By: Rear Admiral William Houston, Director, Undersea Warfare Division, N97
The tragic loss of USS Thresher remains a pivotal moment in U.S. submarine operations, safety and culture.
USS Thresher (SSN 593) got underway with 129 crew and shipyard personnel aboard for her post availability sea trials from Portsmouth Naval Shipyard on April 9th, 1963, accompanied by the Submarine Rescue Ship Skylark (ASR-20). The first day of trials, she submerged and completed her initial shallow trim dive test, surfaced, then performed a second dive to one-half of test depth. She remained submerged overnight, per standard practice, and the following morning she commenced her test depth trials.
During her trial, Thresher remained in contact with Skylark and reported her progress as she incrementally dove to test to pause and check her systems’ integrity. Reaching her test depth at 9:13 a.m., Skylark heard garbled communications from Thresher via the underwater hydrophone. “… experiencing minor difficulty, have positive up-angle, attempting to blow…” Skylark received another communication at 9:15 a.m., this time completely unintelligible. Skylark queries Thresher, “Are you in control?” with no response. At 9:17 a.m. Skylark receives her final communication from Thresher, also garbled, but “…test depth…” was heard. Skylark personnel subsequently detected sounds in the water indicative of an implosion.
When Skylark personnel answered why they thought the sound was an implosion, one of the operators stated: “I had heard a lot of ships breaking up during World War II after having been torpedoed at depth.” Efforts to reestablish contact with Thresher failed, and a search group formed to locate the submarine, unsuccessfully. Rescue and salvage ship Recovery (ASR-43) subsequently recovered bits of debris, including gloves and internal insulation pieces. Photographs taken by bathyscaph Trieste II proved that the submarine had broken up, taking all 112 crewmembers, and 17 civilian personnel aboard to their deaths in 8,400 feet of water, 220 miles east of the Massachusetts coast.
Following the tragedy, a formal Court of Inquiry convened to determine, if possible, the cause. The investigation identified multiple weaknesses in Thresher’s design that contributed to either a flooding casualty or complicated the ship’s ability to combat a casualty to save the ship. Seawater systems predominantly used a silver brazing method for joints, vice welds. Existing nondestructive testing was not adequate for silver brazed joints in seawater systems – visual inspection and successful system testing were the only testing criteria in use. An experimental method of ultrasonic testing was used on Thresher. Still, the testing was not well understood, any failed components did not have rework required if they passed the visual inspection and system testing criteria.
When a casualty occurs, such as flooding, or other system failures, a submarine needs the ability to reach the surface. Unfortunately, Thresher did not have a reliable emergency de-ballasting system in place. As the first in her class, the high-pressure air system had not been tested at full blow conditions from test depth. Follow-on testing on active submarines showed that ice would form in the air piping when the ballast tank was fully blown, preventing airflow to the ballast tanks, and thus preventing the means to emergency surface. Compounding the problem, there was no required air dehumidification to ensure the air was dry and could not ice over. Additionally, Thresher did not have an emergency flood-control system to allow for quick and remote isolation of all sea connected systems during a flooding casualty. Not all emergency equipment was readily accessible, and in some cases, required unbolting of deck tiles to gain access.
As a result of the Court of Inquiry findings and recommendations, the Submarine Force implemented extensive changes to ensure the submarines and their crews’ safety. The newly created Submarine Safety (SUBSAFE) program created strict requirements for the design and construction of all future submarines, and the retrofit and maintenance of existing submarines. Seawater connected systems now require welded joints. Emergency blow systems are required and must function at all depths. Hydraulically operated emergency flood-control systems were installed on all hull openings for seawater connected systems. All emergency equipment storage was redesigned for quick access and requires periodic verification that all equipment can not only be freely accessed but in a specific amount of time to ensure access availability in case of a casualty. For the crew, the Submarine Force increased its emphasis on training requirements and operating procedures.
While the SUBSAFE Program specifically addressed prevention of uncontrolled flooding and the submarine’s ability to return to the surface following a flooding casualty, the Navy wanted one final option to rescue a disabled submarine’s crew in the form of a deployable deep-sea submarine rescue capability. Thus, the Navy created the Deep Submergence Systems (DSS) Project to research and develop two deep submergence rescue vehicles, the Mystic (DSRV-1) and the Avalon (DSRV-2). The two technologically advanced rescue vehicles had a control system twice as complex as NASA’s that landed Apollo 11 on the moon in 1969. Operated by the Deep Submergence Unit, renamed Undersea Rescue Command (URC) in 2008, the DSRVs could deploy anywhere in the world by sea, land and air to reach a disabled submarine. In 2008, the DSRVs were retired and replaced with the Pressurized Rescue Module (PRM) Falcon that can reach a submarine up to 2,000 feet deep and rescue 16 people at a time. URC continues their commitment to worldwide submarine rescue through continuous training and active participation in exercises with the international submarine rescue community.
Thresher lies in six major sections on the ocean floor, with the majority in a single debris field about 400 yards square. The major sections are the sail, sonar dome, bow section, engineering spaces, operations spaces, and the tail section.
Since the implementation of the SUBSAFE Program, not one SUBSAFE-certified submarine has been lost. While the 129 crew and shipyard personnel onboard the Thresher remain on eternal patrol, their legacy reverberates throughout the U.S. Submarine Force to this day.
To learn more about the legacy of USS Thresher, visit Naval History and Heritage Command’s notable ship page here