WASHINGTON — Two U.S. firms awarded a combined $3.5 billion in NASA contracts in December to haul cargo to the international space station have fallen several months behind schedule for demonstrating their vehicles, according to a report released June 16 by the investigative arm of Congress.

“With the approaching retirement of the space shuttle in 2010, NASA faces the difficult challenge of securing sufficient and reliable cargo transportation capabilities to the space station from 2010 to 2015,” the U.S. Government Accountability Office (GAO) wrote in its assessment. “While commercial partners have made progress in developing cargo transportation capabilities, they have recently fallen behind their development schedules. Furthermore, the most critical steps lie ahead, including successfully launching new vehicles and completing integration with the space station.”

The GAO report, entitled “Commercial Partners are Making Progress, but Face Aggressive Schedules to Demonstrate Critical Space Station Cargo Capabilities,” was requested by Congress in a 2008 federal spending bill.

Space Exploration Technologies Corp. (SpaceX), the Hawthorne, Calif.-based startup awarded $278 million under NASA’s Commercial Orbital Transportation Services (COTS) demonstration program in August 2006, is the furthest behind schedule. SpaceX originally proposed conducting the first demonstration flight of its cargo-carrying Dragon spacecraft in September 2008 but early last year alerted NASA that the flight would not occur until this June. With the Falcon 9 rocket the company designed to launch Dragon having yet to make its maiden flight, SpaceX’s first COTS demo has slipped an additional three months to no earlier than September, according to the GAO report.

Orbital Sciences Corp., the Dulles, Va.-based space hardware and launch services firm that won its $170 million COTS award about 16 months after SpaceX, renegotiated its COTS agreement in March to demonstrate a different cargo transport capability than originally planned. That helped delay the flight from December 2010 to March 2011.

Orbital’s winning COTS proposal envisioned flying an unpressurized version of its Cygnus cargo system for the first demo. But those plans changed after NASA awarded Orbital a $1.9 billion space station resupply contract in December, a deal that calls for eight pressurized cargo missions but no unpressurized missions.

According to the GAO report, Orbital also has experienced delays completing design reviews for components of the Russian-heritage AJ-26 engines it is buying from Sacramento, Calif.-based Aerojet for the first stage of the Taurus 2 rocket it is building to launch Cygnus.

SpaceX, which is designing and manufacturing almost all of the components for Falcon 9 and Dragon in-house in a bid to keep costs low, has been forced this year to postpone a March mission readiness review in order to address fabrication, assembly and integration issues with the Falcon 9’s second-stage components, according to the GAO report.

A SpaceX official said the company is optimistic that the first Falcon 9 demonstration launch would occur before the end of the year.

To date, SpaceX has completed 14 of its 22 COTS milestones on time, earning $234 million of the $278 million available to it under the program. The company is behind schedule on three milestones, which include the March mission readiness review, the promised May delivery of a communications flight unit and the first demo mission.

Orbital has completed 7 of its 19 milestones so far, earning $80 million. According to the GAO report, Orbital has yet to complete a COTS system-level preliminary design review that was on the calendar for April.

Orbital spokesman Barry Beneski did not immediately return a call seeking comment.

NASA’s associate administrator for exploration, Doug Cooke, told the GAO in a June 5 letter included in the report that NASA “generally agree[s] and support[s] the conclusions you have reached.”

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...