The RS-27A is a liquid-fuel rocket engine developed by Rocketdyne for use on the first stage of the Delta II and Delta III launch vehicles. It provides 1.05 meganewtons (240,000 lbf) of thrust burning RP-1 and LOX in a gas-generator cycle. The engine is a modified version of its predecessor, the RS-27; its thrust nozzle has been extended to increase its area ratio from 8:1 to 12:1, which provides greater efficiency at altitude. The earlier RS-27 was derived from surplus Rocketdyne H-1 engines used in the Saturn 1B launcher.
The RS-27A main engine is neither restartable nor throttleable. In addition to its main engine, the RS-27A includes two vernier engines to provide vehicle roll control during flight. The RS-27A has a run duration of 265 seconds.