The MA-5 engine(s) has been powering the first stage of the Altas vehicle since the 1960's.  The MA-5 is a gas generator liquid oxygen / kerosene booster engine.  In the Atlas mission sequence, both MA-5 engines are ignited.  After approximately 175 seconds, the MA-5 booster engine is shut down and jettisoned.  The sustainer MA-5 engine burns to core stage propellant deplection, which occurs approximately 280 seconds after lift-off.  The MA-5 is being replaced by the RD-180 engine on the Altas III and Atlas V (EELV) vehicles.