For more than seven decades, the steam-powered catapult has been the standard mechanism for launching airplanes from the decks of aircraft carriers and arresting them on landing.

Without the catapult, aircraft could not reach take-off speeds of over more than 100 knots in just a few seconds and within 100 feet, nor slow to a full stop in a similar time and distance. The steam catapult is a system that works, as proven by how routine aircraft carrier operations have become and how quickly the system can cycle to launch and land planes, often doing so simultaneously thanks to the ships’ angled decks.

But the steam-based catapult suffers from some serious weaknesses. It requires an enormous amount of energy to function and it is inefficient, so it doubly strains a ship's steam boilers (even on nuclear-powered carriers). It also 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, and space is limited even on a modern carrier. A steam catapult also is an open-loop system with no feedback to control its performance. Finally,setting it for different lunch types is a complicated process, and it can handle only a limited range of aircraft types, weights and launch/landing conditions (all of which vary from ship to ship).

As a result, the U.S. Navy has sought an alternative to provide the same sudden, brute-force ramp-up required to launch an aircraft, but with fewer negatives. There were tests using "bottle rockets" attached to individual aircraft but these had many 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 a plane on landing.

The EMALS launch rail, shown at a test facility, powers a series of linear windings on and off, which creates an electromagnetic wave-like action that pulls the launch carriage forward with extraordinary speed and power.The EMALS launch rail, shown at a test facility, powers a series of linear windings on and off, which creates an electromagnetic wave-like action that pulls the launch carriage forward with extraordinary speed and power.Now, however, a new approach called the Electromagnetic Aircraft Launch System (EMALS) is close to deployment. It uses an electromagnetic "rail gun" to launch/arrest aircraft. After delays of between five and 10 years (depending on how you look at the schedule) the system is becoming a reality and is being installed on the carrier Gerald R. Ford (CVN 78) which is to be commissioned in 2016. (Watch a video detailing other innovations of the Ford class of carrier.) Ten so-called Ford-class carriers are planned, with construction continuing through 2058. (The same rail-gun technology is being field tested for shipboard kinetic weapons intended to destroy a target by shooting non-explosive projectiles at Mach 6.)

The EMALS system, with General Atomics as the lead contractor, should provide 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. The entire system is managed by software and is controlled and monitored via a keyboard and display screens: a tap of the appropriate key sequence by the launch lead will initiate the process.

The system also will be able to handle a wider span of aircraft types and loadings, including unmanned aerial vehicles (UAVs) which are too light for the current steam system. The system also offers a closed-loop architecture to provide more accurate adherence to 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.

Power and Energy

Steam catapults typically require more than 1,000 pounds/square inch (psi) of pressure for each launch. On current aircraft carriers they do so using steam produced by the nuclear reactor and delivered via an array of pipes and valves to the catapult control and pistons. In addition, the system has other hydraulic subsystems, a water system to brake the catapult after launch and associated pumps, motors and controls.

In contrast, the EMALS system uses a 100,000-hp electric motor that also functions as a generator driven by a multi-megawatt electric-power system. It is called a motor generator because it is used like a motor while being "charged" by spinning up its rpm; it also functions as a generator when it switches to delivering its energy to the load, thus decreasing the rpm.

This motor-generator assembly weighs more than 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 at 60 peak megawatts. A carrier will require 12 of these energy storage subsystems (motor generator, generator-control tower, and stored-energy power supply) to accelerate a typical aircraft to more than 150 mph in less than a second, on a track less than 100 feet long.

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 aircraft’s front landing gear using the same connection technique as the steam catapult; no changes to the aircraft are needed.

The beams form the physical structure of the linear motor; each has dozens of independent windings arrayed along their length plus wiring to energize them. 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. This action pulls the carriage forward.

After the carriage releases the aircraft, it is brought stop in 20 feet using a sudden reversal of the linear-motor windings at the end of the beams (this replaces the water brake used by today’s steam system). After stopping, the carriage is returned to its starting position by sequencing power through the beam's entire length, but in the opposite direction of the launch phase.

