The traditional and battle-tested steam-powered catapult used to launch aircraft from carriers is being replaced by a powerful, electromagnetic-based, closed-loop linear-motor system — maybe.
For over seven decades, the steam-powered catapult has been the standard mechanism for launching airplanes from the decks of aircraft carriers, with an associated cable-tensioned brake used for arresting them on landing. Without it, aircraft could not reach take-off speeds of over 100 knots in just a few seconds and within a hundred feet, nor slow from landing speed to full stop in a similar time and distance. It’s a system which certainly works, as proven by how routine carrier operations have become and how quickly the system can cycle to launch and land planes, often doing so simultaneously due to their angled decks.
Now, a long-awaited replacement which is based on a radically new approach called the Electromagnetic Aircraft Launch System (EMALS) is closer to full tests and eventual deployment, using an electromagnetic “rail gun” to launch/arrest aircraft. The development has been long and troubled, and there is no assurance it will succeed.
Q: What are some of the specifics of the steam-based catapult launch system?
A: The steam system uses the sudden ingress of high-pressure steam into a piston — simple in concept, very complicated in reality. There is no doubt it works, as has been proven through the decades and in combat. It can launch and retrieve aircraft of varying weights at a rate of about one every minute or two and do so simultaneously due to the angled deck.
Q: So why not continue to use steam?
A: The steam-based catapult has serious weaknesses. It requires an enormous amount of energy to function, and it is energy-inefficient, so it doubly strains the system’s steam boilers (even on nuclear-powered carriers). It requires considerable maintenance and a large crew to keep it functioning, especially in rough seas and potentially in combat conditions. It is bulky and heavy, while space is limited even on a modern carrier; it is an open-loop system with no feedback to control its performance; setting it for different lunch types is a complicated process, and it can only handle a limited range of aircraft types, weights and launch/landing conditions (all of which vary widely).
Replacing traditional steam catapults with the EMALS is meant to eliminate the need for the ship to generate and store steam. That frees up a large area below-deck and requires 25 percent fewer crew members to operate. The Navy has said the savings would be about $4 billion in operating costs over the ship’s expected 50-year lifespan.
Q: What are the other alternatives to steam?
A: In the early days of aircraft carriers, other approaches were tried and failed. There were trials with using falling weights connected by pulleys (a technique used successfully by some of the early Wright Brothers aircraft on land!), but these did not offer the acceleration and force needed. Hydraulic systems were used on some early aircraft carriers but could not provide the power impulse needed for ever-larger aircraft. Even “bottle rockets” attached to individual aircraft were used but these, too, had weakness: they were complicated to tailor to an aircraft’s specifics; difficult to install and ignite; could not be controlled (or stopped) once ignited; added weight to the plane; and were useless for arresting the landing.
Q: Before you explain what’s in the EMALS system, what’s in the steam system (in brief)?
A: Steam catapults typically supply 615 kg (1,350 pounds) of steam at over 1000 psi pressure for each launch, with steam produced by the nuclear reactor (in modern carriers) and delivered via a complex array of pipes and valves to the catapult control and pistons. It’s an open-loop system that, once set-up and the launch initiated, has no closed-loop feedback for real-time control of the cycle.
The physical arrangement of the catapult system on a carrier contrasts with a non-carrier vessel, where the boiler, steam lines, and shaft turbines are in close proximity in the engine room. Also, the steam system has other hydraulic subsystems, a water system to brake the catapult after launch, and many associated pumps, motors, and controls. It is a large, heavy, maintenance-intensive system, and its sudden launch shocks shorten airframe lifespans for carrier-based aircraft.
Q: So why use it at all?
A: Until recently, it was the only viable approach to delivering the high impulse power needed, at the required magnitudes and repetition rates.
Q: So what does EMALS do?
A: EMALS uses an electromagnetic “rail gun” to launch/arrest aircraft. After delays of between five and twenty years (depending on how you look at the schedule) it’s closer to becoming a reality, and is installed on the carrier Gerald R. Ford (CVN 78) which was “commissioned” in 2017 but will not be operational until sometime between 2020 and 2022 (delayed from 2018) (Figure 1). Congress has appropriated funding for EMALS to be installed on the future Ford-class flattops John F. Kennedy and Enterprise, too. (Note that a related rail-gun technology is being used for shipboard kinetic weapons which will destroy the target by shooting non-explosive projectiles at Mach 6, and is in field trials now.)
Q: What are the presumed advantages of EMALS?
A: The EMAL system, with General Atomics as the lead contractor, should provide numerous benefits compared to the classic and long-used steam catapult, including reduced staffing requirements; 25% faster cycle times for more launches/landings; reduced topside weight (critical to carrier stability and roll-resistance); and smaller physical volume (Figure 2). The entire system is managed by software, of course, and is controlled and monitored via a keyboard and display screens; a tap of the appropriate key sequence by the launch leader will initiate launch, (Figure 3).
Q: Are there additional advantages?
