The report from the General Accounting Office, the investigating arm of Congress, also says that the prototype spaceplane, scheduled for test flights from Edwards Air Force Base in July of next year, has faced several technical problems that have led to delays and changes in the mission profile, such as a reduction of the test flight speed.

NASA's comments, included in the report, say "the report highlights some significant differences in opinion on several basic concepts." The agency included six pages of specific rebuttals to the 25 pages of GAO comment.

The X-33 is being designed and built by Lockheed Martin, which has a cost-sharing agreement with NASA. Although NASA's contribution to the program is fixed at $912.4 million, with the responsibility for all cost increases to fall to Lockheed, the report says that the government's share may increase indirectly.

Lockheed can claim some additional costs under the "independent research and development" clause of the government acquisitions contract. Under IRAD, Lockheed could seek to reclaim up to $161 million of its estimated $286.6 million contribution to the project through contracts it has with other agencies, like the Department of Defense.

Delays in the X-33 will also make it difficult for NASA to decide on its level of commitment to the Lockheed VentureStar, a proposed single-stage-to-orbit vehicle to succeed the space shuttle, the report says. NASA has had to delay its decisions on upgrading the shuttle fleet, since the X-33 has not been able to fully demonstrate the reliability of the VentureStar systems it is testing, the report said.