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Intro History Components Operation Malfunctions Future

Intro

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

The launchbar of an aircraft hooked up to the catapult shuttle just before launch

An aircraft catapult is a device used to launch aircraft from ships—in particular aircraft carriers—as a form of assisted take off. The length of an aircraft carrier’s flight deck is only a fraction of that of a commercial runway on land, and catapults provide carrier aircraft sufficient energy to become airborne. Modern catapults consist of a track built into the flight deck, below which is a large piston or shuttle that is attached through the track to the aircraft. When the catapult is activated, it accelerates the aircraft down the catapult track and releases at the end of the deck, providing sufficient airspeed for the aircraft to fly away.

History

Early years

Catapults of various designs have been used ever since aviation pioneer Samuel Langley used a spring operated catapult to launch his successful flying models of 1903. Likewise the Wright Brothers first flight at Kitty Hawk on December 17, 1903 used a small length of track running down a hill with a weight and derrick styled catapult to help the Wright Flyer's small engine in getting the airplane airborne.

In the early years of carrier aviation, most carrier aircraft did not require a catapult to launch. The carrier would simply be turned into the natural wind and the aircraft would have sufficient power to become airborne on their own. As aircraft became heavier, however, supplemental launch energy was required.

LCDR Henry C. Mustin made the first catapult launch from a ship on Nov. 5, 1915, flying an AB-2 flying boat off the stern of USS North Carolina (ACR-12) in Pensacola Bay, Fla. In July 1919, the Naval Appropriations Act provided for the conversion of the collier USS Jupiter (AC-3) into a ship specifically designed to launch and recover airplanes at sea — an aircraft carrier — later to be named USS Langley (CV-1). The engineering plans were modified to include catapults to be fitted on both the forward and after ends of the "flying-off" deck. The first catapult launch from Langley was at anchor in the York River on Nov. 18, 1922 by Cmdr. Kenneth Whiting piloting a PT seaplane.

1930s - 1940s

In 1934, the Naval Aircraft Factory was authorized to manufacture and test the Type H Mark I flush-deck hydraulic catapult. This catapult was designed to use pressurized fluid to launch land planes from carriers, and was to be the primary means of launching planes from carriers for over 20 years. During World War II, 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.

In June 1947, the CNO approved "Project 27A" by which Essex-class carriers were modernized to be able to handle aircraft to 40,000 pounds and included the installation of two H-8 catapults among other modifications. USS Oriskany (CV-34), was the first of nine carriers modernized under this project.

1950s

Hydraulic catapults were unreliable and dangerous, as the explosion of a hydraulic catapult aboard USS Bennington (CV-20) in 1954, which killed nearly 100 crewmen, would show. Hydraulic catapults were also not very adaptable, and as aircraft weights increased they were unable to launch airplanes effectively. Powder-driven catapults were 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. The use of steam to launch aircraft was suggested by Commander Colin C. Mitchell RNVR, and trials on HMS Perseus from 1950 showed its effectiveness. The U.S. Navy followed in 1952, when Project 27A was modified to 27C for three Essex-class carriers for several changes, including higher performance C-11 steam catapults. USS Hancock (CV-19) was the first of these carriers, known as "27 Charlies" capable of launching high performance jets. The same basic design is still in use today.

File:USS Enterprise (CVN 65) underway.jpg

Two bridal catch ramps are clearly visible on the bow in this picture of Enterprise.

Until the 1970s, aircraft were connected to the catapult with a wire rope called a catapult bridle, which attached the catapult shuttle to the aircraft fuselage. Carriers incorporated ramps at the ends of the catapults that were used to catch these ropes at the end of the catapult stroke. The last aircraft to use the bridle system was the T-2 Buckeye, which was retired from carrier service in 2005. The last carrier commissioned with a bridle catcher was USS Carl Vinson; starting with Theodore Roosevelt the ramps were omitted. During Refueling and Comprehensive Overhaul (RCOH) refits in the late 1990s-early 2000s, bridle catchers were removed from the first three Nimitz class aircraft carriers. USS Enterprise is the last operational carrier with the ramps still attached. The bridle system was replaced starting with the A-7 Corsair, which incorporated a launch bar attached to the nose gear.

