Results from the APPU project

Abstract

Results from the APPU project, which investigated the concept of an "Auxiliary Power and Propulsion Unit" (APPU) are presented. The APPU is a hydrogen-driven boundary-layer-ingesting engine at the tail end of a passenger aircraft which replaces the conventional APU and contributes about 15% of total thrust at top of climb. The aim of the configuration is to allow the introduction of hydrogen and BLI technology by upgrading existing aircraft designs. The concept aims to benefit from the advantages of these new technologies as much possible, without requiring the same level of reliability as for conventional propulsion, during times when hydrogen infrastructure is not universally available. The investigation concerns hydrogen tank mass, engine efficiency, operational, aerodynamic and reliability aspects, and finds block CO2 emissions can be reduced by a larger amount than the thrust rating of the auxiliary hydrogen engine may suggest. One reason for this is that the additional engine permits smaller and more efficient designs for the main engines. A still larger benefit is found to arise out of the assumption that the APPU engine and associated H2 fuel systems is less reliable than the conventional underwing engines. This assumption permits different strategies to maximize the utilization of hydrogen over kerosene. CO2 emissions for the design mission are found to be reduced by 23.1% over the A321neo, and by 15.5% over an A321neo fitted with updated turbofan engines.