Eager to incorporate the lessons learned from the first generation of nuclear submarines, the US Navy began to plan a completely new class of fast attack boat in 1957. The new design placed an emphasis on stealth and underwater performance.
S5W Nuclear Reactor and Silencing Techniques
Instead of designing an entirely new power plant, the existing S5W nuclear reactor from the Skipjack class was utilized. The focus instead was placed on methods to negate the excessive noise generated by the plant in the Skipjacks. In a radical departure from previous designs, engineering plant components were mounted on insulated rafts which limited sound transmission to the hull.
To accommodate the rafting, the efficient teardrop hull shape of the Skipjacks was sacrificed. The new hull had to be both widened and lengthened. While this decreased the hydrodynamic efficiency of the boat, external streamlining, which included a reducing the sail profile by over 25%, regained much of the speed that was lost.
Bow Mounted Sonar and HY-80 Steel
In the bow the traditional nest of forward mounted torpedo tubes were replaced with a large sphere of hydrophones. This directional sonar sphere, covered by a rounded dome, maximized sonar capability. The torpedo tubes were moved to a compartment behind the sphere and angled outboard ten degrees to fire through the sides of the hull.
For construction, HY-80 steel became the hull material of choice. While used sparingly in previous designs, the entire Permit pressure hull was constructed with this high tensile steel. Its ability to withstand 80,000 pounds of pressure per square inch radically increased operational depth.
USS Thresher & SUBSAFE
The lead ship of the class, USS Thresher (SSN-593), entered service on August 3, 1961. Her initial trials revealed a vessel whose performance exceeded design expectations. She was the quietest, deepest diving submarine yet built for the US Navy.
Yet expanded abilities brought new challenges. Thresher was lost during a post-overhaul test dive on April 10, 1963. Studies revealed flaws in internal piping design, quality standards, and emergency response procedures. SUBSAFE, a submarine safety program created as a result of the disaster, changed the way all subsequent units were designed and operated. Since its inception, no SUBSAFE compliant boat has been lost.
With the loss of Thresher, the class took the name of the second boat produced, USS Permit (SSN-594). She and her sisters brought a tactical superiority to submarine operations the US Navy would exploit throughout the remainder of the Cold War. And their hydrodynamically shaped hull, bow mounted sonar array, and advanced silencing features would remain the design benchmark for all successive American attack submarine classes.
Reference:Friedman, Norman. U.S. Submarines Since 1945. Annapolis: U.S. Naval Institute Press, 1994.
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