International Space Station (ISS) program managers are currently hard at work preparing for at least one, and potentially two, upcoming and unplanned spacewalks to repair external faults relating to two of the station’s electrical power channels. The spacewalks would both be performed within the next month by experienced spacewalkers Sunita Williams of NASA and Aki Hoshide of Japan, prior to their return to Earth on November 19.
ISS EPS overview:
Power channels are a feature of the ISS United States Orbital Segment (USOS) Electrical Power System (EPS), which generates all of its electrical power 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).
During times when the SAWs are in sunlight (i.e. generating power), the DCSUs send some SAW power to one of four Main Bus Switching Units (MBSUs), and some SAW power to corresponding Battery Charge/Discharge Units (BCDUs) which in turn charge the batteries for use during periods of orbital darkness. During orbital darkness, when the SAWs are not generating power, the DCSUs send battery power to one of the four MBSUs.
These SAW-to-DCSU-to-MBSU feeds are known as power 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 four MBSUs on the ISS each receive power feeds from two power channels, and as such the four MBSUs between them distribute power from all eight power channels and their associated SAWs.
The four MBSUs in turn distribute power from each of their two power channel inputs on to DC to DC Conversion Units (DDCUs), which transform the 160VDC primary power (which can at times vary by around plus or minus 10V, depending on the SAW output) into a steady 124VDC, known as secondary power.
From the DDCUs, the 124VDC 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.
Channel 2B fault/US EVA-20:
The first, and by far most serious, of the two faults relating to ISS power channels is an ammonia leak on channel 2B, which is located on the P6 Truss Integrated Equipment Assembly (IEA).
ISS power channels are cooled by the Photo Voltaic Thermal Control System (PVTCS), which uses ammonia as a coolant to dissipate heat through dedicated Photo Voltaic Radiators (PVRs) on each Truss segment that accommodates SAWs, making for four PVRs on the ISS in total.
The PVRs on the PVTCS are separate from the main Heat Rejection Subsystem Radiators (HRSRs) on the External Thermal Control System (ETCS), which cools the electronics on the main ISS modules not associated with the photo voltaic systems.
In addition to its one PVR, the P6 Truss also contains two Early ETCS (EETCS) radiators that were used to cool the main ISS modules when the P6 Truss was located atop the Z1 Truss during the early life of the ISS.
However, these two EETCS radiators were retracted prior to the P6 Truss being relocated from the Z1 Truss to its current home on the end of the P5 Truss during STS-120 in November 2007, as they were no longer needed since the main HRSRs of the ETCS were fully deployed shortly thereafter.
The P6 Truss was launched on STS-97 in December 2000 with 52 lbs. (pounds) of ammonia coolant on board, however since December 2006 ISS flight controllers have been tracking an ammonia leak on channel 2A on the P6 Truss at a rate of around 1.5 lbs. per year.
Due to this leak, the ammonia lines on channel 2B were topped up by around 8.7 lbs. during an STS-134 EVA in May 2011.
The procedures involved connecting a number of jumpers and opening a number of Quick Disconnect (QD) valves in order to transfer some ammonia from the Ammonia Tank Assembly (ATA) on ETCS loop B on the P1 Truss, to the PVTCS ammonia loop on channel 2B on the P6 Truss. At that time, based on the known leak rate, it was projected that another refill would not be needed until 2015.
Since then, however, the ammonia leak has accelerated from its previous 1.5 lbs. per year to a current rate of 5.2 lbs. per year. Projections show that based on this leakage rate, channel 2B could be offline by the end of 2012 due to lack of sufficient ammonia for cooling, which would present a serious problem, as channel 2B carries “significant” loads for the ISS, which could have impacts across the entire station if they were to go down.
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Thus, an EVA, which will be US EVA-20, has now been planned for November 1, where NASA astronaut Suni Williams and JAXA astronaut Aki Hoshide will make a 6.5 hour excursion to rectify the issue. It is understood that another refill will not be performed however, as analysis has shown that the leak appears to be coming from the P6 Truss PVR, and so refilling the system will simply lead to it being lost again.
Instead, the plan for EVA-20 will involve extending one of the retracted EETCS radiators on the P6 Truss, connecting jumpers to bypass the leaking PVTCS PVR, and instead connect the EETCS radiator in its place, effectively converting the EETCS radiator into a PVTCS PVR. During the ammonia refill EVA of STS-134, the EETCS system on the P6 Truss received an increase of 4.9 lbs. of ammonia, since it was “in the pathway” to get to the channel 2B ammonia system.
The leaking PVTCS PVR could potentially be replaced in future with a spare EETCS radiator from the P6 Truss, which is of the same type as the PVTCS PVRs. The spare radiator located on ExPrESS Logistics Carrier-4 (ELC-4), which was launched on STS-133 in February 2011, is of the HRSR type, and so could not be used to replace a PVR. With the Space Shuttle now retired, no vehicles currently in service or even on the drawing board are capable of launching additional spare radiators, due to their large size.
Channel 3A fault/US EVA-21:
In addition to the channel 2B ammonia leak, another issue exists on channel 3A, related to an overcurrent trip that occurred in the Direct Current Switching Unit (DCSU) of that channel.
