F/A-18 a split second after the red hold-back bar (on far left) has released and the catapult is hurling the aircract down the deck
An aircraft catapult is a device used to launch aircraft from ships — in particular aircraft carriers — as a form of assisted take off. This type of catapult consists of a track built into the flight deck, below which is a large piston or shuttle that attaches up through the track to the nose gear of the aircraft. At launch, a release bar holds the aircraft in place as steam pressure builds up, then breaks (or "releases"; older models used a pin that sheared), freeing the piston to pull the aircraft along the deck at high speed. Within about 4 seconds, aircraft velocity plus wind speed (ship's speed plus head wind) will exceed lift velocity plus 15 knots.
Up to and during World War II most catapults were hydraulic. After the war navies gradually converted to steam catapults, which were the only ones capable of launching the heavier jet fighters. At the beginning of the 21st century, navies started experimenting with catapults powered by linear induction motors and electromagnetics. An important factor driving the conversion to electromagnetic catapults is the steady conversion of warships from steam power to gas turbine power, and the resultant loss of readily-available high-pressure steam.
As of this writing, except in the United States Navy and French Navy, catapults have mostly fallen out of use, as most navies prefer to use the McDonnell Douglas (Boeing) AV-8B Harrier II and its derivatives, which are STOVL aircraft, and can be launched from smaller and cheaper ships than CTOL (Conventional Take Off and Landing) aircraft.
A catapult can be as small as a rubber band slingshot used to skim rocks across a pond or as large as the 90-m (300-ft) long steam units used to launch airplanes from aircraft carriers.
Catapults were used in ancient and medieval warfare until the introduction of the gunpowder cannon in the 14th century.
Hydrauliccatapults used a pressurized fluid to release energy and accelerate an airplane attached to the moving tow bar.
When the catapult is activated, the tow bar pulls the airplane down the catapult track and releases itself at the very end of the deck to allow the airplane to fly off the ship.
Hydrauliccatapults were unreliable and dangerous, and the explosion of a hydrauliccatapult aboard the aircraft carrier USS Bennington in 1954 killed nearly 100 crewmen.
Hydrauliccatapults were also not very adaptable, and as aircraft weights increased they were unable to launch airplanes effectively.
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