The innovation proposed here is a novel multi-scale coupling methodology implemented in the Loci-STREAM CFD code, for developing a high-fidelity, high-performance multiphase combustion modeling capability to enable accurate, fast and robust simulation of unsteady turbulent, reacting flows involving cryogenic propellants (such as LOX/Methane) in liquid rocket engines (LREs). During Phase 1 work, a complete Eulerian-Lagrangian spray modeling methodology will be developed. The key components of this methodology are: (a) Volume-of-Fluid (VOF) method for liquid jet core and primary atomization, (b) Transitional Breakup (TBU) model for treating large drops and ligaments resulting from primary atomization, (c) Secondary Breakup (SBU) models, (d) Lagrangian Particle Tracking Model (LPT) to track the dispersed droplets, and (e) Evaporation models. In Phase 2 work, this spray modeling methodology will be coupled to flamelet-based models in Loci-STREAM to yield a full spray combustion capability. The Transitional Breakup (TBU) model is a novel approach proposed in this project– it will allow a robust transfer of large drops from the VOF model to the LPT model. The key components of this TBU model are: (1) a cloud-of-parcels approach in which the large drops are extracted from the VOF model and injected into the LPT model as a cloud of Lagrangian parcels with a diameter equal to that of the large drop but a fractional number representing the part of the large drop that each parcel represents, and (2) a stochastic model which evaluates the probability of breakup and the size distribution of a large drop using Monte-Carlo methods. The proposed enhancements in Loci-STREAM are anticipated to yield higher fidelity and more reliable analytical/design capability relative to existing capability at NASA for turbulent reacting flows in LREs.