Critical Technologies: Getting On and Off the Boat
What things make carrier-based Naval aviation possible? Actually, a surprisingly few critical technologies set carrier and carrier-capable aircraft design apart from conventional ship and land-based aircraft designs. Most have to do with getting on and off of the ship, and being tough enough to do it over a period of decades.
The Need for Speed: Chasing the Wind
Other than being a lot of fun, speed is essential for aircraft carriers… for two reasons:
• High speed generates artificial wind over the flight deck to assist in the launching and landing of aircraft.
• High sustained speed allows carriers to rapidly transit from one part of the world to another.
Wind over the deck allows some influence over an aircraft's "stall speed"-that is, the minimum speed at which an aircraft can still be controlled without falling out of the sky. The lower an aircraft's stall speed, the easier it will be to launch and land (a consideration that's especially important on the pitching deck of an aircraft carrier). You get wind over the deck, first of all, simply by steering the carrier into the wind. Every knot of wind over the bow acts as a knot of airspeed for an aircraft trying to take off or land, which is why carriers always come into the wind to conduct flight operations. You get even more wind over the deck by cranking up the speed of the carrier. Thus, if you have a fifteen-knot wind and steam into it at twenty-five knots, you can effectively launch and land aircraft at forty knots under their normal stall speed. Putting wind over the deck also maximizes aircraft payload and return weight and reduces stress on the flight deck. All of this means that carriers will be using their maximum speed more often than other ships.
Carriers need more than just a high maximum speed (for launching and recovering aircraft); they need to maintain a high transit speed so CVBGs can move quickly across the oceans. The whole point of forward presence is to have it available now. Building a high, sustained speed into a ship is not easy. While many ships may be capable of "dashing" for short times at high speeds, they are normally designed to cruise at more sane and economical rates. The twelve-knot cruising speed of your average merchant ship is fine for transporting cars or athletic shoes, but it just won't do if you want to move a CVBG in a few days from the South China Sea (say) to the Persian Gulf. That means carrier power plants have to be durable enough to cruise at high speeds for days or weeks at a time, without having to put in for repairs or overhaul. This is one of the reasons why nuclear power plants and their highly reliable machinery have been the gold standard for carriers for going on three decades. Just how fast is fast enough? Most naval analysts believe that carriers require minimum battle/flank speeds of thirty-three knots/ sixty-one kph to operate aircraft in the widest possible wind and weather conditions, and sustained speeds of at least twenty knots/thirty-seven kph to allow for rapid transits to crisis areas.
Catapults and Wires: Getting On and Off the Boat
Though aircraft carriers are very big, there is still very little room on the flight deck to support takeoffs and landings. Since a carrier operates as many aircraft as a small regional airport on just a few acres of flat space (about 4.5 acres on a Nimitz-class (CVN-68) ship), it makes sense to take advantage of some mechanical muscle to assist the aircraft on and off the flight deck. To this end, carrier designers have for many years depended upon the tried-and-true technologies of catapults (to give aircraft the speed to take off) and arresting wires (to give the drag to land).
The current generation of carrier catapults are basically nothing but steam-powered pistons… steam-powered pistons that can throw a Cadillac half a mile (one kilometer). That's a lot of power! But when you're trying to fling a fully loaded aircraft like an F-14 Tomcat or E-2C Hawkeye off a carrier deck, you need that much power. This is how it works. Simply described, the catapult is a pair of several-hundred-foot-long tubes built into the deck, with an open slot along the top (at deck level) that's sealed by a pair of overlapping synthetic rubber flanges. A "shuttle" running above the deck is attached (through the flanges) to pistons at the rear of the tubes; and the nosewheel towbar of the aircraft is attached to the shuttle when it is launched. To accomplish the launch, high-pressure steam, drawn from the carrier's propulsion plant pressurizes the tubes behind the pistons. When the proper pressure is reached, a lock is released, a small, disposable fastener called a "holdback" (it holds the nosewheel to the shuttle) breaks loose, and the pistons (and attached shuttle) fling the aircraft down the deck. At the end of the deck the towbar releases from the shuttle, and the aircraft is airborne. The piston and shuttle assemblies are then run aft (back to the rear of the tubes) in order to prepare for the next launch.
Catapults are high-maintenance, complex, high-risk pieces of equipment that have the ugly habit of failing or breaking if they are not treated with loving care. This is one of the reasons why some nations have chosen to forgo them in their carriers and employ instead vertical/short takeoff and landing (V/STOL) aircraft (like the Harrier/Sea Harrier jump jet), which do not require catapults to operate from ships. Though the technology behind a carrier catapult is relatively simple, the size of the tubes and the magnitude of the forces involved make designing and building them hugely difficult. Very few nations have either the technical or industrial skills to build them. Thus, the very proud and competitive French (who don't like to admit to being second in anything military) are buying American catapult units for their new supercarrier, Charles de Gaulle. The Soviets, after a generation of trying, failed to devise a reliable catapult unit for their carrier, the Kuznetzov.
While taking off from a carrier is difficult, landing on one is almost appalling! Setting down a CTOL (Conventional Take Off and Landing) aircraft like an F/A-18 Hornet strike fighter, for example, has been compared to taking a swan dive out of a second-floor window and hitting a postage stamp on the ground with your tongue. During the Vietnam War, scientists made a study to find out when naval aviators were under their greatest stress during a mission. Their cardiac monitors told the scientists that getting shot at in a bomb run was not even close to the stress of a night carrier landing in heavy weather. In order to make carrier landings easier and less fearsome, the Navy has developed a series of automatic and assisted landing aides to help pilots get their aircraft onto the heaving, pitching deck. But once you're there, how do you stop thirty or forty tons of aircraft that have just slammed down at something over a hundred knots?