CFD codes used to simulate upper stage expander cycle engines are not adequately mature to support design efforts. Rapid and accurate simulations require more versatile grid frameworks to handle complex geometries of multi-element injector configurations. Turbulence models require upgrades to better predict fuel/air mixing with swirl and to predict heat flux. The innovation proposed initiates work towards developing a mature, high-fidelity simulation tool. Geometry complexity and numerical accuracy problems are addressed via a multi-element UNS grid adaptation strategy that builds upon techniques developed for valving problems and scramjet injectors. Turbulent mixing and heat transfer are upgraded by including PDE's that solve for temperature and species variance (yielding local values of Prandtl and Schmidt number), as well as swirl corrections. Finally generalized preconditioning that accounts for stiffness resulting from a large range of Mach numbers, and generalized thermodynamic formulations for real fluids will be matured to yield robust numerics with improved solution convergence. The tools and technology to be developed here would directly impact design efforts for future long duration lunar and Mars missions that require more durable long-life, light weight system components, and address methodology to operate with novel hydrocarbon fuels that may be harvested in-situ.