NASA’s J-2X engine, once considered the pacing item for the next U.S. human-rated rocket, will go on the shelf after development testing wraps up next year because it will be years before the engine is needed to push humans toward Mars.
While the agency is actively seeking other missions for the heavy-lift Space Launch System (SLS) in the planetary science and military arenas, most of the human flights it has in sight for the big new rocket probably can be accomplished with an upper stage powered by the RL-10 engine instead of the J-2X.
“The J-2X for certain [design reference missions] is somewhat overpowered,” said Todd May, NASA’s SLS program manager.
It's that the J-2X engine is "somewhat overpowered," it's that the vision and will of Congress and the Executive is "seriously underpowered" when it coms to funding ambitious human space exploration. To read more, click here.