Electrical issues resolved to allow CRS-17 Dragon launch

by Chris Bergin and Pete Harding

NASA postponed the next Dragon resupply mission to the International Space Station (ISS) after an issue with the Station’s Main Bus Switching Units (MBSUs) – a component of the ISS United States Orbital Segment (USOS) Electrical Power System (EPS) – was observed this week. The CRS-17 launch was slipped to Friday while the issue was being evaluated. However, work to restore the redundant string to the robotic arm that will capture Dragon went to plan, initially with the use of jumpers, followed with a robotic changeout of the faulty MSBU on Thursday.

The CRS-17 Dragon mission was set to launch early on May 1, lifting off on a Falcon 9 rocket from the SLC-40 pad at Cape Canaveral.

The launch vehicle and spacecraft were in the late stage of preparation, with the booster having undergone a successful Static Fire test and rolled back for mating with the Dragon spacecraft. A Launch Readiness Review (LRR) and “Late Loading” were the only outstanding issues in the flow towards the launch day.

However, NASA called off the launch following the issues with the electrical system on the Station, which had the potential to impact on the safe capture of the Dragon once she had arrived later this week.

The MBSUs are essentially electrical distribution boxes for the orbital outpost. Two channels have gone down meaning a loss of redundancy for some key hardware related to Dragon’s arrival.

The MBSU’s have had issues before, with channels 2A and 2B suffering a problem in 2017. Replacement work was conducted using the SSRMS.

US Station electrical power is generated via the station’s eight Solar Array Wings (SAWs) at 160 Volts Direct Current (VDC), which is known as primary power. From each of the eight SAWs, the primary power goes to a corresponding Direct Current Switching Unit (DCSU).

A full overview of the USOS EPS is given in the 182-page ISS EPS training manual, available to download in the ISS section on L2.

During times when the SAWs are in sunlight (i.e. generating power), the DCSUs send some power directly to one of four MBSUs, and some power to a corresponding Battery Charge/Discharge Unit (BCDU) which in turn charge the SAW batteries for use during periods of orbital darkness.

During orbital darkness, when the SAWs are not generating power, the DCSUs send battery power directly to one of the four MBSU via a Utility Transfer Assembly (UTA), which facilitates power transfer across the rotating Solar Alpha Rotary Joint (SARJ). There are two UTAs on the ISS – one for the port SARJ, and one for the starboard SARJ.

Once the power has passed the UTA, the four MBSUs on the ISS each receive power feeds from two DCSUs, and as such the four MBSUs between them distribute power from all eight DCSUs and their associated SAWs. These DCSU power feeds are known as channels, and are denoted as 1A, 1B, 2A, 2B, 3A, 3B, and 4A, 4B – the same as their corresponding SAWs (i.e. SAW 1A feeds channel 1A).

The failure noted on the ISS relates to two of those channels specific to MBSU 3.

“Main Bus Switching Unit (MBSU) 3 Loss of Communication – A critical bus loss occurred on MBSU 3, which brought down loads on channels 3A and 3B,” noted L2 ISS Status information.

“Major impacts included KU-band, Alpha Magnetic Spectrometer (AMS), CMG-3 and Space Station Remote Manipulator System (SSRMS) redundancy. ”

The latter is key to Dragon’s arrival. While one channel remains available for using the SSRMS, NASA likes to have redundancy available in the event of an issue during capture.

SSRMS grapples Dragon – via NASA

Troubleshooting is by no means simple. The MBSUs distribute power from each of their two DCSU input feeds to DC to DC Conversion Units (DDCUs) via Remote Bus Isolators (RBIs). The DDCUs in turn transform the 160 VDC primary power (which can at times vary by around plus or minus 10 volts, depending on the SAW output) into a steady 124 VDC, known as secondary power.

From the DDCUs, the 124 VDC secondary power is sent to the USOS power busses, from where it passes through Remote Power Control Module (RPCM) circuit breakers, from where it can finally be accessed by user loads.

Some MBSUs also send primary power from American to Russian Conversion Units (ARCUs), which transform 160 VDC USOS primary power into standard 28 VDC Russian Segment (RS) power, which is in turn distributed throughout Russian modules to supplement Russian solar array power (a completed task during US EVA-18 involved routing power cables from an MBSU to the RS, for use by the future MLM module). MBSUs can also be cross-tied if necessary in order to effectively re-wire the USOS EPS architecture.

Physically speaking, all four box-like MBSUs are installed onto the S0 Truss, as they have been since the S0 Truss’s launch over a decade ago on STS-110 in April 2002, although MBSUs were designed to be EVA-replaceable as part of the ISS’ modular design philosophy.

NASA has convened a Failure Investigation Team (FIT) to work through the problems.

“Troubleshooting was not successful and teams are assessing the possibility of a power supply failure. A FIT is planned,” added L2 ISS information. “Console teams are working through jumpering plans and alternate power strings to maintain key loads.

As of Monday, the KU-band communication has been regained and AMS has swapped loads to channel 4A and has resumed nominal operations.

Other issues on the Station have also been noted around this event, with the Remote Power Control Module (RPCM) S01A_B Remote Power Controller (RPC) 2 Trip, seemingly in relation to the MBSU issue.

“In response to the MBSU 3 failure and loss of operational power to Control Moment Gyro (CMG)-3, teams attempted to close RPCM Z14B_B RPC 12 in order to provide survival heater power to CMG 3.  Upon closure attempt, the RPC tripped open,” added L2 ISS information.

“This is a Type V external RPCM and is susceptible to hybrid Field Effect Transistor (FET) issues.  Teams are working within thermal clocks and discussing mitigation plans.”

UPDATE: The latest report noted positive news about the ongoing efforts to restore the redundant string that will ultimately allow Dragon to launch on Friday. The key effort will take place on Thursday.

A previous robotic MBSU swapout – via NASA

“Main Bus Switching Unit (MBSU) 3 Loss of Communication – A critical bus loss occurred on MBSU 3 causing all Remote Bus Isolators (RBI) to open and a loss of all loads on channels 3A and 3B.  Troubleshooting was not successful and the FIT determined MBSU 3 is failed,” added L2 ISS Status.

“The crew and ground teams coordinated the install of the Node 1 Y-Jumper and Node 1 Contingency Power Jumper and removal of the Node 1 Channel 2 and 3 isolation.  This jumper plan recovered all loads except those on DDCU P13A.”

The key work came on Thursday when robotic assets worked to changeout the faulty MBSU. This work was deemed successful.

NASA confirmed the launch slip to Friday, but waited until the MBSU changeout to ensure Station assets were ready to support the capture of the CRS-17 Dragon.

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