Rocket-based Combined-Cycle (RBCC) engines offer promise for efficient and flexible propulsion for space transportation over a wide Mach number range by combining rocket and airbreathing propulsion technologies. To achieve this an RBCC engine uses four different modes of operation, i.e., ejector, ramjet, scramjet and rocket modes, during the ascent phase. The engine must thus make effective transition from subsonic to supersonic combustion, i.e., ramjet to scramjet mode during operation. The study is conducted to investigate the effects of turbulence modeling on the simulation of the RBCC combustor flowfield and the prediction of the combustor performance in the scramjet mode. A two-dimensional RBCC configuration has been considered numerically, using hydrogen as the fuel supplied through injectors at the divergent top wall and horizontal floor. Combustion has been found to occur predominantly near the top injector and along the shear layer downstream, with its extent being considerably sensitive to the choice of turbulent Schmidt number and turbulence model. The performance of the combustor crucially depends on these attributes subsequently due to complex flow phenomena and interactions as well as counteracting behavior of pressure and viscous forces.