Aircraft catapult

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 launch bar integrated in the nose gear; instead, a wire rope 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 ropes 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. During Refueling and Comprehensive Overhaul refits in the late 1990s-early 2000s, the bridle catchers were removed from the first three Nimitz class aircraft carriers. 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.

History

Aviation pioneer Samuel Langley used a spring operated catapult to launch his successful flying models and his failed aerodrome of 1903.Fact|date=April 2008 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.

The steam catapult was a British invention [Herbert M. Friedman and Ada Kera Friedman, [http://www.americanheritage.com/articles/magazine/it/2006/4/2006_4_26.shtml "Shot Into the Air"] , "Invention & Technology Magazine", 21, no. 4 (Spring 2006). ] . 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, 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.

The use of steam to launch aircraft was suggested by Commander Colin C. Mitchell RNVR [Friedman ibid.] , and trials on 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) [http://www.naval-technology.com/projects/cvf/cvf5.html] , include electric catapults in their designs.

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 on a carrier deck.

Operation

The commonly-used steam catapult relies on the availability of large quantities of high-pressure steam- found in the vast majority of 20th century capital ships. The steam charges a steam accumulator so that it may be released faster than it can be produced by the ship.

The steam catapult consists of two slotted cylinders similar in principle to those used by the Clegg & Samuda atmospheric railway. The cylinders—typically 18 inches in diameter—contain free pistons connected to 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 damage if not stopped in a controlled fashion. This is done by a water brake, which is 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, the restriction as the water is expelled from the dashpot and the force produced by the expelled water hitting the front of the piston assembly itself serves to absorb the energy of the piston without damage. At that point a bring-back mechanism returns the piston and shuttle for the next launch.

Future

As the 21st century dawns, steam catapults will be replaced by the Electromagnetic Aircraft Launch System (EMALS). Massive systems that require significant manpower to operate and maintain, they are reaching the limits of their abilities, especially as aircraft continue to gain weight. Electromagnetic catapults will require less manpower to operate and improve reliability; they should also lengthen aircraft service life by being gentler on airframes.The amount of steam needed to launch an airplane depends on the craft’s weight, and once a launch has begun, adjustments cannot be made: If too much steam is used, the nosewheel landing gear, which attaches to the catapult, can be ripped off the aircraft. If too little steam is used, the aircraft won’t reach takeoff speed and will tumble into the water. The launch control system for electromagnetic catapults, on the other hand, will know what speed an aircraft should have at any point during the launch sequence, and can make adjustments during the process to ensure that an aircraft will be within 3 mph of the desired takeoff speed. [ [http://www.airspacemag.com/military-aviation/electromagnetic-catapults.html] ]

ee also

*Modern US Navy carrier operations

References