Computational fluid dynamics (CFD) solutions of the full Navier-Stokes equations, including finite-rate chemical kinetics, are used to numerically simulate the reacting in-bore flowfield for a 90-mm (bore diameter) ram-accelerator projectile propulsion system. In this system a gun tube is filled with a high-pressure gaseous mixture of hydrogen, oxygen, and nitrogen. An axisymmetric projectile is launched into this tube, and rails are attached to the tube wall to ensure projectile centering. The shock system that develops around the projectile, along with boundary layers on the projectile and tube surfaces, ignites the mixture on or near the projectile afterbody. The resulting pressure imbalance on the projectile generates thrust. Numerical predictions of this pressure field can be used to evaluate the thrust efficiency of the system, i.e., the projectile design combined with the chemical composition of the propellant gas. Comparison of the predicted pressure field and the measured pressure data is used to judge the accuracy of the CFD code. The choice of chemical kinetics mechanism used in the CFD code is of critical importance in achieving an accurate numerical simulation of the ram accelerator.