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One of two redundant channels in one of two communications cards in that PDU is not working.
NASA announced the problem with the PDU Nov. 30, saying only that it was “troubleshooting the issue.” The problem was first reported by The Verge, which said that presentations by prime contractor Lockheed Martin warned it could take up to a year to replace the PDU because it is located in an adapter between the crew module and service module that is inaccessible now that the two modules are mated to each other.
That time estimate assumed that the crew module would be demated from the service module, the PDU repaired, and the two modules combined again and tested. An alternative option could complete repairs in just four months, but would require removing panels of the adapter to reach the PDU, something the hardware was not designed for.
NASA, in its statement about deciding not to replace the PDU, did not go into details about the repair options, but said that the risks of damaging the spacecraft during the PDU repair outweighed any loss of data should the unit completely malfunction.
Engineers, the agency stated, “determined that due to the limited accessibility to this particular box, the degree of intrusiveness to the overall spacecraft systems, and other factors, the risk of collateral damage outweighed the risk associated with the loss of one leg of redundancy in a highly redundant system.”
“NASA has confidence in the health of the overall power and data system, which has been through thousands of hours of powered operations and testing,” the agency added, noting that the PDU in question was still “fully functional.”
Even before the statement, NASA officials were playing down the possibility of performing an extensive repair to the spacecraft to replace the PDU. “You’ll get a first report, and the first report will show all the worst-case possibilities,” said Ken Bowersox, deputy associate administrator of NASA’s Human Exploration and Operations Mission Directorate, during a Dec. 7 media briefing about a report on the science planned for the Artemis 3 lunar landing mission. “On the best-case side, we may find that it is something we can live with.”
At the time of that briefing, Bowersox said NASA was still studying the problem, but was optimistic it would not have a major effect on the Artemis 1 mission. “As we look at it, we really don’t think that it’s really going to have a big impact on the final schedule,” he said.
With the decision not to replace the PDU, NASA is moving ahead with final closeouts of the spacecraft. In mid-January, it will move from the facility at the Kennedy Space Center where it was assembled to the Multi-Payload Processing Facility there, where it will be fueled and prepared to be installed on the Space Launch System.
The critical path for the Artemis 1 launch, still scheduled for November 2021, remains the SLS core stage, which is undergoing a series of tests called the Green Run at the Stennis Space Center. NASA said Dec. 17 it had restarted preparations for a fueling test called the wet dress rehearsal after it called off a first attempt at that test in early December because liquid oxygen was flowing into the core stage at temperatures slightly higher than desired.