In ramjet and scramjet engines, due to the very high velocity of the incoming flow into the combustion chamber, efficiency and effective fuel–air mixing face significant challenges. One efficient method to increase fuel residence time and create a suitable recirculation zone for stable combustion is the use of cavity flameholders. Despite the widespread application of this method, the effect of cavity geometry on combustion performance in both ramjet and scramjet engines, especially under heated conditions, still requires comprehensive investigation. The aim of this study is to numerically investigate the effect of cavity geometric parameters—including height, length, and ramp angle—on combustion efficiency in both ramjet and scramjet engines. For this purpose, simulations were performed using ANSYS Fluent with the SST k–ω turbulence model in a compressible, hydrogen-fueled combustion configuration. The results indicate that increasing the cavity height in both engines improves combustion efficiency. Reducing the cavity length can also enhance combustion efficiency in both engines. These findings can serve as a basis for optimizing flameholder geometry in future supersonic engines.