Electromagnetic Aircraft Launch System
The Electromagnetic Aircraft Launch System (EMALS) is a system under development by the United States Navy to launch carrier-based aircraft from an aircraft catapult using a linear motor drive instead of the conventional steam piston drive. The main advantage is that this system allows for a more graded acceleration, inducing less stress on the aircraft's airframe.
Other advantages include lower system weight, with a projected lower cost and decreased maintenance requirements. The design includes the ability to launch aircraft that are heavier or lighter than the conventional system can accommodate. In addition, the system requires far less fresh water, reducing the need for energy-intensive desalination.
Design and development
Steam catapults were developed in the 1950s and have been exceptionally reliable. For over 50 years, at least one of the four catapults has been able to launch an aircraft 99.5% of the time. However, there are a number of drawbacks. One group of Navy engineers wrote, "The foremost deficiency is that the catapult operates without feedback control. With no feedback, there often occurs large transients in tow force that can damage or reduce the life of the airframe." The steam system is massive, inefficient (4–6%), and hard to control. These control problems allow Nimitz-class steam-powered catapults to launch heavy aircraft, but not aircraft as light as many UAVs, which is an unacceptable limit for a 21st-century platform.
Linear induction motor
The EMALS uses a linear induction motor (LIM), which uses electric currents to generate magnetic fields that propel a carriage along a track to launch the aircraft. The EMALS consists of four main elements: The linear induction motor consists of a row of stator coils that have the function of a conventional motor’s armature. When energized, the motor accelerates the carriage along the track. Only the section of the coils surrounding the carriage is energized at any given time, thereby minimizing reactive losses. The EMALS' 300-foot (91 m) LIM will accelerate a 100,000-pound (45,000 kg) aircraft to 130 kn (240 km/h; 150 mph).
Energy storage subsystem
During a launch, the induction motor requires a large surge of electric power that exceeds what the ship's own continuous power source can provide. As of 1994, the EMALS energy-storage system design accommodates this by drawing power from the ship during its 45-second recharge period and storing the energy kinetically using the rotors of four disk alternators; the system then releases that energy (up to 484 MJ) in 2–3 seconds. Each rotor delivers up to 121 MJ (34 kWh) from 6400 rpm (approximately one gasoline gallon equivalent) and can be recharged within 45 seconds of a launch; this is faster than steam catapults. A max launch using 121 MJ of energy from each disk alternator slows the rotors from 6400 rpm to 5205 rpm.
Power conversion subsystem
During launch, the power conversion subsystem releases the stored energy from the disk alternators 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 any given moment.
Operators control the power through a closed loop system. Hall effect sensors on the track monitor its operation, allowing the system to ensure that it provides the desired acceleration. The closed loop system allows the EMALS to maintain a constant tow force, which helps reduce launch stresses on the plane’s airframe.
Aircraft Compatibility Testing (ACT) Phase 1 concluded in late 2011 following 134 launches (aircraft types comprising the F/A-18E Super Hornet, T-45C Goshawk, C-2A Greyhound, E-2D Advanced Hawkeye, and F-35C Lightning II)using the EMALS demonstrator installed at Naval Air Engineering Station Lakehurst. On completion of ACT 1, the system was reconfigured to be more representative of the actual ship configuration on board the USS Gerald R. Ford, which will use four catapults sharing several energy storage and power conversion subsystems.
- 1–2 June 2010: Successful launch of a T-45 Goshawk.
- 9–10 June 2010: Successful launch of a C-2 Greyhound.
- 18 December 2010: Successful launch of a F/A-18E Super Hornet.
- 27 September 2011: Successful launch of an E-2D Advanced Hawkeye.
- 18 November 2011: Successful launch of a F-35C Lightning II.
ACT Phase 2 began on 25 June 2013 and concluded on 6 April 2014 after a further 310 launches (including launches of the EA-18G Growler and F/A-18C Hornet, as well as another round of testing with aircraft types previously launched during Phase 1). In Phase 2 various carrier situations were simulated, including off-centre launches and planned system faults, to demonstrate that aircraft could meet end-speed and validate launch-critical reliability.
