NASA has selected a shuttle-derived vehicle with two existing liquid-oxygen/liquid-hydrogen stages as its reference design for the heavy-lift Space Launch System that Congress has ordered it to build for exploration missions beyond low Earth orbit, but it will hold a competition between liquid- and solid-fuel boosters to get it off the pad.

Administrator Charles Bolden on Wednesday endorsed the basic concept developed by launch vehicle experts at Marshall Space Flight Center (MSFC), and sent it on to the White House Office of Management and Budget for confirmation.

Essentially, the latest SLS approach tweaks the earlier “reference vehicle design” selected in January as a starting point for additional analysis. Both stages would use the LOX/hydrogen engines selected in January.

The new wrinkle since then would be a competition for the strap-on boosters needed to get the cryogenic main stage under way, with a kerosene engine added to allow the heavy lifter to “evolve” to the 130-metric-ton capability that Congress ordered in the 2010 NASA reauthorization legislation.

The preliminary SLS reference design called for twin five-segment versions of the four-segment solid rocket boosters built for the space shuttle by ATK. The five-segment variant was developed as the first stage of the defunct Ares I crew launch vehicle, and has been flight tested in the Ares I-X configuration.

The competition would pit that solid-fuel booster against a new liquid-fuel booster powered by an engine to be developed by Aerojet in Sacramento, Calif., and manufactured by Teledyne Brown Engineering in Huntsville, Ala.

According to Julie Van Kleeck, Aerojet vice president for space programs, the new engine would be a U.S.-built version of the Russian-based AJ-26 engine that Aerojet is preparing as the main-stage engine for the Orbital Sciences Corp. Taurus II commercial cargo booster for International Space Station resupply. Built from surplus Soviet-era NK-33 engines, the AJ-26 generates about 340,000 lb. of thrust at sea level.

The new engine — now designated only the -500 — would generate 500,000 lb. of thrust at sea level and be built in expanded Teledyne Brown facilities in Huntsville under a joint venture announced June 3.

“The combination of Aerojet’s leadership in engine design and production and Teledyne’s experience with complex engineered systems and advanced manufacturing creates a strong, unchallengeable offering to customers,” Teledyne Brown President Rex Geveden, a former director of nearby MSFC, said at the time.

For the initial few flights the first stage would be powered by the 15 remaining RS-24 reusable space shuttle main engines — probably five at a time. After that the stage would use a throwaway version designated the RS-25 that Pratt & Whitney Rocketdyne will develop.

The upper stage will be powered by the J-2X engine developed by Pratt & Whitney Rocketdyne for the Ares I crew launch vehicle, which was officially terminated last week along with the rest of the Constellation program. Based on the upper stage engine for the Saturn V, the first developmental J-2X has just been installed in test stand A2 at Stennis Space Center, and could get its first hot-fire test — a 1.9-sec. burn — as early as June 21 (Aerospace DAILY, June 15).

Bolden’s decision was foreshadowed by public letters sent to him by Democratic Sens. Barbara Boxer and Diane Feinstein of California, and Sen. Richard Shelby (R-Ala.) urging him to compete SLS propulsion.

“It was never our intent to foreclose the possibility of utilizing competition, where appropriate,” Shelby wrote to Bolden on June 10, referring to the NASA reauthorization language. “ … I have seen no evidence that foregoing competition for the booster system will speed development of SLS or, conversely, that introducing competition will slow the program down.”