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Thursday, December 1, 2011

Aircraft Carriers

Aircraft carriers
 
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations. They have evolved from wooden vessels used to deploy balloons into nuclear-powered warships that carry dozens of fixed wing and rotary-wing aircraft. Aircraft carriers are typically treated as the capital ship of a fleet and are extremely expensive to build and important to protect: of the nine nations which possess an aircraft carrier, seven of these navies only possess one ship. There are 20 active aircraft carriers in the world, with the U.S. Navy operating 11 out of the total as of June 2011.
 
Modern significance
 
Today's aircraft carriers are so expensive that many countries risk significant political and economic, as well as military, ramifications if they were ever to lose one during operation. Also, observers have opined that modern anti-ship weapons systems, such as torpedoes and missiles, have made aircraft carriers obsolete as too vulnerable for modern combat. Countries appear, however, willing to take the risks in building and fielding aircraft carriers because of the geo-political and military prestige they give by being able to project power at some distance from their national land boundaries. Furthermore, aircraft carriers facilitate quicker projections of military power into local and regional conflicts.
 
Types
 
By role
 
A carrier is intended to operate with the main fleet and usually provides an offensive capability. These are the largest carriers capable of fast speeds. By comparison escort carriers were developed to provide defence for convoys of ships. They were smaller and slower with lower numbers of aircraft carried. Most were built from mercantile hulls or, in the case of merchant aircraft carriers, were bulk cargo ships with a flight deck added on top. Light aircraft carriers are carriers that were fast enough to operate with the fleet but of smaller size with reduced aircraft capacity.
  • Anti-submarine warfare carrier
  • Helicopter carrier
  • Light aircraft carrier
  • Amphibious assault ship
  • Seaplane tender & seaplane carriers
  • Balloon carrier & balloon tenders
By configuration
 
There are three main configurations of aircraft carrier in service in the world's navies, divided by the way in which aircraft take off and land:
  • Catapult Assisted Take-Off But Arrested Recovery (CATOBAR) - these carriers generally carry the largest, heaviest, and most heavily armed aircraft, although smaller CATOBAR carriers may have other limitations (weight capacity of aircraft elevator, etc.). Three nations currently operate carriers of this type: the United States, France, and Brazil for a total of thirteen in service and potentially two more when the Royal Navy's Queen Elizabeth class is complete.
  • Short Take-Off But Arrested Recovery (STOBAR) - these carriers are generally limited to carrying lighter fixed-wing aircraft with more limited payloads. STOBAR carrier airwings, such as the Sukhoi Su-33 and future Mikoyan MiG-29K wings of the Admiral Kuznetsov are often geared primarily towards the air superiority and fleet defense roles rather than strike/power projection tasks which require heavier payloads (bombs, air-to-ground missiles). Currently, only Russia possesses an operational carrier of this type, with India and China each preparing a similar carrier.
  • Short Take-Off Vertical Landing (STOVL) - limited to carrying STOVL aircraft. STOVL aircraft, such as the Harrier Jump Jet family and Yakovlev Yak-38 generally have very limited payloads, lower performance, and high fuel consumption when compared with conventional fixed wing aircraft; however, newer STOVL aircraft such as the F-35 have much improved performance. This type of aircraft carrier is operated by India, Spain, and Italy with five in active service; the UK and Thailand each have one active carrier but without any operational STOVL aircraft in inventory.
By size
  • Super carrier
  • Fleet carrier, the standard sized carrier of a navy.
  • Light aircraft carrier
  • Escort carrier 
More information
 
At its most basic level, an aircraft carrier is simply a ship outfitted with a flight deck - a runway area for launching and landing airplanes. This concept dates back almost as far as airplanes themselves. Within 10 years of the Wright Brothers' historic 1903 flight, the United States, the United Kingdom and Germany were launching test flights from platforms attached to cruisers. The experiments proved largely successful, and the various naval forces started adapting existing warships for this purpose. The new carriers allowed military forces to transport short-range aircraft all over the world.
 
(Below right): The USS George Washington, one of the U.S. Navy's nuclear-powered super aircraft carriers
 
Carriers didn't play a huge role in World War I, but they were central to the air combat of World War II. For example, the Japanese launched the 1941 attack on Pearl Harbour from aircraft carriers. Today, super carriers are a crucial part of almost all major U.S. military operations. While the ship itself isn't especially useful as a weapon, the air power it transports can make the difference between victory and defeat.
 
One of the major obstacles of using air power in war is getting the planes to their destination. To maintain an air base in a foreign region, a nation has to make special arrangements with a host country, and then has to abide by that country's rules, which may change over time. Needless to say, this can be extremely difficult in some parts of the world.
 
