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Aircraft Carrier

An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers are generally the largest ships operated by navies; a Nimitz class carrier powered by two nuclear reactors and four steam turbines is 1092 feet (333 m) long and costs about $4.5 billion. Aircraft carriers are generally accompanied by a number of other ships, to provide protection for the relatively unwieldy carrier, to carry supplies, and to provide additional offensive capabilities. This is often termed a battle group or carrier group, sometimes a carrier battle group.
Aircraft carriers thus allow a naval force to project air power great distances without having to depend on local bases for staging aircraft operations. They have evolved from wooden vessels used to deploy a balloon into nuclear powered warships that carry dozens of fixed and rotary wing aircraft.
World War II saw the first large scale use of aircraft carriers. Escort aircraft carriers built during World War II were converted from merchant ships. Light aircraft carriers represented a larger, more "militarized" version of the escort carrier concept and they had the advantage of higher speed as they had been converted from cruisers under construction rather than civilian merchant ships. Wartime emergencies also saw the creation or conversion of other, unconventional aircraft carriers. Catapult Aircraft Merchantman ships were cargo carrying merchant ships which could launch but not retrieve fighter aircraft from a catapult. These vessels were an emergency measure during World War II. Battle carriers were created by the Imperial Japanese Navy to partially compensate for the loss of carrier strength at Midway. The aft turrets were removed and replaced with a hangar, deck and catapult. Submarine aircraft carriers, were first built in the 1920s, but were generally unsuccessful at war. Modern navies operate aircraft carriers as the capital ship of the fleet, a role previously played by the battleship. The change, part of the growth of air power as a significant part of warfare, took place during World War II. This change was driven by the superior range, flexibility and effectiveness of carrier-launched aircraft. Following the war, the scope of carrier operations continued to increase in size and importance. Supercarriers, typically displacing 75,000 tonnes or greater, have become the pinnacle of carrier development. Most are powered by nuclear reactors and form the core of a fleet designed to operate far from home. Lacking the firepower of other warships, carriers by themselves are considered vulnerable to attack by other ships, aircraft, submarines or missiles and therefore travel as part of a carrier battle group for their protection.
During the Second World War, aircraft would land on the flight deck parallel to the longitudinal axis of the ship's hull. Aircraft which had already landed would be parked on the deck at the bow end of the flight deck. A crash barrier was raised behind them to stop any landing aircraft which overshot the landing area whenever its landing hook missed the arrestor cables. An important development of the early 1950s was the British invention of the angled flight deck by Capt D.R.F. Campbell RN in conjunction with Lewis Boddington, where the runway was canted at an angle of a few degrees from the longitudinal axis of the ship. If an aircraft misses the arrestor cables the pilot only needs to increase engine power to maximum to get airborne again and will not hit the parked aircraft because the angled deck points out over the sea.
The modern steam-powered catapult, powered by steam from the ship's boilers or reactors, was invented by Commander C.C. Mitchell of the British RNVR. The first of the Optical Landing Systems was another British innovation, the Mirror Landing Aid invented by Lieutenant Commander H. C. N. Goodhart RN. This was a gyroscopically-controlled convex mirror (in later designs replaced by a Fresnel lens Optical Landing System) on the port side of the deck. On either side of the mirror was a line of green "datum" lights. A bright orange "source" light was shone into the mirror creating the "ball" which could be seen by the aviator who was about to land. The position of the ball compared to the datum lights indicated the aircraft's position in relation to the desired glidepath: if the ball was above the datum, the plane was high; below the datum, the plane was low; between the datum, the plane was on glidepath. The gyro stabilisation compensated for much of the movement of the flight deck due to the sea, giving a constant glidepath.

In the late 1950s and early 1960s, the UK and the U.S. converted some of their older carriers into Landing Platform Helicopter (LPHs) seagoing helicopter airfields which were used to mitigate against the expensive connotations of the term "aircraft carrier". The arrival of the Sea Harrier fast jet meant they could carry fixed-wing aircraft, despite their short flight deck.
British innovation of the ski-jump ramp as an alternative to contemporary catapult systems. The ski-jump gave the Harriers a STOVL capability, allowing them to take off with heavier payloads. Many carriers equipped with ski-jump ramps do not have an angled landing deck, but this is more a result of the vertical landing capability of the STOVL aircraft embarked.

The US Navy built the first aircraft carrier to be powered by nuclear reactors in the beginning of the nuclear age. The USS Enterprise was powered by eight nuclear reactors and was the second surface warship to be powered in this way. Subsequent supercarriers starting with USS Nimitz took advantage of this technology to increase their endurance utilizing only two reactors. The only other nation to have followed the US lead is France with Charles de Gaulle although nuclear power is used for submarine propulsion by France, Great Britain, China and the former Soviet Union.
In the early 21st century, worldwide aircraft carriers are capable of carrying about 1250 aircraft US carriers account for over 1000 of these. The United Kingdom and France are both undergoing a major expansion in carrier capability, but the United States will still maintain a very large lead. The United States Navy has the world's largest carrier fleet with eleven in service, one under construction, and one on order (all of them supercarriers). It is also the only navy to possess operational supercarriers.
As "runways at sea," modern aircraft carriers have a flat-top deck design that serves as a flight deck for take-off and landing of aircraft. Aircraft take off to the front, into the wind, and land from the rear.A steam-powered catapult is used to propel the aircraft forward, assisting the power of its engines and allowing it to take off in a shorter distance than would otherwise be required. On other carriers, aircraft do not require assistance for take off — the requirement for assistance relates to aircraft design and performance. Conversely, when landing on a carrier, conventional aircraft rely upon a tailhook that catches on arrestor wires stretched across the deck to bring them to a stop in a shorter distance than normal. Other aircraft — helicopters and V/STOL (Vertical/Short Take-Off and Landing) designs — utilize their hover capability to land vertically and so require no assistance in speed reduction upon landing. Conventional ("tailhook") aircraft rely upon a landing signal officer (LSO, sometimes called "paddles") to control the plane's landing approach, visually gauging altitude, attitude, and speed, and transmitting that data to the pilot. Before the angled deck emerged in the 1950s, LSOs used colored paddles to signal corrections to the pilot. From the late 1950s onward, visual landing aids such as mirrors provided information on proper glide slope, but LSOs still transmit voice calls to landing pilots by radio.

The flight deck of an aircraft carrier is one of the world's most dangerous places to work. To facilitate working on the flight deck. aircraft carrier, the sailors wear colored shirts that designate their responsibilities. White shirts are responsible for safety, red shirts handle munitions, purple shirts (grapes) handle jet fuel, green shirts handle the catapult and arresting gear, and so on. Yellow shirts are responsible for directing aircraft.
Several nations which currently possess aircraft carriers are in the process of planning new classes to replace current ones. The world's navies still generally see the aircraft carrier as the main future capital ship, with developments such as the arsenal ship, which have been promoted as an alternative, seen as too limited in terms of flexibility.