NASA will store some rocket engines, slow work on others and study still more as it struggles to squeeze the heavy-lift Space Launch System (SLS) Congress has ordered into a flat, $3 billion annual budget for development.

Early flights of the SLS will use surplus space shuttle main engines (SSMEs) and, as side-mounted strap-ons, the five-segment solid-fuel motors developed for the terminated Ares I crew launch vehicle’s first stage.

The J-2X upper-stage engine, which once was the “pacing item” for Ares I, will be slowed as managers try to maintain enough development momentum to avoid a costly stop and restart in engine development as the big new rocket “evolves” to the 130-metric-ton capability Congress wants.

Early work on the heaviest-lift version of the SLS will come this year, when NASA issues a research announcement for 30-month study contracts on concepts to upgrade the strap-on boosters. In 2014, the agency plans to issue an open request for proposals — not limited to the study-contract winners — for development of new boosters, which can use solid or liquid fuel.

The trick, for NASA and its existing and potential rocket contractors, will be to manage the development within the $3 billion in annual funding the agency hopes to get. NASA managers drummed that point home to contractors during an SLS industry day at Marshall Space Flight Center, but it offers scant solace to companies that have been struggling to keep their teams together while NASA determines its next moves.

Pratt & Whitney Rocketdyne (PWR), for example, built the reusable RS-25D shuttle main engine and has started internal work on a throw-away -25E variant for the SLS. But the company was forced to lay off skilled workers while it awaited a final White House decision on the SLS engine requirements, and it does not have all the answers from the government yet.

“We’re still trying to figure out when to start the -25E work, and that’s a funding-driven discussion within the flat budget,” says Dan Dumbacher, who oversees SLS planning as NASA’s deputy associate administrator for exploration systems development.

The first two SLS launches will use some of the 15 RS-25D SSMEs left over from the shuttle program. Dumbacher says they will be stored in a controlled environment until some are needed for the first flight, no earlier than 2017 under current planning.

The agency is still working out how many it will need to power the big new rocket’s core stage, which will depend on the mission. For the 70-metric-ton first flight, three engines should suffice, Dumbacher says, but for missions needing at least 100 metric tons of lift, NASA is still studying whether four or five core-stage engines is the right number.

The same uncertainty persists on the upper stage, which will be designed to use the same tooling as the core stage but will not be built until after the first flight. Boeing will build the upper stage and avionics ring.