What Killed the Thresher?

The USS Thresher (SSN-593), which sank 60 years ago this April, was the world’s first nuclear-powered submarine to be lost at sea. Amid the public shock over the tragedy, the U.S. Navy grappled for an answer as to what went wrong. Even today, rival theories seek to explain the mystery.

The Thresher got under way on her initial sea trials from the Portsmouth Naval Shipyard in Kittery, Maine, on 30 April 1961. She was the first of a new class of nuclear-powered submarines, combining the attributes of an “attack” submarine and a specialized “hunter-killer” craft. The submarine had a cigar-shaped hull that was derived from the research submarine Albacore (AGSS-569) for efficient underwater performance. In her bow was the massive BQQ-2 sonar, heralded as the most advanced sonar ever fitted to a submarine, being credited with unprecedented passive detection ranges. The sonar “dome” was 15 feet in diameter mounting 1,241 transducers.

The Thresher’s four torpedo tubes were aft of the sonar, two on each side, angled out at ten degrees from the centerline. They could fire the latest antiship and antisubmarine torpedoes. Later she was to be fitted to fire the ASROC—antisubmarine rocket—from those tubes. The ASROC was a combination rocket and torpedo: Launched like a conventional torpedo, it would streak to the surface, leave the water on a ballistic trajectory, and plunge back into the water several miles from the Thresher, where the SUBROC’s nuclear warhead would detonate. (The SUBROC’s W55 nuclear warhead had a variable yield of one to five kilotons. The weapon was in the U.S. arsenal from 1965 to 1988.)

The submarine’s S5W reactor plant would give her unlimited range. Her pressure hull, constructed of HY-80 steel, would enable her to dive to some 1,300 feet—unprecedented for a U.S. submarine.

Her initial sea trials, with Vice Admiral Hyman G. Rickover, head of the Navy’s nuclear propulsion program, on board, were successful, although there were some issues with instrumentation. The Thresher was placed in commission at the Portsmouth shipyard on 3 August 1961, with Commander Dean L. Axene in command. At her commissioning ceremonies Vice Admiral Harold T. Deutermann, the principal speaker, declared: “Thresher is not just another ship. Thresher is totally different!”

Less than two years later, the Thresher lay shattered on the ocean floor with the loss of all 129 men on board.

Following her commissioning, the Thresher continued a lengthy series of trials and tests as well as exercises off the Atlantic coast, sailing as far south as Puerto Rico. While in port in San Juan, the crew encountered problems with her diesel generator and then had difficulty in restarting her nuclear reactor.

Shock tests followed, with the Thresher subjected to a greater intentional pounding than any other submarine in Navy history. She stood up remarkably well, with Commander Axene stating: “There was no question that the Thresher suffered damage. . . . But it was all relatively minor. . . . The damage we sustained did not impair the ship’s ability to operate, and much of it, such as the damage to vital sonar tubes, we could repair ourselves with our store of spare parts.”¹

More tests and trials followed. In mid-July 1962 the Thresher returned to the Portsmouth yard. There she embarked the crew’s wives, parents, and children for a “happy cruise.” Then the submarine was back in the shipyard for major maintenance work. Yard periods always are trying for a ship’s crew. Axene said: “Relations between Thresher and Portsmouth were always extraordinarily good, at least in my experience.” But he quickly added:

It is true that we felt they should have been more efficient, should done better work at times, and move more quickly, and they should have done a better job of cleaning up after themselves. I was told by others that relations did deteriorate toward the end of the [Post Shakedown Availability].

While the Thresher was in the shipyard, Commander Axene received orders to take command of a new-construction Polaris ballistic-missile submarine, the USS John C. Calhoun (SSBN-630). He protested the orders, wanting to see the yard work completed. His request was denied. Axene’s relief, in January 1963, was Lieutenant Commander John Wesley Harvey, age 36.

“Wes” Harvey had studied for a year on a scholarship at the University of Pennsylvania before entering the U.S. Naval Academy. After graduation he served in an aircraft carrier and then in a diesel-electric submarine. He applied for nuclear power training, after which he reported aboard the pioneer nuclear submarine Nautilus (SSN-571) as reactor controls officer. He was on board when the Nautilus became the first ship in history to reach the North Pole.

Harvey had come into the limelight in 1958 when television producers were seeking personable young heroes for quiz shows. John Glenn was a contestant in his pre-astronaut days, when he had just set a new transcontinental air speed record. Harvey, then a lieutenant just back from the Nautilus’s epic cruise, was asked to try out for the NBC game show Concentration.

