A study conducted at the Faculty of Aerospace Engineering in Delft focused on the heat management and performance improvement of aircraft engines by integrating liquid hydrogen. The concept involved replacing the APU of the A321neo with an Auxiliary Power and Propulsion Unit (APPU) powered by a hydrogen-based turboshaft system with boundary-layer ingestion. The use of cryogenic hydrogen served the dual purpose of cooling high-temperature engine parts and preheating the hydrogen to enhance thermal efficiency through an increased lower heating value. By analyzing different scenarios such as cooling bleed air, raising turbine inlet temperature, and intercooling the core flow with varying compressor pressure ratios, researchers found that intercooling with an optimized pressure ratio led to the highest efficiency gain of 3.6%. This method outperformed alternatives like raising TIT (0.7% efficiency gain) and reducing bleed flow (0.2% efficiency gain). The study underscored the importance of maximizing heat extraction by hydrogen to achieve the greatest efficiency benefits. Despite challenges at high operational pressure ratios, the results showcased the potential of liquid hydrogen in significantly enhancing engine performance, particularly through intercooling with pressure ratio optimization.