Editing Submarine (section)

====Pressure hull====
[[File:Bathyscaphe Trieste.jpg|thumb|In 1960, [[Jacques Piccard]] and [[Don Walsh]] were the first people to explore the [[Challenger Deep|deepest part]] of the world's [[ocean]], and the deepest location on the surface of the Earth's crust, in the {{ship|Bathyscaphe|Trieste}} designed by [[Auguste Piccard]].]]
{{See also|Pressure hull}}

The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve, and is separated by watertight [[bulkhead (partition)|bulkheads]] into several [[Compartmentalization (fire protection)|compartments]]. There are also examples of more than two hulls in a submarine, like the {{sclass2|Typhoon|submarine|4}}, which has two main pressure hulls and three smaller ones for control room, torpedoes and steering gear, with the missile launch system between the main hulls, all surrounded and supported by the outer light hydrodynamic hull. When submerged the pressure hull provides most of the buoyancy for the whole vessel.

The [[Submarine depth ratings|dive depth]] cannot be increased easily. Simply making the hull thicker increases the structural weight and requires reduction of onboard equipment weight, and increasing the diameter requires a proportional increase in thickness for the same material and architecture, ultimately resulting in a pressure hull that does not have sufficient buoyancy to support its own weight, as in a [[bathyscaphe]]. This is acceptable for civilian research submersibles, but not military submarines, which need to carry a large equipment, crew, and weapons load to fulfill their function. Construction materials with greater [[specific strength]] and [[specific modulus]] are needed.

WWI submarines had hulls of [[carbon steel]], with a {{convert|100|m|ft|adj=on}} maximum depth. During WWII, high-strength [[alloy]]ed steel was introduced, allowing {{convert|200|m|ft|adj=on}} depths. High-strength alloy steel remains the primary material for submarines today, with {{convert|250|-|400|m|ft|adj=on}} depths, which cannot be exceeded on a military submarine without design compromises. To exceed that limit, a few submarines were built with [[titanium]] hulls. Titanium alloys can be stronger than steel, lighter, and most importantly, have higher immersed [[specific strength]] and [[specific modulus]]. Titanium is also not [[ferromagnetism|ferromagnetic]], important for stealth. Titanium submarines were built by the Soviet Union, which developed specialized high-strength alloys. It has produced several types of titanium submarines. Titanium alloys allow a major increase in depth, but other systems must be redesigned to cope, so test depth was limited to {{convert|1000|m|ft}} for the {{ship|Soviet submarine|K-278 Komsomolets}}, the deepest-diving combat submarine. An {{sclass2|Alfa|submarine|2}} may have successfully operated at {{convert|1300|m|ft}},<ref>{{cite web|url=https://fas.org/man/dod-101/sys/ship/deep.htm|title=Federation of American Scientists|publisher=Fas.org|access-date=18 April 2010}}</ref> though continuous operation at such depths would produce excessive stress on many submarine systems. Titanium does not flex as readily as steel, and may become brittle after many dive cycles. Despite its benefits, the high cost of titanium construction led to the abandonment of titanium submarine construction as the Cold War ended. Deep-diving civilian submarines have used thick [[Poly(methyl methacrylate)|acrylic]] pressure hulls. Although the specific strength and specific modulus of acrylic are not very high, the density is only 1.18g/cm<sup>3</sup>, so it is only very slightly denser than water, and the buoyancy penalty of increased thickness is correspondingly low.

The deepest [[deep-submergence vehicle]] (DSV) to date is [[Bathyscaphe Trieste|''Trieste'']]. On 5 October 1959, ''Trieste'' departed San Diego for [[Guam]] aboard the freighter ''Santa Maria'' to participate in ''[[Project Nekton]]'', a series of very deep dives in the [[Mariana Trench]]. On 23 January 1960, ''Trieste'' reached the ocean floor in the Challenger Deep (the deepest southern part of the Mariana Trench), carrying [[Jacques Piccard]] (son of Auguste) and Lieutenant [[Don Walsh]], USN.<ref>{{cite web|url=http://www.history.navy.mil/danfs/t8/trieste.htm |title=Trieste |publisher=History.navy.mil |access-date=18 April 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100317120249/http://www.history.navy.mil/danfs/t8/trieste.htm |archive-date=17 March 2010 }}</ref> This was the first time a vessel, crewed or uncrewed, had reached the deepest point in the Earth's oceans. The onboard systems indicated a depth of {{convert|11521|m|ft|0}}, although this was later revised to {{convert|10916|m|ft|0}} and more accurate measurements made in 1995 have found the Challenger Deep slightly shallower, at {{convert|10911|m|ft|0}}.

Building a pressure hull is difficult, as it must withstand pressures at its required diving depth. When the hull is perfectly round in cross-section, the pressure is evenly distributed, and causes only hull compression. If the shape is not perfect, the hull deflects more in some places and [[buckling]] instability is the usual [[failure mode]]. Inevitable minor deviations are resisted by stiffener rings, but even a one-inch (25&nbsp;mm) deviation from roundness results in over 30 percent decrease of maximal hydrostatic load and consequently dive depth.<ref>{{cite web|url=http://www.usna.edu/naoe/courses/en200/ch10.pdf|title=US Naval Academy}}</ref> The hull must therefore be constructed with high precision. All hull parts must be welded without defects, and all joints are checked multiple times with different methods, contributing to the high cost of modern submarines. (For example, each {{sclass|Virginia|submarine|0}} attack submarine costs US$2.6 [[1000000000 (number)|billion]], over US$200,000 per [[long ton|ton]] of displacement.)