He managed to solve the rebus puzzle ahead of his competitors often enough to win a car, a set of electric trains, a motion picture projector, two tickets to the coveted Broadway show My Fair Lady, a mink stole, a Swedish-made motorbike, an electric can opener, and a mud bath. Most were welcomed by Harvey’s wife, Irene, and their two sons, then aged four and seven.

Harvey’s star was on the rise. After serving as chief engineer for a land-based submarine reactor at Windsor, Connecticut, he became engineer officer of the hunter-killer submarine Tullibee (SSN-597). In May 1961, as a lieutenant commander, he became executive officer of the Seadragon (SSN-584). That submarine undertook innovative under-ice Arctic operations. Thus, he came to the Thresher well trained and with considerable experience.

Slowly, the Thresher began to look like a fighting ship again as her yard period at Portsmouth neared an end. Within the submarine, opened panels were closed, piping was connected and tested, dials and pointers checked, and the mass of trash and debris that had accumulated during the extensive work was removed.

The submarine underwent a “fast cruise”: While moored to a pier she was sealed and all equipment that could be was tested. Even the Thresher’s propeller was tested while cables held the submarine fast to the pier. More tests were conducted, as well as emergency drills. At last, navigation instruments, food, ashtrays, some personal belongings, mattresses, and bedding were brought aboard as the submarine was prepared for post-overhaul sea trials.

At 0345 on 9 April 1963, the Thresher’s engineers began the four-hour countdown required for starting up the submarine reactor plant. At 0730 the engine room reported that full power was available.

Lines were cast off, and the Thresher was under way. On board were 129 men: 12 officers and 96 enlisted men of the submarine’s crew; a Submarine Force staff officer; 3 officers and 13 civilian employees of the Portsmouth yard; a specialist from the Naval Ordnance Laboratory; and 3 civilian factory representatives.

Four Thresher crew members had remained ashore for various reasons.

The submarine steamed south from the yard and into the open sea off Odiorenes Point. At 0949, east of Boston, she rendezvoused with the submarine rescue ship USS Skylark (ASR-20), which would accompany the submarine on her trials. The 205-foot Skylark carried divers and specialized equipment to enable her to moor over a stricken submarine and rescue personnel with her cable-lowered rescue chamber. Developed in the late 1920s, the rescue chamber had been used operationally only once—in 1939, to rescue 33 men from the stricken submarine USS Squalus (SS-192); another 26 crewmen lost their lives in that tragedy.

The Squalus had sunk in 240 feet of water. The rescue chamber had a rated depth capability of 850 feet.

Although the Skylark was to accompany the Thresher, the rescue ship’s commanding officer had no information as to the capabilities of the submarine, such as her depth and speed. An “underwater telephone” designated UQC was the means of communication with a submerged submarine.

The Thresher made a shallow dive to test equipment and communicated with the Skylark via the UQC. Back on the surface the afternoon of 9 April, Commander Harvey directed the Skylark to rendezvous with the submarine some 200 miles east of Cape Cod for the Thresher’s deep-water tests. At about 1500, the Thresher again slipped beneath the waves and sped toward deeper water.

At 0635 on 10 April, the Thresher came up to periscope depth some ten miles from the Skylark. Just over one hour later, Harvey advised the Skylark that he was taking the Thresher to “test depth”—her maximum operating depth, generally reported as 1,300 feet. (Before her recent yard period, the Thresher had been to her test depth some 40 times.)

0747—The Thresher reported to the Skylark by the UQC underwater telephone that she was beginning the test-depth dive. Five minutes later, Harvey reported being at 400 feet and checking for leaks.

0754—The Thresher told the rescue ship that future references to depth would be made in terms of test depth.

0809—The Thresher reported that she was at one-half test depth.

0835—The Thresher communicated “minus 300 feet.”

0902—The Thresher asked for a repetition of a course reading.

0912—The two ships made a routine check.

The Skylark’s handwritten log—the official record—recorded that at 0912 a “satisfactory” underwater telephone check was made. Then the Thresher reported: “Have position up angle. . . . Attempting to blow up.” The exact time of that final message was not recorded.

Some of the Skylark’s crew remembered the message differently: “Experiencing minor problem. . . . Have positive angle. . . . Attempting to blow.” And, “Experincing minor difficulty. . . . Have positive up angle. . . . Attempting to blow. . . . Will keep you informed.”

