CAPE TOWN, South Africa — NASA plans to open a competition in December for multiple, 30-month contracts to study strap-on booster upgrades for the planned heavy-lift Space Launch System (SLS), including an upgrade for the five-segment, solid-fuel strap-ons baselined as the initial boosters for the big new rocket.

One challenge for NASA engineers will be to design an interface that can link different booster types to the SLS core stage, according to William Gerstenmaier, associate administrator for human exploration and operations. The SLS will be the vehicle NASA uses to send humans beyond low Earth orbit.

“Our vision is we’ll have an interface that’s generic, and we’ll be able to carry potentially different boosters and change them out as needed,” Gerstenmaier told a session of the International Astronautical Congress here Thursday. “So we could go compete in the future, maybe downsize if something’s easier for a mission that requires less thrust. We have some variability there, so if we do our job right, we’ll have the ability to change the boosters that sit on the side. That’s our ultimate goal. We’re not going to pick one.”

NASA plans to build a 70-metric-ton SLS at first, with only the core stage and strap-ons. The vehicle will grow to a 130-metric-ton capability with the addition of an upper stage and upgraded strap-on. The core stage will be powered by surplus RS-25D space shuttle main engines at first, followed by a throwaway version designated the RS25E. The upper stage will be designed to use the same tooling as the core stage, and will be powered by the J-2X engine now in testing at Stennis Space Center in Mississippi.

“If we don’t need an upper stage for certain missions, we don’t have to fly an upper stage,” he said. “We can just add it in for essentially marginal cost for the upper stage. We don’t have to add a new plant, new facilities and new tooling.”

The booster upgrade can be solid-fuel, liquid oxygen (LOX)/kerosene or LOX/liquid hydrogen, Gerstenmaier said. Only the first two SLS flights will be powered by the five-segment version of the space shuttle boosters that were originally developed for the first stage of the terminated Ares I crew launch vehicle.

“It turns out that to get to the 130 metric tons, we’re going to have to redesign the five-segment booster as well,” Gerstenmaier says. “We have to go to potentially a composite case, away from our steel case to save some weight, and we might need to make a propellant change to use the more energetic propellant that sits in the solid rocket motor. So even if we go continuous solids, we’re going to have to make a pretty significant change to the solid-rocket booster segment.”

The competition for an advanced booster will begin in December with study-contract bidding that is likely to include ATK, manufacturer of the current solid-fuel boosters, and a team that includes Aerojet, which has plans to upgrade the Russian-built AJ26 LOX/kerosene engine it modified for the Taurus II launcher that Orbital Sciences Corp. will use to send cargo to the International Space Station.

“We’re not really ready to step up to the booster activity right away with a full-up competition,” Gerstenmaier says. “We think there’s some technology that needs to get explored and understood as we go forward. We think we also need to define a little bit better the core interface with the solid rocket boosters or the liquid rocket boosters, so we have that as a design condition. So we’re going to have kind of a study phase, with potentially multiple contractors participating in that study phase for a period of about 30 months or so, and then we’ll roll right into the actual competition. But the idea is to have the new booster system available, probably in about the 2019 time frame.”