced assistant and trainee in charge of the engine room, which may have been a factor, as he later stated that he probably would have ignored procedure and used that waste heat and steam to keep the engines going as long as possible and expedite the reactor restart, which may have saved the boat.) Compounding the problem was the failure of the emergency blow system, which used high-pressure compressed air (4,500 PSI) routed through a series of manifolds and pipes to blow the water out of the ballast tanks, caused by adiabatic cooling of the air and condensation in the pipes clogging the system with ice, as well as a cramped & awkward engine room design making it difficult for the crew to stop the flooding. At 9:17, a garbled transmission from Thresher said that she sinking below her rated test depth, and Skylark's repeated attempts at further communication after that recieved no response, suggesting that Thresher was crushed by the pressure of the sea shortly thereafter.
All 129 people aboard Thresher were lost with her, including not just her crew of 112 officers and enlisted men, but also a number of naval and a dozen civilian technicians from the shipyard and the submarine program offices.
In the aftermath of Thresher's loss, the Navy was worried as the entire next generation of attack and ballistic missile submarines was based upon the same basic ideas of Thresher's design, and if they were flawed, not only would the submarine program, but a large part of overall naval strategy would be placed in jeopardy. The subsequent inquiry into the disaster found that the basic design of Thresher was sound, but suffered from a number of detail flaws, and resulted in a number of changes. Not only were reactor operating procedures altered to allow for fast emergency restarts, as well as a number of changes to the design and construction of submarines, such as redesigning emergency blow systems so they would vent directly into the ballast tanks through larger pipes to prevent the adiabatic cooling that had helped doom Thresher, treating seawater pipes as if they were part of the hull, making it easier for a single person to shut down any internal piping circuit, and greatly increased attention to quality control and design issues, in what became known as the SUBSAFE program. Submarines under construction and future designs would be changed to reflect these improvements, while existing submarines would be retrofitted to incorporate them to the greatest extent possible as they came in for overhaul.
Thresher's class of submarine was renamed the Permit-class after the next boat of the class, Permit (SSN-594). Although the Thresher/Permit-class subs could be partially retrofitted to incorporate SUBSAFE improvements and correct detail design flaws, fully correcting these issues required extensive re-engineering of the design, and during the process, the Navy took advantage of the opportunity to correct a number of operational deficiencies and provide a roomier submarine that would be more comfortable for the crew and allow greater room for future upgrades; so many changes were made that the modified design became an entirely new class of submarine, the Sturgeon-class.
An animation of the likely course of events that occurred during the loss of Thresher
Further reference:
An online photo gallery with a number of images related to Thresher and her loss.
A listing of the 129 people who were lost with Thresher and remain on eternal patrol.
USS Thresher conducting pre-commissioning trials
The Thresher, the most advanced submarine yet built by the United States, was to be the lead unit of the next class of attack submarines, as well as the engineering prototype for the entire next generation of submarines. Her design combined two of the prior strands of nuclear submarine development, that of the high-speed attack submarine such as the Skipjack-class and the quiet anti-submarine hunter-killer like the Tullibee (SSN-597). Among the innovations in Thresher's design were the use of modified fittings, which combined with the use of the recently developed HY-80 steel alloy in the construction of the hull, allowed a diving depth of 1,300 feet, almost twice that of earlier nuclear submarines, utilized recent advancements in hydrodynamics to streamline the hull to the point where it was only 1 kt slower than Skipjack despite being 15% heavier and over 25' longer while still using the same powerplant, further refined advances in sonar technology such as the spherical bow sonar array first used in Tullibee so she was better able to detect ships and submarines while submerged than any other submarine at the time, and the latest silencing techniques, making her the quietest submarine yet designed.
