High-Mach Scramjet Performance: Interplay of Aerothermodynamics, Kinetics, and Geometry

The design of scramjets for flight at Mach numbers above 9 is constrained by severe aerothermodynamic effects and chemical nonequilibrium. This work utilizes a validated quasi-one-dimensional model with finite-rate chemistry to conduct a systematic parametric investigation of scramjet performance. The study analyzes the influence of inlet contraction ratio, combustor geometry, and flight conditions (Mach number and dynamic pressure) on specific impulse. Results reveal that performance is governed by a critical tradeoff between maximizing combustion completeness, which is favored by high pressure, and mitigating high-temperature energy losses from nitric oxide (NO) formation and product dissociation. An optimal “Max Isp island” is identified in the combustor geometric design space, the location of which is highly sensitive to the operating point. Increasing flight Mach number forces the optimal geometry toward more aggressive expansion to manage thermal loads, while increasing dynamic pressure shifts it back toward less expansion to capitalize on the suppression of fuel dissociation. These findings demonstrate that an optimal scramjet configuration is not fixed but is a dynamic function of the flight conditions, highlighting the necessity of multipoint optimization for designing hypersonic vehicle propulsion systems.