The RS-27 engine has been powering the first stage of the Delta vehicle since the 1960's.  Today, the RS-27A is employed on the first stage of the Delta II and Delta III launch vehicles.  The RS-27 is a gas generator liquid oxygen / kerosene booster engine.  In the Delta II and III mission sequence, the RS-27A is ignited several seconds before ignition of the solid GEM strap-ons and burns to core stage propellant deplection, which occurs approximately 220 seconds after lift-off.  The RS-27's close relative, the MA-5, powers the first stage of the Atlas I and II vehicles.  Once the current EELV designs have established themselves in the marketplace, the RS-27 and MA-5 will be phased out of production.