| SUBSIM Review |
Thresher Down Reprinted from Mechanical Engineering Magazine, February 1987 |
The date is Tuesday, April 10, 1963. The time is 3:00 P.M. The gray form of the U.S.S. Thresher slips beneath the surface of the Atlantic and begins a run for deep waters, where the continental shelf drops precipitously to the ocean floor some 8000 feet below. Within her sleek, cigar-shaped hull, the nuclear submarine's crew labors to work out the kinks from an overhaul completed two days before at the Portsmouth, New Hampshire, naval yard. The vessel cruises throughout the night toward a rendezvous with Skylark, a Navy submarine rescue vessel. If Thresher should have to blow ballast and surface quickly, Skylark could clear the area of passing ships. She could also attempt a rescue of Thresher's crew if the submarine became marooned in several hundred feet of water. In seas almost two miles deep, however, Skylark really plays only a nominal role.
April 10, 6:35 A.M. Thresher rises to periscope depth, spots Skylark, and reports to the surface vessel by acoustic telephone. Captain John Harvey is ready to take Thresher down for testing at her maximum serviceable depth--about 1000 feet. The descent is made in stages of several hundred feet at a time. At 400 feet Thresher's crew checks for leaks in the hull, fittings, and piping system. Any rupture could be disastrous, blasting water into the interior at 600 psi.
7:54 A.M. Harvey notifies Skylark that future references to his depth will be encoded--"half test depth,three quarters test depth," and so on--because of the numerous Russian trawlers that cruise along the U.S. coastline. 8:09 A.M. Thresher is at one-half her test depth.
9:02 A.M. The submarine requests Skylark's navigator to repeat a course reading.
9:03 A.M. The following message is received from Thresher "Experiencing minor problem. Have positive angle." And then: "Attempting to blow." Skylark's telephone picks up the sound of air under high pressure as Thresher attempts to push sea water from her ballast tanks. Then there is silence. For the next ten minutes Skylark attempts to make contact with Thresher, but there is no reply.
9:17 A.M. Skylark receives a garbled message. It is mostly unintelligible, but it ends with the distinct and ominous words: "...test depth." Thresher's acoustic phone has remained open, and Skylark's navigator, a veteran of naval combat in World War 11, is astounded by what follows. He hears the distinctive groans and clanks of a doomed ship. Thresher is breaking up.
Skylark crisscrosses the coordinates where Thresher dived, calling for a response. None arrives. The Navy quickly dispatches scores of planes and ships to the site. Skylark spots an oil slick at the place where the submarine was last reported. Other vessels retrieve floating debris: yellow gloves of the type used on nuclear submarines, plastic, and several pieces of the cork used to insulate submarine hulls from the ocean's chill. Most mysterious is a report (which was never verified) from one of the searching submarines that her UQC (underwater telephone) is picking up what sound like garbled voices. Later, an investigator will speculate that the sub's forward section might have been blown clear to drift momentarily on a cold layer of water. The voices, he suggests, were those of crew members trapped inside, calling for help on Thresher's forward UQC. Within a few hours it is clear that a disaster of major proportions has occurred. Thresher exceeded her maximum test depth and imploded. Her hull gave way and a torrent of sea water burst into the crippled sub. The impact of the wall of air and water may have been enough to explode the vessel's store of diesel fuel.
To the Navy, the disaster meant more than the loss of 129 crew members and civilians. Thresher had been the most advanced submarine in the world, capable of reaching depths and speeds unimaginable a decade before. With a destructive power unequaled by the Navy's entire submarine force in World War II, she had helped to neutralize the growing threat of Soviet submarines. In the words of Vice Admiral Elton Grenfell, commander of the Atlantic fleet's submarine force, in the two years that Thresher was active with the fleet, she "fully confirmed the anticipated quantum advance in attack submarine capabilities which the designers had promised."
