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.

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The concept of an "Auxiliary Power and Propulsion Unit" (APPU) is introduced, which consists of a Boundarylayer ingesting (BLI) propulsor with an engine mounted at the rear of an passenger aircraft fuselage, replacing the Auxiliary Power Unit (APU) and contributing around 10% of total cruise thrust, as well as auxiliary power. This APPU unit is using hydrogen provided by an additional tank installed in the tailcone of the aircraft. The concept is aimed at lowering the threshold to installing both hydrogen-driven propulsion and BLI propulsors on aircraft in the short term, while minimizing resulting operational risk. The concept has been investigated using a preliminary aircraft synthesis tool and further component-level mass estimates. Operational aspects,
sensitivities and limits to the design have been investigated. Estimates of mission fuel burn find that CO2 emissions emissions reduce roughly proportionally to the APPU thrust share, with additional savings due to improved overall efficiency. Further improvements are deemed feasible and are the topic of ongoing research.@en