A troubled on-orbit satellite-servicing experiment led by the U.S. Defense Advanced Research Projects Agency (DARPA) has run into further cost-growth problems that could lead to its cancellation.

Difficulties with the Orbital Express propulsion system and fuel-transfer hardware, as well as the need to replace possibly defective computer chips on the two spacecraft, have contributed to the continued cost growth on the project, said Jan Walker, a DARPA spokeswoman .

When DARPA awarded the Orbital Express prime contract to Boeing Phantom Works in 2002, the on-orbit demonstration was expected to cost $100 million. Walker declined to divulge the latest cost estimate for the experiment, but an industry source said the price tag has tripled.

Daryl Stephenson, a spokesman for Boeing Phantom Works of Huntington Beach, Calif., referred all questions regarding the Orbital Express program to DARPA.

The experiment is intended to examine the feasibility of using a semi- autonomous spacecraft to service other satellites to extend their orbital lifetime, including performing upgrades, fixing broken equipment and refueling . DARPA officials also wanted to see if a satellite based on Orbital Express’ technology could help move another spacecraft built by Boeing to evade potential threats.

The demonstration, which still is slated to launch in September 2006, includes a refueling satellite called Astro that will rendezvous with a second satellite called NextSat, built by Ball Aerospace.

After awarding the initial contract, DARPA had added a microsatellite to the mission whose role was to observe the servicing experiment. But the third spacecraft subsequently was scrapped as it was not considered crucial to the experiment and would have made it more complicated, Walker said.

The addition of the observing spacecraft was blamed in the spring of 2004 for contributing to a doubling of Orbital Express’ projected cost. But its elimination has not brought the cost back under control.

Developing the technology for autonomous refueling in space is “an inherently risky undertaking with potentially significant payoffs for future space operations,” Walker said. The fuel-transfer and propulsion systems proved more challenging than DARPA expected, she said.

Those systems recently completed a series of tests with mission flight-control software and hardware at subcontractor Northrop Grumman Space Technology’s facilities in Redondo Beach, Calif., and were delivered to Boeing in mid June, Walker said.

The problems with the computer chips were an industry-wide issue that affected a variety of U.S. government space programs, Walker said. The problem devices, known as field programmable gate arrays, were built by Actel Corp. of Mountain View, Calif. The devices can be programmed to manage a variety of spacecraft functions.

DARPA was forced to replace a “large number” of the field programmable gate arrays on “nearly complete flight hardware,” Walker said.

Most of the Astro and NextSat satellite hardware has been delivered, and work on the program now is focused primarily on test and integration, Walker said. As with other DARPA efforts, the agency will reassess the experiment before making a final decision on whether to launch it, she said. Walker did not respond to a question on when that decision will be made.

The fate of Orbital Express could be tied to a new demonstration in DARPA’s portfolio that also examines orbital rendezvous, according to industry and congressional sources. The Spacecraft for the Universal Modification of Orbits (SUMO) may be a more attractive candidate for a space demonstration to Pentagon officials, the sources said.

DARPA’s 2006 budget justification materials describe SUMO as a spacecraft capable of assisting in orbital salvage, repair, rescue, reposition and debris removal.

Walker said SUMO, a new start in the 2006 budget request, is not intended as a substitute for Orbital Express. DARPA is asking Congress for $12.6 million for SUMO in 2006, and plans to spend $22 million on the effort in 2007, according to the budget justification documents.

SUMO may be a more attractive option for DARPA in part because it is early in development and has not been tainted by the technical difficulties that have plagued Orbital Express, the sources said.

The program also may be better designed to meet the Pentagon’s needs, the sources said. Satellites would have to be specially designed to be handled on orbit by the types of hardware and techniques to be demonstrated with the Orbital Express mission, the sources said. SUMO, on the other hand, would be capable of grappling any satellite.