At 0914 the Skylark requested that the Thresher give her course and bearing. There was no reply. Then the Skylark asked: “Are you in control?” There was no reply. The question was asked again—several times.

Nothing more was heard from the submarine until 0917, when there was a garbled message that ended with the distinct words “test depth.” Seconds later, the Skylark’s commanding officer, Lieutenant Commander Stanley W. Hecker, heard what sounded like “a ship breaking up . . . like a compartment collapsing.”

Subsequently, the Skylark began dropping grenades, the prearranged signal for a submarine to surface in the event that UQC communications failed.

The Skylark advised shore commands of the situation, and soon massive air and surface forces were searching for the missing undersea craft. The Thresher was gone, in waters some 8,400 feet deep.

At about 1900 on 10 April, the Navy began notifying the families of the men on board the Thresher that the submarine was overdue and presumed missing. There was no use of the terms “sunk” or “lost,” as optimists hoped that the submarine was intact and suffering a communications failure.

That hope quickly faded.

A massive search began for the remains of the Thresher. In time, some floating debris from the submarine was recovered. An automobile and then a discarded diesel-electric submarine, the Toro (SS-422), were sunk in the area to determine how local currents would affect a sinking object. Soon ships searched the ocean floor with sonar and deep-towed sleds with cameras, magnetometers, and Geiger counters. Some debris was detected, and small pieces were recovered.

Next, the bathyscaph Trieste II was brought to the area. She was the only U.S. craft that could reach the 8,400-foot depth of the Thresher’s remains. On 27 June, she located the main Thresher debris field. The Trieste II was able to recover a twisted pipe with the markings “593 Boat.” The bathyscaph returned to dive on the debris field in 1964, 1967, 1977, and 1979.²

What killed the Thresher?³ On 6 June 1963, the Navy’s court of inquiry into the loss of the Thresher completed its work after almost two months of hearings. The court recorded 1,700 pages of testimony from 120 witnesses and collected some 225 charts, drawings, letters, photographs, and other exhibits related to the disaster. The first two paragraphs of the court’s subsequent public statement presented the official Navy opinion of what killed the Thresher:

A flooding casualty in the engine room is believed to be the “most probable” cause of the sinking of the nuclear submarine SS THRESHER, lost April 10, 1963, 220 miles east of Cape Cod with 129 persons aboard.

The Navy believes it most likely that a piping system failure had occurred in one of the THRESHER’s saltwater systems, probably in the engine room. The enormous pressure of sea water surrounding the submarine subjected her interior to a violent spray of water and progressive flooding. In all probability water affected critical electrical circuits and caused loss of power. THRESHER slowed and began to sink. Within moments she had exceeded her collapse depth and totally flooded. She came to rest on the ocean floor, 8,400 feet beneath the surface.

But midway in the statement was a qualifier: “The record states that it is impossible, with the information now available, to obtain a more precise determination of what actually happened.”

Still, an analysis of the last messages from the Thresher, coupled with the later analysis of acoustic recordings by the Navy’s seafloor Sound Surveillance System (SOSUS) indicated that a piping failure was not the cause of her loss. Rather, it appears that at 0912 on 10 April, the Thresher was at 1,100 to 1,200 feet, approaching her test depth in what so far had been a normal, safe, and seemingly routine test dive.

Then came the message “experiencing minor problem.” What is a minor problem in a complex, nuclear-propelled submarine? At a depth of 1,000 feet, approximately 100,000 pounds of seawater would flood through a six-inch pipe in just one minute. Even flooding through a half-inch pipe hardly would be considered “minor,” as the noise and the powerful spray that would cause confusion within the submarine.

Commander Axene, the Thresher’s first commanding officer, believed that even a half-inch pipe failure at those depths would not be described as a “minor” problem. The breaching of a pipe between two and five inches in diameter would have led to water rushing into the submarine with a velocity of about 1,800 miles per hour at a depth of 1,300 feet. Had there been major flooding at test depth it would have generated extreme noise levels that would have prevented effective UQC communications with the Skylark. No such interference was evident during those communications. Further, the noise generated by such flooding would have been detected by SOSUS. It was not detected.

“Experiencing minor problem” indicates that no sudden, catastrophic event befell the Thresher.

For four minutes—as the Thresher was “attempting to blow up” by emptying her ballast tanks with compressed air—there was no communication. Then, at 0917, the submarine transmitted a message ending with “test depth.” It generally was supposed that “exceeding” was one of the words preceding “test depth.”

