WASHINGTON — More than a month after NASA awarded Boeing Space Exploration and Space Exploration Technologies Corp. contracts to build the first U.S. crewed spacecraft since the space shuttle, the Republican leaders of the House Science Committee are fuming that they have yet to be briefed on the logic behind NASA’s selections.

“To date, the Committee has not been briefed on the source selection, nor has it received the source selection statement, despite the fact that the offerors have been briefed, details were released to the press, the [Government Accountability Office] is now involved; and NASA has decided to proceed with the contracts,” Reps. Lamar Smith (R-Texas) and Steven Palazzo (R-Miss.) wrote in a letter to NASA Administrator Charles Bolden dated Oct. 21. Smith and Palazzo chair the House Science Committee and House Science space subcommittee, respectively.

“We hope that NASA will not expect taxpayers to blindly fund billion-dollar programs absent any information related to the procurement or contract,” wrote Smith and Palazzo, who are ardent supporters of the Space Launch System, major contractual details of which were only finalized this year and have not yet been made public.

NASA has not publicly released the 29-page source selection document for the Commercial Crew Transportation Capability procurement, in which William Gerstenmaier, the agency’s associate administrator for human exploration and operations, detailed his reasons for giving Boeing and SpaceX contracts respectively worth $4.2 billion and $2.6 billion while rejecting Sierra Nevada Corp.’s (SNC) $3.3 billion proposal to build a lifting-body spacecraft called Dream Chaser. Gerstenmaier was the source selection authority on the program.

Nor does it plan to — at least not until the Government Accountability Office completes its review of the bid protest filed by SNC Sept. 26. The GAO has until Jan. 5 to complete its investigation.

“We’re not releasing the source selection statement at this time due to the litigation,” NASA spokeswoman Stephanie Schierholz wrote in an Oct. 24 email. “We are confident in our evaluation and selection process.”

A copy of the document was obtained by SpaceNews. In it, Gerstenmaier said that while price was the single most important of three factors he considered, the combined importance of the other two — mission suitability and past performance — gave human spaceflight veteran Boeing a clear edge, despite the fact that SNC’s $3.3 billion proposal was some $900 million lower than Boeing’s winning bid.

Gerstenmaier also disagreed with the members of the source evaluation board about the importance of some planned Dream Chaser features. For example, Gerstenmaier gave less weight to Dream Chaser’s ability to land on runways than did the evaluation board, and was more troubled than the board over some of the remaining technical hurdles in SNC’s proposal.

“Significant design trades remain to be made, complex hardware and software development remains, and there is a lengthy ground and flight test approach,” Gerstenmaier wrote in the document.

Gerstenmaier said he was troubled by “a critical design decision yet to be made regarding different main propulsion systems” for Dream Chaser, which resembles a mini space shuttle. The spaceplane, which would launch on a United Launch Alliance Atlas 5 rocket, originally was supposed to use a hybrid rocket motor with solid fuel and liquid oxidizer for onboard propulsion. The motor would have been similar to what SNC was providing for Virgin Galactic’s SpaceShipTwo suborbital space tourism vehicle, before that company switched to another SNC-provided engine that will use a different solid propellant.

SNC announced June 19 that it had acquired Madison, Wisconsin-based ORBITEC, a provider of liquid rocket propulsion systems. That fueled speculation that the company was planning to switch Dream Chaser to an all-liquid onboard engine.

Meanwhile, with Gerstenmaier’s decision under review at GAO, SNC has doubled down on the public relations campaign it has staged around Dream Chaser since 2012, the last time the company won any commercial crew funding from NASA.

At an Oct. 23 meeting of the American Society for Gravitational and Space Research in Pasadena, California, John Roth, vice president of business development for SNC, introduced “Dream Chaser 4 Space,” a version of the seven-seat spacecraft that could act as an independent science laboratory. Researchers using Dream Chaser 4 Space “will maintain scientific intellectual property rights, free from Federal Research Laboratory regulations that govern the International Space Station,” Roth said.

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.