The Adaptable Gas Turbine
Whether creating electricity or moving planes, this engine continues to inspire innovation
It is now hard to remember when the aviation gas turbine—the jet engine—was not part of aircraft flight. Before jet engines, an aviation piston engine manufacturer could expect to sell 20 to 30 times the original cost of the engines in aftermarket parts. With the advent of the jet engine, this aftermarket figure dropped to three to five times the original cost (an important reduction that made air travel affordable and reliable, and airlines profitable, although engine manufacturers have had to alter their business models). In recent years, technology and market demands have resulted in even longer lasting engine components, dropping the aftermarket figure to increasingly lower levels.
A well-managed airline will try to keep a jet-powered plane in the air as much as 18 hours a day, 365 days a year. If well maintained, the airline expects the engines to remain in service and on the wing for 15,000 to 30,000 hours of operation, depending on the number of takeoffs and landings experienced by the plane. After this period, the jet engine will be taken off and overhauled, usually with replacement of parts that experience heating, such as the combustor and turbine. (Currently the in-flight shutdown rate of a jet engine is less than 1 per 100,000 flight hours. In other words, on average, an engine fails in flight once every 30 years.)
Aircraft jet engines make up about 25 percent of the cost of the airplane. In 2011 the worldwide aviation gas turbine market amounted to $32 billion, of which $27 billion was for commercial aircraft, with the remainder for military applications. Currently there are about 19,400 airplanes in the worldwide air transport fleet. Both major airplane manufacturers, Boeing in the United States and Airbus in Europe, project that there will be 34,000 aircraft in world fleets by 2030.
This promising market is stimulating jet engine development for commercial airlines, with an emphasis on fuel economy. Currently, 40 to 60 percent of airline operating expenses are jet fuel costs. The Pratt & Whitney turbofan engine shown in the second figure is currently being developed for new, single-aisle, 90- to 200-passenger aircraft. This engine has a hub-mounted gearing system that drives the front mounted fan at lower speeds, permitting as much as 16 percent less fuel consumption and much reduced engine noise. Later, the geared-fan technology may be applied to higher-thrust engines for larger airplanes.
Although military jet engines represent a smaller segment of the gas turbine market, the technology developed there has historically resulted in benefits for commercial aviation. The new U.S. F135 Joint Strike Fighter engine, at 40,000 pounds of thrust, is a case in point. It powers three variants of aircraft: An Air Force fighter that takes off conventionally, a carrier-based Navy jet and a short takeoff/vertical landing aircraft for the Marines.
Temperatures in the Joint Strike Fighter engine run 3,600 degrees Fahrenheit (1,982 degrees Celsius). How do the cobalt-nickel alloy turbine airfoils survive such running conditions? The vanes and blades are cooled to some eight-tenths to nine-tenths of their alloy melting temperatures (2,200 to 2,600 degrees Fahrenheit). Each high- temperature turbine airfoil is formed from an elaborate casting to accommodate the intricate internal passages and surface hole patterns necessary to channel and direct cooling air (bled from the compressor) within and over its external surfaces. An error in hole location or cooling air pressure ratios could cause airfoil gas path inhalation rather than cooling exhalation, which at such high temperatures would be catastrophic. The cooling design is based on some 30 years of research and unequivocally pushes forward the state-of-the-art of turbine performance and durability.