The innovation proposed here is a high performance, high fidelity simulation capability to enable accurate, fast and robust simulation of unsteady turbulent, reacting flows involving cryogenic propellants (such as LOX/LH2 and LOX/LCH4). The key features of this proposed capability are: (a) Hybrid RANS-LES (HRLES) methodology, and (b) flamelet modeling for turbulent combustion incorporated in a proven existing solver called Loci-STREAM which has been developed by the proposing personnel under funding from NASA over the last several years. The proposed enhancement in Loci-STREAM is anticipated to yield an order of magnitude improvement in simulation turnaround times relative to existing capability for turbulent reacting flow applications. The work proposed here will ultimately result in a state-of-the-art design and analysis tool to enable the accurate modeling of for multiphase combustion in solid and liquid rocket engines, combustion stability analysis, etc. which constitute critical components of versatile space propulsion engines part of NASA's deep space missions.