A High-Tech LaunchSystem for Carriers

Linear induction motors will replace steam catapults

In the fall, the U.S. Navy began testing two competing systems that will revolutionize the way planes are launched from the decks of aircraft carriers. The Navy's idea is to use a linear induction motor, driven into explosive motion by energy from an advanced storage system, to catapult aircraft into the air. The motor's concept is similar to that used in magnetic levitation (maglev) train systems.

Photo: Travis Simmons / U.S. Navy

Models of the two systems had been delivered by September to the U.S. Naval Air Systems Command site in Lakehurst, N.J., where evaluations of the Electromagnetic Aircraft Launch System (EMALS) technology are being conducted on simulation test beds. The prototype systems are completely functional but are equipped with a half-length launch track, about50 meters long, as opposed to the full-scale 103-meter system required on carriers. Two California companies, General Atomics in San Diego and Northrop Grumman Marine Systems in Sunnyvale, are vying to become the sole contractor for the launcher. It will first be used on the upcoming CVN-21 aircraft carrier, a next-generation vessel intended to be part of the Navy's "all-electric, all-electronic" fleet.

GO! At sea on the USS John F. Kennedy, two flight deck catapult officers signal to launch a T-45C Goshawk jet from Training Squadron 7.

Development of the catapult is part of the Navy's overall plan to replace electromechanical systems with electric ones, which will be more reliable and easier to operate and maintain. The initial contract to develop, test, and build two prototype electric catapults--eventually there will be four systems, each costing about US $26.5 million, for each CVN-21 carrier--is valued at $355 million.

Even sophisticated nuclear aircraft carriers today use steam propulsion, a decades-old technology, to accelerate their aircraft--typically F-14 Tomcats and F/A-18 Hornets, weighing up to 33 000 kg--to 240 km/h in 2 to 3 seconds.

The so-called steam "cat" is a complicated maze of steam pipes, valves, and hydraulics requiring scores of trained technicians. Its explosive force comes from a very large, steam-driven piston whose engine is big, heavy, and very inefficient.

EMALS promises to de-liver better performance and launch control that is tailored for a broader range of aircraft. It will accelerate aircraft weighing roughly 4500 to45 000 kg to launch speeds of 100 to 370 km/h, with a controlled deftness that the inflexible, brute-force steam cats can't manage. Adjusting to the weight of the craft will mean less stress on the airframe. This will be especially crucial as the military relies increasingly on unmanned aircraft for surveillance, as well as combat, like the Predators and Global Hawks that proved their worth in Afghanistan and Iraq.

The heart of the EMALS is the 103-meter-long linear induction motor, which propels an armature, with the aircraft attached. In contrast to the linear motors developed for maglev trains [see "Faster than a Speeding Bullet Train," IEEE Spectrum, August 2003, pp. 30-34], in which the moving element floats in the air, the EMALS' armature is attached to rollers and a carriage running in channels along the launch track. In both the General Atomics and the Northrop designs, the armature is essentially a piece of U-shaped aluminum fitted on three sides of the stators.

The linear motor of the EMALS is powered by energy from rotational storage devices that draw electric power from the ship's electrical distribution system. Initial EMALS design concepts included flywheels and pulse disk alternators, later eschewed in favor of more traditional systems that store energy in the rotors of generators, according to Richard Bushway, the U.S. Navy's EMALS program director. The energy storage systems are an "advance in the state of the art," he says, but he declines to discuss technical details for fear of disclosing proprietary information during the competitive evaluation phase.

Kinetic energy from the rotating system is converted into electric energy, and asolid-state power-conditioning system delivers a tremendous 2- to 3-second pulse of power to the stator. The system must deliver the pulse as often as every 45 seconds to match the capability of the current steam catapult.

The power-conditioning system feeds a current with a rising voltage and frequency, through inverters, to the linear motor. A moving electromagnetic wave is created by powering just the coils in the portion of the motor where the traveling armature is located. The drive comes from high-power silicon controlled rectifiers capable of tens of thousands of amperes at many thousands of volts. The electromagnetic field set up by the current in the stators accelerates the armature. The field simultaneously pushes and pulls the armature and its carriage down the length of the motor until the attached aircraft reaches launch speed, disengages, and flies off.

In addition to allowing for greater control flexibility, EMALS should eventually be half the size and weight of steam cat systems

A closed-loop system, it constantly monitors itself, continuously adjusting the speed and power to create a launch profile tailored to each type of aircraft. Steam catapults are open-loop systems, with no sensors or feedback once the launch sequence is initiated.

In addition to allowing greater control flexibility, EMALS should eventually be half the size and weight of steam cat systems, Bushway says. Eliminating the maze of high-pressure, high-temperature steam pipes and valves will also lessen the risk to the crew during routine operations and battle.

It's also expected that with its electric and electronics subystems, EMALS could be serviced and maintained by an estimated 30 percent fewer crew members.

Further, the EMALS is modular, so that components and subsystems can be swapped in and out both for maintenance and if, as expected, different elements of the system are improved. Advances won't come just from defense contractors, either. Commercial companies are developing systems for civilian backup energy storage and even for hybrid automobiles, Bushway notes. "Commercial demand for things like regenerative energy storage in hybrid automobiles will push the state of the art to the point where we can adopt fairly mature technologies for EMALS," he predicts.

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