The USS Thresher (SSN-593) was the first in a new class of deep diving fast attack nuclear submarines. She was constructed in the Ports mouth Naval Shipyard in Kittery, Maine. By early April 1963 Thresher had completed her “Post Shakedown Availability” (PSA) and went to sea for PSA sea trials.
As part of those sea trials a dive to test depth was scheduled. Thresh er did not recover from the deep dive and sank to the ocean floor at a depth of just over 8000 feet. All 129 persons on board died instantly when the ship’s hull collapsed/imploded at a depth estimated to be at least 150% of test depth.
Extensive investigations were conducted in the following weeks and months, including by a Navy Board oflnquiry, which was promptly established at the shipyard, and later extensive congressional hearings were held.
The only other US nuclear submarine that has been lost at sea is the USS Scorpion (SSN-589), which sank in the mid-Atlantic on 22 May 1968. While there has been much research and speculation, the cause of the loss of Scorpion is still a mystery. A battery explosion is often cited as a factor, but the cause of the battery explosion is unknown. The cause of the loss of Thresher is much better understood for two principal reasons: Thresher had an escort ship and a SOSUS (off shore Sound Surveillance System) array was located in relatively close proximity. In Thresher’s case that array was on the ocean floor south of Nova Scotia, and about 30 nautical miles from the accident site.
Virtually all information about the tragedy was quickly classified, including the report by the Board of Inquiry, congressional hearings, SOSUS data, as well as the underwater telephone (UQC) communica tions with the escort ship, USS Skylark (ASR20). This information was de-classified (slowly and piecemeal) over time and now only minor de tails remain classified.
Much has been written about the tragedy, especially since the de-classification of the records began. In the view of the three co-authors of this article, there is of yet no short, easy to comprehend account that is both as factual as possible and unbiased. The three co-authors have studied and discussed the available history extensively and have sought to achieve just such an account. Here is our analysis of Thresher’s deep dive and loss.
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Thresher’s final 25 minutes:
0853 Thresher reports her depth (test depth -300 feet), going to test depth, to her escort ship, USS Skylark (ASR-20), on the Under Water Telephone (UQC).
0902 Skylark reports to Thresher on the UQC that she is changing course to 090 degrees.
At about this time, plus or minus a few minutes, Thresher reaches test depth.
0909 The Sound Surveillance System (SOSUS) detects what ana lysts describe as “line frequency instability”. This most likely means that Thresher’s Ships Service Turbine Generators (SSTGs) were not stable at 60 Hz, as they normally are.
0909.8 SOSUS detects what some analysts describe as a main bal last tank (MBT) blow of 1.5 minutes duration.
0910 Thresher reports a course change in what appears to be a rou tine UQC communication. Since the report was made without alarm the classification of the noise detected by SOSUS at 0909.8 as a MBT blow is questionable. Additionally, Skylark did not report detecting MBT blow noise at this point, but both SOSUS and Skylark did detect and reported MBT blow noise three minutes later.
As has been described in other sources (Note 1), attempts by Thresh er to blow main ballast tanks were so ineffective that VERY little water was being displaced from the ballast tanks, and the air flow through the ship’s airlines was only a small fraction of designed air flow. Thus, the noise transmitted to sea could be very different than the noises detected by SOSUS in other MBT blow situations.
0911 SOSUS detected Thresher’s Main Coolant Pumps (MCPs) stopping. (SOSUS had detected MCPs running in fast speed earlier in the dive). The loss of MCPs in fast speed could be due to instabilities in SSTG output, as noted by SOSUS two or so minutes earlier, or due to a reactor scram. In any event, a reactor scram almost certainly occurred at this point.
The abrupt stoppage of Main Coolant Pumps (without the normal procedure of first shifting the pumps to slow speed) would cause an im mediate reactor scram as the reactor protection circuitry detected a com plete loss of flow. The reactor scram could also have been caused by salt water spray from leakage from any one of the MANY sea water valves
/ fittings / flanges / sil-braze joints / valve stem packings / etc. located in the machinery space. The machinery space on Thresher was located between the reactor compartment and the engine room and contained extensive sea water system piping. Reactor control electronics and the AC/DC motor generators were located in this space.
The ship has now lost all propulsion. Shifting propulsion to the ship’s Emergency Propulsion Motor (EPM), a low powered electric mo tor wrapped around the shaft to the propeller, was a slow process. Shift ing to the EPM required dis-engaging a massive clutch in the shaft so that the EPM would be turning just the propeller and not the main turbine rotors and the reduction gears. On Thresher this had to be done locally, at the very stern of the ship. And, at that point in history, Naval Reactors procedures did not make provisions for use of residual heat in the reactor for propulsion.
0912 Thresher responded to a UQC transmission check from Sky lark without expressing alarm. Both ships used the code word “KILO” meaning that communications were satisfactory. At that point, however, the Captain and Officer of the Deck, both in the Control Room, would very likely have been getting alarming and, at least at first, confusing reports from the Maneuvering Room, located aft in the engine room.
0913 Thresher reports on the UQC: “Experiencing minor difficul ties. Have positive up angle. Attempting to blow up.” This transmission is thought to have been made by the Commanding Officer.
The ship was using coast down speed after the scram to plane up and was likely able to get at least a few feet above test depth. The ship was heavy from sea water leaks and possibly from being out of trim—not noticeable at 10 or 12 knots, but very noticeable at 2 or 3 knots.
