Lockheed Martin Corp.'s assembly of a next-generation space shuttle has stumbled on yet another technical glitch, potentially delaying the prototype's first test flight an additional six months or more, sources said.

Development of the X-33 rocket plane suffered the latest setback Wednesday night as a critical component underwent testing at NASA's Marshall Space Flight Center in Huntsville, Ala. A large section of the outer wall of one of the vehicle's two liquid hydrogen fuel tanks separated about two hours after it had completed pressure and structural tests at the facility, sources said.

Technical snags and cost overruns are common in space projects, but glitches that have plagued the X-33 for more than a year are threatening a program that NASA deems pivotal for its future. The space agency has placed a huge wager on Lockheed Martin's technologically ambitious design, committing $941 million to the X-33's development.

Under the National Aeronautics and Space Administration's plan, the prototype would lead to the development of a reusable launch vehicle called Venture Star that would dramatically cut the cost of space flight and take the place of the fleet of shuttles, which NASA aims to replace after 2012.

For Lockheed Martin, the latest delay all but ensures that it will be unable to have an operational Venture Star by the end of 2004, as it had promised to more than a dozen states competing to serve as the space port for the vehicle.

"We've been very honest about how tough this is," said one Lockheed Martin official, speaking on the condition of anonymity. When the states understand the details of the latest problem, he said, "they'll see this is the nature of a technology development program."

Lockheed and NASA engineers have started to survey the damage to the tank, and will erect scaffolding around it this week to permit a closer inspection, officials said.

Wednesday's glitch was the second time one of the graphite-reinforced plastic tanks has run into trouble in the manufacturing process. The inner skin of the X-33's other liquid hydrogen fuel tank separated while it was being bonded at a temperature of 350 degrees in December.

Examiners with the General Accounting Office, the investigative arm of Congress, have criticized the project for its rising costs and have warned that delays in the prototype's development would harm NASA's plans for a full-scale reusable vehicle.

GAO officials said in a report released in August that technical setbacks up to that point had increased the cost to taxpayers by $317.6 million--even though the prototype will use an internal liquid-oxygen tank made of aluminum, instead of lightweight composites as envisioned earlier. The full-scale Venture Star will probably need to have a composite tank in order to be light enough to reach orbit, the GAO said. Examiners questioned whether the X-33 will effectively lead to the development of the more advanced full-scale vehicle.