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Within the past decade, concerns over the environmental impact of civil aviation have pushed the research community towards the development of more efficient propulsion technology, which delivers a lower carbon and NOx footprint. The current progress achieved in the various specialised disciplines creates the need to redefine the performance barrier achievable by 2025 state-of-the-art aero-engines. This paper summarises some of the latest advancements within the gas turbine research community on the performance modelling and analysis of very low dspecific thrust direct-drive turbofan engines for EIS 2025. Engine and aircraft performance models were used to predict the extent of fuel burn reduction at aircraft level that could be achieved by reducing the engine specific thrust level , increasing operating pressure and temperature levels and applying technology factors representing a step beyond current state-of-the-art. The models represented modern three-spool direct-drive turbofans powering a typical A350XWB-type aircraft. The outputs of the engine design of experiments (DoE) exercise resulted in three most promising candidates. Targeting EIS in 2025, the final optimum design showed 14.81% block fuel improvement for a representative long (7000nm) range mission, accompanied by 30.9% penalty on engine weight. These results propose that with current technology level, at the lower end of the specific thrust range, there is still available design space for the direct-drive turbofan architecture. INTRODUCTION In order to achieve and maintain a leadership position within the aerospace sector, engine manufacturers are continuously in pursuit of more efficient engines in a commercial environment where even an improvement of 0.1% in terms of SFC is considered by airlines to be of significant benefit in the long-run (Pitt and Norsworthy, 1999). Additionally, with the aerospace sector under scrutiny due to its contribution to GHG emissions, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) put forward its ‘vision’ for 2020 (European Commission, 2001) setting a target of 50% reduction in fuel-burn and perceived noise, and 80% reduction in landing/take-off NOx emission compared to a typical EIS in 2000 aircraft. This further restricts the design space of the conventional engine architecture which is already at its limit in terms of efficiency. The overall efficiency of an engine is a product of the propulsive, thermal and transfer efficiency. Thermal efficiency depends on component efficiencies, overall pressure ratio (OPR) and turbine entry temperature (TET) which in turn depends on advancements in material. Propulsive efficiency dictates how well useful work is achieved by the propulsive jets relative to their kinetic energy. With higher jet velocity, with respect to the airspeed, propulsive efficiency decreases. This has led to the development of high by-pass ratio turbofan engines which led to large fan diameters and weight implications due to the size of the fan and the nacelle. Transfer efficiency dictates the extent of which the energy supplied by the core reaches the

Assessment of the performance boundaries of very low specific thrust direct-drive turbofan engines at aircraft level for EIS 2025