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The last submarines of the THRESHER (SSN-593)/ PERMIT (SSN-594) class are in the process of being decommissioned. These were the U.S. Navy’s first deep-diving, quiet, multi-purpose submarines of the nuclear era. The previous U.S. SSNs- the pioneer NAUTILUS (SSN-571} and SEAWOLF (SSN-575), the four SKATEs (SSN-578}, and the six SKIPJACKs (SSN-585) – did not exceed conventional subma-rines in most operational parameters; rather, it was in sub-merged endurance that these undersea craft excelled.

With THRESHER, authorized in the fiScal year 1957 budget, the Navy took revolutionary steps beyond the submarine’s propulsion system. The principal changes were in three key areas: (1) depth, (2) quieting, and (3) combat systems. The basic THRESHER design, project No. 188, was an elongated version of the SKIPJACK, which had combined nuclear propul-sion with the high-speed ALBACORE (AGSS-569} hull concept The THRESHER had a greatly enlarged amidships section to accommodate the quieting of the propulsion plant and changes in machinery systems.

The THRESHER was probably the deepest-diving submarine built to that time by any navy. HY-80 steel had been used in the previous SKIPJACKs, but that class was rated at an operat-ing depth of 750 feet, the same as other U.S. post-World War.

The THRESHER employed HY-80 and ad-vanced welding techniques to provide an operating depth of some 1,300 feet. Beyond the operating advantages of greater depth (e.g., going below sound layers), greater depth increased the margin for control error or malfunction during high-speed maneuvers. Thus, the THRESHER and her sister ships would be capable of almost twice the operating depth of previous U.S. submarines — nuclear and diesel-electric.

The increase in depth became one of the most controversial aspects of the THRESHER design. While the lead ship was still under construction, the following exchange occurred between Admiral H.G. Rickover, head of Navy’s nuclear-propulsion program, and Representative George Mahon, a member of the House Appropriations Committee:

Mahon. Are you over designing these ships? I am talking now mostly about submarines. Are you putting on refinements that are really not necessary? You spoke of the THRESHER diving to a very great depth. Rickover. Yes, sir.

Mahon.  How deep are you going?

Rickover. The World War ll submarines were designed for [400] feet. Right after World War ll we developed the present [750]-foot submarines. Now we are going to [classified) feel The reason is that the deeper a ship goes, the less it is possible to detect it. It can take advantage of various thermal layers in the ocean. It also is less susceptible to damage by various types of depth charges and other anti-submarine devices. The greater depth gives it greater invisibility, greater invulnerability. We would like to go deeper if we could, but a point comes where existing hull steel may not be able safely to withstand the greater pressure…. However, there is considerable military advantage, Mr. Mahon, to be able to go deeper; it is somewhat analogous to having airplanes which can fly higher.

Three years later, after the THRESHER was lost at sea, the issue of going deep again came up in Congress, before the Joint Committee on Atomic Energy. At the 1964 hearing, Vice Admiral Lawson P. Ramage, a much-decorated submariner and at the time a Deputy Chief of Naval Operations, explained the advantage of going deeper, including added safety in certain maneuvering situations. Rickover responded to the committee:

Sure, you can intuitively say — as Admiral Ramage said in the comparison he made – you would like to go deeper. It is good to have a machine that can perform better. However, I claim we have to be realistic and find out how important this is first, because right now we are incurring considerable expenses in building these ships….

The THRESHER design also advanced machinery quieting to new levels, reducing the submarine’s narrow-band acoustic signature. Previously little effort had been made to reduce machinery noises in U.S. nuclear submarines, with the noise of coolant pumps, fluids in the coolant piping, and turbine noises. THRESHER introduced turbines and related gearing mounted on a sound-isolating raft to reduce self-generated noises. The concept was successful, but increased the volume required for machinery and hence the overall submarine size, contributing to a two-foot increase in hull diameter over the SKIPJACKs.

Both classes had the same SSW reactor/twin turbine plant producing some 15,000 shaft horsepower. The larger size of ‘niRESHER, however, resulted in a speed loss of several knots over the SKIPJACKs. One of the later submarines of this design, JACK (SSN-605), was provided a modified propulsion system with a direct-drive (ungeared) turbine. She had a large-diameter, sleeve-like outer propeller shaft housing a smaller-diameter inner shaft, providing an arrangement of counter-rotating propellers on essentially a single shaft. The scheme was used to increase efficiency and reduce turbulence. There was no increase in speed and the machinecy spaces were ten feet longer than in other units of the class. The concept was not repeated in other submarines and, reportedly, JACK suffered problems with her turbine arrangement, with the unique arrangement being eventually replaced by a conven-tional, single-shaft arrangement.

