THE FATAL THRESHER DEEP DIVE: DIFFERENT OCEAN, DIFFERENT OUTCOME?

Captain John F.O’Connell, USN (Ret.) was commissioned from the United States Naval Academy. He served in USS BON HOMME Richard (CVA-31) and USS ROCHESTER (CA-124) before attending Submarine School. He served in USS PERCH (ASSP-313), USS CA/MAN (SS-323), GMU Ten, Squadron One staff, USS BARBERO (SSG-317), XO USS PICKEREL (SS-524), and ComSubPac staff. He commanded USS SPIN AX (SS-489) and Submarine Division 41. He was a Branch Head in the Submarine Warfare Division of OpNav (OP-31) and Chief Staff Officer of Submarine Flotilla Seven. He served as ComSubPac N3. and then as Defense and Naval Attache Tokyo. He has published five books, three dealing with air power and two with submari11e operational effective11ess in the 2th century.

On 9 April 1963 USS THRESHER (SSN-593) got underway from Portsmouth Naval Shipyard and proceeded to sea for Post Shakedown Availability (PSA) sea trials. She rendezvoused with her sea trial escort, submarine rescue vessel USS SKYLARK (ASR-20), and completed a shallow dive that day. They separated and headed for the deep test dive area. The maximum depth capability of SKYLARK’s McCann Rescue Chamber was 850 feet. The assigned area for THRESHER’s deep dive was approximately 200 miles east of Cape Cod with a depth of water of about eight thousand feet. THRESHER’s design test depth, the deepest depth to which the Navy Bureau of Ships (BuShips) authorized her to descend was 1300 feet.1 Test depth is based on hull thickness and material. THRESHER’s hull was constructed of HY 80 steel, used previously only in diesel-electric USS ALBACORE (SS 569) and SKIPJACK (SSN 585) class SSNs. Improved hull welding techniques allowed THRESHER to operate as deep as 1300 feet compared to the Skipjack class test depth of 700 feet.

THRESHER began her descent from periscope depth to test depth at 0747. Norman Polmar asserts in Death of USS THRESHER that SKYLARK had not been provided an agenda by the commanding officer of THRESHER for its first PSA deep dive. I have made certain assumptions based on normal submarine practice for test depth dives during sea trials, and has drawn on material developed during the Naval Court of Inquiry proceedings into the loss of USS THRESHER on 10 April 1963.

There was a sub-surface communications channel available between THRESHER and SKYLARK during the sea trial. It was the AN/UQC underwater telephone, referred to as Gertntde in the Navy. Submarines are equipped with Gertrude, as are antisubmarine warfare vessels and submarine rescue ships. Gertrude is similar to tactical radio in that the originator voices a message and ends with over or out. The former implies that an answer or acknowledgment is expected from the other party, while a message ending in out does not require a reply. Unlike a radio message being transmitted through air, the Gertrude message travels through water. Both air and water are fluids but the speed of transmission in them differs significantly.

THRESHER and SKYLARK established Gertrude communications while THRESHER was at periscope depth. THRESHER indicated that she was beginning her dive to test depth. The normal procedure would be to go from periscope depth to test depth in a series of steps, leveling off at each new depth level and checking throughout the boat for leaks. Flooding would be reported immediately.3 Flooding, depending upon its severity, might call for emergency surfacing of the submarine. In order to avoid a possible collision between the submarine carrying out an emergency surfacing operation and the ASR, they would operate offset horizontally from each other. About half way to test depth THRESHER would probably have rigged for deep submergence, setting certain valves and fittings for increased safety at deeper depths.

Within the THRESHER all compartments were connected by sound powered telephones, with the control center phone talker acting as controller for the circuit. Given the importance of the sea trial, an officer in the control center might have been monitoring the circuit also. At each level of submergence depth the control center telephone talker would poll all compartments to check for leaks or any other problems. As THRESHER proceeded deeper her hull compressed with increasing sea pressure and that action normally brought forth a series of mechanical creaks and groans, somewhat alarming to crew members who had spent the past nine months in the shipyard. During builder’s trials of a new class of submarine, a telescoping rod is positioned in the interior hull of the submarine at its greatest breadth, and the actual hull compression is measured as the submarine proceeds to test depth.

