The U.S. Navy experienced its worst submarine disaster when the nuclear-powered attack submarine USS Thresher (SSN-593) was lost on 10 April 1963 with 129 men on board. Although the specific cause was never established, the Navy theorized that a flooding casualty short-circuited electrical equipment that caused the nuclear reactor to shut down. Operating at a depth of about 1,300 feet, the crew tried to blow her ballast tanks. At that point, a phenomenon known as Venturi cooling took place: moisture in the deballasting systems accumulated and froze in the boat's reducing valve strainers, blocking the passage of compressed air into the ballast tanks. These strainers, installed by the manufacturer to protect sensitive reduction valves from particulate matter, proved the sub's undoing.1 Unable to blow her tanks or restart her reactor in time, the Thresher passed beyond collapse depth and imploded.
Three candidates for negligence emerge from the loss of the Thresher: substandard workmanship in constructing or overhauling the boat, inadequate restart procedures for the onboard reactor, and the use of inefficient deballasting technology in a new, higher-performance submarine class.
In the years following the Thresher's loss, several lawsuits were filed in civil court by spouses of the deceased. These legal actions claimed negligence on the Navy's part in designing, building, and operating the submarine, which could dive deeper and go faster than any of her predecessors. In a letter to the Admiralty and Shipping Section of the U.S. Justice Department, J. W. Howay wrote: "[I]f this matter were to be litigated, using only the information already made public, the Navy would be found to have been negligent in the construction and/or operation and maintenance of THRESHER, or the vessel herself would be found to have been unseaworthy for her assigned test mission, or both." The Navy's response was to avoid litigation, since it would "require submission of voluminous classified information over and above that already released."2 The negligence claims were settled out of court by the Navy, which characterized one claim for $99,000 as "extremely favorable to the Government, considering the potential exposure."3
Was the Navy—specifically, the Bureau of Ships (BuShips) and the Nuclear Propulsion Branch—negligent in the loss of 129 lives and a very expensive submarine? In its court of inquiry held on the sinking, the Navy examined thoroughly the issue of negligence on its part. Many problems were uncovered in the areas of hull and pipe construction, deballasting technology, the transience of technical and management personnel involved with the Thresher, flooding casualty training, and perhaps most important, a lack of appreciation for the dangers presented by a faster, deeper operational environment.
Flooding Casualties
Rear Admiral Dean Axene, the Thresher's first commanding officer, wrote in November 1962 that "the most dangerous condition that exists in the THRESHER is the danger of salt water flooding while at or near test depth."4 A week before she sank, the Thresher underwent drydock repairs, which included work on her main circulating water valve. Yet no procedures were included in the boat's fateful test dive to operate her sea valves at shallower depths. The first post-drydock operation of her sea valves was scheduled to occur at test depth.5
Much has been made of the silver-brazed piping joints in the Thresher and the percentage of joints with substandard bonding that were discovered during the boat's overhaul at Portsmouth Naval Shipyard. Yet Admiral Axene has noted that "too many people got mesmerized by the salt water joints and the piping and the ultrasonic testing, and jumped to the conclusion that . . . the most probable thing was a salt water leak which led to . . . the loss of the ship. But I don't necessarily subscribe to the fact that it was a failed silver-brazed joint."6
The Portsmouth Naval Shipyard was allowed to deviate from specifications in building the Thresher; furthermore, deficiencies in the shipyard's pipe shop were documented subsequently.7 Despite these and many other indicators of substandard work in the boat's sea water systems, it is quite possible that she received her first flooding casualty only after she lost power, and after that attempted to blow main ballast. The last clear message from the Thresher indicated that she was experiencing a "minor difficulty." Could such a phrase describe a flooding casualty at 1,300 feet? The court of inquiry noted "the increasing operating depths of submarines has compressed the time available in which to take effective damage control action with respect to flooding. The shortness of time available to control flooding is not well recognized."8
Nuclear Reactor Operations
Sound surveillance system (SOSUS) recordings indicate that at 0911 on the morning of 10 April, the Thresher's main coolant pumps either stopped or slowed down. This situation caused an automatic shutdown or "scram" of the boat's reactor. The length of time it took to restart a shipboard reactor meant the crew had no chance of restoring main power before the boat passed beyond her collapse depth about seven minutes later.9
What caused the Thresher's pumps to slow down or stop altogether? Could the reactor have scrammed without assistance from a flooding casualty? Although the boat's crew had drilled strenuously for accidental reactor scrams at deep depths, a real one apparently never had taken place.10 While the court of inquiry made no specific recommendations for nuclear propulsion systems in submarines, the subject was discussed extensively throughout the proceedings. Two problems became apparent: the amount of time required to recover from a reactor scram was too long, given the new environment for submarine operations. Second, reactor operations needed to be modified in a way that would make use of latent heat in the cooling system for emergency propulsion.11
