Engineers have elected to replace a vital control unit inside the steering system of an Orbital Sciences Minotaur rocket, delaying Friday's launch of the vehicle and its Air Force spacecraft cargo by several weeks.

The booster will fly from Vandenberg Air Force Base in California to deliver the Experimental Satellite System-11 (XSS-11) craft into orbit for demonstrations of autonomous operations and close maneuvering with other space objects.

But concerns over reliability of some capacitors within a box called the Thrust Vector Controller (TVC) led to the postponement, the Air Force said Tuesday. These boxes control the steering of the Minotaur's engine nozzles during ascent.

"The capacitors in the TVC come from two different vendors. It was determined in testing and physical examination after the tests that one vendor's capacitors are significantly more robust than another's," according to officials at the Air Force Space and Missile Systems Center.

"Concerns over capacitors have existed for some months, and all TVCs received additional box-level testing as a result. The intent of the additional testing was to screen out questionable capacitors or workmanship.

"Testing over the last several days of individual capacitors revealed that box-level testing might not catch all potential defects, and that one vendor's capacitor was more robust. It was therefore decided to take a conservative approach and change to the more robust capacitor."

The controllers used on Minotaur rockets are the same carried aboard Orbital Sciences' air-launched Pegasus vehicles. A circuit card or possibly an entire TVC featuring the better capacitors will be removed from the Pegasus earmarked to launch the Air Force's C/NOFS satellite later this year for installation on this Minotaur.

Plans are still being developed for the replacement work. But the Air Force says initial estimates suggest the Minotaur could be ready for the $80 million launch in late April.

This will be the third Minotaur flight, following two successful missions in 2000. The rocket incorporates decommissioned first and second stages from a Minuteman 2 ICBM missile and solid-fuel motors from Pegasus for third and fourth stages. The vehicle is designed to provide the U.S. government with reliable access to space for small satellites.

The 319-pound XSS-11 spacecraft was built by Lockheed Martin at its facilities near Denver for the Air Force Research Laboratory.