Lost Subs - USS Thresher (SSN-593)
From Wikipedia, the free encyclopedia. Compiled July 2, 2008, by David Barth.
The second USS Thresher (SSN-593) was the lead ship of her class of nuclear-powered attack submarines in the
United States Navy. Her loss at sea during deep-diving tests in 1963 is often considered a watershed event in the
implementation of the rigorous submarine safety program SUBSAFE.
The contract to build Thresher was awarded to Portsmouth Naval Shipyard on 15 January 1958, and her keel was laid
on 28 May 1958. She was launched on 9 July 1960, was sponsored by Mrs. Frederick B. Warder (wife of the famous
Pacific War skipper), and was commissioned on 3 August 1961, with Commander Dean L. Axene in command.
Early Career
Thresher conducted lengthy sea trials in the western Atlantic and Caribbean Sea areas in 1961 and 1962. These tests
provided a thorough evaluation of her many new and complex technological features and weapons. Following these trials,
she took part in Nuclear Submarine Exercise (NUSUBEX) 3-61 off the northeastern coast of the United States from
September 18 to September 24, 1961.
On October 18 Thresher headed south along the East Coast. While in port at San Juan, Puerto Rico on 2 November 1961,
her reactor was shut down and the diesel generator was used to carry the "hotel" electrical loads. Several hours
later the generator broke down, and the electrical load was then carried by the battery. The generator could not be
quickly repaired, so the captain ordered the reactor restarted. However, the battery charge was depleted before the
reactor went critical. With no electrical power for ventilation, temperatures in the machinery spaces reached 60
degrees C (140 degrees F), and the boat was partially evacuated. Cavalla (SS-244) arrived the next morning and
provided power from her diesels, enabling Thresher to restart her reactor.
Thresher conducted further trials and fired test torpedoes before returning to Portsmouth on November 29. The boat
remained in port through the end of the year, and spent the first two months of 1962 evaluating her sonar and
Submarine Rocket (SUBROC) systems. In March, the submarine participated in NUSUBEX 2-62 (an exercise designed to
improve the tactical capabilities of nuclear submarines) and in antisubmarine warfare training with Task Group
ALPHA.
Off Charleston, SC, Thresher undertook operations observed by the Naval Antisubmarine Warfare Council before she
returned briefly to New England waters, after which she proceeded to Florida for more SUBROC tests. However, while
moored at Port Canaveral, Florida, the submarine was accidentally struck by a tug which damaged one of her ballast
tanks. After repairs at Groton, Connecticut, by the Electric Boat Company, Thresher went south for more tests and
trials off Key West, Florida, then returned northward and remained in dockyard for refurbishment through the early
spring of 1963.
Loss of Thresher
On April 9, 1963, after the completion of this work, Thresher, now commanded by Lieutenant Commander John Wesley
Harvey, began post-overhaul trials. Accompanied by the submarine rescue ship USS Skylark (ASR-20), she sailed to
an area some 350 kilometers (220 statute miles or 190 nautical miles) east of Cape Cod, Massachusetts, and on the
morning of April 10 started deep-diving tests. As Thresher neared her test depth, Skylark received garbled
communications over underwater telephone indicating "... minor difficulties, have positive up-angle, attempting
to blow." When Skylark's queries as to if Thresher was under control were answered only by the ominous
sound of compartments collapsing, surface observers gradually realized Thresher had sunk. All 129 officers,
crewmen and military and civilian technicians aboard her were killed.
After an extensive underwater search using the bathyscaphe Trieste, oceanographic ship Mizar and other ships,
Thresher's remains were located on the sea floor, some 8,400 feet (2560 m) below the surface, in six major sections.
The majority of the debris is in an area of about 134,000 square meters (160,000 square yards). The major sections
are the sail, sonar dome, bow section, engineering spaces section, operations spaces section, and the
stern planes.
Deep sea photography, recovered artifacts, and an evaluation of her design and operational history permitted
a Court of Inquiry to conclude Thresher had probably suffered the failure of a weld in a salt water piping
system, which relied heavily on silver brazing instead of welding. Earlier tests using ultrasound equipment
found potential problems with about 14 percent of the tested brazed joints, most of which were determined not to pose
a risk significant enough to require a repair. High-pressure water spraying from a broken pipe joint may have
shorted out one of the many electrical panels, which in turn caused a shutdown ("scram") of the reactor, with a
subsequent loss of propulsion. The inability to blow the ballast tanks was later attributed to excessive moisture
in Threshers high-pressure air flasks, which froze and plugged its own flowpath while passing through the valves.
This was later simulated in dock-side tests on the Thresher's sister ship, USS Tinosa (SSN-606). During a test to
simulate blowing ballast at or near test depth, ice formed on strainers installed in valves; the flow of air
lasted only a few seconds. (Air driers were later retrofitted to the high pressure air compressors, beginning
with Tinosa, to permit the emergency blow system to operate properly.)
