In the beginning, submarines were introduced with the sole purpose of fighting wars. Submarines were extensively used in World War I and II. Military usage range from protecting aircraft carriers, for blockage running for patrolling a country's boundaries and many more. Nowadays, the use of submarine has widely increased including marine science, salvage, exploration and facility inspection or maintenance. Submarines can also be used as search and rescue, undersea cable repair and even for tourism and academic research.
Before and during World War II, the primary role of the submarine was anti-surface ship warfare. Submarines would attack either on the surface or submerged, using torpedoes or (on the surface) deck guns. They were particularly effective in sinking Allied transatlantic shipping in both World Wars, and in disrupting Japanese supply routes and naval operations in the Pacific in World War II.
Mine-laying submarines were developed in the early part of the 20th century. The facility was used in both World Wars. Submarines were also used for inserting and removing covert agents and military forces, for intelligence gathering, and to rescue aircrew during air attacks on islands, where the airmen would be told of safe places to crash-land so the submarines could rescue them. Submarines could carry cargo through hostile waters or act as supply vessels for other submarines.
Retractable 7.5 cm submarine gun produced by the Krupp company circa 1900
Submarines could usually locate and attack other submarines only on the surface, although HMS Venturer managed to sink U-864 with a four torpedo spread while both were submerged. The British developed a specialized anti-submarine submarine in WWI, the R class. After WWII, with the development of the homing torpedo, better sonar systems, and nuclear propulsion, submarines also became able to hunt each other effectively.
The development of submarine-launched nuclear missiles and submarine-launched cruise missiles gave submarines a substantial and long-ranged ability to attack both land and sea targets with a variety of weapons ranging from cluster bombs to nuclear weapons.
The primary defense of a submarine lies in its ability to remain concealed in the depths of the ocean. Early submarines could be detected by the sound they made. Water is an excellent conductor of sound (much better than air), and submarines can detect and track comparatively noisy surface ships from long distances. Modern submarines are built with an emphasis on stealth. Advanced propeller designs, extensive sound-reducing insulation, and special machinery allow a submarine to be as quiet as ambient ocean noise, making them difficult to detect. It takes specialized technology to find and attack modern submarines.
A model of Günther Prien's U-47, German WWII Type VII diesel-electric hunter
Active sonar uses the reflection of sound emitted from the search equipment to detect submarines. It has been used since WWII by surface ships, submarines and aircraft (via dropped buoys and helicopter "dipping" arrays), but it gives away the position of the emitter and is susceptible to counter-measures.
A concealed military submarine is a real threat, and because of its stealth, can force an enemy navy to waste resources searching large areas of ocean and protecting ships against attack. This advantage was vividly demonstrated in the 1982 Falklands War when the British nuclear-powered submarine HMS Conqueror sank the Argentine cruiser General Belgrano. After the sinking the Argentine Navy recognised that they had no effective defense against submarine attack, and the Argentine surface fleet withdrew to port for the remainder of the war, though an Argentine submarine remained at sea.
[edit] Civil uses
Submarine getting ready to take tourists for a dive in Mauritius
Although the majority of the world's submarines are military ones, there are some civil submarines. They have a variety of uses, including tourism, exploration, oil and gas platform inspections and pipeline surveys. The first tourist submarine was launched in 1985, and by 1997 there were 45 of them operating around the world.[1]
A semi-civilian use was the adaptation of U-boats for cargo transport during World War I and World War II.
Like all surface ships, submarines are in a positively buoyant condition using Archimedes Principle of buoyancy which weighing less than the volume of the water displace if it fully submerges. In order to submerge hydrostatically, a ship must have negative buoyancy either by increasing the weight of the ship or decreasing its displacement of water. To control this, ballast tanks is used which can be filled with outside water or pressurized air.
Submarines use Main Ballast Tanks (MBTs) that use the forward and aft tanks which are filled with water to submerge, or filled with air to surface. Under submerged conditions, MBTs generally remain flooded, which simplifies their design, and on many submarines these tanks are a section of interhull space. Submarines use smaller Depth Control Tanks or DCTs, also called hard tanks due to their ability to withstand higher pressure for more precise and quick control of depth. Depth control tanks can be located either near the submarine's center of gravity or separated along the submarine body to prevent affecting trim.