Current use

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.

Catapult Main components

The principle component of the steam catapult is a cylinder-piston assembly with two power cylinders and two pistons per catapult. The spear-shaped pistons are solidly interconnected and, upon catapult initiation, are forced by steam pressure at high speed through the cylinders. The pistons are attached, through a slot in the flight deck, to a shuttle to which the aircraft is attached. The shuttle is a small roller-mounted car which moves (during the launch) on tracks installed just under the flight deck.

Power to drive the shuttle and its aircraft load comes from expanding steam piped to the catapult from the ship's main steam system. This steam is placed under pressure in large tanks (called accumulators or receivers) located under the launching engine. Steam is admitted to the receivers through the flow control valve (sometimes called steam charging valves). From the receivers, the steam is transferred at the moment of launch into the power cylinders. Steam acts directly on the pistons and propels the piston-shuttle assembly through the cylinders. A sealing strip closes the slot in each cylinder as the pistons are driven forward, thus preventing the escape of steam from the cylinder slots through which the connector moves.

Steam

There are two basic steam systems associated with steam catapults. They are the dry receiver system and the wet receiver (constant-pressure) system. The main difference between the two is that the constant-pressure system uses a capacity selector valve (CSV) to control the steam pressure to the catapults for launching while the dry receiver system must have the pressure selected for each launch.

Retraction

A rotary type retraction engine system provides the power to retract the shuttle and the launching engine pistons after the catapult has been fired. It is also used to advance and maneuver the grab forward and aft.

Operation

Elements of the catapult of Charles de Gaulle disassembled during her refit in 2008

SEM no 49 catapulting off Charles de Gaulle in June 2007. Teams of the catapult crew signal to the pilot the stage of the preparations.

Ship perspective

Prior to use, the launching cylinders must be preheated to prevent thermal shock to the launching engine when superheated steam is admitted through the launching valves into the launching cylinders. This process is called "soaking" and can take many hours to accomplish. Steam catapults draw their power directly from the heat of the ship’s engines in the form of high-pressure super-heated steam. The steam charges a steam accumulator located in the catapult room (one for each catapult) just below the flight deck. 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. Upon catapult firing, the steam is quickly released from the accumulator into the cylinder, propelling the piston, shuttle, and aircraft forward. 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, aircraft structural damage can occur. If too little steam is used, the aircraft will not achieve sufficient flying speed.

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.

From its four catapults, an aircraft carrier can launch an aircraft every 20 seconds. The catapults are about 300 feet long and consist of a large piston underneath the deck. Above the deck, only a small device engages the aircraft nose gear. The catapult has two rows of slotted, cylindrical piping in the trough beneath the flight deck. When the planes are ready for takeoff, the aircraft handlers on the flight deck guide the plane onto the catapult and hook up the catapult to the plane's nose gear. On each plane's nose gear is a T-bar which pulls the plane down the catapult. This bar on the nosegear of the aircraft attaches to a shuttle protruding from the flight deck and connects to a pair of pistons in the trough. A holdback device installed on the nosegear holds the aircraft in place as tension is applied. After a final check, the pilot increases the aircraft engines to full power. When the engines are steady at full power, the catapult is fired , which accelerates the plane from 0 to 160 knots in under two seconds. On a signal from the catapult safety observer on the flight deck, steam is admitted to the catapult by opening the launching valves assembly. (The length of time the valves remain open is determined by the weight of the aircraft and the wind over the deck.) Steam surges into the cylinders, releasing the holdback and forcing the pistons and shuttle forward while accelerating the aircraft along the 300-foot deck. A 60,000-pound aircraft can reach speeds in excess of 150 mph in less than two seconds. The shuttle is stopped when spears on the pistons plunge into waterbrake cylinders. A cable and pulley assembly then pulls the shuttle back down the catapult for the next launch.