THRESHER also introduced major improvements in submarine combat systems. She combined the features of a high-speed attack submarine (SKIPJACK) with those of a hunter-killer submarine (SSKISSKN). For the latter role THRESHER was fitted with the AN/BQQ-2 sonar system that had a 15-foot diameter bow sphere mounting the active/passive BQS-series sonars. The BQR-7 conformal array of passive hydrophones was also carried within the large forward dome.

The sonar sphere forced the location of the four 21-incb torpedo tubes amidships, angled out two to each side. Signifi-cantly, previous U.S. combat submarines dating to 1941 had at least six bow tubes, the exceptions being specialized hunter-killer (SSK), guided missile (SSG/SSGN), and radar picket (SSRN) submarines. It was believed that THRESHER’s quieting and sonar effectiveness could permit the detection of Soviet undersea craft long before they could make a detection; this, coupled with U.S. torpedo effectiveness, reduced the number of launch tubes required.

No stem tubes were provided in the SKIPJACK, THRESHER, and later submarines because of the tapering stem configuration and the guidance/homing capabilities of modem torpedoes.

Beyond torpedoes, the THRESHER design was the first to provide for the SUBROC anti-submarine weapon. With the long-range detection capabilities of the BQQ-2 exceeding existing torpedoes, and the proliferation of tactical nuclear weapons in the 1950s, the torpedo-tube launched SUBROC could fling a nuclear depth bomb (W55 warhead) to a distance of some 20 nautical miles.

The amidships, angled torpedo tube configuration, which originated with German U-boat designs of World War II, have followed in all subsequent U.S. nuclear attack submarine designs.

THRESHER was commissioned on 3 August 1961. Speaking at her commissioning, Vice Admiral Harold T. Deutermann, declared that in the THRESHER “We see a weapons system so advanced in concept and design that no other submarine in the world today can equal her range and fire power for anti-submarine weapons. We see the inclusion of a sonar system so sensitive and so powerful that the ocean around her for greater distances than ever before become her territory.”

She was rapidly followed by four similar submarines that had been ordered as guided miSsile submarines (SSGN) to carry the Regulus missile. The Navy’s long-range plan for the 1970s had called for 12 of these SSGNs – the earlier HALmUT {SSGN·and 11 of the PERMIT (SSGN-594) class. The first four SSGNs — project No. 166A — were reordered as attack subma-rines following cancellation of the Regulus II land.attack cruise missile in favor of the Polaris missile project in December 1958. As SSGNs these boats were to each carry four of the surface launched, supersonic missiles. As attack submarines they became the PERMIT (SSN-594), PLUNGER (SSN-595), BARB {SSN·596), and DACE (SSN-607). Subsequently, through fiscal year 1961 the Congress authorized nine additional THRESHER-class submarines for a class total of 14 units. Production of the design was curtailed in favor of the similar but larger STURGEON (SSN-637) class; the latter submarines had still quieter machinery, were provided with an under-ice capability, and their larger sail structures permitted the fitting of additional electronic suiVeillance equipment, a shortcoming in the THRESHERs.

THRESHER immediately established new records for operating depths as her crew, under Commander Dean Axene, put the submarine through her paces. There was a major problem in November 1961, when TIIRESHER moored at San Juan and the submarine suffered a diesel breakdown and was unable to restart the reactor until CAVALIA (SS-244), moored alongside, was able to provide electricity for a reactor restart.

From July 1962 until9 April 1963, THRESHER underwent a major overhaul at the Portsmouth Naval Shipyard. When she returned to sea on trials, under Lieutenant Commander John W. HaiVey, she carried a crew of 108, a SubLant staff officer, 3 officers and 13 civilians from the shipyard, a specialist from the Naval Ordnance Laboratory, and 3 civilians representing contractors – a total of 129.

On the morning of 10 April, while conducting trials to her maximum operating depth, THRESHER was lost with all hands. Apparently a reactor scram and piping failure doomed the submarine. In the aftermath of her loss, the Navy revised reactor restart procedures. Modifications were also made to all U.S. nuclear submarines to increase their emergency high-pressure air blow capabilities, and in some cases additional ballast tanks were added. Known as the SUBSAFE procedures, these modifications delayed the completion of the later subma-rines of this. class, which was officially renamed the PERMIT class after the THRESHER loss.

The remaining submarines of the class were completed through 1967. The 13 submarines of the PERMIT class have provided valuable service to the fleet during the ensuing years. DACE, commissioned in 1964, was the first submarine of the class to stand down, being formally decommissioned on 2 December 1988. Others have followed; by mid-1992 only two remained in commission, GREENLING (SSN-614) and GATO (SSN-615). They too will be gone shortly, marking the end of an important milestone design in the history of submarine development.

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