The water pressure on submarine's hull can reach 4 MPa (580 psi) for steel submarines and up to 10 MPa (1,500 psi) for titanium submarines during submerged while interior pressure remains relatively unchanged.This difference results in hull compression, which decreases displacement. As the pressure are higher water density also increases with depth but this incompletely compensates for hull compression, so buoyancy decreases as depth increases. A submerged submarine is in an unstable equilibrium, having a tendency to either fall or float to the surface. Keeping a constant depth requires continual operation of either the depth control tanks or control surfaces
Submarines in a neutral buoyancy condition are not intrinsically trim-stable. To maintain desired trim, submarines use forward and aft trim tanks. Pumps can move water between these, changing weight distribution, creating a moment pointing the sub up or down. A similar system is sometimes used to maintain stability.
The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several surfaces, which can be moved to create hydrodynamic forces when a submarine moves at sufficient speed. The stern planes, located near the propeller and normally horizontal, serve the same purpose as the trim tanks, controlling the trim, and are commonly used, while other control surfaces may not be present on many submarines. The fairwater planes on the sail and/or bow planes on the main body, both also horizontal, are closer to the centre of gravity, and are used to control depth with less effect on the trim.[5]
When a submarine performs an emergency surfacing, all depth and trim methods are used simultaneously, together with propelling the boat upwards. Such surfacing is very quick, so the sub may even partially jump out of the water, potentially damaging submarine systems.
Submarine hull
Overview
Modern submarines are cigar-shaped. It reduces the hydrodynamic drag when submerged, but decreases the sea-keeping capabilities and increases drag while surfaced. Since the limitations of the propulsion systems of early submarines forced them to operate surfaced most of the time, their hull designs were a compromise. Because of the slow submerged speeds of those subs, usually well below 10 knot (18 km/h), the increased drag for underwater travel was acceptable. Late in World War II, when technology allowed faster and longer submerged operation and increased aircraft surveillance forced submarines to stay submerged, hull designs became teardrop shaped again to reduce drag and noise. On modern military submarines the outer hull is covered with a layer of sound-absorbing rubber to reduce detection.
The occupied pressure hulls of deep diving submarines are spherical instead of cylindrical. This allows a more even distribution of stress at the great depth. A titanium frame is usually affixed to the pressure hull, providing attachment for ballast and trim systems, scientific instrumentation, battery packs, syntactic flotation foam, and lighting.
A raised tower on top of a submarine accommodates the periscope and electronics masts, which can include radio, radar, electronic warfare, and other systems including the snorkel mast. In many early classes of submarines, the control room was located inside this tower.
[edit] Single/double hull
U-995, Type VIIC/41 U-Boat of WWII, showing the typical combination of ship-like non-watertight outer hull with bulky strong hull below
Modern submarines and submersibles, as well as the oldest ones, usually have a single hull. Large submarines generally have an additional hull or hull sections outside. This external hull, which actually forms the shape of submarine, is called the outer hull (casing in the Royal Navy) or light hull, as it does not have to withstand a pressure difference. Inside the outer hull there is a strong hull, or pressure hull, which withstands sea pressure and has normal atmospheric pressure inside.
As early as World War I, it was realized that the optimal shape for withstanding pressure conflicted with the optimal shape for seakeeping and minimal drag, and construction difficulties further complicated the problem. This was solved either by a compromise shape, or by using two hulls; internal for holding pressure, and external for optimal shape. Until the end of World War II, most submarines had an additional partial cover on the top, bow and stern, built of thinner metal, which was flooded when submerged. Germany went further with the Type XXI, the general predecessor of modern submarines, in which the pressure hull was fully enclosed inside the light hull, but optimized for submerged navigation, unlike earlier designs that were optimized for surface operation.
Type XXI U-Boat, late WWII, with pressure hull almost fully enclosed inside the light hull
After World War II, approaches split. The Soviet Union changed its designs, basing them on German developments. All post-World War II heavy Soviet and Russian submarines are built with a double hull structure. American and most other Western submarines switched to a primarily single-hull approach. They still have light hull sections in the bow and stern, which house main ballast tanks and provide a hydrodynamically optimized shape, but the main cylindrical hull section has only a single plating layer. The double hulls are being considered for future submarines in the United States to improve payload capacity, stealth and range.[6]
[edit] Pressure hull
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. There are also examples of more than two hulls in a submarine, like the Typhoon class, 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.
