For numerical simulations of reacting, viscous nozzle flows in advanced rocket nozzles, a numerical method has been developed. The code is based on the well established DLR Euler/thin layer Navier-Stokes code NSHYP that was originally written for hypersonic re entry how simulations. The code solves the three-dimensional, time-dependent Euler and thin-layer Navier-Stokes equations in conservation law form, Hydrogen and oxygen are considered as fuel and oxidizer, respectively. The passive scaler approach accounts for the multicomponent diffusion effects. Turbulent transport is described with an algebraic eddy-viscosity, and a k-epsilon two-equations model. Calculations of conventional bell nozzles were performed, Results of Vulcan Mark 1 nozzle flow simulations are shown and compared in detail with another well-established numerical approach, based an the method of characteristics. A modified Vulcan engine is proposed, where the gas generator exhaust gases are injected into the main nozzle. Results of an engine analysis are presented. Based on this engine analysis, numerical flowfield calculations of the modified nozzle were performed and are presented.