The innovation proposed here is a high-performance, high-fidelity framework in the computational fluid dynamics (CFD) code called Loci-STREAM to enable accurate, fast and robust simulations of unsteady multiphase flows such as combustion involving liquid-gas phases in liquid rocket injectors and solid-gas phases in solid rocket motors, and cryogenic cavitation in delivery systems of liquid rocket engines. This framework will provide a state-of-the-art multiphase unsteady turbulent flow simulation capability employing Hybrid RANS-LES (HRLES) methods which are a blend of Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches. Robust primary atomization models for liquid jet breakup and both phenomenological and stochastic secondary droplet breakup models will be developed. Lagrangian particle tracking and Eulerian multiphase models will be coupled to enable simulation of multiphase combustion involving solid particles or liquid droplets. The work proposed here will result in a state-of-the-art design and analysis tool to enable the accurate modeling of: (a) multiphase combustion in solid and liquid rocket engines, (b) combustion stability analysis (c) acoustic fields of space propulsion systems in near-ground operation, (d) small valves and turbopumps, etc. which constitute critical components of versatile space propulsion engines part of NASA's space near- and long-term space programs.