At 0752 THRESHER reported being at 400 feet. At 0809 THRESHER reported being at 650 feet, and at 0835 at 1,000 feet. At 0853, THRESHER reported she was descending to her test depth of 1,300 feet.

Events apparently proceeded normally until about 0913. At that time THRESHER reported “Have positive up angle … attempting to blow up”. SKYLARK’s commanding officer recalled the transmission from THRESHER as “Experiencing minor problem … Have positive angle … Attempting to blow.” At 0917 a further garbled report was noted. The SKYLARK officer of the deck later testified that he heard the words test depth followed by breaking up sounds similar to ship sinking sounds he had heard during WW II.

At about 1058 SKYLARK began attempting to establish radio communications with the Submarine Operational Control Authority in New London, Connecticut to report to Commander Submarine Flotilla Two about the problems with the THRESHER sea trial. There were difficulties in getting the message through, including the relatively low priority assigned to the message by SKYLARK.4 Finally at about 1256 SKYLARK’s alarming information reached Captain J. S. Schmidt, USN, and subsequently the world including Assistant Commander Submarine Force Atlantic Fleet (New London), Commander Submarine Force Atlantic Fleet (ComSubLant) (Norfolk), Commander in Chief Atlantic Fleet (Norfolk), the Chief of Naval Operations, the Secretary of the Navy, and President John F. Kennedy were informed that there was a very serious problem with one of the most advanced nuclear-propelled submarines in the U.S. Navy.

SUBMISS was declared indicating an overdue and unaccounted for submarine. It was followed by SUBSUNK when it was clear that THRESHER had gone to the bottom during the course of her sea trial and was lost forever, along with her entire crew of officers and enlisted men, plus a contingent of officials, navy and civilian, from Portsmouth Naval Shipyard, the Bureau of Ships (BuShips ), and SubLant staff, some 129 men in all.

A Court Of Inquiry was rapidly convened under the leadership of Vice Admiral Bernard L. Austin, USN as President of the Court. It included five senior submarine officers of ranks from Vice Admiral to Captain. It began its proceedings on 13 April 1963 and took testimony from a number of individuals connected with the PSA and subsequent sea trials. The Court of Inquiry Report totaled 1700 pages. In summary it concluded that the loss of THRESHER was due to three factors:

• Failure of a silver-brazed fitting in the engine room, with immediate flooding, and subsequent emergency shutdown of its nuclear reactor (scram) due to spray in the engine room affecting electrical control panels (Portsmouth Naval Shipyard)
• Freezing of entrapped water in blow valve strainers. This blocked air from the flasks going in to the main ballast tanks to expel their water and push the submarine hull to the surface (BuShips Submarine Design section)
• Inability of the engineering watch to restore the reactor to normal operation from its scrammed status to provide normal propulsion power within a short time period (B11Ships Code 08-Naval Reactors Branch) The author has indicated in parentheses and italics, after each factor-the responsible organization in the train of events that led to the loss of THRESHER.
• Portsmouth Naval Shipyard had clear evidence a full year before the loss of THRESHER that the qualities of its silver braze joint work was suspect, when at 600 feet USS BARBEL (SS 580) had a silver braze joint in her engine room break loose. BARBEL conducted an emergency surface operation and reached the surface safely but with her engine room flooded up to the deck plate level.6 The Court of Inquiry Report, Part II, item 111 noted that ” … prior to THRESHER’s post shakedown availability, there had been reports of serious failures of sil-braze joints in BARBEL, SKATE, SNOOK, SCULPIN, ETHAN ALLEN and THRESHER.”
• THRESHER’s main ballast tank blow system was a new design by BuShips. It had not been tested as a complete system in a simulated emergency situation. The valve manufacturer had added a strainer to the valves. When the emergency blow started, the flow of air became very cold due to adiabatic effect and the strainer orifices were rapidly blocked with ice. Subsequent to loss of THRESHER, a dockside test of an identical system was conducted in USS TINOSA (SSN 606) and the exact same circumstances were observed. The blow started, and then stopped because of ice formation.
• Reactor plant operation was rigidly governed by the Reactor Plant Manual issued by Admiral Rickover’s branch of Naval Reactors within BuShips. It allowed for no discretion or individual judgment in procedural matters. Once the reactor scrammed, that is the control rods were automatically inserted to moderate neutron flow and slow the reactor down, the specified restart procedure was lengthy-too long to allow THRESHER to regain normal propulsion power before she sank below crush depth, given the flooding in the engine room.

