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One documentary I saw blamed the debris screens on the emergency blow valve outlets. Tests revealed that they could ice up, given the wet environment and the rapid expansion of compressed air.

I read an anniversary (30th?) article that included an interview with a guy that was a submariner back then that said that there were problems with silver soldered pipe joints at the time, and that a joint failure may have been the initial cause of the boat’s problems.

FWIW, the Thresher Class was renamed to Permit Class.

I lived aboard the USS Patrick Henry SSBN 599, originally a Thresher class boat later that class name was changed.
There is no mystery as to what happened to the Thresher, we all knew exactly what happened.

During sea trials they did a normal surface procedure. A problem developed with the aft ballast tanks, actually the high pressure air valve that allowed air into the tanks to blow out the water to give the boat positive buoyancy. The forward group operated normally and the boat developed a hard up angle which allowed the air to spill out of the tanks. The boat then lost buoyancy provided by forward motion and began to go backwards quickly gathering speed in reverse. The ship then cycled all main ballast vents allowing the air in the forward tanks out in an attempt to level the boat and added full planes angles to level the ship to some success. After the ship was leveled they again tried to blow ballast but the aft blow valves again failed. They were already deep on this last attempt when again the forward group filled enough to develop a severe up angle.
Again the forward motion was stopped and the ship started losing depth in reverse. Again the Main vents were cycled but it was too late, another attempt to blow was made but at the depth they were there was not enough high pressure air left in the air banks and the boat went below crush depth in reverse.

The Surface craft above heard and recorded all the sounds that pretty much tell the story. Later testing showed that the high pressure air valves developed ice as the HP air expanded and clogged up the valve keeping the aft group from being able to fully blow.

One of the most important features of the SubSafe program was to include emergency blow valves that were huge ball valves that could not clog with ice. Another feature was to provide more drying of compressed air to reduce the amount of moisture that could cause problems. The program caused there to be high pressure air actuated from mechanical “T” handled valves at the diving/ballast control panel to make it easily accessible to the diving officer, BCP operator or even the Conning officer and planesmen.

After most every upkeep these boats would go to test depth and do an emergency blow to test the system and train on it’s use.

SubSafe was an excellent program. Many of us owe our lives to those who went down on the Thresher by opening the eyes of the submarine service to a very dangerous situation.

The Scorpion is an entirely different situation of which I suspect will be kept under wraps for a long time because of several unanswered questions.