Electromagnetic Aircraft Launch System

Electromagnetic Aircraft Launch System (EMALS) is a system under development by the United States Navy to launch aircraft from carriers using a linear motor drive instead of steam pistons used in conventional aircraft catapults. This technology has the advantage of gradually increasing the aircraft's speed, thus reducing the stress the plane's airframe has to support. The EMALS is currently being developed for the U.S. Navy's newest "Ford "class aircraft carriers. It was also considered for the Royal Navy's new "Queen Elizabeth" class aircraft carriers (CVF), but the Royal Navy opted for a Vertical/Short Takeoff and Landing (VSTOL) configuration instead.

How EMALS works

The EMALS in based on the concept of the linear induction motor (LIM). This system uses large electric currents to generate strong magnetic fields, which in turn propel the carriage down a track to launch the aircraft.cite news
first=Bill
last=Schweber
title=How It Works
url=http://www.edn.com/contents/images/207108.pdf
format=PDF
publisher=EDN Magazine
date=2002-04-11
accessdate=2008-02-18 ] The EMALS consists of [http://www.ga.com/atg/EMS/m1346.php four main elements] :

*Linear induction motor
*Energy-storage subsystem
*Power-conversion subsystem
*Control consoles

Linear induction motor

The linear induction motor (LIM) is the main component of the EMALS. It consists of a row of stator coils that, when energized, accelerate the carriage (equivalent to a conventional motor’s rotor) down the track. Only the section of the stator coils surrounding the carriage is energized at any given time, minimizing reactive losses. The EMALS will use a convert|300|ft|m|abbr=on long LIM capable of launching a convert|100000|lb|abbr=on aircraft to convert|130|kn|km/h.

Energy storage subsystem

To power the LIM, the EMALS requires a large amount of electric energy to be used in a short amount of time. The ship’s power source can’t provide this kind of immediate energy, so the energy-storage subsystem is designed to accumulate power from the ship and store it kinetically on rotors of four disk alternators. cite news
author=Doyle, Samuel, Conway, and Klimowski
title = Electromagnetic Aircraft Launch System - EMALS
url = http://www.lakehurst.navy.mil/nlweb/ieeerevc.pdf
format=PDF
date=1994-04-15
accessdate=2008-03-12] Each rotor can store more than 100 MJ, and they are able to recharge within 45 seconds of a launch.

Power conversion subsystem

At the time of launch, the power conversion subsystem releases the stored energy from the disk alternators in a controlled manner by using a cycloconverter. The cycloconverter provides a controlled rising frequency and voltage to the LIM, energizing only the small portion of stator coils that affect the launch carriage at each instant in time.

Control consoles

The power used by the EMALS is controlled through a closed loop system to give the operators complete control over the system's performance. A number of Hall effect sensors positioned on the track provide feedback to the control consoles, allowing the system to monitor itself and ensure that it provides the desired acceleration. The closed loop control system allows the EMALS to maintain a constant tow force, which helps reduce the launch stresses on the plane’s airframe.

Advantages

The EMALS has a number of significant advantages over the conventional steam driven catapult system, including reduced weight, volume, and maintenance, and increased controllability, availability, reliability, and efficiency.

One of the most important advantages of the EMALS is its simplicity, compared to the existing catapult system. The steam catapult system, which uses about 614 kg of steam per launch, requires extensive mechanical, pneumatic, and hydraulic subsystems that complicate the launch system as a whole. With the EMALS, launching, braking, and retraction would be performed by the linear induction motor, eliminating a substantial amount of auxiliary components. The EMALS is an entirely stand alone electric system, requiring no steam to be pumped around the ship.cite web
url=http://www.defensetech.org/archives/003410.html
title=Defense Tech: EMALS: Next Gen Catapult
accessdate=2008-02-27
last=Lowe
first=Christian] This characteristic makes the EMALS easily suitable for integration into the all-electric ships the U.S. Navy is currently researching. Its simplicity also reduces the manpower necessary to operate and maintain the system.

The EMALS is also a much more flexible system. Its feedback control system allows operators to control the launch performance with greater precision. This ability makes the EMALS capable of launching a wide variety of aircraft, from heavy fighter jets to lightweight unmanned aircraft. The EMALS could also be easily incorporated into a ramp, which would reduce the aircraft’s required end speed and consequently the launch energy.

The EMALS offers the increased energy capability necessary to launch the next generation of carrier based aircraft. The current steam catapult system can deliver about 95 MJ of energy, while the EMALS is capable of 122 MJ, a 29% increase. The EMALS is also being designed to be much more efficient than the current system, which is only about 5% efficient. With a cycle time of only 45 seconds, the EMALS can launch planes much more quickly than the conventional launch system.

References

Further reading

* [http://www.globalsecurity.org/military/systems/ship/systems/emals.htm "Electromagnetic Aircraft Launch System - EMALS"] , "GlobalSecurity.org"