Powered by liquid oxygen and liquid hydrogen, the J–2X is an evolved variation of two historic predecessors — the powerful J–2 upper stage engine that propelled the Apollo-era Saturn IB and Saturn V rockets to the moon in the 1960s and 1970s, and the J–2S, a simplified version of the J–2 developed and tested in the early 1970s but never flown. The J–2X builds on the legacy of the Apollo-Saturn Program and relies on nearly a half-century of NASA spaceflight experience, heritage hardware and technological advances.