The dive depth cannot be increased easily. Simply making the hull thicker increases the weight and requires reduction of onboard equipment weight, ultimately resulting in a bathyscaphe. This is acceptable for civilian research submersibles, but not military submarines.
World War I submarines had hulls of carbon steel, with a 100-metre (330 ft) maximum depth. During World War II, high-strength alloyed steel was introduced, allowing 200-metre (660 ft) depths. High-strength alloy steel remains the primary material for submarines today, with 250-400-metre (820-1,300 ft) 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 can be stronger than steel, lighter, and is not 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 need to be redesigned to cope, so test depth was limited to 1,000 metres (3,300 ft) for the Soviet submarine Komsomolets, the deepest-diving combat submarine. An Alfa-class submarine may have successfully operated at 1,300 metres (4,300 ft),[7] 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 during 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 acrylic pressure hulls.
The deepest Deep Submergence Vehicle (DSV) to date is Trieste. On October 5, 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 January 23, 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.[8] This was the first time a vessel, manned or unmanned, had reached the deepest point in the Earth's oceans. The onboard systems indicated a depth of 11,521 metres (37,799 ft), although this was later revised to 10,916 metres (35,814 ft) and more accurate measurements made in 1995 have found the Challenger Deep to be slightly shallower, at 10,911 metres (35,797 ft).
The task of building a pressure hull is very difficult, as it must withstand pressures up to that of 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 is bent, with several points heavily strained. Inevitable minor deviations are resisted by stiffener rings, but even a one inch (25 mm) deviation from roundness results in over 30 percent decrease of maximal hydrostatic load and consequently dive depth.[9] 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 Virginia-class attack submarine costs US$2.6 billion, over US$200,000 per ton of displacement.)
[edit] Propulsion
HMCS Windsor, a Victoria-class diesel-electric hunter-killer submarine
Originally, submarines were human propelled. The first mechanically driven submarine was the 1863 French Plongeur, which used compressed air for propulsion. Anaerobic propulsion was first employed by the Spanish Ictineo II in 1864, which used a solution of zinc, manganese dioxide, and potassium chlorate to generate sufficient heat to power a steam engine, while also providing oxygen for the crew. A similar system was not employed again until 1940 when the German Navy tested a hydrogen peroxide-based system, the Walter turbine, on the experimental V-80 submarine and later on the naval U-791 and type XVII submarines.[10]
Until the advent of nuclear marine propulsion, most 20th century submarines used batteries for running underwater and gasoline (petrol) or diesel engines on the surface, and for battery recharging. Early submarines used gasoline, but this quickly gave way to kerosene (paraffin), then diesel, because of reduced flammability. Diesel-electric became the standard means of propulsion. The diesel or gasoline engine and the electric motor, separated by clutches, were initially on the same shaft driving the propeller. This allowed the engine to drive the electric motor as a generator to recharge the batteries and also propel the submarine. The clutch between the motor and the engine would be disengaged when the submarine dove, so that the motor could drive the propeller. The motor could have multiple armatures on the shaft, which could be electrically coupled in series for slow speed and in parallel for high speed. (These connections were called "group down" and "group up", respectively.)
[edit] Electric transmission
[edit] Diesel-electric
All early submarines used a direct mechanical connection between the engine and propeller. It switching between diesel engines for surface running, and electric motors for submerged propulsion.
In 1929, diesel-electric transmission is created. Instead of driving the propeller directly while running on the surface, the submarine's diesel would drive a generator which could either charge the submarine's batteries or drive the electric motor
This meant that motor speed was independent of the diesel engine's speed, and the diesel could run at an optimum and non-critical speed. This also meant the submarine continued to run using battery power while one or more of the diesel engines could be shut down for maintenance.
The advantages of this arrangement were that a submarine could travel slowly with the engines at full power to recharge the batteries quickly, reducing time on the surface. It was then possible to insulate the noisy diesel engines from the pressure hull, making the submarine quieter. Additionally, diesel-electric transmissions were more compact.
