Developments in advanced materials, more than anything else, have contributed to the spectacular progress in thrust-to-weight ratio of the aero gas turbine. This has been achieved in the main through the substitution of titanium and nickel alloys for steel, fig 1. Aluminium has virtually disappeared from the aero engine, and the future projection illustrates the potential for composites of various types. The aero engine designer requires a much wider range of materials than the airframe designer because the temperature range is large, whereas a civil airframe, even that of Concorde, lies entirely within the capability of aluminium. Materials also supply the enabling technology for equally significant improvements in performance and reliability.
The RB211 and Trent families of engines provide good illustrations of the link between material capabilities and engine performance. Civil engine programmes are becoming the drivers for materials development, replacing the military programmes that were the leaders at the beginning of the gas turbine era. The earlier approach of technology transfer from military to civil is tending to switch direction.
The turbine entry temperatures of modern civil engines are now approaching those of the latest military combat engines, and the longer operational lives expected by airlines place greater demands on materials technology.
Design Parameters
The key design parameters are fan airflow, which is related directly to thrust, particularly at take-off, and the pressure ratio and flow size of the core, which determine the fuel consumption and climb thrust for a given engine size. Take-off thrust is determined by the airflow, with a direct relationship to fan diameter. Increasing physical size places considerable importance on design, not only for low weight but also for structural stiffness.
Core engine size is equally important The power output to drive the fan is determined by core mass flow and combustor temperature rise. Component development provides increased temperature capability, but the physical size of the compressor is not easily changed and mass flow through the core can only be increased by supercharging to higher overall pressure ratios.
Three examples of aerospace components - the fan blade, the rear of the high-pressure compressor and the high-pressure turbine illustrate how materials are responding to the required performance and design parameters. They also highlight the potential of advanced materials such as titanium and nickel alloys, plus the possibilities for composite materials. In the production of larger diameter, low weight fan blades, the contribution of advanced materials is vital, not only in terms of density but also through advanced methods of fabrication. New materials must also be able to withstand the demands for increasing compressor delivery temperature and turbine entry temperature. Specific fuel consumption depends on thermal as well as propulsive efficiency. Thermal efficiency depends in turn on the maximum temperature of the cycle, as with any heat engine. Maximising efficiency within the design compromise on each component is clearly important for fuel consumption.
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