A: In principle, it will also offer benefits in capabilities, as it will be able to handle a much wider span of aircraft types and loadings, including unmanned aerial vehicles (UAVs) which, ironically, are too light for the steam system; and its closed-loop architecture provides more-accurate adherence to the intended launch profile. Finally, it allows precise tuning of the launch profile to the specific aircraft, load, and wind conditions, which will minimize stress on aircraft during takeoff and so lengthen their service lives.
Q: What’s the power source at the core of the EMAL system?
A: The aircraft carrier has two nuclear reactors which together produce 600 MW of power. EMALS begins with a 100,000-hp electric motor which also functions as a generator, driven by a multi-megawatt electric-power system. This motor-generator functions as a motor while being “charged” by spinning up to 6400 rpm; it functions as a generator when it switches to deliver its energy to the load (thus decreasing the rpm as it gives up its energy).
To launch, this rotor-based kinetic energy is drawn off and converted to electrical power in a two- to three-second pulse. The generator needs just 45 seconds between launches to “recharge” the rotors by spinning them back up to capacity, ready to deliver another burst of power. This motor-generator assembly weighs over 80,000 pounds, is about 13.5 feet long, 11 feet wide and 7 feet tall, and can deliver up to 60 megajoules of electricity and 60 megawatts at its peak. A carrier will require twelve of these energy storage subsystems (motor generator, the generator-control tower, and the stored-energy power supply) to accelerate a typical aircraft to over 150 mph in less than a second, on a track less than 100 feet in length.
Q: What about the launch rail “motor”?
A: The linear induction motor has three main parts: a pair of 300-foot-long parallel, stationary beams separated by a few inches and acting as a track, plus a 20-foot-long carriage (shuttle) that is sandwiched between the two beams and can slide back and forth along their length. The shuttle attaches to the front landing gear of the aircraft, using the same connection technique as the steam catapult uses; no changes to the aircraft are needed.
The beams forms the physical structure of the linear motor; each has dozens of independent windings arrayed along their length plus the wiring needed to energize them (Figure 4). By turning windings on and off in sequence along the beam, a magnetic wave is generated. This produces an attracting magnetic force at the carriage’s leading edge and a repelling one at its trailing edge, thus pulling the carriage forward.
Q: What happens after the aircraft launches?
A: After the carriage releases the aircraft, it is brought stop in only 20 feet using a sudden reversal of the linear-motor windings at the end of the beams (this replaces the water brake used by the steam system). After stopping, the carriage is smoothly returned to its starting position by sequencing power through the beam’s entire length, but in the opposite direction of the launch phase.
Q: What’s the power-supply chain look like?
A: The pulse-power and overall energy needs of the linear motor are well beyond what batteries or a conventional generator could deliver. Instead, the power produced by the generators is stored kinetically in rotors spinning at 6,400 rpm. To launch, this rotor-based kinetic energy is drawn off and converted to electrical power in a two- to three-second pulse. As the kinetic energy is drawn from the rotors, they slow down and their remaining available energy drops. The generator needs 45 seconds between launches to “recharge” the rotors by spinning them back up to capacity, ready to deliver another burst of power.
Part 2 of this FAQ will look at the actual installation and the related progress and problems.
References (both technical and “news”)
- Popular Mechanics, “Watch the Navy’s Railgun Catapult Skip a 4-Ton Cart Like a Stone”
- Popular Mechanics, “Trump Tells U.S. Navy to Go Back to Steam Catapults”
- com, “Engineering Destruction: The Terrifying and Awesome Power of The USS Gerald R. Ford”
- Ars Technica, “Trump, steamed over delays, pulls plug on electric carrier catapults”
- Defense Industry Daily, “EMALS/ AAG: Electro-Magnetic Launch & Recovery for Carriers”
- International Journal of Mechanical And Production Engineering, June 2017, “A Brief Review on Electromagnetic Aircraft Launch System”
- Naval Technology, “EMALS – launching aircraft with the power of the railgun”
- Next Big Future, “US Navy Readying Electro-Magnetic Launch for New Carriers Which Will Also be Ready for New Lasers and Railguns Later”
- Smithsonian Air & Space, “How Things Work: Electromagnetic Catapults”
- Breaking Defense, “Navy’s Troubled Ford Carrier Makes Modest Progress”
- General Atomics, “Electromagnetic Aircraft Launch System (EMALS)”
- Navy Times, “Report: EMALS might not be ready for the fight”
- Navy Times, “Why Trump asked the Wasp’s crew ‘electric or steam?’ “
- Navy Times, “Why the Navy thinks the carrier Gerald R. Ford will work after all”
- Business Insider, “The Navy’s new $13 billion supercarriers have a high-tech feature that is apparently driving Trump crazy”
- Strategy Page, “Naval Air: EMALS In The Age Of Error”
- Power Electronics Technology (Informa), “Carrier-Based Launch of Aircraft to Use Power Electronics Instead of Steam Catapult”
- S, Naval Institute, USNI News, “Experts: Navy Would Spend Billions to Answer Trump’s Call to Return Carriers to Steam Catapults”