Unlike diesel submarines, nuclear subs relied on speed and deck angle (that is, driving the ship towards the
surface) rather than deballasting to surface. Ballast tanks were almost never blown at depth; this could cause
the ship to rocket to the surface out of control. Normal procedure was to drive the ship to periscope depth,
raise the periscope to verify the area was clear, then blow the tanks and surface the ship.
At the time, reactor-plant operating procedures precluded a rapid reactor restart following a scram, or even the
ability to use steam remaining in the secondary system to "drive" the ship to the surface. After a scram, standard
procedure was to isolate the main steam system, cutting off the flow of steam to the turbines providing propulsion
and electricity. This was done to prevent an over-rapid cool-down of the reactor. Thresher's Reactor Control Officer,
Lt. Raymond McCoole, was not at his station in the maneuvering room, or indeed on the ship, during the fatal dive.
McCoole was at home caring for his wife who had been injured in a freak household accident - he had been all but
ordered ashore by a sympathetic Commander Harvey. McCoole's trainee Jim Henry, fresh from nuclear power school,
probably followed standard operating procedures and gave the order to isolate the steam system after the scram,
even though Thresher was at or slightly below her maximum depth and was taking on water. Once closed, the large
steam system isolation valves could not be reopened quickly. In later life, McCoole was sure he would have delayed
shutting the valves, thus allowing the ship to "answer bells" and drive herself to the surface, despite the
flooding in the engineering spaces. Admiral Rickover later changed the procedure, allowing steam to be withdrawn
from the secondary system in limited quantities for several minutes following a scram.
There was much (covert) criticism of Rickover's training after Thresher went down, the argument being his "nukes"
(nuclear reactor operators) were so well conditioned to protect the nuclear plant that they would have shut the main
steam stop valves by rote - depriving the ship of needed propulsion - even at great depths and with the ship
clearly in jeopardy. Nothing enraged Rickover more than this argument. Common sense, he argued, would prove this
to be untrue.
It's more likely that the engine room crew was simply overwhelmed by the flooding casualty, or took too long to
contain it. In a dockside simulation of flooding in the engineroom, held before Thresher sailed, it took the
watch in charge 20 minutes to isolate a simulated leak in the auxiliary seawater system. At test depth, taking
on water, and with the reactor shut down, Thresher would not have had anything like 20 minutes to recover. Even
after isolating a short-circuit in the reactor controls it would have taken nearly 10 minutes to restart the
plant.
Thresher imploded (that is, one or more of her compartments collapsed inwards in a fraction of a second) at a depth
somewhere between 1,300 and 2,000 feet (400 and 600m). All on board were killed nearly instantly (1 or 2 seconds
at most).
Over the next several years, the Navy implemented the SUBSAFE program to correct design and construction problems
on all submarines (nuclear and diesel-electric) in service, under construction, and in planning. During the formal
inquiry, it was discovered record-keeping at the Portsmouth Naval Shipyard was far from adequate. For example, no
one could determine the whereabouts of hull weld X-rays made of Thresher's sister ship Tinosa, nearing completion
at Portsmouth, or, indeed, whether they had been made at all. It was also determined Thresher's engine room layout
was awkward, in fact dangerous, as there were no centrally-located isolation valves for the main and auxiliary
seawater systems. Most subs were subsequently equipped or retrofitted with flood control levers, which allowed
the Engineer Officer of the Watch in the maneuvering room to remotely close isolation valves in the seawater
systems from a central panel, a task necessarily performed by hand on Thresher. Hand-power valves might not
even have been accessible during a flooding casualty. At such depths, the blast of water from even a small
leak (a "water spike") can dent metal cabinets, rip insulation from cables, and even cut a man in half. (Water
pressure at 1,000 feet (300 m) is about 450 psi (3,100 kPa).)
SUBSAFE would prove itself to be a crucial part of the Navy's safe operation of nuclear submarines, but was
disregarded just a few years later in a rush to get another nuclear sub, Scorpion ready for service as part of
yet another program meant to increase nuclear submarine availability. The subsequent loss of Scorpion reaffirmed
the need for SUBSAFE and apart from Scorpion, the U.S. Navy has suffered no further losses of nuclear
submarines.
The Navy has periodically monitored the environmental conditions of the site since the sinking and reported the
results in an annual public report on environmental monitoring for U.S. Naval nuclear-powered ships. These
reports provide specifics on the environmental sampling of sediment, water, and marine life which were taken to
ascertain whether Thresher's nuclear reactor has had a significant effect on the deep ocean environment. The
reports also explain the methodology for conducting deep sea monitoring from both surface vessels and submersibles.
The monitoring data confirms that there has been no significant effect on the environment. Nuclear fuel in the
submarine remains intact.
