One way to achieve that capability would be with a “dual-use upper stage” carrying three or four RL-10s. All of them would ignite to loft 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 translunar injection to send the payload toward the Moon.
NASA has not ruled out using the J-2X for that portion of the trip, but it may 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) to reach lunar orbit. “To try to save costs and accelerate mission capability, [we've looked at] combining the functions of our upper stage and the CPS so that we just have to have one stage,” says Creech.
Development of the J-2X started under the George W. 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 J-2X. Initially, the J-2X was expected to be the most time-consuming element of the Ares I, although its Saturn heritage was selected 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, Miss. Those tests are scheduled to end next year, and then work on the J-2X will halt “until we're ready” to integrate the engine with an SLS upper stage, 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 explains, 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.”
That make-the-best-of-a-bad-situation approach is standard these days in NASA's Human Exploration and Operations (HEO) directorate, which is forging ahead with plans for human spaceflight in cislunar space despite the funding uncertainty. Planning is underway to send the first two tests of the initial 70-ton SLS variant—the core stage, powered by surplus RS-25 space shuttle main engines, with a Delta IV upper stage and Orion capsule—to a distant retrograde orbit (DRO) around the Moon (see illustration). That is also where NASA wants astronauts to meet an asteroid nudged there by a solar-electric tug (AW&ST Sept. 23, p. 26). The test flights—one with a crew—are set for 2017 and 2021, and the asteroid redirect mission would follow by 2025.
If the asteroid mission is canceled, it will still have served a valuable function as a focus for flight engineering, according to William Gerstenmaier, associate administrator for HEO and a master of the make-do approach to human-exploration development. Just as the basic principles of operating in LEO were worked out in the early days of spaceflight, the prospect of working in DRO or at the Earth-Moon libration points L-1 and L-2 is driving planning that can be useful for decades to come.
“We developed all these safety techniques to operate with this huge gravity vector from the Earth,” he says. “So now we're in this different region where we don't have that huge gravity vector. What are those rendezvous and prox-ops techniques that we ought to be developing in that environment?”
An Orion crew in DRO might have to wait as long as five days to return to Earth in an emergency, and Gerstenmaier says the use of lunar gravity assists will become “routine,” not just in such cases, but also to move around cislunar space. Other ideas for the asteroid mission—including the possibility of snatching a boulder from the surface of a large asteroid instead of grabbing a small space rock and nudging it into DRO—were to have been discussed at an invitation-only Asteroid Initiative Ideas Synthesis Workshop in Houston last week, hosted by the Lunar and Planetary Research Institute.