It has now been 55 years since the USS Thresher (SSN-593) sank east of Cape Cod, Massachusetts, during a routine test dive—the first nuclear-submarine disaster and, in lives lost, the worst. Lessons were learned and improvements made—but the Navy has yet to declassify important documents on the loss of the world’s most advanced submarine.

A Naval Court of Inquiry investigated the submarine’s loss and concluded that the probable cause had been major flooding from ruptured 2–5-inch piping in the engine room.1 However, as shown by the evidence discussed here, this was probably not the underlying cause. Substantiating this assertion is the Thresher’s acoustic signature, recorded on low-frequency analysis and recording grams (LoFARGrams). These paper-based time-vs.-frequency plots are produced by Sound Surveillance System (SOSUS) passive sonar hydrophones of Array Fox terminating at the Canadian Naval Station HMCS Shelburne (Nova Scotia).2 The data reveal that the Thresher very likely had already sunk below her 1,300-foot test depth limit when she reported minor difficulties. The result was a hull collapse that could have been avoided with more testing and better planning.

The Most Advanced Sub

The first nuclear-powered submarines, slow and radiating excessive noise, retained the traditional diesel-electric hull design. The USS Albacore (AGSS-569), a diesel-electric research submarine, was commissioned in 1953 to improve submerged performance. Based on the Albacore, the Navy determined that the teardrop-shaped hull, with the single screw aft of the rudder and stern planes, was the best design for the Skipjack (SSN-585), commissioned in 1959. These innovations, combined with Naval Reactors’ director Admiral Hyman Rickover’s more powerful S5W reactor plant—easier to maintain and operate—created a high-speed (29.5 knots) but noisy submarine.

Commissioned 3 August 1961, the Thresher featured 22 percent more displacement and a beam two feet wider than the Skipjack’s. The torpedo room was moved from the bow to amidships to fit a large, spherical active and passive sonar in the bow. Machinery sound dampening drastically reduced radiated noise, increased sonar detection capabilities, and reduced passive sonar counter- detection ranges, making the Thresher a real threat to Soviet submarines. The test depth nearly doubled, from 700 to 1,300 feet, providing more protection from antisubmarine weapons, making active surface-ship sonars less effective, and increasing the margin of safety for depth excursions during control-surface casualties.

The Nuclear System Failed

In sound trials, weapons testing, and submarine exercises, the Thresher’s performance was outstanding, but she had not yet deployed overseas to face Soviet submarines. After explosive-shock testing, she entered Portsmouth Naval Shipyard (PNSY) in Kittery, Maine, on 16 July 1962 for a nine-month post-shakedown availability (PSA). On 9 April 1963, the Thresher departed for post-PSA sea trials in tactical command of, and escorted by, the submarine rescue vessel USS Skylark (ASR-20). The ships rendezvoused the next morning just beyond the continental shelf for a deep-dive test, during which the Thresher sank. Until communications were lost, they had been maintained with the Skylark through an underwater telephone system (Gertrude).

In 1962–63, Lieutenant Bruce Rule was the analysis officer for the SOSUS Evaluation Center in Norfolk, Virginia; he analyzed the LoFARGrams and testified to the Naval Court of Inquiry. After leaving the Navy in September 1963, Rule spent the next 42 years as the lead acoustic analyst for the Office of Naval Intelligence.3 Key information from Rule’s LoFARGram analysis that was redacted from the released portions of the inquiry’s report includes the following points:

The Thresher ran main coolant pumps (MCPs) in fast speed until they stopped at 0911. If power from steam-driven ship’s service turbine generators (SSTGs) failed, slow-speed MCPs could run using power generated by the ship’s service motor generators. Fast-speed MCPs did not have this capability.

The Thresher’s MCPs gradually varied in speed up to 24 revolutions per minute (rpm) about five times over a two-minute period, from 0909 until 0911. This resulted from a change of up to four-tenths of a Hertz in the 60-cycle power supplying the fast-speed MCPs from the SSTGs. 4

Since SOSUS did not detect blade rate (screw rpm), the Thresher did not exceed 12 knots.

