 The engine's mass is approximately 2,470 kg (5,450 Lb), significantly heavier than its predecessors.The J-2X was intended to be based on the J-2 used on the S-II and S-IVB stages of the Saturn rockets used during the Apollo program, but as required thrust for the Ares I increased due to weight problems it became a clean-sheet design.It entered development in 2007 as part of the cancelled Constellation program.