## 4 Conclusions  
This study aims to enhance our understanding of the performance of a hybrid solid oxide fuel cell-gas turbine (SOFC/GT) system when applied to aircraft. The investigation focuses on evaluating a specific system configuration, considering the intricate interdependence among the system's components and their influence on each other's performance. Continual parameter adjustments within an ever-changing operational context are necessary to optimize system performance. Of particular importance are the HPC outlet air split ratio (referred to as the AS-ratio) and its direct impact on SOFC fuel utilization and electric power generation. Consistent fine-tuning of the AS-ratio is crucial, as failure to do so adversely affects the hybrid system's performance. The study also assesses the influence of the CPOx O/C ratio on the overall system performance. The study also investigates the effect of varying the SOFC fuel utilization (FU) on system performance. To fully comprehend the impact of these parameters on system performance, the study evaluates the system's performance when these parameters deviate from the design point parameters. Under specified design conditions (altitude: 11 km, Mach: 0.8, standard day, aircraft speed: 849.6 km/hr. ), the system achieves a total power output of 1.96 MW, with 102 kW generated by the SOFC and 7.9 kN (1.86 MW) of thrust. The SOFC's electric efficiency at this point is 45.89%, determined by the specific AS-ratio configuration and power demand. System off-design performance is assessed under variable parameters, particularly the O/C ratio, SOFC fuel utilization (FU) and the HPC outlet air split ratio (AS-ratio). The key findings of the off-design analysis are as follows:  
1. O/C ratio variation: By varying the fuel flow rate into the CPOx, its influence on the hybrid system performance is assessed. It is observed that it influences SOFC electric efficiency as the fuel utilization in the stack is compromised by suboptimal O/C ratios. An O/C ratio between 0.85 and 0.9 leads to the production of reformed hydrogen, resulting in a fuel utilization of 75 \(\pm\) 10%. In this same range, a balance between optimal fuel generated, carbon deposition and reactor temperature are obtained. The variable O/C ratio is also observed to yield performance variation in the GT, where there is a modest thrust decrease from 7.92 to 7.86 kN. The TSFC is also observed to increase from 308.5 to 311 mg/s/N. 2. SOFC FU variation: The effect of a variable SOFC FU on the hybrid system is assessed by varying the FU from 60% to 95%. It is observed that the SOFC performance parameters (SOFC power, electric efficiency, Power density) increase within this same variation range. Subsequently, its influence on the GT is assessed, where it is observed that there is a slight decrease in thrust from 7.907 to 7.887 kN, prompting a slight decrease in the GT overall efficiency. The TSFC also increases slightly from 309 to 309.8 mg/s/N. This showed that the SOFC FU bears a greater influence on the SOFC than it does the GT. 3. AS-ratio variation: As the hybrid system is operating in flight, due to the variable engine inlet air mass flow rate, one critical parameter is influenced, and therefore variable, and that is the AS-ratio. The three fractions to the reformer, cathode and combustor are perturbed to assess their influence on the hybrid system performance parameters. The reformer, cathode and combustor fractions are perturbed from 0.001 to 0.4, 0.2 to 0.9 and 0.05 to 0.7, respectively. From these perturbations, it is noted that it is imperative to always optimize the AS-ratio at any point in the aircraft operation. This analysis provides insight to optimize these variable operational parameters for optimal hybrid system performance. The obtained results significantly enhance our understanding of the application of SOFC/GT systems in aircraft, particularly by identifying the crucial parameters necessary for achieving optimal performance.