Nearly a quarter century after the loss of Thresher, we have come to take for granted the high standard of performance of nuclear submarines. In 1963, however, they represented a breakthrough in design and capability. The so-called "pig boats" of World War II were designed for surface running and submerged only when preparing to attack or to escape detection. Their bow sweep was similar to that of a destroyer, and their decks were flat and carried heavy-caliber guns. While such innovations as the snorkel and nuclear power enabled submarines to remain submerged for long periods, their design remained the same as a surface ship's. Even Nautilus, hailed as a great advance because she was nuclear powered, had many characteristics in common with older vessels. Then, in the early 1950s, naval architects and designers developed the cigar-shaped sub. The first of these to be tested was Albacore, the prototype design from which Thresher and today's designs evolved. Compared with previous submarines, Albacore was a true submersible, more at home under water than on the surface, where she tended to wallow like a stricken whale. Propelled by nuclear reactors, the new submarines could reach speeds of 30 knots and better. While gliding through water instead of through air, these vessels shared many of the liabilities of airplanes, as well as their advantages. They were able to bank and turn at the merest nudge of the sail or stern plane. But at the high speeds that they now could attain, a wrong flick of the stick could send them careening to the surface or headlong past their test depth.
James L. McVoy, a former submariner and now the editor of the Naval Engineering Journal, suggests that Thresher may have been a victim of the same advancing technology that brought her into being. The Navy's ability to design subs that could attain great depths and speeds, he claims, was ahead of its ability to cope with the problems inherent in those attainments. Until a few years ago, for example, currents and other conditions far below the surface were not significant considerations where submarines were concerned. Columbus O.D. Iselin, of the Woods Hole Oceanographic Institution, has speculated that Thresher may have been lost because of a huge underwater swirl and wave. A large storm, which had moved across the Gulf of Maine on April 8 and created a subsurface eddy, could also have caused gigantic, 300-foot underwater waves. If Thresher submerged at the exact spot where these undersea disturbances took place, she might have been first caught in the eddy and then been pummeled by the waves. This would have considerably hastened the vessel's descent, sweeping her down to close to crush depth before the crew had time to respond. Iselin suggests that if Thresher then experienced a failure in her ballast-blow system, she would not have had time to recover before imploding.
Of course, no one will ever know what caused the loss of Thresher, but the Navy's version of what happened is far more prosaic. After months of study, a naval board of inquiry concluded that a failure in a segment of the vessel's internal piping system, probably in the engine room, was responsible. According to Admiral Grenfell, writing in the March, 1964, issue of the U.S. Naval Institute's Proceedings, "The casualty must have occurred when the ship was at or near test depth, which subjected the interior to a violent spray of water and progressive flooding. In all probability, water and spray shorted, out vital electrical circuits, causing a loss of propulsion power. The Thresher presumably blew main ballast, started to rise, and began to sink. Shortly thereafter, she undoubtedly exceeded her collapse depth and plunged to the bottom." Modern submarines rely heavily on power to propel them to the surface, especially from great depths. If Thresher's nuclear reactor shut down because of the rupture and short-circuiting, she would have had only her auxiliary diesel system to churn the propeller. As the ship filled with water, the auxiliary's power would have been insufficient to push Thresher toward the surface, and her ballast system was inadequate to offset the tremendous counter pressure exerted at great depth.
The gruesome recreations of Thresher's last moments describe the crew struggling to plug the rupture and restart the reactor. As the interior filled with mist from the jet of water and the auxiliary vainly turned the propeller, Thresher rose slowly and then began its fatal descent. The hull twisted and rippled like rubber, the ship groaned and creaked, then literally blew apart. Debris was scattered across the ocean floor and several large chunks of the hull drifted down intact.
Captain McVoy admits that, "When the Navy tried to determine the cause of Thresher's loss we found so many things wrong it was almost a good thing we didn't know what happened." Had the cause been obvious, he explains, investigators might have overlooked numerous flaws in construction and safety procedures that could have led to future disasters.