Rule’s analysis of LoFARGrams from SOSUS stations as remote as Argentia, Newfoundland, and Antigua, British West Indies, produced a time-difference fix on where the Thresher imploded. This time-difference fix resulted in a four-by-eight nautical-mile ellipse, with the major axis oriented 040 degrees to 220 degrees true.

The Naval Court of Inquiry justified its finding of major flooding by citing a then-recent history of silver-brazed pipe joint failures on six submarines, including the Thresher. But even though all the silver-brazed joints that had been worked during the Thresher’s overhaul had been ultrasonically tested, only 145 of the submarine’s unworked joints had been tested, with a 14 percent failure rate. This left 2,855 silver-brazed joints untested.5

Major flooding creates streams of high-velocity seawater striking the pressure hull and internal structures, producing broadband noise and narrowband resonances. Rule reported a LoFARGram showing a compartment flooding at great depth producing more than 100 individual strong narrowband resonances detectable from more than 700 nautical miles. SOSUS Array Fox, 30 nautical miles from the Thresher, detected MCPs and main ballast tank (MBT) blow events, but not flooding.

The inquiry reported that the Thresher’s MCPs had stopped, which would have caused an automatic reactor shutdown (scram) or a shift to slow speed. While Rule was positive the MCPs stopped, Naval Reactors said the acoustic data were inconclusive.6 Two commanders—not members of the Naval Court of Inquiry and likely acting as agents for Naval Reactors—tried to intimidate Rule during his classified testimony before the court into saying that the MCPs were in slow speed, not fast. Slow-speed MCPs were a more reliable lineup, but Ronald Estes, a reactor operator who served 14 months on the Thresher, recalls that it was normal to run fast-peed MCPs during deep dives to ensure immediate availability of flank speed to go shallow.

In a 1987 interview with Fred Korth, Secretary of the Navy when the Thresher was lost, and his executive assistant, Vice Admiral Marmaduke Bayne, both said Rickover had altered portions of the Naval Court of Inquiry’s report, and had probably done so because wording on MCPs was left as “inconclusive.”7 This deflected blame for the sinking away from Naval Reactors by creating doubt that there had been a scram.

Both then–Secretary of the Navy Fred Korth and his executive assistant, then–Vice Admiral Marmaduke Bayne, testified at the inquiry. Both later maintained that portions of the findings were altered. U.S. Navy

Anatomy of a Rush Job

The final ten minutes of the Thresher are detailed in the March 2018 Proceedings (p. 87). Here the analysis begins at 0853, 16 minutes earlier, because it is at this point that the inadequacy of testing becomes clear. At 0853, the Thresher reported going to test depth: 1,300 feet. Why did she not use planes, angle, and speed to go shallow during the next 18 minutes, while the propulsion plant was capable of flank speed?

Acoustic data rule out major flooding, but control-surface casualties were relatively common. Stern planes stuck in a dive position would require the Thresher to stop to prevent a large down angle and downward depth excursion. Only de-ballasting by blowing MBTs and pumping variable ballast tanks to sea would enable ascent. One admittedly speculative, yet entirely plausible, scenario is such a stern planes failure.

During an NBC News interview in his official capacity on 10 April 1963, hours after the Thresher was lost, Captain James Calvert (later Vice Admiral), commanding a division of nuclear submarines, implicitly speculated about a control-surface casualty as a cause for the loss. Rear Admiral Charles Curtze, a deputy commander at the Bureau of Ships, testified before a congressional hearing that the electro-hydraulic control valves and piping for ship control surfaces were under review because failures had caused submarines to take large trim angles and lose depth control. Curtze also said a review of the reliability of ship control systems was Task No. 10 of the Submarine Safety (SubSafe) Program.

The following timeline delves more deeply into the information presented in the March 2018 Proceedings:

• 0909: SOSUS detects SSTGs changing speed slowly by measuring MCP frequency—a symptom of an ongoing problem in the engine room, probably caused by seawater-leak isolation of the main seawater system that supplied cooling to the main condensers. It should have prompted shifting MCPs to slow to prevent a reactor scram and to go shallow. Instead, an MBT blow was detected starting 48 seconds later, indicating that main propulsion was not usable, and the Thresher was sinking.

