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.

An upgrade of the Saturn V upper-stage engine, the all-cryogenic J-2X generates 294,000 lb. of thrust with its gas-generator cycle. While it almost certainly will be needed to send men and women to Mars, the equally venerable RL-10 is beginning to look like a better power plant for the SLS upper stages that will be needed before that far-off mission.

Congress ordered an SLS able to lift 130 metric tons to low Earth orbit (LEO), which is a generally accepted requirement for launching a Mars mission. But for missions to the Moon, where a lot of Mars-precursor missions are being planned, a 105-ton SLS is probably sufficient, according to Steve Creech, May’s deputy, who is responsible for finding other applications for the SLS.

One way to get to that capability would be with a “dual-use upper stage” carrying three or four RL-10s. All of them would ignite to get the payload — an Orion crew capsule, in-space habitat or lunar lander — into LEO, and then some subset of that number would fire for the trans-lunar injection to send the payload toward the Moon.

NASA hasn’t ruled out using the J-2X for that portion of the trip, but it could be faster to develop the dual-use stage than the originally planned SLS upper stage powered by the J-2X, and a cryogenic propulsion stage (CPS) for getting into lunar orbit.

“What we’ve looked at to try to save costs and accelerate mission capability [is] combining the functions of our upper stage and the CPS so that we just have to have one stage,” Creech says.

Development of the J-2X started under the Bush administration’s Constellation program, which envisioned a human-rated launcher called the Ares I that used a shuttle-derived solid-fuel first stage, and an upper stage powered by the Saturn-heritage J-2X. At the beginning of the Constellation effort, the J-2X was considered the most time-consuming element of the Ares I, even though its Saturn heritage was chosen to minimize development complexity.

Now the engine has been built, using drawings and some hardware retained by NASA and Aerojet Rocketdyne, and is in development testing at Stennis Space Center in Mississippi. Those tests are scheduled to end next year, and after that work on the J-2X will halt “until we’re ready” to integrate the engine with an SLS upper stage, probably for the Mars mission, May says.

“Under constrained funding the number of simultaneous developments is limited, and that’s why we’ve essentially ended up with the architecture we did, because we only have the core to develop,” he says, referring to the SLS first stage. “And if you can do a dual-use upper stage you can actually get to a very capable rocket with only one more major development — not an upper stage and then a CPS.”