Aviation has a significant contribution to climate change, which is poised to increase in the coming years due to increasing demand in air travel. The A321 APPU aircraft could offer a significant improvement as it offers a synergistic combination of two interesting technologies-a fuel-flexible hydrogen combustion system combined with boundary layer ingestion, by introducing a hydrogen-powered auxiliary power and propulsion unit (APPU). This turboshaft engine is located in the tail cone and powers a boundary layer ingestion propulsor, producing approximately 15% of the thrust. To improve the efficiency of the APPU, the feasibility of the steam ijection and recovery (SIR) cycle is evaluated. This semi-closed water cycle can reduce fuel consumption and NOx emissions. Both the baseline and the SIR APPU are modelled in pyCycle, an open-source gas turbine parametric analysis tool. The baseline APPU engine was found to have a thermal efficiency of 45% and a mass of around 500 kg. The SIR cycle can reduce fuel consumption by up to 7% and decrease NOx emissions by approximately 33%, with an increase in engine mass of approximately 15%.

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.