• 0909.8 to 0911.3: SOSUS detected an MBT blow. Flank speed, about 28 knots, would have surfaced the Thresher in under two minutes, but speed stayed below 12 knots. Rickover reported to Congress on the sub’s inadequate MBT blow capacity. By design, new submarine classes relied on nuclear propulsion to surface in case of emergency, rather than on the MBT blow system. On the Thresher, the MBTs were made smaller to increase speed, which reduced reserve buoyancy and the effectiveness of the MBT blow. Crews became complacent about being negatively buoyant.8

Rickover rode the Thresher during the first deep dive after new construction and was concerned with flooding recovery at depth. He demanded that the submarine stop every 100 feet to check for leaks, cycle valves, and test critical equipment.9 Lieutenant Commander John Harvey, commanding officer (CO) of the Thresher for just three months, approved the PNSY sea-trial agenda for a two-hour deep dive—not enough time for tests, and with brief stops at 400, 650, 1,000, and 1,300 feet to check for leaks. The CO and his executive officer had no experience on the S5W reactor plant or deep-diving high-speed submarines with fairwater planes on the sail.

Excessive leakage from multiple sources would not cause high-pressure streams of water, although it could make the Thresher negatively buoyant. A Thresher-class submarine gets 1,000 pounds heavier for every 100-foot increase in depth, as hull compression reduces ship volume. If variable ballast was not pumped to sea to compensate, and with normal increases in weight, such as sanitary tanks filling, the Thresher might have reached test depth at least 12,000 pounds heavy. This is consistent with testimony that neutral buoyancy was no longer a priority with reliable nuclear propulsion.10

There were many potential sources of leakage, as even sanitary flushing was directly connected to the sea. As the Thresher continued her descent, leakage rates increased, and new sources of leakage developed. The cumulative effect of multiple leaks could have exceeded the capacity to deballast by pumping to sea or blowing MBTs.

• 0910: The Skylark acknowledged the Thresher’s course change to match the Skylark’s course—standard procedure to minimize the chance of collision, as the expected result of the MBT blow was to surface.

After the Thresher loss, the Naval Court of Inquiry directed a pierside test of the MBT blow system on the Tinosa (SSN-606), a Thresher-class submarine in the final stages of construction at PNSY. Lieutenant Zack Pate (later Captain), the Tinosa auxiliary division officer and damage control assistant, directed this test. The inquiry’s Finding of Fact 50 inaccurately describes its results: “Strainers in the reducers of Tinosa were blocked and ruptured by the formation of ice in about 30 seconds.” The Marotta Company manufactured the 4,500–3,000-pounds-per-square-inch (PSI) reducing valves used for ship’s service air, including MBT blow. Pate reported that conical strainers and orifice plates were installed upstream of these reducers, and that the strainers were collapsed, not ruptured, as shown in this photo that he took.

This photo shows one of two collapsed conical strainers and orifice plates found in the Tinosa’s inlet to the 4,500–3,000 PSI air reducers. The 1.3-inch diameter strainer is mounted on a 1.5-inch diameter backing plate with a 0.3-inch orifice held in place by a coupling in a 2-inch airline. Courtesy Zack Pate

The airflow stopped and restarted sporadically every few seconds for six minutes, until the test was stopped.11 Ruptured strainers would be less restrictive than collapsed strainers, and orifice plates (not mentioned in the Naval Court of Inquiry’s report) significantly restricted air flow to the MBTs. This inaccurate description of an important test was referred to many times in testimony before the court and Congress. Marotta provided the orifice plates and strainers, and PNSY installed them, without anyone on the Thresher or the Tinosa knowing that to prevent construction debris from damaging the reducing valves, they should have been removed when the work was completed. Ice forming on the strainers from adiabatic cooling from the Venturi effect blocked air flow until the ice melted or broke free, allowing reduced air flow until ice formed again. This explains the intermittent interruptions during the Thresher’s MBT blow. Pate provided one set of the orifice plates and strainers to the court and retained the other. Yet a letter from Marotta’s president and chief executive officer states that there were no records or corporate memory of this issue.12

• 0911: SOSUS detected MCPs stopping, causing a reactor scram. Alternating-current power was not lost, as the Gertrude was still working. The reactor operator shut main steam stop valves (MS-1 and 2) per the scram procedure, which secured steam supply to the main engines and SSTGs.

