The paper discusses some major fuel-related issues which will influence the development of aviation over the next 50 years. Provided that global economic development is not halted by worldscale war or crisis, the demand for civil air transport is likely to continue to expand. The consequent rising requirement for aviation fuel is considered in relation to the projection that the total oil extraction rate from relatively accessible fields will reach a peak and then decline. The fuel options for aviation, against such a scenario, are explored. It is concluded that if the aviation demand cannot be met economically from conventional oilfield sources, supplementation by kerosene-like fuel synthesized from other feedstocks is much more likely, in the timeframe considered, than the radical step of a move to liquid hydrogen. The latter could be delayed until much later unless necessitated by constraints on carbon emissions. Other issues relating to aero-engine emissions are discussed, including the present uncertainties regarding the environmental significance of injection into the atmosphere at aircraft operating altitudes. The implications of restricting operations to the troposphere, to avoid the lower mixing rates of the stratosphere, are considered briefly.
The major factors determining the choice of engine cycle for a combat aircraft are the requirements of the design mission and those of aircraft speed and agility. The requirement for jet-borne flight in short take-off vertical landing (STOVL) aircraft imposes further demands on cycle and configuration. The changing nature of combat aircraft requirements is the reason for changes in engine design. Specific thrust is shown to be the major parameter defining engine suitability for a particular role. An examination of mixed turbofan characteristics shows that specific thrust is also the key to understanding the relationships between engine characteristics. The future development of combat engines is discussed, in particular the implications of stoichiometric limits on cycle temperatures and the benefits of variable cycle engines are examined. Recent work on advanced STOVL (ASTOVL) aircraft is reviewed and aircraft/engine concepts designed to meet the requirements of the role are assessed. Experience shows that the technology for these advanced engines must be fully demonstrated before production to minimize the risks and costs of the development programme.
The Variable Pitch fan has a number of features which make it attractive as the basis for ultra-high bypass ratio ducted fans designed primarily for STOL aircraft. The variability imposes certain design constraints, particularly on fan pressure ratio, and leads to differences in engine geometry relative to equivalent fixed pitch engines. The merits of such engines are discussed under the headings of Performance, Noise, Engine Control, Thrust Modulation, Provision of Air Bleed for High Lift, Reverse Thrust and Development Flexibility.
The prospects for the longer-term evolution of aircraft and their propulsion systems are explored, taking a fifty year time-frame. It is considered that the most likely global scenario features continued economic development, aided by the introduction of fuel from alternative sources to offset a progressive shortfall in the availability of oil from relatively accessible fields. Pollution and other environmental problems are foreseen as exerting an increasing influence on decision making. Assuming such a scenario, the aviation fuel prospect is considered. It is concluded that the most likely alternative fuel for aviation in the next fifty years is some form of manufactured liquid hydrocarbon resembling current aviation kerosine, rather than liquid hydrogen, which is a candidate for later introduction. An assessment is offered of the classes of flight vehicle that will be required for civil and military purposes in the time-frame under review. The implications with regard to propulsion systems are then considered, with a discussion of design objectives, technological challenges and research requirements.
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