HUNTSVILLE, ALA. — NASA managers are relying on hardware developed for other human-spaceflight programs to flatten the development budget on the planned heavy-lift Space Launch System (SLS) to squeeze it within a spending cap of $1.2 billion a year.

Todd May, the SLS program manager here at NASA’s Marshall Space Flight Center, said Nov. 4 that the big new rocket is being designed with affordability and sustainability considerations at least as important as performance.

“Our challenge is not so much technological,” he told Aviation Week in an interview. “It’s how to field the biggest and most affordable rocket in an affordable way, and not just affordable but sustainable—affordable meaning we live within our means; sustainable meaning we create a production and operations wedge so we can build the other things for the architecture to be able to explore in earnest.”

NASA is planning on $3 billion a year for the SLS, the Orion-based multi-purpose crew vehicle (MPCV) and the ground infrastructure that will be needed to use it to send humans beyond low Earth orbit. The MPCV and SLS each receive $1.2 billion of that for development, on a schedule that would use a first-generation SLS by the end of 2017 to send an unmanned Orion capsule around the Moon to generate relevant velocities to validate the thermal protection system for an atmospheric reentry from deep space. A manned lunar flyaround would follow in 2021.

Just as NASA opted to use the Orion, which has already cost taxpayers $5 billion, as the basis for the MPCV, May’s organization is using existing hardware wherever possible to hold down development costs.

“You start looking around for what you have in the toolshed,” he says. “When you develop a large, complex vehicle you typically want a development curve [where] you peak out at critical design and then you come down. Under a flat budget you have to do some fairly innovative things to avoid that large hump. One of the things you can do is use things that have already been through that hump.”

In the SLS design selected in June and cleared by the White House budget office in September, the vehicle will use the 15 remaining RS-25D space shuttle main engines (SSMEs) to power the first stage in early flights, the J-2X upper-stage engine in development—based on the Saturn V upper-stage engine—for the defunct Ares I crew launch vehicle, and a core and upper-stage structure based on the space shuttle external tank. The first two flights will use the five-segment solid-fuel rocket developed as the Ares I first stage as twin strap-on boosters.

All of that heritage hardware will help keep development costs for the new vehicle flat. Although it was once the pacing item for the Ares I, for example, the J-2X development will be slowed to match the flat budget. Once it is ready, May says, engine-development resources will be shifted to develop the throwaway RS-25E version of the SSME.

About 700 civil servants and another 450 contractor support personnel are working on the SLS. One issue they are tackling is how many engines to mount on each stage. The core stage probably will be built to carry five engines, but may use only three for some missions. The upper stage may carry one or two J-2Xs, May says, stressing that the trade-offs are still being worked.

And while affordability is a major driver in the SLS development, it isn’t the only one, he says, citing crew safety as the top priority.

“There were a number of driving objectives that led us to . . . the architecture itself,” he says. “Obviously safety is paramount. In the configuration we have it’s not going to be like the shuttle where we’re flying five to seven times a year. We may get an opportunity once a year, maybe at peak two a year, so we want it to be safe.”