MCPs were not started in slow speed to remove decay heat from the reactor core, a procedural violation. With MS-1 and 2 shut and no running MCPs to transfer decay and residual heat from the reactor to generate steam, the main engines were useless. Keeping MS-1 and 2 open after a reactor scram for emergency propulsion was a well-known procedure, but had not yet been approved by Naval Reactors.13 Naval Court of Inquiry board member Captain James Osborn (later Rear Admiral) had served as the first CO of the USS George Washington (SSBN-598). His standing orders, approved by Rickover, were to allow MS-1 and 2 to stay open and to answer the ordered propulsion bell if the reactor scrammed while below 500 feet when experiencing a 20-degree down angle, or during the maneuvering watch.14 This class of submarine had a test depth of 700 feet. With MS-1 and 2 shut, the only means of propulsion was the emergency propulsion motor, a large, direct-current motor powered by the ship’s battery that could propel the ship at speeds of up to five knots.

• 0913: The court derived this underwater-telephone report using testimony from four witnesses:

“Experiencing minor difficulties”

“Have positive up angle”

“Am attempting to blow up”

“Will keep you informed.”15

By 0913 the Thresher had exceeded test depth, maybe by as much as 600 feet if the 0917 report, “900 North,” is interpreted as 900 feet below test depth or 2,200 feet—reasonable, given that the submarine was reporting depth relative to test depth. There were no running MCPs; the reactor had scrammed; MS-1 and 2 were shut; main propulsion was lost; the boat was accelerating downward in an uncontrolled descent that the MBT blow could not stop; and the crew could hear the creaking and groaning sounds of the pressure hull compressing. The Thresher’s difficulties were by no means “minor” at 0913.

One possible explanation is that “experiencing minor difficulties” in the 0913 report was from a delayed transmission describing an earlier event considered minor at the time, such as a control surface failure.

An up angle was expected to enable the Thresher to go shallow. “Have positive up angle” implies that the submarine had recovered from a down angle, possibly caused by a stern plane’s jam dive. The report of “attempting to blow up” confirms that the MBT blow was ineffective.

• 0913.5 to 0914: SOSUS and the Skylark detected a 30-second MBT blow. This was probably a restart of the blow started at 0909.8.

• 0916: The Thresher made a garbled report believed to include the words “test depth,” possibly preceded by the word “exceeding.”

• 0917: The Thresher sent a garbled report interpreted to include the phrase “900 North.”

• 0918.4: SOSUS and the Skylark detected hull collapse at a calculated depth of 2,400 feet, 450 feet below the crush depth of 1,950 feet (150 percent of test depth), creating a bubble pulse with an energy release equivalent to 22,500 pounds of TNT. The hull collapsed in 47 milliseconds (~1/20th of a second), too fast to be cognitively recognized by those on board.16

An Avoidable Disaster; A Better Future

A simple decision to schedule more time for the deep dive could have saved the Thresher. Her loss resulted in the creation of the SubSafe program, which mandated the redesign of and strict quality control procedures for the manufacture, repair, and testing of critical systems on submarines. The prevention of unauthorized alterations in critical systems—including the hull, seawater systems, high-pressure air, and control surfaces—was prioritized. New SubSafe systems, such as a separate emergency MBT blow system and an emergency remote hydraulic seawater hull valve closure system, were installed on all submarines. Until a boat was SubSafe-certified, she was restricted to operating at half her test depth.

Like radar at the start of World War II, SOSUS data were not fully trusted or used in the investigation. If they had been, the Naval Court of Inquiry’s report on MCPs would not have been deemed “inconclusive.” Relying on a single normally reliable system—the nuclear propulsion plant—without a designed and tested backup proved catastrophic for the Thresher.

Even though the SubSafe program has already brought lifesaving changes, there may be more to be learned. It is time for the Navy to fully share all the evidence pertaining to this historic watershed naval loss.

No SubSafe-certified submarines have been lost. The only other sinking of a U.S. nuclear submarine was the USS Scorpion (SSN-589) after a battery explosion in May 1968. Release of the Naval Court of Inquiry’s report, declassifiable per Executive Order 13526, would give valuable insight.