WASHINGTON -- Haunted by the prospect of huge cost overruns on a module to help boost the International Space Station (ISS) in orbit, NASA managers instead have decided to fashion the component largely from test hardware now in storage at a NASA center.

The component -- known as Node X -- will have a central core that looks identical to the silver-colored U.S.-built node Unity on the ISS. Unity serves as the docking port for visiting space shuttles.

Both packages would be detachable so they could ferried to Earth for refilling via the shuttle. NASA expects those refill trips could happen every two years or so, depending on how often the propulsion package is used.

Node X is scheduled to be flown to the ISS aboard a space shuttle in June 2004 and will be positioned in front of the European-built laboratory Columbus.

"This module does increase the overall robustness of the system," Mike Hawes, deputy associate administrator for spaceflight development, told reporters Thursday.

A key selling point was its cost. Node X uses existing hardware already tested at the Marshall Space Flight Center in Huntsville, Alabama. Managers expect it can be built within the $540 million limit NASA had budgeted for the Boeing-designed Propulsion Module.

The Propulsion Module, which is only about 10-percent finished, had escalated in cost "on the order of $100 million to $200 million over our cost estimates," Hawes said.

"What concerned us most was we felt there was still residual risk in the [cost] proposals we were getting," Hawes said. "The large uncertainty range was one of my concernsI had seen options that went over $750 million."

NASA ordered Boeing to stop work on the Propulsion Module in late June or early July. Managers now expect to issue a new work order on Node X in March.

Node X was built about the same time as Unity and had been scheduled to go to the ISS as an identical twin to Unity. But NASA ended up in a "barter arrangement" with the European Space Agency (ESA), Hawes said, in which ESA agreed to provide two nodes as a trade for the costs of launching the Columbus module aboard the space shuttle.

That left NASA free to do what it wanted with its own node.

Node X "met all the structural and pressure tests" required of the original Propulsion Module, Hawes said. "It basically has been sitting there without use."

Once in orbit, Node X will serve as a sort of insurance policy to help keep the ISS in a steady orbit above Earth. NASA now uses the space shuttle to reboost the station and Russia has promised to deliver automated Progress spacecraft to do the same thing.

But Node X by itself will be able to provide up to half of the 15,430 pounds (7,000 kilograms) of fuel needed each year for reboost capability.

The central core of Node X "will still be a passageway for the crew," Hawes said. "It will be treated as habitable volume with lighting and will house the avionics boxes and computers."