Design and analysis of an ideal scramjet flowpath

The current work presents a novel conceptual framework for the fluid and gasdynamics that govern the design and performance of an ideal scramjet flowpath. These include a theoretical comparison between ram and scram modes, the physics of thrust loss during inlet unstart, and the design of an optimal scramjet flowpath. We present a unique explicit, closed-form relation for the wall divergence of an ideal scramjet combustor. The accompanying derivations and discussions, which leverage this formulation, seek to address uncertainties and misconceptions regarding the dominant fluid processes present in these engines. It is shown that scram and ram modes exhibit theoretical similitude for maximum thrust potential at conditions beyond the one-dimensional Rayleigh choking limit but can diverge below the global choking threshold. Additionally, it is shown that even for an ideal scramjet heat engine cycle, thermodynamic efficiencies at various flight conditions deviate from those of the classical Brayton cycle. These insights and accompanying theoretical analyses are meant to establish a foundation for the thermodynamics and gasdynamics relevant to the performance of dual-mode scramjet engines. The resulting work offers an intuitive technical perspective on supersonic combustion and the fundamentals of dual-mode scramjet operation that can be applied across a wide range of scramjet-related experimental and computational studies and design efforts in the future.