"I think what has happened is that as we have worked on the X-33, the design of the VentureStar has morphed," Payton said. "Some technologies have become more important and others haven't They have more confidence in their thermal protection system now." To reduce the weight of the vehicle, Lockheed is designing the internal tanks of the VentureStar, for greater "volumetric efficiency," reducing the need for weighty, wasteful supporting structures.

"They've also learned in the last three years that there's other technology that we have to push harder," Payton said, "specifically lighter weight material being used in the rocket engineuntil we prove those technologies, which have never been part of the X-33, I don't think you'll see either NASA or Lockheed Martin willing to work on a robust Venturestar program." The engine chosen for both the X-33 and the Venturestar is the Boeing-built Aerospike. The engine, using a radical new chamber-less design, has never flown in space before.

NASA will probably research those technologies, such as ceramic casings, as part of its space transportation programs. But Payton said only small pilot programs exist now to develop those materials.

"What [Lockheed] want[s] to do is wait for us to finish proving these lightweight propulsion components, preferably in flight, before they really have a lot of confidence in all the technological bugaboos" with VentureStar, Payton said.

"To one way of thinking, they have started VentureStar," because the company has already started designing it, Payton said. "It's just not going to be a billion-dollar a year program" yet.

Payton said that media reports on X-33 cost overruns have been misleading. The General Accounting Office report does not say that there will be a direct cost to the government, he says, rather that there will be more of Lockheed's independent research and development money, which it receives from every government contract, will go to the X-33.

Payton said that should be seen as a contribution by Lockheed instead of a cost, because the company would have received the money in any case and could spend it in any of the areas it does government work. Lockheed is one of the Defense Department's major contractors.

He said the program has been very fiscally responsible.

"What I like to remind people is that when Congress appropriates money for the X-33 in fiscal year 2000, the amount of money they are appropriating is the same amount of money that we predicted at the start of the program in 1996."

The GAO report also claimed that delays on the X-33, and subsequent delays to the Venture Star, could force NASA to undertake expensive upgrades on the shuttle fleet in order to keep regular supplies headed up to the International Space Station. NASA would theoretically lease time on Lockheed Martin's Venture Star to resupply the station if the privately developed vehicle is built.

"There is absolutely no relationship between the X-33 or any operational vehicle that may come after the X-33, to the space station," Payton said. "We're going to use the space shuttle in its current configuration for assembly of the station. Any operational vehicle will be finished after the station is assembled."

Decisions to upgrade the shuttle fleet are based primarily on safety concerns, he said. Anything after that "would have to be balanced against the payback, and that's almost independent of any other vehicle program."

Payton also said that reducing the X-33's maximum test speed from Mach 15 to Mach 13.8 would make any difference to the program. The goal of the high-speed atmospheric flight was to test the craft's thermal protection system, and it can actually be fully tested at speeds below Mach 13.8.