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Steam Catapult

The Electromagnetic Aircraft Launch System (EMALS) has repeatedly been criticised by President Donald Trump. In May 2019 Trump once again called for equipping US aircraft carriers with traditional steam powered catapults rather than newer electromagnetic systems which he claimed may malfunction during warfare. “We’re spending all that money on electric and nobody knows what it’s going to be like in bad conditions. So I think I’m going to put an order — when we build a new aircraft carrier, we’re going to use steam”, he told sailors and marines on board the USS Wasp during his visit 27 May 2019 to the Yokosuka Naval Base south of Tokyo.

He upheld his view by insisting that steam catapults work much better that higher-tech Electromagnetic Aircraft Launch System (EMALS). “Steam’s only worked for about 65 years perfectly. And I won’t tell you this because it’s before my time by a little bit, but they have a $900 million cost overrun on this crazy electric catapult. They want to show — next, next, next. And we all want innovation, but it’s too much”, Trump added.

Trump repeatedly expressed doubt over EMALS’ efficiency, especially in combat conditions, something that he apparently wanted to underscore during a Thanksgiving Day call with a US sailor in 2018, when the US president wondered whether the sailor wants to “go with steam or electromagnetic” aircraft catapults.

Planes that require flying speed are launched from an aircraft carrier by a catapult using high pressure steam. Theoretically, a steam catapult is not complex. Fresh water is needed to generate the steam. As the catapult moves the pressure drops. The initial "kick" is very high and then the acceleration drops off. The plane and pilot may be subjected to as much as 5G's at the start to get enough speed to get airborne. This is why aircraft carriers always turn into the wind and increase speed to launch planes. To conserve fresh water and reduce stress to pilots and planes.

Aircraft launchers on today's aircraft carriers generally propel the aircraft to be launched using steam that is generated by the same system that provides steam for propulsion of the vessel.

The US active duty C-13-1-type and C-13-2 type steam catapults have design defects, resulting in the extremely demanding requirements for materials, manufacturing is extremely difficult and costly. The Soviet Union struggled but failed to produce qualified steam catapult. Steam catapults on the French aircraft carrier Charles de Gaulle are a US procurement.

Such use of the steam available on seaborne vessels produces a significant weight increase. In the first place, the steam generating machinery must be larger. Secondly, the piping, valves, accumulators and other machinery needed to store steam at high pressure in readiness for application to a catapult launch add weight. Also the conventional steam catapult requires the apparatus for steam launching to be positioned relatively high on the ship. This reduces in a large measure the desirable upper level space required for other purposes. Also the added weight produces a higher center of gravity. Neither of these is a desirable feature in any vessel.

Another drawback of the steam catapult is that it forces, from an overall life-cycle cost standpoint, the use of steam for propulsion rather than such alternatives as gas turbines and diesel engines. Still another drawback is that existing steam catapults are open loop systems: Once the steam launching valve is opened, no further control of the system is provided.

Launching systems for aircraft on aircraft carriers include a steam catapult having one or more cylinders below deck and one or more pistons arranged to accelerate a shuttle along a longitudinal slot in the deck. The cylinders are supplied with a common high pressure steam supply to drive the pistons which in turn are connected by a transverse coupling member. The latter member extends through longitudinal slots in the cylinders to drive the shuttle along its slot. Although the slots in the cylinders are sealed, the seals are only partially effective and as a result low pressure steam escaping from the cylinders rises through the shuttle slot in the deck. This steam has been found to be undesirable not only on the part of the deck crew of the aircraft carrier but in addition has seriously degraded the performance of the jet engines as the same are catapulted through the steam during the critical launch run thereby endangering the aircraft and crew.

Cylindrical members having elongated longitudinal slots therein have been used to permit connection between a pressure-driven piston member that travels within the cylinder and a shuttle that carries or tows the aircraft to be launched. Such a slotted cylinder requires a means for sealing the slot in order to maintain operating pressure within the cylinder, and a variety of sealing devices have been devised for this purpose. One such type of seal devised for use in the slotted cylinder comprises essentially a single, elongated, ribbon-like strip held in place along the slot by tension applied to the ends of the strip. During operation of the catapult, the sealing strip is guided internally of the cylinder as the piston and shuttle travel therethrough by a cam-like surface on the moving piston.

While such a flexible, dynamic seal has proven satisfactory in the slotted cylinders of aircraft catapults for lower shuttle speed and shorter operating strokes, problems have arisen in their use in catapults generating high shuttle speeds over extended operating strokes because of an increased tendency of the ribbon-like strips to whip forward of the moving piston under such conditions. Whipping of the strip forward of the piston produces a random wave motion in the strip that adversely effects the operation of the pressurized cylinder.

The launching systems employed on aircraft carriers today usually include a steam catapult having one or more cylinders installed below deck and one or more pistons arranged to accelerate a shuttle along the deck. The aircraft is attached to this shuttle by means of a bridle which hooks over the shuttle and through which the launching load is exerted. Full catapult launching power is insured by temporarily restraining the aircraft with a holdback cable that secures it to the carrier deck. When full catapult power is reached, the holdback is released and the aircraft is catapulted forward, pulled by the shuttle. The steps of positioning the aircraft over the shuttle and securing the tow and holdback cables slow up launching to the point where it is a discontinuous, ineificient operation.

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Page last modified: 29-05-2019 19:04:21 ZULU