The NSF article discussed a timing issue when several things were coming together at ISS at the same time in mid-August 2017. Of course ISS scheduling is always complicated, but this particular issue seemed to be somewhat unique in its combinations.
The Dragon CRS-12 cargo resupply mission was already having to work around an Atlas TDRS launch. NASA was the customer on both launches, so they were the ones who set the priority. In this case priority was being given to the TDRS (tracking and data relay satellite) launch. The TDRS launch date had moved a few times as they dealt with replacing an S-Band antenna that had been damaged during final processing of the satellite in July.
However Dragon wound up getting some breathing room when the TDRS launch was postponed again by a few days. The unusual “single-shot” restriction that was mentioned in the article was because of another timing factor, a planned Russian EVA on August 17th, 2017. Normally Dragon would just need to avoid arriving on the day of the EVA, but in this case the Russian cosmonauts would be manually deploying several nanosatellites.
Russian Cosmonaut Fyodor Yurchikhin with two Russian university nanosatellites that he and cosmonaut Sergey Ryazansky deployed on August 17th, 2017 along with three other satellites. (Photo: Roscosmos)
While the satellite deployment would be safe for ISS or any vehicles already attached to ISS, the time period immediately after deployment would not be safe for an approaching spacecraft until the orbits of the nanosats were reliably calculated. This meant that Dragon needed to be safely berthed to ISS prior to the EVA. Since it normally takes a couple of days for cargo Dragon to reach ISS, it had to launch as scheduled on August 14th or else delay for several days.
Meanwhile another potential conflict had cropped up because of a problem with the power supply for one of the S-Band transponders on ISS used for communication. The power supply experienced issues on August 9th, just five days before Dragon's scheduled launch. The Dragon launch criteria required that both of the two S-Band "strings" are functioning for redundancy. String-1 is located on the S1 truss, the problem was on String-2 located on the P1 truss. This loss of redundancy would have also affected the upcoming EVA. The problem with the power supply seemed to be caused by a Field Effect Transistor (FET) failure, located within an RPCM (Remote Power Controller Module). The RPCM is described in this Boeing document about the ISS Electric Power System (EPS).
The workhorse of the secondary power distribution system is the RPCM, an Orbital Replacement Unit (ORU), which contains solid-state or electromechanical switches, known as Remote Power Controllers (RPCs). RPCs can be remotely commanded, by on-board computers, to control the flow of power through the distribution network and to the users. There are different types of RPCMs, containing varying numbers of RPCs and varying power ratings.
However the S-Band problem was soon cleared as an impediment to the Dragon launch because on August 10th the teams on the ground did several tests using Ku-Band to perform the needed communication functions, and it was decided that Ku-Band could serve as a backup for both the Dragon arrival and the EVA.
As it turned out everything worked out fine, CRS-12 launched on August 14th, 2017 and was berthed to ISS on August 16th. The EVA was conducted on August 17th, and the Atlas TDRS launch took place on August 18th.
An interesting comment was made in the NSF article which noted that the replacement of the RPCM by Dextre was not restricted by the EVA, so there was the potential that the two operations might be taking place at the same time. RPCM’s are typically replaced by the Canadian built Dextre robot (Special Purpose Dexterous Manipulator).
Dextre (Photo: NASA)
In fact the very first hardware replacement performed by Dextre, which arrived at the station in 2008, was the replacement of an RPCM in August 2011. A NASA document about that replacement stated:
Known by the technical term “Remote Power Control Modules,” (RPCMs) circuit-breaker boxes control the flow of electricity through the ISS’s secondary power distribution system, and tend to fail occasionally. Up to now, exchanging the boxes was done by spacewalkers, which always carries a certain level of risk. Dextre was designed to reduce the need for astronauts to conduct spacewalks for routine maintenance
The NSF article pointed out the possibility of both robotic and human "workers" performing separate tasks simultaneously outside the station, which does sound a bit futuristic when you think about it that way, even though Dextre is not autonomous but controlled by the ground.
As it turned out, the RPCM replacement was scheduled for August 24th, a week after the Russian EVA. However on August 23rd during preparations for the procedure, which would have included unstowing Dextre, one of the Latching End Effectors on Canadarm2 experienced a problem that caused it to be unable to provide a power and data bridge to whatever it was grasping, which would include Dextre. This problem also prevented the arm from being able to walk from one location to another. However it did not prevent the arm from unberthing the Dragon CRS-12 capsule, which it did on September 17th.
The Latching End Effector was replaced during a U.S. spacewalk on October 5th, and the RPCM was replaced on October 17th, exactly two months after the Russian EVA. However even the RPCM replacement didn't go quite smoothly according to the NASA blog:
On Monday night RPCM P12B-B was extracted and installed into empty truss slot P11A-D and the replacement RPCM was removed from slot P13A-G, however it could not be inserted into the P12B-B at that time. Robotics activities resumed on Tuesday afternoon and teams were able to successfully install the replacement RPCM in the P12B-B slot after 47 wiggle and push attempts. The new P12B-B RPCM powered up nominally.
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