A European Space Agency (ESA) inquiry points to a still-unexplained problem with a computer communications interface as the culprit in a temporary power loss that followed the recent docking of the cargo- and propellant-laden Automated Transfer Vehicle-3 with the International Space Station.

The incident, which followed a flawless March 28 docking of the 35-ft.-long freighter with the station’s Russian segment, had the station’s six-member crew scrambling three days later to off-load the most crucial cargo in case a power-drained ATV-3 had to be prematurely jettisoned on April 1 (Aerospace DAILY, April 3).

Though unexplained, the difficulty has not further interrupted ATV-3 operations. The freighter’s own solar panels should provide steady power through at least May, and the ATV-3’s stay may be extended beyond the originally scheduled Sept. 3 departure, said Massimo Cislaghi, the ATV-3 mission manager, in an April 12 telephone interview from the European Space Research and Technology Center in Noordwijk, The Netherlands.

“ATV-3 does not have any kind of power issue,” Cislaghi stresses. “It was a failure of an internal ATV communications bus. It was never a crisis situation.”

The post-docking difficulty appears to have occurred in the data interplay between the ATV’s multi-channel Russian Equipment Control System (RECS) and the freighter’s central computer, formally known as the Fault Tolerant Computer. The FTC shut down the RECS channel one in response to a fault message detected in the RECS, which is an ATV component.

The channel one shutdown interrupted other RECS functions, including a supplementary, low-level power feed from the station to the ATV. At the same time, the solar beta angle on the station’s orbital plane was increasing, prompting concerns within NASA’s Mission Control that the steady flow of electricity from the 4-5 kw of power generated by the ATV’s solar arrays would be interrupted as well by April 1. On March 30-31, flight control teams, joined by the astronauts, scrambled to successfully establish a power connection through the RECS channel 2, while the station crew off-loaded high-priority cargo.

A Russian air filtration fan activated 1 hr., 40 min. before the problem has been partially absolved of responsibility, Cislaghi says. The fan is used to remove contaminants from the air inside the freighter before the astronauts are permitted to enter the capsule.

RSC Energia, which supplies the RECS unit, is assisting in independent ESA fault-tree investigation, Cislaghi says.

“We need to make sure we do not encounter the same problem again in case the investigation reveals a design deficiency,” Cislaghi says. “You never know. It is the third ATV, but it’s a quite complex machine.”

ESA is committed to the flight of two more ATVs through 2015, a period crucial to NASA’s efforts to establish a post-shuttle U.S. commercial resupply capability for the orbital lab.