A nuclear submarine is a ship powered by atomic energy that travels primarily under-water, but also on the surface of the ocean. Previously, conventional submarines used diesel engines that required air for moving on the surface of the water, and battery-powered electric motors for moving beneath it. The limited lifetime of electric batteries meant that even the most advanced conventional submarine could only remained submerged for a few days at slow speed, and only a few hours at top speed. On the other hand, nuclear submarines can remain under-water for several months. This ability, combined with advanced weapons technology, makes nuclear submarines one of the most useful warships ever built.
Submarines provide unique warfighting capabilities including a stealthy platform with great range, mobility, endurance, payload potential, and survivability. In many hostile environments, the submarine may be the only survivable platform. Future submarines will offer a significant degree of flexibility and reconfigurability, both internally and through the use of off-board vehicles, sensors, and weapons; they also will accommodate rapidly emerging technology to improve current capabilities and to enable new roles and missions.
Advanced battle management systems that enable cooperative engagement with other naval forces will enhance the effectiveness of submarine participation in complex missions including antisubmarine warfare, strike operations, theater and national missile defense, and the deployment of ground forces for specialized warfare. The greater relative survivability (based on stealth, mobility, and endurance) of the submarine and the potential for expanding the range and depth of mission effectiveness suggest a greater role for submarines in the Navy of 2035
One of the technology focus areas is Submarine Architecture, including hull structure, shaping, and materials. It encompasses the use of innovative design, materials selection, and total systems integration to significantly improve submarine performance, payload capacity, and stealth while improving manufacturability and reducing costs. The goals of advances in architecture include greater speed for the same power input by reducing drag, greater stealth through the reduction of acoustic and nonacoustic signatures, and simplified fabrication using creative structural design and advanced materials.
An integrated approach is required because changes to individual architectural components affect hydrodynamic and operational performance. Various geometries and materials have been identified that could provide improvements in hydrodynamic performance and reduced target strength and, in the long term, provide space and surface area for embedded sensors. Improved sail shaping could reduce life-cycle cost by facilitating maintenance.
The main material used in manufacturing a nuclear submarine is steel. Steel is used to make the inner hull that contains the crew and all the inner workings of the submarine, and the outer hull. Between the two hulls are the ballast tanks, which take in water to make the submarine sink and eject water to make the submarine rise.
All small modern submarines and submersibles, as well as the oldest ones, have a single hull. However, for large submarines, the approaches have separated. All Soviet heavy submarines are built with a double hull structure, but American submarines usually are single-hulled. They still have light hull sections in bow and stern, which house main ballast tanks and provide hydrodynamically optimized shape, but the main, usually cylindrical, hull section has only a single plating layer.
A double hull submarine has two major components, the light hull and the pressure hull. The light hull (casing in British usage) of a submarine is the outer non-watertight hull which provides a hydrodynamically efficient shape. The pressure hull is the inner hull of a submarine that maintains structural integrity with the difference between outside and inside pressure at depth.
The double hull approach also saves space inside the pressure hull, as the ring stiffeners and longitudinals can be located between the hulls. These measures help minimise the size of the pressure hull, which is much heavier than the light hull. Also, in case the submarine is damaged, the light hull takes some of the damage and does not compromise the vessel’s integrity, as long as the pressure hull is intact.
Materials Requirements for Hull
The minimum thickness of the pressure hull required for a submarine can be reduced by using material with higher yield strength. A lesser thickness would be advantageous in reducing the weight, but comes at a cost of higher price
Depth is one of the most important and deciding structural design criteria. The pressure hull is the primary structural element of the submarine, and is designed to be able to withstand the external hydrostatic pressure. It is designed for a particular collapse depth, at which complete failure is expected within a very narrow range. The collapse depth is actually calculated by multiplying the maximum operable depth (MOD) or service depth with a factor of safety. The hydrostatic pressure at this depth is considered as the design pressure for all the pressure hull calculations.
A submarine is designed to withstand the loads generated by underwater detonations (for example, mine explosions, pressures generated by bursting of large underwater gas bubbles). Apart from the direct shock load imparted from the explosion, each shockwave from a single underwater explosion causes a wave of vibration to propagate along the pressure hull. Vibratory loads not only reduce the fatigue life, but can cause resonance resulting in major structural failure.
The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve, and is separated with watertight bulkheads into several compartments. The constructions of a pressure hull requires a high degree of precision. This is true irrespective of its size. Even a one inch (25 mm) deviation from cross-sectional roundness results in over 30 percent decrease of hydrostatic load. Minor deviations are resisted by the stiffener rings, and the total pressure force of several million longitudinally-oriented tons must be distributed evenly over the hull by using a hull with circular cross section. This design is the most resistant to compressive stress and without it no material could resist water pressure at submarine depths. A submarine hull requires expensive transversal construction, with stiffener rings located more frequently than the longitudinals. No hull parts may contain defects, and all welded joints are checked several times with different methods.
Russia designing submarine that will use composite materials for the hull and other parts
Russia will start building multi-purpose nuclear-powered submarines of the fifth generation in 2020. Companies of the United Shipbuilding Corporation (USC) are ready to start the work in 2020, when 885 Yasen project is completed. The Russian Husky submarine will be the follow up to the Yasen submarine.
Russia will also incorporate composite structures in its next-generation follow-on to the Project 855M Yasen-class in the 2020s. The next-generation Russian nuclear submarines may use composite structures in an attempt to drastically reduce their acoustical signatures.
The new composite materials are still in testing, but Russia will test its first composite propeller design in 2018. “This is one of our institute’s most promising projects,” Polovinkin said. “This trend reduces vibration in the blades and increases the efficiency of the screw. These various effects will help improve the ship’s acoustic signature.”
In addition to steel, various parts of a nuclear submarine are made from other metals, such as copper, aluminum, and brass. Other materials used to manufacture the thousands of components which make up a fully equipped nuclear submarine include glass and plastic. Electronic equipment includes semiconductors such as silicon and germanium. The nuclear reactor that powers the submarine depends on uranium or some other radioactive element as a source of energy.
Active mounts, which employ piezoelectric materials or other types of actuators to actively cancel mechanical vibration, can greatly attenuate major noise paths from the machinery to the hull. Such mounts can be incorporated into a system of shipwide active noise control techniques that will work together to maximize the effect of this technology at minimal cost.
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