The first stage of the Delta III is powered by a Rocketdyne RS-27A main engine which has a 12:1 expansion ratio and employs a turbine/turbopump, a regeneratively cooled thrust chamber and nozzle, and a hydraulically gimbaled thrust chamber and nozzle that provides pitch and yaw control. Two Rocketdyne vernier engines provide roll control during main-engine burn, and attitude control between main-engine cutoff (MECO) and second-stage separation. High repeatability of mixture ratio ensures very accurate propellant utilization for the engines. The Rocketdyne RS-27A main and vernier engines are both unchanged from Delta II.