<p>An approach to combine two different equation-of-state models is described, which expands an ideal gas model to a range that captures both cryogenic and combustion conditions for any generalized mixture. This proposed model includes a real gas Helmholtz energy semi-empirical model combined with NASA thermophysical property polynomial fits that transitions to a pure ideal gas mixture model that uses only NASA polynomial fits. Capturing cryogenic and supercritical states are important for rotating detonation rocket engines that have high pressure and cryogenic inlet conditions into the combustion chamber where downstream combustion conditions can be reasonably modeled as an ideal gas. A parahydrogen and oxygen mixture is selected and presented where chosen shock tube simulations display improvements due to higher fidelity thermophysical property calculations. Operational conditions approximating the operation of NASA’s SWORDFISH engine represents one of the test cases for the shock tube study and showed that such improvements are important to consider when capturing the entire flow regime throughout its combustion chamber in a simulation. When implemented in a CFD program, this new model shows an excellent match to all selected cases of the shock tube study.</p>