The U.S. Navy constructs submarines to withstand one and one-half times the pressure of their designed test depth. That is a safety factor. The collapse depth is based on both mathematical calculations and model tests. It thus seemed reasonable to assume that the collapse depth of the Thresher was approximately 1,950 feet.

The sequence of casualties suffered by the Thresher apparently caused a fateful depth increase of some 600 feet—from test depth to collapse depth—in five agonizing minutes. As she neared collapse depth, the fittings and pipes would have begun to give way, admitting powerful jets of water that pushed aside men as they struggled to plug them and shorted out electrical systems, making corrective action impossible. The additional weight of water thus admitted would have driven the Thresher still deeper at an ever-increasing speed. The submarine’s hull would groan under the increasing pressure trying to crush her air-filled interior.

There probably were no serious personnel casualties to that point. But all in the submarine would now have sensed that they were rushing toward disaster and groped frantically for some means of escape or survival. The insulating cork that lined the submarine’s interior would begin to crack and possibly flake off. Pipes would pull apart as the water pressure began to “pull” the submarine’s hardened steel—like taffy. The hull would then implode. Complete destruction would occur in 1/20th of a second, too fast to be cognitively recognized by the men within the submarine.

Millions of pounds of water under tremendous pressure would smash the submarine’s hull, breaking it open, twisting portions, disintegrating other parts. The theory that water filled the plummeting Thresher before she could implode is generally discounted because the additional weight of water would have accelerated the downward rate and caused an implosion before the hull could fill with water.

What was the “minor” difficulty that the Thresher encountered at about 0913 that led to the worst submarine disaster in history? Even though that problem was called “minor,” the Thresher’s commanding officer soon attempted to blow his main ballast tanks to reach the surface. Thus it appears that the minor difficulty probably was one that caused the submarine to become “heavy.” That most likely would occur as a result of losing propulsion or taking on water. As noted above, taking on water could have led to a loss of propulsion.

More probable is that the loss of propulsion was the initial difficulty. With the resulting loss of forward motion, the Thresher began to sink deeper as a result of her heavy water ballast. An effort to expel ballast would have been difficult because of the increasing external water pressure. Before the downward motion of the submarine could be halted, she exceeded her collapse depth, with the subsequent failure of pipes and fitting admitting water, driving her still deeper, until the hull imploded.

Multiple lines of evidence confirm that the Thresher did not experience major flooding before the collapse of her pressure hull at a depth of 2,400 feet. Most significant, acoustic analysis of SOSUS recordings “confirmed a loss of electrical power and the resulting reactor scram (shut-down) . . . near test-depth (1,300 feet), about nine minutes before collapse of the pressure hull.”⁴

And Commander Axene had stated that the message “attempting to blow up” indicated that propulsion was lost because the submarine should have come up faster by planing up than by blowing ballast. He also could envision that a reactor scram could be described as a “minor” problem, even at those depths.

Thus died the USS Thresher. Six months later, the Navy announced that all 31 nuclear-powered submarines then under construction in the United States were being delayed. The Navy noted that “deficiencies in equipment and workmanship are contributing factors” to the delays. Follow-on Thresher-class submarines were being lengthened to provide more buoyancy, with a SUBSAFE program providing additional safety features. Every operational nuclear submarine was restricted to 50 percent of test depth until SUBSAFE modifications could be made to them.

Also, the restarting procedures following a reactor scram were being revised to reduce the startup time.

After the Thresher disaster revealed the Navy’s limitations for deep-ocean search and rescue, in 1964 the Navy established the Deep Submergence Systems Project (DSSP) to develop and manage advanced ocean systems. That effort included plans to construct six rescue submersibles that could operate to submarine collapse depths and two deep-ocean search vehicles that could operate to 20,000 feet, i.e., 90 percent of the ocean depths. The DSSP portfolio eventually included deep-sea salvage capabilities, seafloor habitation down to 600 feet, the location and recovery of small objects from the ocean floor, and the nuclear-powered submersible NR-1.  (Two 6,000-foot-capability rescue submersibles were built—and later were retired without replacement; the search vehicles were not built.)

Still, when the nuclear-powered submarine Scorpion (SSN-589) was lost in 1968 with all 99 men on board, there was no possibility of rescue or salvage. Subsequently, the Soviet Union and Russia suffered the loss of five nuclear submarines, with some crewmen being saved from four of them as they reached the surface before sinking. But the USS Thresher was the first nuclear submarine to be lost—and the worst disaster in terms of people lost.