Single-engine Centaur for Atlas IIIA. The Lockheed Martin manufactured Centaur IIIA upper stage is powered by one Pratt & Whitney RL10A-4-1 turbopump-fed engine burning liquid oxygen and liquid hydrogen. Using the proven flight design of the Centaur IIAS stage, the only changes to Centaur for IIIA are in the aft region of the stage. For Centaur IIIA, one of Centaur IIAS's two RL10 engines is removed. The remaining engine is re-positioned to a center-mount, and electro-mechanical thrust vector control actuators replace the hydraulically actuated system previously in use. Guidance, tank pressurization, and propellant usage controls for both Atlas and Centaur phases are provided by the inertial navigation unit (INU) located on the forward equipment module. The first Centaur burn lasts about nine minutes after which the Centaur and its payload coast in a parking orbit. During the first burn, approximately ten seconds after ignition, the payload fairing is jettisoned. The second Centaur ignition occurs about 23 minutes into the flight, continues for about three minutes, and is followed several minutes later by the separation of the spacecraft from Centaur.