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F/A-18 a split second after the red hold-back bar (on far left) has released and the catapult is hurling the aircraft 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. It consists of a track built into the flight deck, below which is a large piston or shuttle that is attached through the track to the nose gear of the aircraft.

Older aircraft did not have a tow bar integrated in the nose gear; instead, a metal framework called a catapult bridle was attached to the aircraft and the catapult shuttle. The ramps at the catapult ends on older carriers were used to catch these frameworks so they could be reused; bridles have not been used on aircraft since the end of the Cold War and all carriers commissioned since then have not had the ramps. The last carrier commissioned with a bridle catcher was USS Carl Vinson; starting with Theodore Roosevelt the ramps were deleted. USS Enterprise is the last operational carrier with the ramps still attached.

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 four seconds, aircraft velocity plus apparent wind speed (ship's speed plus "natural" wind) will be sufficient to allow an aircraft to fly away, even after losing one engine.

The steam catapult was a British invention^[1]. Up to and during World War II most catapults were hydraulic, and there were a number of armed merchantmen, known as CAM ships ("catapult armed merchantmen"), that had rocket-driven catapults. Some carriers were completed before and during World War II with catapults on the hangar deck that fired athwartships, but they were unpopular due to their short run, low clearance of the hangar decks, and lower clearance of the water (the latter two qualities afforded pilots far less margin for error in the first moments of flight). They were mostly used for experimental purposes, and their use was entirely discontinued during the latter half of the war.

The use of steam to drive aircraft forward was suggested by Commander Colin C. Mitchell RNVR^[2], and trials on the HMS Perseus from 1950 showed its effectiveness. Navies introduced steam catapults, capable of launching the heavier jet fighters, in the mid-1950s. Powder-driven catapults were also contemplated, and would have been powerful enough, but would also have introduced far greater stresses on the airframes and may have been unsuitable for long use. At the beginning of the 21st century, navies started experimenting with catapults powered by linear induction motors and electromagnetics. These electromagnetic catapults would be more energy efficient on nuclear powered aircraft carriers and would alleviate some of the dangers posed by using pressurized steam. On gas-turbine powered ships, an electromagnetic catapult would eliminate the need for a separate steam boiler for generating catapult steam. Some upcoming carrier designs, including the U.S. Navy's Gerald R. Ford class and the Royal Navy's new Queen Elizabeth class aircraft carriers (CVF)[1], include electric catapults in their plans.

Nations that have retained large aircraft carriers and high performance CATOBAR (Catapult Assisted Take Off But Arrested Recovery) or CTOL (Conventional Take Off and Landing) aircraft (the United States Navy, Brazilian Navy, and French Navy) are still, out of necessity, using catapults. Other navies operate STOVL aircraft, such the Sea Harrier or AV-8B Harrier II, which do not require catapult assistance, from smaller and less costly ships. The Russian Su-33 "Flanker-D" can take off from aircraft carriers without a catapult, albeit at a reduced fuel and armament load. U.S. Navy tactical aircraft use catapults to launch with a heavier warload than would otherwise be possible. Larger planes, such as the E-2 Hawkeye and S-3 Viking, require a catapult shot, inasmuch as their thrust-to-weight ratio is too low for a conventional rolling takeoff in the short distance available on a carrier deck.

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The commonly-used steam catapult relies on the ready availability of large quantities of high-pressure steam- a condition satisfied by the vast majority of 20th century capital ships. The steam is used to charge a steam accumulator so that it may be released even faster than it can be generated by the ship's main boilers.

The catapult itself comprises one or more slotted cylinders similar in principle to those used by Brunel's atmospheric railway. The cylinder- typically several feet in diameter- contains a free piston to which is attached a shuttle which protrudes through a slot in the flight deck. The nosewheel of the aircraft to be launched is attached to the shuttle by a launch bar.

On completion of the launch the piston is travelling at high speed and would cause catastrophic damage if not brought to rest in a controlled fashion. This is typically done by a water brake which is basically a horizontal dashpot into which sea water is pumped with a swirling action as fast as it can flow out of the open end, the combination of the slight compressibility of the aerated water and the elasticity of the dashpot itself serves to absorb the energy of the piston without damage. At that point a bring-back mechanism returns the piston and shuttle ready for the next launch.