Major components of the EMALS are housed below decks at the start of the launch track, with primary power from the ship's reactors accumulated and stored during a brief rotor-recharge period, and released in a one-second burst of power.Major components of the EMALS are housed below decks at the start of the launch track, with primary power from the ship's reactors accumulated and stored during a brief rotor-recharge period, and released in a one-second burst of power.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. (Keep in mind that energy is the ability to do work, while power is the rate at which energy is delivered; they are mathematically related but different, although the two terms are often used interchangeably in casual or less-technical conversation.)

To launch, this 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 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.

EMALS System Architecture

The EMALS is more than the power source (motor-generator) itself. The overall design has six major functional blocks.

  1. The Prime Power Interface, which is the interconnect to the ship's electrical distribution system (which is sourced by nuclear reactors) and delivers power to drive the energy-storage rotors;
  2. The Launch Motor (the linear-induction motor discussed above);
  3. The Power-Conversion Electronics, which takes the energy stored in the rotors and converts it to the timed wave to energize a series of windings within the launch motor. The power switches that control the windings are located below decks; the switching for each winding is controlled by a module built of solid-state SCR and IGBT devices;
  4. The Launch Control, which manages the current delivered to the launch motor windings for smooth, tailored acceleration, with closed-loop feedback for precision as conditions vary;
  5. The Energy Storage motor-generator rotors (also discussed above);
  6. The Energy Distribution System, which includes the cables, disconnects, and terminations needed to deliver the energy from the power-conversion system to the launch motor.

The list of technology advances and innovations needed to build a system such as EMALS is extensive. On the electrical side, the key component is the multi-component IGBT/SCR module that switches megamps to the windings of the motor (one per winding) in milliseconds, as well as the components of the circuitry which controls those modules. Mechanically, the EMALS needs high-strength, defect-free materials for the motor-generator's rotor, along with advanced bearings for the rotor.

Clearly, in a system of this complexity and power levels, no detail is minor. Every "wire" (actually, huge bus bars), every connection, every action is of a physical size and electrical magnitude where any mistake, imperfection or oversight has serious and potentially disastrous consequences. Any action or "fix" must be carefully evaluated, as there is no tolerance for hasty, improvised or temporary solutions.

Start-Up Pains

On May 15, 2015, the Navy conducted its first shipboard full-speed EMALS catapult test shots, called “no-loads,” since there was no weight attached to the launching shuttle. The test aimed to verify the integration of the catapult system. The next test phase began several days later with a series of “dead load” launches. Here, wheeled steel vessels weighing up to 80,000 pounds were used to simulate the weight of an aircraft and verify that the catapult and each of its components was working properly. (Watch a video of the dead-load testing.)

The first successful dead load weighed 15,000 pounds and launched at 140 knots; the second one was 8,000 pounds and traveled at 180 knots. Over the three days, 15 sleds of varying weights and speeds were launched. (As a testament to "stuff happens", the media was present for that first test launch; when the button was pushed, absolutely nothing happened—apparently some miscommunication between various subsystems and software. All the successful tests occurred after the media departed.)

The related landing-arresting subsystem, called the Advanced Arresting Gear (AAG) sub-program, will replace the present hydraulic-ram based system and provide adjustable firmness and flexibility in managing the shock absorption and retarding of carrier’s arresting wires. It uses energy-absorbing water turbines and a large induction motor to provide the adaptable, fine-tuned control of the forces and payout on the wires. It is designed to handle a wide range of aircraft (including some which are heavier than the present arresting system can accommodate), and reduce staffing and maintenance. The AAG also adds self-diagnosis, performance analysis and detailed system-status indicators. Although it will likely not be ready in time for the commissioning of the carrier Gerald R. Ford, it will be retrofitted to that ship, and also be retrofitted to the existing fleet of Nimitz-class carriers.

References

  1. http://www.airspacemag.com/military-aviation/how-things-work-electromagnetic-catapults-14474260/?no-ist
  2. http://www.ga.com/emals
  3. http://www.navytimes.com/story/military/2015/06/16/emals-fails-in-first-public-launch/28808761/