The main stage of NASA’s planned heavy-lift Space Launch System exploration rocket will carry four surplus RD-25D space shuttle main engines, as NASA and main-stage prime contractor Boeing move toward preliminary design review (PDR) on the big new rocket by the end of the year.

With the first flight scheduled in 2017, Jim Chilton, Boeing vice president and program manager for exploration launch systems, calls the development schedule “sporty.” But selection of the basic four-engine configuration is a significant step in developing the launch vehicle NASA plans to use for exploration beyond low Earth orbit in the 2020s and beyond.

Engineers considered three- and five-engine versions of the main stage, and settled on four for “money, time and performance” reasons. After the supply of RD-25Ds is used up, plans call for a throwaway version of the reusable shuttle engine designated the RD-25E. “A five-engine version takes another engine, so it costs more; it drives mass into the vehicle, so you end up flying a little higher, you carry a little more prop,” Chilton said at the National Space Symposium here. “A three-engine version, for some of the later missions, might have had to be upgraded too soon.”

Other design features selected in the run-up to PDR include an 8.4-meter-dia. for the stage, which will be extended to the SLS upper stage when its development begins at an as-yet-undetermined point in the future. Chilton says the diameter was selected to minimize ground-infrastructure modifications at Kennedy Space Center, where the SLS will use the Vehicle Assembly Building and launch pad facilities originally built for the Saturn V Moon rocket and later the space shuttle.

Still to be determined is precisely how the main stage will accommodate the advanced strap-on booster that NASA plans to build later to increase the vehicle’s initial 70-metric-ton capability to 130 metric tons. Chilton explained that the structure must be able to accommodate the five-segment, solid-fuel boosters that will be used initially, and whatever advanced system emerges from the development process that is just getting started at NASA. Options include kerosene-fueled engines, and the attach points for solid- and liquid-fueled strap-ons probably will be different, Chilton says, noting that Boeing has submitted its own booster proposals for the risk-reduction contracts currently being selected at NASA.

The main stage will be assembled at the government-owned Michoud Assembly Facility in New Orleans, using friction-stir welders and other tools that will be owned by the government, Chilton says. It probably will be constructed of aluminum lithium like the space shuttle external tanks also built at Michoud, although Chilton says engineers are still considering other aluminum alloys as well.

Initial production will be two units per year, with the manufacturing plan optimized for that low rate with multiskilled workers and heavy use of automation to hold down costs. Only two flights are currently scheduled — an unmanned initial test in 2017, and a 2021 manned flight with the Orion multipurpose crew vehicle that Lockheed Martin is developing.

Those flights will use an “interim cryogenic upper stage,” probably a Delta IV heavy upper stage. NASA is testing the J-2X engine it is developing for the SLS upper stage, but Pratt & Whitney Rocketdyne executives here say that engine will be mothballed after development is complete, and probably won’t go into production until the late 2020s. Chilton says the upper stage may not be developed until after the strap-on boosters, with the interim stage filling in until a more capable stage is needed.