Although the BuShips had suggested to the Nuclear Reactors Branch that reactor recovery procedures should be modified, no such action was taken before the Thresher sank.12 After the sub's loss, Admiral Hyman Rickover ordered changes that reduced substantially the time needed to recover from a reactor scram. Procedures also were developed to use latent heat in reactor cooling systems for emergency propulsion.13
Deballasting Capability
Acoustic data from the Thresher sinking indicate the sub may have attempted to blow her tanks twice: once from 0909.8 to 0911.3, and again from 0913.5 to 0914.14 Between these two time periods, to the best recollection of witnesses on the bridge of the tender Skylark (ASR-20) the last intelligible message from the Thresher was: "Experiencing minor difficulties. Have positive up angle. Am attempting to blow. Will keep you informed."15 About five minutes after this message was received, the submarine's hull imploded.16
Before the Thresher sank, BuShips had "no requirements relative to the mechanical design of systems which would prevent the formation of blockages due to ice which may form during an extended blow."17 In fact, the Navy had not promulgated any test results on the workability of deballasting components in a deep-submergence environment "with frozen moisture from the air system complicating the situation."18 Subsequent tests conducted in the submarine USS Tinosa (SSN-606) demonstrated that the Thresher's strainers in her reducing valves probably froze about 30 seconds into her first attempt to blow ballast.19
The truth is, the design of the Thresher class was never reviewed thoroughly in the area of deballasting capability,20 despite concerns expressed by Admiral Rickover.21 At the time, the operational mind-set of the submarine community accepted the situation as a mission requirement. "It was well understood by all of us," Admiral Axene explained, "that the capacity to deballast at depth was severely limited. That never was considered, by me at least, to be a major source of recovery potential."22
At greater depths, it took the Thresher about seven or eight minutes before she responded to a low-pressure blow.23 Another operational characteristic of the submarine was that, after a loss of electrical power, the blow system would shut down for 10 to 50 seconds.24 Difficulties with the boat's high-pressure air system (including leakage in the reducing valves) had been well documented during her time in drydock. Much of the trouble was traced to the existence of minute particles in the system.25 Strainers were placed near the reducing valves, apparently to protect them from such particles. Although these strainers were not included in the Navy's design specifications, they remained in the boat and clogged the blow system during the Thresher's final dive.26
It should come as no surprise that the court of inquiry found the submarine's tank blowing system to be deficient.27 Just as the Nuclear Reactors Branch ignored warnings from BuShips about time needed for reactor recovery, BuShips ignored Admiral Rickover's concerns about the boat's reduced blow capacity. As for the reducing valve strainers, who was negligent? Was it the manufacturer, for installing them in violation of Navy specifications? Or was it the Navy, for failing to have them removed.
With the rise of nuclear-powered submarines in the 1950s, a new weapon system was introduced—one for which new operating manuals would have to be written. Admiral Axene felt the Navy may have been moving a bit too fast in those days: "[T]he decision to go into uncharted territory sometimes occurs before the technology that'll get you there is fully developed. . . . [W]hen you move ahead of the technology, you have an accident."28
In the meantime, commanding officers of nuclear-propelled submarines got into the habit of operating their boats with negative trim. They knew that no matter how much ballast their submarines had, the force of nuclear power would drive them back to the surface.29 According to a former crewman, Keith Johnson, the Thresher often came up from a dive with its ballast tanks full. The boat would get a 40°–45° up angle, then simply drive herself to the surface. The boat also would drop below test depth from time to time. Johnson noted that such habits might seem a bit lax, "but the crew knew what they were doing. We felt invincible. We never thought we were going to die."30
The Thresher loss sobered the naval community and made submarine crews realize they could not rely on their reactors to deliver them safely from any "minor difficulties" that might be encountered during a dive. As naval historian and former submarine commander Captain Edward L. Beach Jr. has noted, submariners at the time knew that problems might be encountered as the result of increased depth and speed, but they did not accept the reality of such dangers until the Thresher went down.31 The court of inquiry agreed, concluding that shortcomings
have developed incident to the rapid changes in materials, workmanship and operating conditions of submarines during the last decade and to the accelerated pace of the submarine program. They can be blamed on no individual or individuals, and many would not have come to notice had THRESHER not been lost.