[edit] Air-independent propulsion
Main article: Air-independent propulsion
During the Second World War, German Type XXI submarines were designed to carry hydrogen peroxide for long-term, fast air-independent propulsion, but were ultimately built with very large batteries instead. At the end of the War, the British and Russians experimented with hydrogen peroxide/kerosene (paraffin) engines which could be used surfaced and submerged. The results were not encouraging; although the Russians deployed a class of submarines with this engine type (codenamed Quebec by NATO), they were considered unsuccessful.
German Type 212 submarine with AIP propulsion in dock at HDW/Kiel
Today several navies use air-independent propulsion. Notably Sweden uses Stirling technology on the Gotland-class and Södermanland-class submarines. The Stirling engine is heated by burning diesel fuel with liquid oxygen from cryogenic tanks. A newer development in air-independent propulsion is hydrogen fuel cells, first used on the German Type 212 submarine, with nine 34 kW or two 120 kW cells and soon to be used in the new Spanish S-80 class submarines.[12]
[edit] Nuclear power
Main article: Nuclear submarine
Steam power was resurrected in the 1950s with a nuclear-powered steam turbine driving a generator. By eliminating the need for atmospheric oxygen, the length of time that a modern submarine could remain submerged was limited only by its food stores, as breathing air was recycled and fresh water distilled from seawater. Nuclear-powered submarines have a relatively small battery and diesel engine/generator powerplant for emergency use if the reactors must be shut down.
Nuclear power is now used in all large submarines, but due to the high cost and large size of nuclear reactors, smaller submarines still use diesel-electric propulsion. The ratio of larger to smaller submarines depends on strategic needs. The US Navy, French Navy, and the British Royal Navy operate only nuclear submarines,[13][14] which is explained by the need for distant operations. Other major operators rely on a mix of nuclear submarines for strategic purposes and diesel-electric submarines for defence. Most fleets have no nuclear submarines, due to the limited availability of nuclear power and submarine technology.
Diesel-electric submarines have a stealth advantage over their nuclear counterparts. Nuclear submarines generate noise from coolant pumps and turbo-machinery needed to operate the reactor, even at low power levels.[15] Some nuclear submarines such as the American Ohio class can operate with their reactor coolant pumps secured, making them quieter than electric subs. A conventional submarine operating on batteries is almost completely silent, the only noise coming from the shaft bearings, propeller, and flow noise around the hull, all of which stops when the sub hovers in mid water to listen. Commercial submarines usually rely only on batteries, since they never operate independently of a mother ship.
[edit] Alternative propulsion
Oil-fired steam turbines powered the British K-class submarines, built during the first World War (and later), to give them the surface speed to keep up with the battle fleet. The K-class subs were not very successful, however.
Toward the end of the 20th century, some submarines, such as the British Vanguard class, began to be fitted with pump-jet propulsors instead of propellers. Although these are heavier, more expensive, and less efficient than a propeller, they are significantly quieter, giving an important tactical advantage.
Magnetohydrodynamic drive (MHD) was portrayed as the operating principle behind the titular submarine's nearly silent propulsion system in the film adaptation of The Hunt for Red October. However, in the novel, the Red October did not use MHD. Although experimental surface ships have used this system, speeds have been below expectations.[citation needed] In addition, the drive system can induce bubble formation, compromising stealth, and the low efficiency requires high powered reactors. These factors make it unlikely for military usage.[citation needed]
[edit] Armament
A sequence of photos showing the decommissioned Australian warship HMAS Torrens sinking after being used as a target for a submarine-launched torpedo.
The success of the submarine is inextricably linked to the development of the torpedo, invented by Robert Whitehead in 1866. His invention is essentially the same now as it was 140 years ago. Only with self propelled torpedoes could the submarine make the leap from novelty to a weapon of war. Until the perfection of the guided torpedo, multiple "straight running" torpedoes were required to attack a target. With at most 20 to 25 torpedoes stored onboard, the number of attacks was limited. To increase combat endurance most World War I submarines functioned as submersible gunboats, using their deck guns against unarmed targets, and diving to escape and engage enemy warships. The importance of guns encouraged the development of the unsuccessful Submarine Cruiser such as the French Surcouf and the Royal Navy's X1 and M-class submarines. With the arrival of ASW aircraft, guns became more for defence than attack. A more practical method of increasing combat endurance was the external torpedo tube, loaded only in port.