Subsequently Portsmouth Naval Shipyard paid much more attention to the quality of silver braze joint work, coupled with BuShips’ new found interest in welding rather than silver-brazing certain fittings. The Submarine Safety (Sub Safe) program was instituted at great expense and delay in delivering new submarines to ensure that a loss like THRESHER would never occur again. Presumably BuShips submarine system designers learned a lesson about thoroughly testing newly designed systems before sending them to sea where people’s lives are at stake. Admiral Rickover quickly issued modifications to the Reactor Plant Manual to allow a much quicker restart of naval submarine reactors in emergency situations.

However there was another factor involved in the complete loss of THRESHER that has not previously been reported or explored, at least publically. It was the failure of Commander Submarine Force Atlantic (ComSubLant), the submarine operational commander in the Atlantic Fleet, to specify that deep dives conducted after new construction, post shakedown availability or shipyard overhaul-be conducted in specified safe deep dive areas.

In 1963, the year of THRESHER loss, Commander Submarine Force Pacific Fleet (ComSubPac) had a standing requirement in effect, specified in its ComSubPac 30 I-Year operation order, that initial deep dives after completion of new construction, conversion or overhaul, would be conducted only in approved areas.8 There were separate areas specified for each class of submarine by its test depth limitation.

The first, shallow dives to periscope depth or several hundred feet would be conducted in 200-300 feet of water, as was THRESHER’s initial shallow dive. Most of the diesel powered Guppy and Fleet Snorkel boats that made up the main part of the ComSubPac inventory were 400 foot test depth boats, although there were a few 300 foot thin skin diesel-electric boats left in service.9 The post-WW II TANG (SS 563) and Barbel class diesels were 700-foot boats, as were the later nuclear powered Skates (SSN 578) and Skipjacks.

For a 400-foot boat, the deep dive area would probably have been no deeper than 500 feet. The relationship of crush depth to test depth is roughly 1.5.11 Thus a 400-foot boat’s crush depth would be conservatively estimated at 600 feet. 12 The existence of a bottom at 500 feet ensured that if the submarine conducting a deep dive had a flooding casualty and sank to the bottom, it would be at a depth above the estimated crush depth of the hull-and thus the submarine would not automatically be lost forever. The McCann Rescue Chamber could operate as deep as 850 feet. In theory at least, a 400-foot boat, with several compartments flooded and on the bottom at 500 feet and unable to surface, might still have survivors of the flooding rescued by the ASR.

For THRESHER, a 1300-foot boat, its crush depth would be estimated at 1,950 feet. Using the same logic that motivated ComSubPac in its standing operations order, the assigned ComSubLant deep dive area for THRESHER should have been no deeper than a 1500-1700 feet range. Instead, THRESHER was sent to an area so deep, about 8,000 feet, that she was doomed to be crushed by sea pressure if she sustained a major flooding casualty and could not surface.