Chief among the suspected causes of the disaster was an improperly bonded silver-brazed fitting in the sub's piping system. All of Thresher's pipes under four inches in diameter were joined by a silver ring inserted between the two ends; joints were bonded with a torch. Pipes above four inches were welded. Investigators theorized that a fitting came loose, spraying water into the engine room. The water short-circuited a control panel, causing the reactor to close down and leaving Thresher without power.
In subsequent investigations of other submarines, the Navy found that, although these pipe fittings were tight, some of the joints revealed no silver when the pipes were knocked apart. The brazing had been neglected. This startling revelation led to the implementation of a quality assurance program at naval shipyards, in which teams of inspectors checked thousands of fittings. As an extra precaution, fitters had to fill out and sign a card, verifying that they had brazed each joint, and submit it to an inspector, who also checked the fittings and signed the card. This information was stored in a computer, which kept a record of all submarine fittings. In Thresher's day, X rays were sometimes used to check the strength of fittings, but they could miss many flaws and cracks. Today, ultrasound is used to inspect fittings and verify the integrity of at least 60 percent of the bond.
An open sea valve was also proposed as the culprit in Thresher's loss. Sea valves are openings in the hull that allow water to be taken in for the cooling systems. If the sub's electrical system had closed down, the valve would have had to be closed manually. This could have allowed enough water into the vessel to seal its fate. Later submarines employed a redundant hydraulic system that could close the valves immediately without the need for electricity.
Investigation also revealed that the aluminum-bronze used in castings for the sub's torpedo tubes, which passed through its hull, had a tendency to leach and lose strength. A weakened tube might have been the source of a fatal leak. Also under suspicion was the vessel's ballast-blow system. To rise to the surface, a submarine gains buoyancy by filling its ballast tanks with air. In Thresher, the air was blown through an extensive system of pipes and manifolds before reaching the ballast tanks. If Thresher had taken on water faster than her ballast tanks had filled with air, she would have begun to sink.
One of the most important improvements in submarine construction resulting from the loss of Thresher was in the blow rate, or the speed with which the ballast tanks fill with air. Submarines now employ an emergency blow system that introduces air directly into the ballast tanks at a rate seven times faster than on Thresher. To a stricken submarine, such a system can mean the difference between life and death. Another possible, albeit unlikely, cause of Threshers loss was a jammed diving plane. At high speeds and great depths, a locked stern or sail plane could drive a vessel below its crush depth before its crew could respond.
According to Daniel Savitsky, director of the Davidson Laboratory (which played a major role in developing Albacore) at Stevens Institute of Technology in Hoboken, New Jersey, designers are constantly seeking ways to increase the speed of a submarine without sacrificing control. For example, smaller fins and planes reduce drag but can also compromise controllability. "Threading the needle" is what Savitsky calls the balancing act between ensuring the safety of submarines and increasing their speed and depth.
The Thresher disaster also demonstrated the Navy's need for improved deep submersibles for search and rescue operations. Skylark carried a McCann submarine rescue chamber, developed in the 1920s, which was capable of rescue operations down to 850 feet. The Navy also had the bathyscaph Trieste, which, in 1960, dived to 35,800 feet in the Marianas Trench. But Trieste had none of the maneuverability of today's deep submersibles, such as the remotely controlled Argo, which last year discovered the Titanic in 13,000 feet of water. Trieste was essentially an elevator that could reach great depths but could maneuver only slowly. She submerged at two knots, could remain on the bottom for only four hours, and had a search width of only 100 feet.
With such limited capabilities, it is not surprising that it took weeks for the submersible to locate Thresher.
With the creation of the Deep Submergence Systems Program in May of 1964, the Navy made the development of deep submersibles a top priority. Among the vehicles that have come out of this program are such manned submersibles as the highly maneuverable and agile Alvin, which has an operating depth of 13,000 feet. Another type of craft is the deep submergence rescue vehicle, or DSRV, which can rescue submarine crews at depths of about 3500 feet. A third type of submersible is the NR-1, a nuclear-powered vehicle with an array of sensors and work devices. The NR-1 has space for seven and can remain submerged for long periods.