Upper stage / space tug - in production. Twin engined Centaur for Atlas V, powered by two Pratt & Whitney RL10A-4-2 turbopump-fed engines burning liquid oxygen and liquid hydrogen.

For heavy payload, low earth orbit missions, Centaur would use two RL10 engines to maximize boost phase mission performance. Guidance, tank pressurization, and propellant usage controls for both Atlas and Centaur phases were provided by the inertial navigation unit (INU) located on the Centaur forward equipment module.