NASA may therefore need to commit to costly overhauls of the Space Shuttle fleet instead, said House space subcommittee Chairman Dana Rohrabacher, R-Calif.

The report by the General Accounting Office, the investigative arm of Congress, said, "Delays in the X-33 Program may affect NASA's investment plan for future space-launch programs, including decisions on whether and when to upgrade the space shuttle fleet or rely on a new launch vehicle, such as the Venture Star (sic) RLV."

The ranking Democrat on the committee, Bart Gordon, D-Tenn., was unavailable for comment, as was Ralph Hall, D-Tex, ranking member on the Science Committee. Lockheed Martin referred space.com to NASA. NASA did not return repeated calls for comment.

But in a formal commentary on the report, NASA strongly rebutted the claims, saying that, "While there is a delay in the flight test element of the program, the overall X-33 program remains within the original schedule," which ends on Dec. 31, 2000. "Consequently, the flight delays should not impact decisions associated with investing in Space Shuttle upgrades."

For now, the shuttle fleet is temporarily grounded as NASA checks for frayed wires in the engine control system. Inspection crews have found damage on all four shuttles.

The X-33 is being developed by Lockheed Martin under a cost-sharing agreement with the civil space agency. It is the prototype for the VentureStar, a proposed single-stage-to-orbit vehicle to be built entirely by Lockheed. NASA plans on possibly using a proposed fleet of VentureStars to ferry crew and supplies up to the International Space Station.

Rohrabacher said the problems lay not with the X-33, which he called, "a good first step towards cheaper access to space," but with NASA's whole approach to reusable launch vehicle development.

"The Administration put all its technology eggs in one fragile basket and then told it to start flying. The result is that we still see NASA officials planning to upgrade and operate a government-owned Space Shuttle for another forty years," he said.

Although the report was also critical of cost overruns in the project that could eventually wind up as a taxpayer burden, Rohrabacher also attacked the White House for not funding the X-33 and X-plane programs.

"Our efforts [to increase funding] have been met with bureaucratic food fights, unending studies, funding impoundments, and even a Presidential veto," Rohrabacher said.

The White House was still coordinating its response as of late Thursday afternoon, and had no official comment.

Rohrabacher also mentioned a section of the GAO report detailing how technical problems have forced Lockheed to revise some of its goals for the X-33.

"The required top speed has been reduced by almost 10%, and important technology demonstrations for the VentureStar RLV have been eliminated," he said. The X-33 will use an aluminum liquid oxygen tank rather than a tank made of lighter, composite materials, the report said.

NASA's very clearly disagreed with this conclusion in its official comments, saying that the reduction of test flight speed from Mach 15 to Mach 13.8 will not affect the technical aspects of the program, and that all other major requirements of the NASA-Lockheed agreement are being met.

"We believe it is therefore inappropriate to imply that the X-33 performance objectives have been reduced," NASA said.