Dual Drive Booster for a Two-Spool Turbofan: Performance Effects and Mechanical Feasibility

A novel two-spool turbofan engine configuration is described which uses a booster powered by both the low an high pressure spools. Design and off-design performance analysis shows the operating characteristics of the configuration, and a mechanical feasibility study of the gearbox is presented. The trends toward ever higher engine overall pressure ratio and bypass ratio have resulted in a combination of higher pressure ratio and lower blade speed in the booster compressor of conventional two-spool turbofans. This combination gives rise to many stages in the booster and/or lower booster efficiency and also a higher degree of off-design mismatch between the core compressors. The current paper describes an engine architecture which aims to alleviate both these issues by powering the booster compressor from both low and high pressure spools through an epicyclic gear system. We have called this engine architecture the dual drive booster. The concept gives the engine designer greater flexibility to optimize component performance and work split, resulting in the potential for lower cruise specific fuel consumption and higher hot-day takeoff thrust capability than current engine configurations. The gear system is described along with the mathematical derivation of the booster rotational speed in terms of LP- and HP-spool speeds. Both the design point and off-design performance modeling have been conducted and comparison is made between a conventional turbofan and a turbofan fitted with the dual drive booster. The results show a significant enhancement in takeoff thrust due to the better speed match of the booster. The paper also describes the results of a preliminary study into the design and mechanical feasibility of the engine architecture and gear system. The presented concept is an alternative to the conventional turbofan and should be considered during the conceptual design of future aircraft engines.