The responsibility for the loss of THRESHER cannot be charged to neglect or dereliction on the part of any individual or group of individuals.32
Then who or what killed the Thresher? Did the workers at Portsmouth not pay enough attention to their welds and brazes? Was it the overhaul schedule that cut short testing of the Thresher's pipe joints? How about the manufacturer who installed reducing-valve strainers against Navy specifications—who failed to consider the effects of Venturi cooling on wire mesh? Was it Admiral Rickover, who refused to compromise his procedures for the safe operation of a nuclear reactor inside a submarine? What about the people at the Nuclear Propulsion Branch, who never thought about using latent heat for emergency propulsion? Or should blame go to the commanding officer for running with negative trim? If the Navy felt negligence could be proved in a court of law, why did it come to the opposite conclusion in its own court of inquiry?
The "minor difficulties" had become a horrible disaster when the boat's reactor scrammed and reducing valves froze. We may never know the order in which those malfunctions occurred. But if the sound heard at 0909 was an attempt to blow ballast, why was the crew doing so two minutes before losing nuclear power? If it was not an attempt to blow tanks, then what was it? If a flooding casualty preceded loss of power, why would it be described as "minor" at 1,300 feet?
The Navy ran computer simulations based on SOSUS recordings of the boat's final minutes. Later, Admiral Rickover and others found the most probable scenario had not been run. The computer and acoustic experts ran the scenario again at Admiral Rickover's insistence and concluded the submarine could not have sunk under the conditions postulated.33 Perhaps the acoustic data should be modeled again with state-of-the-art computer analysis.
If the Navy were found to be negligent in the Thresher loss by a civil court of law, how would such a verdict translate into the world of combat environments, safety versus mission tradeoffs, and operational confidence in crew and equipment? The verdict could not translate, because civilians operate by different rules; they are not expected to live day-by-day in a high-risk environment. Why, then, should certain deficiencies in a high-risk environment be branded as negligence? If anyone was negligent, it was a corporate entity known collectively as the U.S. Navy; its bureaucratic truths allowed too many compromises and variations in developing a new weapon system that required stricter attention.
After the Thresher sank, the Navy improved the way it built and operated nuclear-powered submarines. Most important, an emergency blow system was developed to increase overall survivability in deep waters. Certain aspects of the Thresher loss may always remain a mystery. Perhaps the best way to remember the boat and her crew is to remind ourselves of the Navy's outstanding track record in submarine safety. Lessons learned from the Thresher disaster have not been forgotten.
1. Occupational Safety Observer, June 1994 (Office of Safety and Quality Assurance, Department of Energy), p. 5.
2. J. W. Howay, assistant admiralty counsel for the Judge Advocate General, letter of 9 November 1971. JAG:11: RFR: 1st, serial #9843.
3. J. W. Howay letter of 9 November 1971.
4. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 91.
5. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 139.
6. Philip M. Callaghan, Effects of the USS Thresher Disaster Upon Submarine Safety and Deep-Submergence Capabilities in the United States Navy, master's thesis, Virginia Tech, 1987, p. 7.
7. U.S. Navy court of inquiry findings on the loss of the USS Thresher, facts 11, 13, and 152.
8. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 47.
9. U.S. Navy court of inquiry findings on the loss of the USS Thresher, opinion 45.
10. Phone conversation with former Thresher crew member Keith Johnson, 12 October 1996.
11. Callaghan, Effects of the USS Thresher Disaster . . ., p. 9.
12. Callaghan, Effects of the USS Thresher Disaster . . ., p. 18.
13. Callaghan, Effects of the USS Thresher Disaster . . ., p. 18.
14. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 18.
15. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 16.
16. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 19.
17. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 48.
18. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 50.
19. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 50.
20. Callaghan, Effects of the USS Thresher Disaster . . . p. 11.
21. Callaghan, Effects of the USS Thresher Disaster . . . p. 22.
22. Callaghan, Effects of the USS Thresher Disaster . . . p.20.
23. Phone conversation with Keith Johnson, 12 October 1996.
24. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 51.
25. U.S. Navy court of inquiry findings on the loss of the USS Thresher, fact 124.
26. Occupational Safety Observer, June 1994, (Office of Safety and Quality Assurance, Department of Energy), p. 5.
27. U.S. Navy court of inquiry findings on the loss of the USS Thresher, opinion 8.
28. Callaghan, Effects of the USS Thresher Disaster . . ., p. 59.
29. Callaghan, Effects of the USS Thresher Disaster . . ., pp. 19–20.
30. Phone conversation with Keith Johnson, 12 October 1996.
31. Callaghan, Effects of the USS Thresher Disaster . . ., p. 21.
32. U.S. Navy court of inquiry findings on the loss of the USS Thresher, opinion 55.
33. Theodore Rockwell, The Rickover Effect (Annapolis, MD: Naval Institute Press, 1992), p. 319.