WASHINGTON — U.S. Sen. Richard Shelby (R-Ala.) has joined two Democratic colleagues in calling for NASA to solicit bids for work on propulsion elements of the Space Launch System (SLS), the heavy-lift rocket Congress ordered the agency to build last year.

“I strongly encourage you to initiate a competition for the Space Launch System booster,” Shelby wrote in a June 10 letter to NASA Administrator Charles Bolden. Shelby said he had “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.”

Shelby also bluntly critiqued some of the technology NASA is considering for the SLS. He said he was “particularly concerned” that NASA is considering space shuttle-style boosters fueled by solid-rocket propellant.

“Designing a Space Launch System for heavy lift that relies on existing Shuttle boosters ties NASA, once again, to the high fixed cost associated with segmented solids,” Shelby wrote.

Shelby’s letter follows by about two weeks a similar letter from Sens. Barbara Boxer (D-Calif.) and Dianne Feinstein (D-Calif.). The Senators from California urged NASA to bid out work for the SLS propulsion system rather than doling out the work under existing contracts awarded under the now-defunct Constellation program.

Both Shelby’s letter and the May 27 letter from the California delegation noted that the NASA Authorization Act of 2010 required the use of existing contracts only “to the extent practicable.”

The NASA Authorization Act of 2010, signed into law by U.S. President Barack Obama in October, requires the agency to build a heavy-lift vehicle capable of sending 130 tons of payload into low Earth orbit along with a Multi-Purpose Crew Vehicle (MPCV) by 2016. The law directs NASA to use contracts awarded under Constellation program, and existing space shuttle infrastructure, whenever possible.

NASA is using an existing contract with Lockheed Martiin Space Systems, Denver, to build the MPCV. The agency has not said whether existing contracts will be used to build SLS.

NASA is expected to present lawmakers with its preferred design for SLS sometime before the final space shuttle mission launches. That launch is scheduled for July 8.

Boeing Space Exploration, Houston, ATK Aerospace Systems, Magna, Utah, and Pratt & Whitney Rocketdyne, Canoga Park, Calif., hold contracts to build key components of the now-canceled Ares 1 rocket, which was designed under Constellation to launch the crew-carrying Orion capsule. Depending on the design NASA selects for the SLS, this work might be directly applicable to the heavy launcher.

Aerojet of Sacremento, Calif., has expressed interest in building the propulsion elements of the SLS and recently announced an alliance with Teledyne Brown Engineering of Huntsville, Ala., to compete for that business. NASA officials have expressed concern that awarding the SLS work on a sole-source basis could draw a bid protest from Aerojet and like-minded companies.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.