The Court of Inquiry Report did not make any statements about test dive area depths at all. If ComSubLant had no restrictions in effect, that omission is understandable. What is less understandable is the failure of the Court of enquiry to take note of the different practices of the two Submarine Force Commanders in force at that time, and to recommend to ComSubLant that it adopt the same practice as its Pacific counterpart.

Apparently OPNAV, presumably the Submarine Warfare Division (OP-31 ), took note of the difference. During the hearings of the Joint Committee on Atomic Energy, on Wednesday July I, 1963, Vice Admiral Ramage, Deputy Chief of Naval Operations for Fleet Operations and Readiness (OP-03) testified, ” … some new requirements include the following: Test dives are made in depth of water limited to 11/2 times the test depth … ”

If THRESHER had been a unit of the Pacific Submarine Force and had undergone sea trials off San Francisco, after a PSA at Mare Island Naval Shipyard, she would have been assigned a deep dive area in the 1500 -1700 foot bottom range. Assuming the same tragic sequence of events, she would have flooded her engine room and gone to the bottom, but would still be intact at say 1600 feet. TRIESTE could have surveyed her off San Francisco and reported on her condition.

She would not have been crushed and laid strewn about the bottom as scrap metal at 8,000 feet. It is possible that a salvage effort might subsequently have raised THRESHER’s hull allowing a factual determination to be made of the exact cause of her flooding rather than educated conjecture about possible causes.

What might have come next is in the realm of speculation. At 1600 feet the sea pressure is approximately 726 pounds per square inch. The engine room would have flooded almost completely except for a small volume of compressed air (at 726 psi) in the upper part of the compartment.

If a few engine room personnel remained alive in the air bubble in the overhead of flooding compartment, they would soon be breathing air at 726 psi and consequently they would be poisoned by the toxic effects of oxygen and nitrogen at high partial pressures. They would be quickly rendered unconscious and then die.

In any case the spray and flooding would have disabled all electrical equipment and circuits in the engine room. The maneuvering room, located in the forward starboard part of the upper level of the engine room, would not be capable of directing steam to the propeller shaft. All motor-generator sets would be shorted or flooded out, and the unflooded compartments forward would be left with only battery-powered emergency lighting.

Carbon dioxide removal equipment would be unable to operate. In addition to the existing air in them, the forward compartments would have whatever pure oxygen was stored in tanks, which could be bled into the compartments, as well as carbon dioxide removal chemicals which could be spread around to limit the natural increase of C02 as surviving personnel breathed. However, asphyxiation would inevitably ensue as the 02 supply was exhausted and C02 removal chemicals were used up.

Presumably all engine room personnel evacuated forward. Attempts to limit flooding of the engine room by admitting high pressure air from the main air flasks using internal salvage air valves might have encountered the same freeze up that prevented THRESHER from blowing its main ballast tanks to get the surface. In this alternative scenario THRESHER then sits on the bottom in 1600 feet of water, possibly still in Gertrude’s communication with her ASR escort, and that escort is informed about her situation. Without the nuclear reactor plant in operation, THRESHER’s sole power source is her battery, with about half the power of a standard diesel-electric Guppy submarine battery.

THRESHER was not designed with enough reserve buoyancy to flood a large compartment like the engine room and still surface. With the engine room flooded, she would be stuck on the bottom. Even if the ice that had formed to block the main ballast tank blows, melted-and the main ballast tanks blew dry, there would probably not be enough buoyancy to lift the hull to the surface.

The end result would be the death of all on board due to asphyxiation. At 1600 feet in early 1963 there were no rescue assets available that could have taken any of the surviving crew off THRESHER.

However, and this is a big difference, THRESHER would not have been torn apart. She would have remained intact on the ocean floor until such time that Navy salvage efforts might have raised her, using large flotation devices and external salvage air connections.

As the title of the article implies, the final outcome of THRESHER’s deep dive, if conducted in the Pacific instead of the Atlantic, would have been different from the actual outcome, albeit her crew were doomed by the advances that submarine operating depth technology had made over rescue depth capability.