The ducted rocket is a supersonic flight propulsion system that takes the exhaust from a solid fuel gas generator, mixes it with air, and burns it to produce thrust. To develop such systems, the use of numerical models based on computational fluid dynamics (CFD) has been increasing, but to date only simplified treatments of the combustion within ducted rockets have been reported, likely due to the difficulties in characterizing and accurately modeling the partially reacted, particle-laden fuel exhaust from the gas generator. Through a careful examination of the governing equations and experimental measurements, a CFD-based methodology that properly accounts for the influence of the gas generator exhaust, particularly the solid phase, has now been developed to predict the performance of a ducted rocket combustor using a simulated solid fuel. It uses an equilibrium-chemistry probability density function combustion model with two separate streams, one gaseous and the other of 75-nm-diam carbon spheres, to represent the exhaust products from the gas generator. After extensive validation with direct-connect combustion experiments over a wide range of geometries and test conditions, this CFD-based method was able to predict, within a good degree of accuracy, the combustion efficiency of a ducted rocket combustor.