- June 2014: The Navy completed EMALS prototype testing of 450 manned aircraft launches involving every fixed-wing carrier-borne aircraft type in the USN inventory at Joint Base McGuire-Dix-Lakehurst during two Aircraft Compatibility Testing (ACT) campaigns.
- May 2015: First full speed shipboard tests conducted.
Compared to steam catapults, EMALS weighs less, occupies less space, requires less maintenance and manpower, is more reliable, recharges more quickly, and uses less energy. Steam catapults, which use about 1,350 lb (610 kg) of steam per launch, have extensive mechanical, pneumatic, and hydraulic subsystems. EMALS uses no steam, which makes it suitable for the Navy's planned all-electric ships.
Compared to steam catapults, EMALS can control the launch performance with greater precision, allowing it to launch more kinds of aircraft, from heavy fighter jets to light unmanned aircraft. Each one of the four disk alternators in the EMALS system can deliver 29 percent more energy than a steam catapult's approximately 95 megajoules; each disk alternator can supply up to 121 megajoules. The EMALS will also be more efficient than the 5-percent efficiency of steam catapults.
Systems that use or will use electromagnetic aircraft launch systems
EMALS is a design feature of the Ford-class carrier. Such a launch system was also considered as a retrofit for carriers of the Nimitz class, but was not workable due to the high electrical power requirements of the EMALS catapults, requirements that the two Westinghouse nuclear reactors on board the ships of this class could not provide. John Schank stated: "The biggest problems facing the Nimitz class are the limited electrical power generation capability and the upgrade-driven increase in ship weight and erosion of the center-of-gravity margin needed to maintain ship stability."  Therefore, the newer Ford class' carriers were equipped with power plants that produce more power than the ship actually needs as of now. This allows unforeseen technological advances to be implemented later, something which was not possible with the Nimitz-class when the possibility for EMALS was considered.
Converteam UK were working on an electro-magnetic catapult (EMCAT) system for the Queen Elizabeth-class aircraft carrier. In August 2009, speculation mounted that the UK may drop the STOVL F-35B for the CTOL F-35C model, which would have meant the carriers being built to operate conventional takeoff and landing aircraft utilizing the UK-designed non-steam EMCAT catapults.
In October 2010, the UK Government announced it had opted to buy the F-35C, using a then-undecided CATOBAR system. A contract was signed in December 2011 with General Atomics of San Diego to develop EMALS for the Queen Elizabeth-class carriers. However, in May 2012, the UK Government reversed its decision after the projected costs rose to double the original estimate and delivery moved back to 2023, cancelling the F-35C option and reverting to its original decision to buy the STOVL F-35B.
The Indian navy has shown interest in installing EMALS for its planned CATOBAR Supercarrier INS Vishal. The Indian government has shown interest to produce the Electromagnetic Aircraft Launch System locally with the assistance of General Atomics.
The concept of a ground carriage is intended for civilian use and takes the idea of an electromagnetic aircraft launch system one step further, with the entire landing gear remaining on the runway for both takeoff and landing.
- Schank, John. Modernizing the U.S. Aircraft Carrier Fleet, p. 80.
- Doyle, Michael, Douglas Samuel, Thomas Conway, and Robert Klimowski. "Electromagnetic Aircraft Launch System - EMALS". Naval Air Engineering Station Lakehurst. 1 March. p. 1.
- Doyle, Michael, "Electromagnetic Aircraft Launch System - EMALS". p. 1.
- Schweber, Bill (2002-04-11). "How It Works" (PDF). EDN Magazine. Retrieved 2014-11-07.
- Doyle, Samuel & Conway, Klimowski (1994-04-15). "Electromagnetic Aircraft Launch System – EMALS" (PDF). Archived from the original (PDF) on 2004-10-25.Doyle, Samuel & Conway, Klimowski. "Electromagnetic Aircraft Launch System – EMALS" (PDF).
