The Centaur is a family of rocket propelled upper stages that has been in use since 1962. It is currently produced by U.S. launch service provider United Launch Alliance, with one main active version and one version under development. The 3.05 m (10.0 ft) diameter Common Centaur/Centaur III flies as the upper stage of the Atlas V launch vehicle, and the 5.4 m (18 ft) diameter Centaur V has been developed as the upper stage of ULA's new Vulcan rocket. Centaur was the first rocket stage to use liquid hydrogen (LH2) and liquid oxygen (LOX) propellants, a high-energy combination that is ideal for upper stages but has significant handling difficulties.

Vulcan Centaur is a two-stage-to-orbit, expendable, heavy-lift launch vehicle created and operated by United Launch Alliance (ULA). It is principally designed for the United States Space Force's National Security Space Launch (NSSL) program, which launches satellites for the Defense Department and U.S. intelligence agencies. It will replace ULA's existing heavy-lift Atlas V and Delta IV Heavy rockets. Vulcan Centaur will also be used for commercial launches, including an order for 38 launches from Kuiper Systems.