Upper stage / space tug - out of production. Dual-engine Centaur for Atlas IIIB.

The Lockheed Martin manufactured Centaur IIIB upper stage was powered by two Pratt & Whitney RL10A-4-2 turbopump-fed engines burning liquid oxygen and liquid hydrogen. The changes to Centaur for Atlas IIIB were a stretched tank (1.68 m) and the addition of the second engine. Guidance, tank pressurization, and propellant usage controls for both Atlas and Centaur phases were provided by the inertial navigation unit (INU) located on the forward equipment module. The first Centaur burn lasts about five minutes, after which the Centaur and its payload coast in a parking orbit. During the first burn, approximately eight seconds after ignition, the payload fairing was jettisoned. The second Centaur ignition occurs 27 minutes into the flight, continues for about three minutes, and was followed several minutes later by the separation of the spacecraft from Centaur.

For Centaur IIIA, one of Centaur IIAS's two RL10A-4 engines was removed. The remaining engine was 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 were 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 was jettisoned. The second Centaur ignition occurs about 23 minutes into the flight, continues for about three minutes, and was followed several minutes later by the separation of the spacecraft from Centaur.