Authorized in the defense appropriations for 1958, Thresher was built by the Portsmouth Naval Shipyard in Kittery, Maine, being laid down on May 28, 1958, launched on July 9, 1960, and commissioned on August 3, 1961. After completing her trials, shakedown cruise, and crew training, Thresher spent over a year participating in a series of exercises designed to fully explore the capabilities of the new submarine, as well as possible ways to counter then-notional Soviet submarines of similar capability. Following the excercises, Thresher underwent an overhaul, that in addition to taking care of accumulated maintenance needs and correcting defects, would carry out a number of systems upgrades. After completing the overhaul in the spring of 1963, Thresher began conducting a series of post-overhaul trials. On April 10, 1963, Thresher began a deep-submergance test some 220 miles east of Cape Cod, with the submarine-rescue vessel Skylark (ASR-20) standing by.
As can best be reconstructed from a series on unclear underwater telephone transmissions between Thresher and Skylark, as well as the subsequent investigation, shortly after 9:10 AM local time, while Thresher was operating at a depth of approximately 1,000 feet, a seawater pipe joint in the engine room failed, causing the submarine to take on water, and her captain attempted to surface the boat.
However, the flooding likely caused a short circuit which caused the reactor to enter emergency shutdown, and the emergency battery could not provide enough power to drive the submarine to the surface. Furthermore, the standard operating procedures of the era would have prevented the crew from using the waste heat & remaining steam to keep the main engines running or speed up the restart of the reactor, which going by the book, would have taken over 7 minutes. (Thresher's chief engineer had been granted compassionate leave just before her sailing to care for his sick wife, leaving his inexperienced assistant and trainee in charge of the engine room, which may have been a factor, as he later stated that he probably would have ignored procedure and used that waste heat and steam to keep the engines going as long as possible and expedite the reactor restart, which may have saved the boat.) Compounding the problem was the failure of the emergency blow system, which used high-pressure compressed air (4,500 PSI) routed through a series of manifolds and pipes to blow the water out of the ballast tanks, caused by adiabatic cooling of the air and condensation in the pipes clogging the system with ice, as well as a cramped & awkward engine room design making it difficult for the crew to stop the flooding. At 9:17, a garbled transmission from Thresher said that she sinking below her rated test depth, and Skylark's repeated attempts at further communication after that recieved no response, suggesting that Thresher was crushed by the pressure of the sea shortly thereafter.
All 129 people aboard Thresher were lost with her, including not just her crew of 112 officers and enlisted men, but also a number of naval and a dozen civilian technicians from the shipyard and the submarine program offices.
In the aftermath of Thresher's loss, the Navy was worried as the entire next generation of attack and ballistic missile submarines was based upon the same basic ideas of Thresher's design, and if they were flawed, not only would the submarine program, but a large part of overall naval strategy would be placed in jeopardy. The subsequent inquiry into the disaster found that the basic design of Thresher was sound, but suffered from a number of detail flaws, and resulted in a number of changes. Not only were reactor operating procedures altered to allow for fast emergency restarts, as well as a number of changes to the design and construction of submarines, such as redesigning emergency blow systems so they would vent directly into the ballast tanks through larger pipes to prevent the adiabatic cooling that had helped doom Thresher, treating seawater pipes as if they were part of the hull, making it easier for a single person to shut down any internal piping circuit, and greatly increased attention to quality control and design issues, in what became known as the SUBSAFE program. Submarines under construction and future designs would be changed to reflect these improvements, while existing submarines would be retrofitted to incorporate them to the greatest extent possible as they came in for overhaul.
Thresher's class of submarine was renamed the Permit-class after the next boat of the class, Permit (SSN-594). Although the Thresher/Permit-class subs could be partially retrofitted to incorporate SUBSAFE improvements and correct detail design flaws, fully correcting these issues required extensive re-engineering of the design, and during the process, the Navy took advantage of the opportunity to correct a number of operational deficiencies and provide a roomier submarine that would be more comfortable for the crew and allow greater room for future upgrades; so many changes were made that the modified design became an entirely new class of submarine, the Sturgeon-class.
An animation of the likely course of events that occurred during the loss of Thresher
Further reference:
An online photo gallery with a number of images related to Thresher and her loss.
A listing of the 129 people who were lost with Thresher and remain on eternal patrol.