The big government rocket Congress has insisted be built for deep-space human exploration is on track for a 2017 first flight. So far, there are no serious technical issues in sight and it is garnering growing interest from other potential users, according to the NASA managers responsible for developing the heavy-lift vehicle known as the Space Launch System (SLS).

The U.S. space agency and its international partners are basing plans for sending humans beyond low Earth orbit on the SLS under a schedule that will be set more by the funding available for development work than by developing hardware. The first flight-version 70-ton core stage is due at Stennis Space Center in Mississippi for ground testing in 2016, and NASA will pace subsequent development of the advanced boosters and upper stage needed to reach the final 130-ton capability on how those elements would be used.

A mission to capture a small near-Earth asteroid and nudge it into the distant retrograde orbit (DRO) around the Moon would require a more powerful restartable upper stage for in-space propulsion, while other missions could be accomplished with the initial upper stage and the advanced strap-on boosters now under competitive study. The SLS program is canvassing potential government and commercial users to expand the set of SLS missions beyond the two on the books—an unmanned trip to DRO in 2017 and another with a crewed Orion capsule in 2021.

“As people see that it is more and more real and progress is being made, the notion that this is a paper rocket is being quickly dispelled,” says Dan Dumbacher, deputy associate administrator for exploration systems development at NASA headquarters.

The SLS program already is bending metal at NASA's Michoud Assembly Facility in New Orleans, where the big rocket and the Orion capsule's pressure vessel will be built using the friction-stir welding process for lighter weight, greater strength and lower cost. Pathfinder propellant tank sections are being built, and the tooling required to stack and weld them into full-length tanks is en route from Sweden. Two to four pathfinders will be built in order to perfect manufacturing processes before the first flight article is built, says Dumbacher.

“The major cost comes in rework to flight hardware, so the investment you make up front that avoids the rework on flight hardware is money well spent,” he says.

For added thrust off the launch pad, the first two SLS vehicles will use five-segment solid-fuel strap-on boosters derived from the four-segment versions built for the space shuttle fleet. NASA and its booster prime contractor, ATK, have changed the production process in an effort to lower recurring costs. However, after these alterations, unacceptable voids appeared in the first two propellant castings for a ground-qualification, requiring rework and delays. While reluctant to blame the process changes for the problem until a search for the root cause is complete, Dumbacher says there is plenty of time to find and fix the problem before the first flight late in 2017.

Another issue that must be resolved before the first flight involves the head pressure at the pump inlets on the four surplus RS-25 Space Shuttle Main Engines baselined to power early SLS versions. Propellant temperature also is a problem. It turns out that the SLS configuration delivers higher pressures and colder temperatures than was the case on the shuttle, so engineers must find ways to accommodate the engine start sequence to the new conditions.

The engines also will use the new controller developed for the J-2X upper-stage engine, which turns out to be the only part of the J-2X that will fly in the early SLS variants. While it was the pacing item for the terminated Ares I crew launch vehicle, the J-2X delivers more power than is needed for any of the SLS variants except the 130-ton variant planned for human missions to Mars (AW&ST Oct. 7, p. 28).

In a Nov. 18 interview, Dumbacher said work on the SLS core stage was five months ahead of schedule. Preliminary design review was completed in July, and 70% of the detailed design drawings—as measured by the mass of the hardware they represent—were complete. Although “the hard part is coming,” when the tankage sections are stacked and welded together, he says, “we're on track for core stage critical design review late spring into summer next year.”

Work is just beginning on the upper stages planned for future missions beyond the first. That mission—Exploration Mission I (EM I), with an instrumented Orion on top—probably will use an interim cryo propulsion stage (ICPS), which is basically the cryogenic second stage from the Delta IV. However, because the second Exploration Mission (EM II in 2021) will carry a heavier Orion to accommodate the crew, the SLS program at Marshall Space Flight Center in Huntsville, Ala., is considering stretching the liquid hydrogen tank by 18 in. for more performance, according to Chris Crumbly, manager of the advanced development office in the SLS program.

“Because we're doing two test flights, we may go ahead and configure the first ICPS to be similar to the second,” he says. “That is still under study.”

Also under study is a 120,000-lb.-thrust “dual-use upper stage” (DUUS) that Dumbacher says will have enough capability to give the program a choice of developing it or the advanced booster first. Although unfunded and fairly notional in concept (see illustration), it would use four RL-10 engines or perhaps two of Japan's proposed 60,000-lb.-thrust MB-60s.

Dennis Tito has asked NASA to accelerate development of the DUUS because his Inspiration Mars manned planetary flyby mission needs it (AW&ST Nov. 25, p. 13). But its median 40-ton capability to trans-lunar injection could also help with other missions beyond the DRO trajectory that EM I and II would follow.

“It really has to do with the capability we're required to put in, the destination,” says Crumbly of the decision to develop the boosters or the upper stage first. “The asteroid-redirect mission would be better served if we had more in-space transportation capability, which would lean you toward an upper stage first.”

Ultimately, says Dumbacher, the what-next choice will be “a budget-driven trade.” The core stage, too, is at the mercy of the funding available, and in the current budget environment the SLS program is “basically working on a daily basis” (AW&ST Nov. 25, p. 46). The program's current funding stems from a continuing resolution based on its enacted funding for fiscal 2013—$1.4 billion, including launch infrastructure—that expires Jan. 15. If that level falls under a new continuing resolution, or additional funding sequestration to trim the budget, the development schedule will change.

“If the appropriations beyond January are at the same levels we are at, then we'll proceed to making progress toward December 2017 if we can, recognizing we'll just have to play through and sort all this out,” Dumbacher says. “I don't think I have ever seen a program where any of us have had to manage in this environment, where I get funding on a quarterly basis. Even in fiscal '13 we didn't have an approved operating plan until August.”