LOS ANGELES — Boeing is expediting work to define a larger, more capable exploration upper stage (EUS) for NASA’s Space Launch System (SLS) that it says could, if approved, reduce overall costs as well as expand potential early deep-space mission possibilities to include everything from Moon or Mars missions to asteroid recovery flights.

NASA’s next-generation heavy-lift SLS is being developed to launch the Lockheed Martin-made Orion Multi-Purpose Crew Vehicle as well as other exploration payloads. The larger upper stage would provide a payload capacity of 120 metric tons to low Earth orbit (LEO), compared to the 70 metric ton capacity of the initial Block 1 SLS, which is due to launch for the first time on a test flight named Exploration Mission 1 (EM-1) in 2017.

Although the SLS plan has always included growth to more capable Block 2 designs, the current schedule calls for a second mission (EM-2) in 2021 that would use the same Interim Cryogenic Propulsion Stage that Boeing is supplying for the 2017 mission. EM-2, as currently envisioned, will test a crewed Orion on a flight to rendezvous with a captured asteroid near the Moon in 2021. Boeing now believes that accelerating the development of a higher lift capacity EUS for use from EM-2 onwards will enable more mission possibilities to open up sooner, creating more demand for a vehicle that critics say is currently short on missions.

The EUS, originally named dual-use upper stage, is provisionally designed to be powered by four Aerojet Rocketdyne RL-10s rather than the single RL-10 used in the interim upper stage. The SLS, and particularly a more capable version, is gaining traction among science mission planners, says John Elbon, vice president and general manager of space exploration for Boeing Defense, Space and Security. "There’s starting to be more people interested in using SLS," says Elbon, who cites a potential robotic mission to the Jovian moon Europa as an example. Such a mission "could get to Europa in 2.5 years if launched on SLS. If it was launched on an EELV it would take about seven years to get there. Shortening the duration of that is a significant saving because you have the whole program running for those five extra years at around $50 million per year," he adds.

"As we worked through the definitization of the upper core stage it became apparent there would be a lot of synergy if we could do the design of the upper stage right on the heels of the core stage," Elbon says. "So we proposed to NASA that there would be cost reductions on the core and upper stage compared to if there was a gap. That seems attractive to NASA and they’ve given us a task order to start better defining the upper stage. Assuming that plays out we will have a systems design review which is the first step of the systems design process this fall. We would negotiate with NASA to add that to the contract, so it hasn’t been added yet," he adds.

Elbon says potential architectures involving launching Orion on a EUS have not yet been laid out. However he adds that it "probably makes sense with cargo and Orion to get the most out of the launch that way." In the meantime contract negotiations are underway with United Launch Alliance for an interim upper stage, with the request for proposals currently including two units. Boeing is already on contract for two core stages, the development of which is currently running five months ahead of schedule, Elbon says. "It’s an unusual thing for a difficult development program for a big rocket like SLS to be ahead of schedule. So we are on track for launch in 2017."