Aerothermodynamic trade-offs in Mach 12 scramjets: Coupled effects of geometry, mixing and kinetics

Extending the operational envelope of air-breathing propulsion to Mach 12 introduces a regime in which severe aerodynamic losses and high-temperature dissociation restrict the effectiveness of conventional scramjet design approaches. This study investigates the aerothermodynamic performance trade-offs of a hydrogen-fuelled scramjet at flight Mach number 12 and dynamic pressure 60 kPa using a quasi-one-dimensional framework with finite-rate chemical kinetics. A systematic exploration of the combustor geometry shows that the largest sampled specific impulse does not coincide with the largest sampled combustion efficiency. A shorter combustor can preserve net momentum by limiting skin-friction losses, even though a portion of the fuel chemical energy remains unreleased. The mixing survey reveals a related competition between chemical conversion and the conversion of released energy into axial kinetic energy. Specific impulse favours more front-loaded mixing, whereas combustion efficiency reaches its largest sampled value at a longer mixing length. The thermal-efficiency fields quantify this difference, while the Rayleigh heat-addition relation provides a local thermodynamic interpretation of the heat-release location. Chemical freezing is not imposed at a prescribed station, but can arise as the chemical time scale becomes long relative to the remaining convective time. These results show that reaction completeness alone is insufficient to identify the preferred design within the sampled parameter ranges.