Systems and methods of controlling solid propellant gas pressure and vehicle thrust are provided. Propellant gas pressure and a vehicle inertial characteristic are sensed. Propellant gas pressure commands and vehicle thrust commands are generated. A propellant gas pressure error is determined based on the propellant gas pressure commands and the sensed propellant gas pressure, and vehicle thrust error is determined based on the vehicle thrust commands and the sensed vehicle inertial characteristic. Reaction control valves are moved between closed and full-open positions based on the determined propellant gas pressure error and on the determined vehicle thrust error. The system and method allow the reaction control valves to operate at variable frequencies or at fixed frequencies. The system and method also allows propellant pressure to be commanded to follow a predetermined pressure profile or commanded to vary "on-the-fly."