0913.5 Skylark and SOSUS both ” … detected noise thought… ” to be from Thresher’s attempt to blow Main Ballast Tanks. The noise had a duration of 30 seconds. With no propulsion and an ineffective MBT Blow System — and heavy — the ship was slowly sinking,
0916 Thresher reported “exceeding test depth” on the UQC. (In this transmission the words “test depth” were clear while the first word was garbled; but widely thought to be “exceeding” or “passing”.)
0918.1 Skylark and SOSUS heard the hull implode, instantly killing 129 souls.
—-end—-
Notes: (1) In a letter and statement of analysis to the president of the Submarine Museum and Library, written on the 50th anniversary of the loss of Thresher, retired Captain Zack T. Pate describes experience on Thresher’s sister ship at the shipyard, the USS Tinosa (SSN-606), which provides data and physical evidence that convincingly demonstrate that Thresher’s Main Ballast Tank blow system was seriously degraded. The principal cause of the degradation was “temporary” strainers in the air system that collapsed. Fine mesh conical strainers, backed by a thick bronze plate with a small orifice, were installed by the shipyard to protect air system valves from minute particles of debris during construction. The strainers were not removed before the ship went to sea, as intended,and the conically shaped mesh collapsed into the back-up orifice, block ing air flow to the main ballast tanks. See the photo below. The aforemen tioned letter and statement are included in this issue of The Submarine Review and are also available at the Submarine Library and Museum Association in Con-necticut.
- The authors do not believe that Thresher suffered a major flooding ca sualty, as suggested by the Navy Board of Inquiry. SOSUS experts believe that the noise from ma jor flooding at test depth would have been unmistakable at the (fairly near by) SOSUS array and, in addition, the report by the Cap tain of “experienc ing minor difficul ties” is inconsistent with major flood ing. No submariner considers flooding a minor We do
believe, however, that it is likely that Thresher experienced sea water leakage and, that even a relatively small leak at test depth could cause considerably spraying of salt water onto electrical and electronic equip ment. We consider that a probable cause of the frequency instability de tected by SOSUS and, directly or indirectly, of the reactor scram.
- In this article the authors have not sought to assign responsibility / accountability. We believe, however, that there is plenty to go around,and that the shipyard is certainly not exempt.
All three co-authors served in the Navy’s nuclear submarine pro gram as the principal part of their career. VADM Emery served as Com mander Submarine Force Atlantic before retiring. RADM Goebel served as Commander Submarine Group Two and now, in retirement, serves as President of the Submarine Library and Museum Association in Con necticut. CAPT Pate served as a special assistant to Admiral Rickover before retiring.
CAPT Pate’s first assignment on a nuclear submarine was as Auxilia ry Division Officer on USS Tinosa (SSN-606), the sister ship to Thresher at the Portsmouth Naval Shipyard. After Thresher’s loss then LT Pate was given a special assignment by his commanding officer to serve as liaison to the Navy Board of Inquiry and with shipyard engineers in the search for the cause of Thresher’s loss; AND in the intensive quest to make sure Tinosa did not suffer the same fate. Tinosa did extensive tests alongside her pier at the request of the Board oflnquiry, especially of the Main Ballast Tank Blow System. (see Note [l] above).
CAPTAIN Zack Pate’s letter to RADM Goebel on the subject of the Main Ballast Tank blow system investigation following the loss of the USS Thresher
May 31, 2013
RADM David Goebel 77 l Pequot Avenue
New London, CT 06320 Dear Dave,
The purpose of this letter is to forward a strainer (and its associated orifice) which was removed from the 4500/3000 psi air system on the USS Tinosa (SSN-606), shortly after the loss of USS Thresher (SSN- 593) in 1963. Tinosa was Thresher’s sister ship, under construction at the Portsmouth Naval Shipyard, when Thresher was lost at sea on 10 April 1963. The Thresher was the first ship of a new class.
The Court of Inquiry, formed to investigate the loss of Thresher, asked Tinosa to conduct tests of the main ballast tank blow system short ly after the court was formed. At the time I was the Auxiliary Division Officer (aka Damage Control Assistant, or DCA) on Tinosa.
When the main ballast tank blow system failed to operate as de signed, auxiliarymen working for me soon located collapsed strainers just upstream of both 4500 to 3000 psi (Marotta) reducing valves. The collapsed strainers were quite clearly blocking most air flow to the main ballast tanks, as is evident from a visual inspection of the enclosed strain er, one of the two removed after the test. The two strainers were virtually identical in appearance and condition.
After the (failed) air system tests on Tinosa, I was interviewed by CAPT Osborne, a member of the Court of Inquiry. During our discus sions I showed him the two collapsed strainers and where they came from in Tinosa’s systems. At the end of the interview I gave one of the strainers to CAPT Osborne to be part of the court’s evidence. With the approval of my commanding officer I kept the other one “until further notice.”
Now, 50 years later, and near the 50th anniversary of the tragic loss of Thresher, our sister ship, I am pleased to donate this second strainer to the Submarine Force Library and Museum Association.
Immediately following my signature on this letter is my “testimo nial”, as you requested, describing key aspects of the special tests on Tinosa, which I personally supervised, and other directly related obser vations.
The strainers were supposed to be a temporary installation, intended to protect the Marotta reducers during construction. There is abundant evidence that the strainers found on Tinosa were still installed on Thresh er on the fateful day of her loss.
In my view the strainers were a significant, if not critical, factor in Thresher’s inability to recover from problems encountered during her deep dive. After reading the testimony below, I believe most readers will agree.
Warmest Regards,
(orig. signed)
Zack T. Pate
CAPT. USN Retired
