AS A jet fighter screams away from the deck of an aircraft-carrier a swirl of vapour trails from the steam-driven catapult that launched it into the air. Catapults are an ancient technology, developed from the crossbow for increased range and firepower. By the Middle Ages, they could hurl rocks as big as 75kg (170 pounds) to batter castle walls. From using the kinetic energy stored in twisted ropes and sinews to launch projectiles, catapults were developed using hydraulics, gravity and air as propellants. Steam became a favourite with naval architects because it was on tap, generated by the engines of ships. Now catapults are going electronic and finding new military and civilian roles.
Despite their punch, the steam-driven catapults on aircraft-carriers are not as powerful as some would like. Even with their engines roaring, catapulted aircraft still need the extra airspeed provided by turning the carrier into a headwind. If there is no wind, you must “crank the ship up” to generate one by sailing faster, says a retired commander of a US Navy warship.
The US Navy is so impressed with the push delivered by its new catapult, the Electromagnetic Aircraft Launch System (EMALS), that its next aircraft-carrier, the Gerald R. Ford, is in effect being built around it, says Captain James Donnelly, manager of the launcher. EMALS can accelerate a heavy warplane to 180 knots (333kph)—about 30 knots faster than a steam catapult. As the acceleration can be finely adjusted every millisecond, it produces smoother launches, which are better for pilots and aircraft.
The system is being fine-tuned by General Atomics, a defence contractor, at an airfield in Lakehurst, New Jersey. Just under the runway lies a nearly 100-metre array of electromagnets straddled by a sliding, conductive armature. Precisely timed pulses of electricity create a wave of magnetism which rapidly pushes the armature along. The armature is connected to a shuttle on the runway above, to which the aircraft’s nose wheel is hitched.
The technology is similar to the linear-induction motors employed in some high-speed trains—except, of course, trains are not expected to take off. The Lakehurst system can propel the shuttle to the other end of the runway in just 2.4 seconds, says Mike Doyle, the programme’s chief technology officer. But it takes a lot of energy, more even than a nuclear-powered aircraft-carrier can suddenly muster. Hence energy is stored kinetically in rapidly spinning rotors and released to power generators whenever the catapult is fired.
Such kit is not cheap. The four-catapult system for the Gerald R. Ford has a price tag of some $750m. But it eliminates all the tentacular plumbing of steam catapults and should cut crewing and upkeep expenses by about $250m over its expected 50-year life, the retired commander estimates. Being much lighter it will also make the aircraft-carrier more stable, manoeuvrable and cheaper to propel.
EMALS is costly partly because it has to be squeezed into the confines of an aircraft-carrier. Building such a system on land would be much cheaper. This leads some to wonder whether catapults could be used to cut the costs of commercial flying. The engines on airliners guzzle fuel on take-off. Scott Forney, head of General Atomics’ electromagnetics business, says that he has been approached by cargo airlines considering this. But could it be used to launch passenger aircraft too?