An electrical distribution unit aboard the International Space Station, taking down two of the lab's eight solar power channels. After extensive troubleshooting, NASA managers opted to replace the component using the station's robot arm, delaying launch of a SpaceX Dragon cargo ship, agency officials said Tuesday.
Loaded with nearly 5,500 pounds of equipment and supplies, the Dragon had been expected to blast off atop a Falcon 9 rocket from the Cape Canaveral Air Force Station early Wednesday. Launch now has been delayed to Friday at 3:11 a.m. EDT, setting the stage for the Dragon's capture and berthing early Sunday.
Unlike Russian Progress supply ships that fly themselves all the way in to docking at one of four Russian ports at the station, the Dragon relies on the U.S. segment's 50-foot-long robot arm to capture it and pull it in for berthing at the Earth-facing port of the forward Harmony module.
The Canadian-built robot arm can be operated with a single power source, but NASA flight rules require two for redundancy to guard against the possibility of a power failure during a critical operation. As it turns out, one of the two power circuits supporting arm operations was knocked out by the main bus switching unit -- MBSU -- problem that cropped up Monday.
Flight controllers at the Johnson Space Center initially hoped to restore main bus switching unit No. 3 to normal operation or if not, to re-route power from channels 3A and 3B, the two affected by the failure, into the station's other operational power channels.
They were able to re-route power as needed to keep critical systems operating normally, but were unable to provide a redundant power source for the robot arm. NASA managers then opted to replace MBSU 3 with a spare mounted on the station's main solar power truss.
The work will be carried out by flight controllers operating the arm remotely from the Johnson Space Center in Houston. While redundant power is required for capturing visiting cargo ships, the MBSU will be replaced using the single available power circuit.
The MBSU is critical to space station operations. Four such electronics boxes mounted on the central segment of the station's main truss distribute solar array power to eight primary circuits, or channels, that deliver electricity to myriad downstream systems across the lab complex. Jumpers can be used to divert power from one channel to another in the event of failures.
Spacewalking astronauts replaced a failed MBSU during two back-to-back excursions in 2012. Then, in 2017, flight controllers used the station arm to replace a second MBSU, proving the task could be accomplished robotically.
If all goes well, the installation and retest of a replacement for MBSU 3 should be completed by mid week, clearing the way for the Dragon's pre-dawn launch Friday.