The forward torpedo tubes in HMS Ocelot
The ability of submarines to approach enemy harbours covertly led to their use as minelayers. Minelaying submarines of World War I and World War II were specially built for that purpose. Modern submarine-laid mines, such as the British Mark 6 Sea Urchin, are designed to be deployed by a submarine's torpedo tubes.
After World War II, both the US and the USSR experimented with submarine launched cruise missiles such as the SSM-N-8 Regulus and P-5 Pyatyorka. Such missiles required the submarine to surface to fire its missiles. They were the forerunners of modern submarine launched cruise missiles, which can be fired from the torpedo tubes of submerged submarines, for example the US BGM-109 Tomahawk and Russian RPK-2 Viyuga. Ballistic missiles can also be fired from a submarine's torpedo tubes, for example missiles such as the anti-submarine SUBROC, and versions of surface to surface anti-ship missiles such as the Exocet and Harpoon, encapsulated for submarine launch. With internal volume as limited as ever and the desire to carry heavier warloads, the idea of the external launch tube was revived, usually for encapsulated missiles, with such tubes being placed between the internal pressure and outer streamlined hulls.
The strategic mission of the SSM-N-8 and the P-5 were taken up by submarine-launched ballistic missile beginning with the US Navy's Polaris missile, and subsequently the Poseidon and Trident missiles.
Germany is working on the short-range IDAS (missile) which is launched from a torpedo tube and can be used against ASW helicopters as well as surface ships and coastal targets.
Sensors
A submarine will have a variety of sensors determined by its missions. Modern military submarines rely almost entirely on a suite of passive and active sonars to find their prey. Active sonar relies on an audible "ping" to generate echoes to reveal objects around the submarine. Active systems are rarely used, as doing so reveals the sub's presence. Passive sonar is a set of sensitive hydrophones set into the hull or trailed in a towed array, generally several hundred feet long. The towed array is the mainstay of NATO submarine detection systems, as it reduces the flow noise heard by operators. Hull mounted sonar is employed to back up the towed array, and in confined waters where a towed array could be fouled by obstacles.
Submarines also carry radar equipment for detection of surface ships and aircraft. Sub captains are more likely to use radar detection gear rather than active radar to detect targets, as radar can be detected far beyond its own return range, revealing the submarine. Periscopes are rarely used, except for position fixes and to verify a contact's identity.
Civilian submarines rely on small active sonar sets and viewing ports to navigate. Sunlight does not penetrate below about 300 feet (91 m) underwater, so high intensity lights are used to illuminate the viewing area.
Navigation
Early submarines had few navigation aids, but modern subs have a variety of navigation systems. Modern military submarines use an inertial guidance system for navigation while submerged. An inertial navigation system (INS) is a navigation aid that uses a computer, motion sensors (accelerometers) and rotation sensors (gyroscopes) to continuously calculate via dead reckoning the position, orientation, and velocity (direction and speed of movement) of a moving object without the need for external references. This system drift error unavoidably builds up over time. To counter this, the Global Positioning System will occasionally be used to obtain an accurate position. The periscope - a retractable tube with prisms allowing a view to the surface is only used occasionally in modern submarines, since the range of visibility is short.
Communication
Military submarines have several systems for communicating with distant command centers or other ships. One is Very low frequency(VLF) radio reder to radio frequencies (RF) in the range of 3 kHz to 30 kHz, which can reach a submarine either on the surface or submerged to a fairly shallow depth, usually less than 250 feet (76 m). Extremely low frequency (ELF) frequencies from 3 to 30 Hz can reach a submarine at much greater depths, but have a very low bandwidth and are generally used to call a submerged sub to a shallower depth where VLF signals can reach. A submarine also has the option of floating a long, buoyant wire to a shallower depth, allowing VLF transmissions to be made by a deeply submerged boat.
By extending a radio mast, a submarine can also use a "burst transmission" technique. A burst transmission takes only a fraction of a second, minimizing a submarine's risk of detection.
To communicate with other submarines, a system known as Gertrude is used. Gertrude is basically a sonar telephone. Voice communication from one submarine is transmitted by low power speakers into the water, where it is detected by passive sonars on the receiving submarine. The range of this system is probably very short, and using it radiates sound into the water, which can be heard by the enemy.
Civilian submarines can use similar less powerful systems to communicate with support ships or other submersibles in the area.