A. Disk Alternator
The average power from the prime power is rectified and then fed to inverters. With power from the inverters, the four disk alternators operate as motors and spin up the rotors in the 45 seconds between launches. The disk alternator is a dual stator, axial field, permanent magnet machine (see Fig. 1). The rotor serves both as the kinetic energy storage component and the field source during power generation and is sandwiched between the two stators. There are two separate windings in the stators, one for motoring and the other for power generation. The motor windings are placed deeper in the slots for better thermal conduction to the outside casing. The generator windings are closer to the air gap to reduce the reactance during the pulse generation. The use of high strength permanent magnets allows for a high pole pair number, 20, which gives a better utilization of the overall active area. The rotor is an inconel forging with an inconel hoop for prestress. The four disk alternators are mounted in a torque frame and are paired in counter-rotating pairs to reduce the torque and gyroscopic effects. The rotors operate at a maximum of 6400 rpm and store a total of 121 MJ each. This gives an energy density of 18.1 KJ/KG, excluding the torque frame.
Each disk alternator is a six phase machine with phase resistance and reactance of 8.6 mΩ and 10.4μH, respectively. At max speed, the output of one of the disk alternators would be 81.6 MW into a matched load. The frequency of this output is 2133 Hz and drops to 1735 Hz at the end of the pulse, for a max launch. Machine excitation is provided by the NdBFe 35 MGOe permanent magnets, which are housed in the rotor. These magnets have a residual induction of 1.05 T at 40° C and create an average working air gap flux density of 0.976 T, with tooth flux densities approaching 1.7 T. The stator consists of a radially slotted laminated core with 240 active slots and liquid cold plate. The maximum back EMF developed is 1122 V. Maximum output voltage is 1700 V (L-L) peak and current is 6400 A peak per phase. The disk alternator's overall efficiency is 89.3%, with total losses of 127 KW per alternator. This heat transfers out of the disk alternator through a cold plate on the outside of each stator. The coolant is a WEG mixture with a flow rate of 151 liters/minute. The average temperature of the copper is 84° C, while the back iron temperature is 61° C.
- Bender, Donald (May 2015). "Flywheels" (PDF). Sandia Report (SAND2015–3976): 21.
The system is sized to charge fully in 45 s. During a launch event, the energy stored in the rotors is released in a pulse lasting about 2 s. Peak alternator output is 81.6 MW when discharged into an impedance matched load. When fully charged, the EMALS rotors store 121 MJ (33.6 kW·h) of extractable energy at a maximum speed of 160 Hz (6 400 RPM). The total stored energy is much higher as the rotor speed only decreases by about 25% during a launch event.
- EMALS launches first Goshawk
- EMALS successfully launches first Greyhound
- "USN undertakes first EMALS Hornet launch". Air Forces Monthly. No. 275. Key Publishing Ltd. March 2011. p. 18. ISSN 0955-7091.
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- "Navy Announces Successful Test of Electromagnetic Catapult on CVN 78". www.imperialvalleynews.com. PEO Carriers. 15 May 2015. Retrieved 16 May 2015.
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- AVIATIONWEEK.COM Carrier Launch System Passes Initial Tests
- Schank, John. Modernizing the U.S. Aircraft Carrier Fleet: Accelerating CVN 21 Production Versus Mid-Life Refueling. Santa Monica: Rand Corporation, 2005. p. 76.
- "Converteam develops catapult launch system for UK carriers" By Tim Fish, Jane's. 26 July 2010
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- "Indian Navy seeks EMALS system for second Vikrant-class aircraft carrier".
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- defence secretary to visit India in May to push aircraft carrier technologies The Times of India 5 April 2015
- "Chinese aircraft carrier should narrow the gap with its U.S. counterpart". english.peopledaily.com.cn. People's Daily. 18 October 2013. Retrieved 18 October 2013.
- Rohacs, Daniel; Voskuijl, Mark; Rohacs, Jozsef; Schoustra, Rommert-Jan (2013). "Preliminary evaluation of the environmental impact related to aircraft take-off and landings supported with ground based (MAGLEV) power". Journal of Aerospace Operations (2): 161.
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