A lead ship of the 14 nuclear submarines USS Thresher was commissioned in 1961. A follow on to the Skipjack class Thresher introduced several key innovations that would be repeated on most of the follow on classes of US subs. The main innovation was the midship torpedo tubes, freeing the bow for the huge sonar dome. The other innovation was use of the stronger steel for the hull allowing for deeper dives and mounting of the machinery on special raft for sound dampening. Streamlining of the hull and slimmer sail allowed the sub to be able to keep the same speed as the smaller Skpijack despite being bigger and using the same reactor.
USS Thresher had a very short career. After two years of intensive training it went in to a shipyard for refit and 1963 and after completion of that refit it left on the acceptance trials. On 9 April 1963 it submerged for a deep diving test and was never seen again. The accompanying rescue ship Skylark received a garbled message "... minor difficulties, have positive up-angle, attempting to blow." 1 hour and 26 minutes after the submarine dived. That was the last anyone heard from them. The actual cause of the sinking was never established but the theory goes that a weld on one of the pipes in the hull failed and flooded the engine room shorting out the reactor controls, causing the reactor to be shut down. The ballast tanks were not able to blow because the highly pressurized air caused the valves to freeze as it was going through them. Unable to blow its tanks and without propulsion the sub reached its crush depth and was destroyed.
The accident caused the US navy to create a SUBSAFE program that in addition to rectifying the known problems also put together a number of procedures to make sure the future problems do not appear and are detected early. The program was very successful making sure that there were no US submarine losses after the 1960s. With the loss of the head ship, the Thresher class was renamed Permit class after the second submarine in the class and went on to serve until retired after the end of the cold war in the early 1990s.
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