Pilot perspective

The aircraft is prepared for launch. As the aircraft is taxied to the catapult, the pilot indicates the aircraft's gross weight to a catapult operator. The aircraft's weight is then combined with the relative headwind to calculate the catapult's firing force, which is entered into the catapults operating machinery. Ideally, the aircraft will be given sufficient speed by the catapult such that it could fly away with only one engine, if required. Applying too much energy to the catapult would result in unnecessary stresses. Under the precise direction of an aircraft director using hand signals, the pilot guides the aircraft's launch bar into the catapult shuttle. A "holdback" fitting, designed to hold the aircraft in place prior to initiation of the catapult, is attached to the aft of the nose landing gear. Upon direction from the aircraft director, the pilot advances the engines to full power while the catapult crew puts "tension" on the catapult. The pilot does a final check of aircraft systems while ground crews check the exterior of the plane. Once the pilot indicates that he is ready for launch, the Catapult Officer (also known as the "shooter") does a final check of catapult systems and initiates catapult firing. Upon catapult firing, the holdback fitting is no longer able to hold the aircraft (by design), and it "releases". Some older aircraft holdback fittings utilize a one-time-use steel pin that breaks. Within a few seconds, aircraft velocity plus apparent wind speed (ship's speed plus "natural" wind) is sufficient to allow an aircraft to fly away, even after losing one engine.

The aircraft to be launched is spotted just aft of the launching shuttle at the battery position. The aircraft is attached to the shuttle, and a holdback unit is installed to hold the aircraft in place during tensioning. The tensioner is then activated to apply pressure and move the shuttle forward to tension the aircraft. Prior to a launch, the engines of the aircraft must be operating at full power. A holdback device is utilized to prevent the aircraft from being moved forward by the thrust of its own engines, until the time of launch. The holdback device hooks into a fitting in the flight deck.

The catapult is fired. After tensioning is completed, the catapult is fired by opening the launching valve assembly and permitting steam to surge into the cylinders. The force of the steam pushes the pistons in the cylinders, breaking the holdback. The steam then forces the pistons forward, towing the shuttle and aircraft at ever- increasing speed until takeoff is accomplished.

The piston-connector-shuttle assembly is stopped at the end of its launching run by a water brake. The brake consists of two cylinders of water located co-axially with the power cylinders at the forward end of the catapult. The spear tips of the pistons ram into the water-filled cylinders. As the spear tips penetrate the water, pressure builds up and stops the assembly. The pistons and shuttle are halted by the water brake. The forward motion of the shuttle is stopped when tapered spears attached to the pistons plunge into the water-filled cylinders of the water brake.

The retraction engine returns the shuttle/pistons to the battery position. The catapult has completed a full cycle and is in position to launch another aircraft. A hydraulic retraction engine returns the shuttle/pistons to the pre-launch position after firing.

Malfunctions

A "suspend" is initiated whenever an abnormal condition occurs after tension, but prior to catapult firing. During this time, the aircraft engines are kept at full power in anticipation of a launch until the launchbar is released from the shuttle. A "hangfire" is a condition where the catapult partially fires, but the holdback fitting does not release. A "cold cat" is a condition where the catapult fires and the holdback releases, but the system does not deliver sufficient energy for the aircraft to fly away. A "spit shuttle" is where the launchbar becomes disengaged from the shuttle

Future

The future of aircraft catapults is likely to involve As the 21st century dawns, steam catapults will be replaced by the Electromagnetic Aircraft Launch System (EMALS).

Because of the dangers posed by using pressurized steam and the significant manpower required to operate and maintain the massive steam systems, navies have been experimenting with catapults powered by linear induction motors and electromagnetics. Electromagnetic catapults will require less manpower to operate, will improve reliability, and they should reduce aircraft fatigue by being gentler on airframes.

The launch control system for electromagnetic catapults will be able to modulate power delivered to an aircraft at any point during the launch sequence, in order to ensure that it is within 3 mph of the desired takeoff speed. 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)[2], include electromagnetic catapults in their designs.

See also

• Modern US Navy carrier operations

References

Catapult Category:Aircraft carriers

de:Flugzeugkatapult fr:Catapulte (avion) it:Catapulta per aerei ko:항공모함 캐터펄트 he:מעוט hu:Gőzkatapult ja:カタパルト pl:Katapulta startowa samolotu zh:飞机弹射器

Sources: Principles of Naval Engineering, NAVEDTRA 12960, Catapults, NAVAIR 51-15ABD-1, USS Nimitz (CVN 68) Ship Information Book Vol 2 Pt 1 Bk 3 Piping Systems