Deck-level view of the catapult track aboard museum ship USS Hornet, with an A-4 in launch position.
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, or in some cases a wire rope called a catapult bridle is attached to the aircraft and the catapult shuttle.
The ramps at the catapult ends on some aircraft carriers are used to catch the ropes so they can be reused; bridles have not been used on U.S. aircraft since the end of the Cold War, and all U.S. Navy carriers commissioned since then have not had the ramps. The last U.S. carrier commissioned with a bridle catcher was USS Carl Vinson; starting with USS Theodore Roosevelt the ramps were deleted. During Refueling and Complex Overhaul refits in the late 1990s–early 2000s, the bridle catchers were removed from the first three Nimitz-class aircraft carriers. USS Enterprise was the last U.S. Navy operational carrier with the ramps still attached before her decommissioning in 2012.
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 two to four seconds, aircraft velocity by the action of the catapult plus apparent wind speed (ship's speed plus or minus "natural" wind) is sufficient to allow an aircraft to fly away, even after losing one engine.
Samuel Pierpont Langley's catapult, houseboat & unsuccessful man-carrying Aerodrome (1903)
Aviation pioneer and Smithsonian Secretary Samuel Langley used a spring-operated catapult to launch his successful flying models and his failed aerodrome of 1903. Likewise the Wright Brothers beginning in 1904 used a weight and derrick styled catapult to assist their early aircraft with a takeoff in a limited space.
On 31 July 1912, Theodore Gordon Ellyson became the first person to be launched from the experimental catapult system. The U.S. Navy had been perfecting an air-compressed catapult system and mounted it on the Santee Dock in Annapolis, Maryland. The first attempt nearly killed Lt. Ellyson when the plane left the ramp with its nose pointing upward and it caught a crosswind, pushing the plane into the water. Ellyson was able to escape from the wreckage unhurt. On 12 November 1912, Lt. Ellyson made history as the Navy’s first successful catapult launch, from a stationary coal barge. On 5 November 1915, LCDR Henry C. Mustin made the first catapult launch from a ship underway.
In the late 1920s the passenger liners SS Europa and the SS Bremen of the Norddeutscher Lloyd line experimented with catapult-launched mailplanes. Up to and during World War II, most catapults were hydraulic. Some carriers were completed before and during World War II with catapults on the hangar deck that fired athwartships, but they were unpopular because of their short run, low clearance of the hangar decks, inability to add the ship's forward speed to the aircraft's airspeed for takeoff, and lower clearance from the water (conditions which 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.
In the 1920s and 1930s, many naval vessels apart from aircraft carriers carried float planes, seaplanes or amphibians for reconnaissance and spotting. They were catapult-launched and landed on the sea alongside for recovery by crane. There were submarine aircraft carriers, and some Japanese submarines used them for offensive operations also. The first launch off a Royal Navy battlecruiser was from HMAS Australia on 8 March 1918. Subsequently many RN ships carried a catapult and from one to four aircraft; battleships or battlecruisers like the HMS Prince of Wales carried four aircraft and HMS Rodney carried two, while smaller warships like the cruiser HMNZS Leander carried one. The aircraft carried were the Fairey Seafox or Supermarine Walrus. Some like HMS Nelson did not use a catapult, and the aircraft was lowered onto the sea for takeoff. Some had their aircraft and catapult removed during WWII e.g. HMS Duke of York, or before (HMS Ramillies).
During World War II a number of ships were fitted with rocket-driven catapults, first the Fighter catapult ships of the Royal Navy, then armed merchantmen known as CAM ships from "catapult armed merchantmen." These were used for convoy escort duties to drive off enemy reconnaissance bombers. CAM ships carried a Hawker Sea Hurricane, dubbed a "Hurricat" or "Catafighter", and the pilot bailed out unless he could fly to land.
Like Brunel's Atmospheric Railway the tube of the catapult has a longitudinal valve covering the slot except where it is displaced to allow the connection to the internal piston to pass through. Tubes may be paired. Although Heinkel-manufactured seaplane catapults in use by the German Deutsche Luft Hansa's South Atlantic Air Mail service utilized, as early as 1929, high-pressure air to launch their mail delivery aircraft from specially equipped vessels such as the SS Westfalen, the SS Bremen, and the SS Europa, the modern steam catapult as we know it was a British invention. The use of steam to launch aircraft was suggested by Commander Colin C. Mitchell RNVR, and trials on HMS Perseus, flown by pilots such as Eric "Winkle" Brown, from 1950 showed its effectiveness. Navies introduced steam catapults, capable of launching the heavier jetfighters, 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 might have been unsuitable for long use.
Nations that have retained large aircraft carriers and high performance CATOBAR (Catapult Assisted Take Off But Arrested Recovery) (the United States Navy, Brazilian Navy, and French Navy) are still using catapults for their superior power and flexibility. 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, since their thrust-to-weight ratio is too low for a conventional rolling takeoff on a carrier deck.
The size and manpower requirements of steam catapults place limits on their capabilities. A newer approach is the Electromagnetic Aircraft Launch System (EMALS). Electromagnetic catapults place less stress on the aircraft and offer more control during the launch by allowing gradual and continual acceleration. Electromagnetic catapults are also anticipated to require significantly less maintenance through the use of solid state components.
Linear induction motors have been experimented with before, such as Westinghouse's Electropult system in 1945. However at the beginning of the 21st century, navies again started experimenting with catapults powered by linear induction motors and electromagnets. EMALs 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. The U.S. Navy's upcoming Gerald R. Ford class carrier includes electromagnetic catapults in its design.
From 1929, the German Norddeutscher Lloyd-liners SS Bremen and Europa were fitted with catapults to launch mail-planes. These ships served the route between Germany and the United States. The aircraft, carrying mail–bags, would be launched while the ship was still many hundreds of miles from its destination, thus speeding mail delivery by about a day. Initially, Heinkel HE 12 aircraft were used before they were replaced by Junkers Ju 46, which were in turn replaced by the Vought V-85G. The catapults were powered by compressed air.
After World War II, Supermarine Walrus amphibian aircraft were briefly operated by a British whaling company, United Whalers. Operating in the Antarctic, they were launched from the factory ship FF Balaena, which had been equipped with an ex-navy aircraft catapult.