50 years ago today, USS THRESHER (SSN-593) departed on her ill-fated voyage after having undocked and completed an overhaul period at Portsmouth Naval Shipyard. She transited to a point about 200 miles off Cape Cod with 129 crew and civilian shipyard engineers and technicians, and commenced a series of dive tests to verify her work in the shipyard. She successfully completed a series of tests on 9 April and remained submerged overnight.
On the morning of 10 April, THRESHER commenced another series of dive tests. The approximate timeline of that fateful day is as follows:
07:47 Thresher begins its descent to the test depth of 1,000 ft (300 m).
07:52 Thresher levels off at 400 ft (120 m), contacts the surface, and the crew inspects the ship for leaks. None are found.
08:09 Commander Harvey reports reaching half the test depth.
08:25 Thresher reaches 1,000 ft (300 m).
09:02 Thresher is cruising at just a few knots (subs normally moved slowly and cautiously at great depths, lest a sudden jam of the diving planes send the ship below test depth in a matter of seconds.) The boat is descending in slow circles, and announces to Skylark she is turning to “Corpen [course] 090.” At this point, transmission quality from Thresher begins to noticeably degrade, possibly as a result of thermoclines.
09:09 It is believed a brazed pipe-joint ruptures in the engine room. The crew would have attempted to stop the leak; at the same time, the engine room would be filling with a cloud of mist. Under the circumstances, Commander Harvey’s likely decision would have been to order full speed, full rise on the fairwater planes, and blowing main ballast in order to surface. The pressurized air rapidly expanding in the pipes cools down, condensing moisture and depositing it on strainers installed in the system to protect the moving parts of the valves; in only a few seconds the moisture freezes, clogging the strainers and blocking the air flow, halting the effort to blow ballast. Water leaking from the broken pipe most likely causes short circuits leading to an automatic shutdown of the ship’s reactor, causing a loss of propulsion. The logical action at this point would have been for Harvey to order propulsion shifted to a battery-powered backup system. As soon as the flooding was contained, the engine room crew would have begun to restart the reactor, an operation that would be expected to take at least 7 minutes.
09:12 Skylark pages Thresher on the underwater telephone: “Gertrude check, K [over].” With no immediate response (although Skylark is still unaware of the conditions aboard Thresher), the signal “K” is repeated twice.
09:13 Harvey reports status via underwater telephone. The transmission is garbled, though some words are recognizable: “[We are] experiencing minor difficulty, have positive up-angle, attempting to blow.” The submarine, growing heavier from water flooding the engine room, continues its descent, probably tail-first. Another attempt to empty the ballast tanks is performed, again failing due to the formation of ice. Officers on Skylark could hear the hiss of compressed air over the loudspeaker at this point.
09:14 Skylark acknowledges with a brisk, “Roger, out,” awaiting further updates from the SSN. A follow-up message, “No contacts in area,” is sent to reassure Thresher she can surface quickly, without fear of collision, if required.
09:15 Skylark queries Thresher about her intentions: “My course 270 degrees. Interrogative range and bearing from you.” There is no response, and Skylark’s captain, Lieutenant Commander Hecker, sends his own gertrude message to the submarine, “Are you in control?”
09:16 Skylark picks up a garbled transmission from Thresher, transcribed in the ship’s log as “900 N.” [The meaning of this message is unclear, and was not discussed at the enquiry; it may have indicated the submarine’s depth and course, or it may have referred to a Navy “event number” (1000 indicating loss of submarine), with the “N” signifying a negative response to the query from Skylark, “Are you in control?”]
09:17 A second transmission is received, with the partially recognizable phrase “exceeding test depth….” The leak from the broken pipe grows with increased pressure.
09:18 Skylark detects a high-energy low-frequency noise with characteristics of an implosion.
09:20 Skylark continues to page Thresher, repeatedly calling for a radio check, a smoke bomb, or some other indication of the boat’s condition.
11:04 Skylark attempts to transmit a message to COMSUBLANT (Commander, Submarines, Atlantic Fleet): “Unable to communicate with Thresher since 0917R. Have been calling by UQC voice and CW, QHB, CW every minute. Explosive signals every 10 minutes with no success. Last transmission received was garbled. Indicated Thresher was approaching test depth…. Conducting expanding search.” Radio problems meant that COMSUBLANT did not receive and respond to this message until 12:45. Hecker initiated “Event SUBMISS [loss of a submarine]” procedures at 11:21, and continued to repeatedly hail Thresher until after 17:00.
Recent reports suggest that the last transmission from THRESHER at 0917 might have meant that the boat was 900 feet greater than her designed test depth of 1300 feet, or 2200 feet. It’s estimated that she imploded in less than a tenth of a second at a depth of 2400 feet, nearly double her test depth and greater than her design crush depth of 2000 feet.
The loss of the boat was due to several factors:
–Joints subjected to sea pressure were often silver-brazed joints, which were susceptible to leaking.
–The scram (shutdown) of the reactor also caused the main steam stops to close, preventing use of residual steam being produced to be used for propulsion.
–In the EMBT (Emergency Main Ballast Tank) blow system, 4500 psi air is taken from tanks located inside the ballast tanks, through “knocker valves” and into the main ballast tanks themselves. Prior to underway, THRESHER used “shore air” to pressurize their EMBT cylinders. This air had very high moisture content. When the emergency blow was conducted, this air was redirected through piping as small as one inch in diameter, and the reduction in pressure and lowering of temperature caused the moisture to freeze and plug the EMBT piping, preventing more air from getting into the main ballast tanks.
What was learned from the loss of THRESHER was put to use in what is called the SUBSAFE system. Welds replaced brazed joints. New procedures allowed propulsion to be maintained even in the event of a plant shutdown. More robust piping for the EMBT systems, and many other changes were implemented. From 1915 through 1963, 16 submarines were lost due to non-combat conditions. Since THRESHER, only one has been lost: USS SCORPION (SSN-589), which was not a SUBSAFE boat and whose cause for her loss has never been conclusively determined.
Another reminder for submariners that “There is no such thing as peacetime underway” and the inherent hazards of the sea.
USS Thresher’s name was retired, but she was never struck from the commissioning list. She remains “on eternal patrol.”
Sailors rest your oars.