Under international laws, aircraft carriers and other warships are recognized as sovereign territories in almost all of the ocean. As long as a ship doesn't get too close to any nation's coast, the crew can carry on just like they're back home. So, any nation can freely move a carrier battle group (an assembly of an aircraft carrier and six to eight other warships) all over the globe. Bombers, fighters and other aircraft can fly a variety of missions into enemy territory, and then return to the relatively safe home base of the carrier group. In most cases, the Navy can continually resupply the carrier group, allowing it to maintain its position indefinitely.
 
Carriers can move in excess of 35 knots (40 mph, 64 kph), which gives them the ability to get anywhere in the ocean in a few weeks. The United States currently has six carrier groups stationed around the world, ready to move into action at a moment's notice.
 
Parts of an Aircraft Carrier
 
With about a billion individual pieces, the U.S. Nimitz-class super carriers are among the most complex machines on earth. But on a conceptual level, they're pretty simple. They're designed to do four basic jobs:
  • Transport a variety of aircraft overseas
  • Launch and land airplanes
  • Serve as a mobile command centre for military operations
  • House all the people who do these things.
To accomplish these tasks, a carrier needs to combine elements of a ship, an air force base, and a small city. Among other things, it needs:
  • A flight deck, a flat surface on the top of the ship where aircraft can take off and land
  • A hangar deck, an area below deck to stow aircraft when not in use
  • An island, a building on top of the flight deck where officers can direct flight and ship operations
  • Room for the crew to live and work
  • A power plant and propulsion system to move the boat from point to point and to generate electricity for the entire ship
  • Various other systems to provide food and fresh water and to handle things that any city has to deal with, like sewage, trash and mail, as well as carrier-based radio and television stations and newspapers
  • The hull, the main body of the ship, which floats in water.
The hull of the ship is made up of extremely strong steel plates, measuring several inches thick. This heavy body is highly effective protection against fire and battle damage. The ship's structural support largely comes from three horizontal structures extending across the entire hull: the keel (the iron backbone on the bottom of the ship), the flight deck and the hangar deck. (Right): A diagram showing the various parts of an aircraft carrier.
 
The hull portion below the water line is rounded and relatively narrow, while the section above water flares out to form the wide flight-deck space. The lower section of the ship has a double bottom, which is pretty much what it sounds like -- there are two layers of steel plating: the bottom plating of the ship and another layer above it, separated by a gap. The double bottom provides extra protection from torpedoes or accidents at sea. If the enemy hits the bottom of the ship, smashing a hole in the outer steel layer, the second layer will prevent a massive leak.
 
Building an Aircraft Carrier
 
Since the 1950s, almost all U.S. super carriers have been constructed at Northrop Grumman Newport News in Newport News, Virginia. To make the construction process more efficient, most of each super carrier is assembled in separate modular pieces called superlifts. Each superlift may contain many compartments (rooms), spanning multiple decks, and they can weigh anywhere from 80 to 900 tons (~70 to 800 metric tons). A super carrier is made up of almost 200 separate superlifts.
 
Before placing a superlift module into the ship, the construction crew assembles its steel body and hooks up almost all wiring and plumbing. Then they use a giant bridge crane to lift the module and lower it precisely into its proper position inside the ship; then they weld it to the surrounding modules. Near the end of construction, the crew joins the last module, the 575-ton island, to the flight deck.
 
Just like a motor boat, an aircraft carrier propels itself through the water by spinning propellers. Of course, at about 21 feet (6.4 meters) across, a carrier's four bronze screw propellers are in a very different league than a recreational boat's. They also have a lot more power behind them. Each propeller is mounted to a long shaft, which is connected to a steam turbine powered by a nuclear reactor.
 
The carrier's two nuclear reactors, housed in a heavily-armoured, heavily restricted area in the middle of the ship, generate loads of high-pressure steam to rotate fan blades inside the turbine. The fans turn the turbine shaft, which rotates the screw propellers to push the ship forward, while massive rudders steer the ship. The propulsion system boasts something in excess of 280,000 horsepower. 
The four on-board turbines also generate electricity to power the ship's various electric and electronic systems. This includes an on-board desalination plant that can turn 400,000 gallons (~1,500,000 litres) of saltwater into drinkable freshwater every day -- that's enough for 2,000 homes. 
 
Unlike the old oil-boiler carriers, modern nuclear carriers don't have to refuel regularly. In fact, they can go 15 to 20 years without refuelling. The trade-offs are a more expensive power plant, a longer, more complicated refuelling process (it takes several years) and the added risk of a nuclear disaster at sea. To minimize the risk of such a catastrophe, the reactors inside a super carrier are heavily shielded and closely monitored. The pictures under "Building An Aircraft Carrier" all shows the various stages of building an aircraft carrier.
 
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