According to newly declassified information and statements made by the lead scientist himself, Dr Robert "Bob" Ballard,
the successful search
for the wreck of RMS Titanic was actually part of a secret mission to map and collect data on both the Thresher and
the Scorpion wrecks. Ballard approached the Navy in 1982 for funding to find the Titanic with his new deep-diving
robot submersible. The Navy saw the opportunity and granted him the money on the condition he first inspect the two
submarine wrecks. Ballard's robotic survey discovered that the Thresher had sunk so deep it imploded, turning into
thousands of pieces. His 1985 search for the Scorpion, which was thought to be a victim of a Soviet attack, revealed
such a large debris field that it looked "as though it had been put through a shredding machine." The survey data
revealed the most likely cause of the loss of the Scorpion was one of its own torpedoes going rogue and hitting the
sub after firing. Once the two wrecks had been visited, and the radioactive threat from both was established as small,
Ballard was able to search for Titanic. Due to dwindling funds, he had just 12 days to do so, but he used the same
debris-field search techniques he'd used for the two subs, which worked and the Titanic was found.
U.S. submarine classes are generally known by the hull number of the lead ship of the class - for instance, Los
Angeles-class boats are called 688s because the hull number of USS Los Angeles was SSN-688. The Thresher-class
boats should thus be called 593s, but since Thresher's sinking they have been referred to as 594s
(Permit class).
Details of the disaster
Time accelerated sequence of events during the disasterThe following is from the 1975 book The Thresher
Disaster by John Bentley. Times are in 24-hour notation.
0747: Thresher begins its descent to the test depth of 1,300 feet (400 m).
0752: Thresher levels off at 400 feet (120 m), contacts the surface, and the crew inspects the ship for leaks.
None are found.
0809: Commander Harvey reports reaching half the test depth.
0825: Thresher reaches 1,000 feet (300 m).
0902: Thresher is cruising at just a few knots (subs normally moved slowly and cautiously at great depths, lest a
sudden jam of the diving planes send the ship below test depth in a matter of seconds.) The boat is descending in
slow circles, and announces to Skylark she is turning to "Corpen [course] 090." At this point, transmission quality
from the Thresher begins to noticeably degrade, possibly as a result of thermoclines.
0909: It is believed a brazed pipe-joint ruptures in the engine room. The crew would have attempted to stop the
leak. At the same time, the engine room would be filling with a cloud of mist. Under the circumstances, Commander
Harvey's likely decision would have been to order full speed, full rise on the sail planes, and blow main
ballast in order to surface. Due to Joule-Thomson effect, the pressurized air rapidly expanding in the pipes
cools down, condensing moisture and depositing it on strainers installed in the system to protect the moving
parts of the valves; in only a few seconds the moisture freezes, clogging the strainers and blocking the air
flow, halting the effort to blow ballast. Water leaking from the broken pipe most likely causes short circuits
leading to an automatic shutdown of the ship's reactor, causing a loss of propulsion. The logical action at this
point would have been for Harvey to order propulsion shifted to a battery-powered backup system. As soon as the
flooding was contained, the engine room crew would have begun to restart the reactor, an operation that would be
expected to take at least 7 minutes.
0912: Skylark pages Thresher on the underwater telephone: "Gertrude check, K [over]." With no immediate response
(although Skylark is still unaware of the conditions aboard Thresher), the signal "K" is repeated twice.
0913: Harvey reports status via underwater telephone. The transmission is garbled, though some words are
recognizable: "[We are] experiencing minor difficulty, have positive up-angle, attempting to blow." The submarine,
growing heavier from water flooding the engine room, continues its descent, probably tail-first. Another attempt to
empty the ballast tanks is performed, again failing due to the formation of ice. Officers on the Skylark could hear
the hiss of compressed air over the loudspeaker at this point.
0914: Skylark acknowledges with a brisk, "Roger, out," awaiting further updates from the SSN. A follow-up message,
"No contacts in area," is sent to reassure Thresher she can surface quickly, without fear of collision, if
required.
0915: Skylark queries Thresher about her intentions: "My course 270 degrees. Interrogative range and bearing from
you." There is no response, and Skylark's captain, Lieutenant Commander Hecker, sends his own gertrude message to the
submarine, "Are you in control?"
0916: Skylark picks up a garbled transmission from Thresher, transcribed in the ship's log as "900 N." [The meaning
of this message is unclear, and was not discussed at the enquiry; it may have indicated the submarine's depth and
course, or it may have referred to a Navy "event number" (1000 indicating loss of submarine), with the "N" signifying
a negative response to the query from Skylark, "Are you in control?"]
0917: A second transmission is received, with the partially recognizable phrase "exceeding test depth...." The
leak from the broken pipe grows with increased pressure.
0918: Skylark detects a high-energy low-frequency noise with characteristics of an implosion.
0920: Skylark continues to page Thresher, repeatedly calling for a radio check, a smoke bomb, or some other
indication of the boat's condition.
1104: Skylark attempts to transmit a message to COMSUBLANT (Commander, Submarines, Atlantic Fleet): "Unable to
communicate with Thresher since 0917R. Have been calling by UQC voice and CW, QHB, CW every minute. Explosive
signals every 10 minutes with no success. Last transmission received was garbled. Indicated Thresher was
approaching test depth.... Conducting expanding search." Radio problems meant that COMSUBLANT did not receive
and respond to this message until 12:45. Hecker initiated "Event SUBMISS [loss of a submarine]" procedures at
1121, and continued to repeatedly hail the Thresher until after 1700.
On April 11, at a news conference at 10:30, the Navy officially declared the ship as lost.