On September 1, while the ISS was operating with the loss of two power channels due to installation difficulties surrounding MBSU-1 during US EVA 18 and then EVA 19, another fault also occurred on power channel 3A; specifically an overcurrent trip in Remote Bus Isolator-1 (RBI-1) within the DCSU on channel 3A, leading to the loss of that entire channel.
Internal ISS on-orbit status notes from the time, available exclusively on L2, provided detail on the event: “The trip indicates current flowing from the DCSU toward the Sequential Shunt Unit (SSU). Review showed a fault current >250A out of RBI-1. The trip limit on RBI-1 is >25A for 40ms (milliseconds)”.
While MBSU-1 and its two power channels were eventually recovered during US EVA-19 in early September, channel 3A remained offline, and with it 12.5 per cent of the station’s power, while ISS flight controllers tried to determine the source of the trip in DCSU-3A. Leaving channel 3A offline was not desired, especially when the fact that channel 2B could be offline by yearend was considered.
In order to aid in the failure investigation, the ISS crew were requested to take imagery of components surrounding the P4/P4 Truss segment Integrated Equipment Assembly (IEA), where DCSU-3A is located, in order to determine if any visible damage had occurred on that area.
Using a camera pointed out of the forward-port facing hatch porthole on the Russian Docking Compartment-1 (DC-1) “Pirs” module, the crew photographed the 3A SSU, Bearing Motor Race Ring Module (BMRRM), and DCSU, the full set of which are available to download on L2.
Since the photos did not appear to show any obvious signs of damage, the failure investigation team came up with a plan to gradually bring channel 3A back online, and to try to determine where the fault lay by re-powering all suspect components sequentially.
According to exclusive L2 ISS on-orbit status notes, starting October 11, the first step performed was to supply power to the suspect area of channel 3A from the batteries, followed by supplying power to the suspect area from the SAW itself.
After a period of monitoring, with no further RBI-1 trips observed, the next step was to gradually change the position of SAW-3A, by rotating the Beta Gimbal Assembly (BGA) to point the array more directly at the Sun, in order to increase power generation by the SAW. The SAW had previously been angled slightly away from the Sun in order to limit voltages during the initial re-powering of channel 3A.
After further monitoring, the next step performed was to re-close RBI-1, which resulted in the re-powering of DCSU-3A. With this step performed successfully, and with a period of seven days of monitoring completed with no further RBI-1 trips experienced, on October 17 power channel 3A was successfully re-integrated into the ISS EPS architecture, enabling channel 3A to re-power its usual loads.
The re-integration was successful, with channel 3A providing its usual voltage level of between 150-160VDC, and a current of around -30A, resulting in a power load of around 4.7kW.
Although channel 3A is now successfully re-integrated into the ISS, the efforts to determine the original cause of the DCSU-3A RBI-1 trip are still on-going, with efforts now appearing to be concentrating on Sequential Shunt Unit-3A (SSU-3A), which is where the current flowed to during the original overcurrent trip in RBI-1.
SSUs provide a constant level of voltage to components which run off ISS primary power, since primary power, which is generated directly by the SAWs at a typical level of 160VDC, can vary by a significant amount depending on the level of sunlight, which affects SAW voltage output.
As such, SSUs ensure that all components using primary power are always supplied with a constant level of voltage, which is typically 160VDC.
SSUs are rectangular box-like components located at the base of the Mast Canister Assembly (MCA) of their associated SAW, which in the case of SSU-3A is SAW-3A.
While it is now looking unlikely due to the successful recovery of channel 3A, reports indicate that, should it be determined to be required, US EVA-21, which would again be performed by Suni Williams and Aki Hoshide sometime before November 19, would be devoted to the Removal and Replacement (R&R) of SSU-3A on the P3/P4 Truss.
Even though the potential R&R of SSU-3A was originally pencilled in for US EVA-20, the more urgent P6 ammonia leak issue bumped the SSU-3A R&R to US EVA-21.
The desire to perform the two EVAs sooner rather than later is complicated by the fact that Suni Williams and Aki Hoshide will return to Earth aboard their Soyuz TMA-05M/31S spacecraft less than a month from now, on November 19, although some slight relief has been added to the schedule due to the Russian decision to delay the Soyuz TMA-05M undocking and landing by one week, since its previous landing date was November 12.
This decision to delay the landing is unrelated to the EVAs however, and is driven by the desire to decrease the amount of time between the Soyuz TMA-05M undocking and the Soyuz TMA-07M docking on December 21, and thus reduce the amount of time that the ISS is at three crewmembers.
While Soyuz TMA-06M/32S will launch to the ISS on October 23 and dock two days later on October 25, upon the undocking of Soyuz TMA-05M on November 19, only one USOS crewmember will be aboard the ISS – NASA astronaut Kevin Ford – and EVAs require at least two USOS crewmembers to be present.
Additional USOS crewmembers in the form of NASA astronaut Tom Marshburn and Canadian astronaut Chris Hadfield will not arrive at the ISS until December 21 aboard Soyuz TMA-07M/33S, and that would leave only ten days over the Christmas period in order to perform the EVA to refill the P6 ammonia lines before the channel could go off-line due to insufficient ammonia levels by New Years.
Additionally, both Suni Williams and Aki Hoshide are already experienced at performing challenging EVAs together, as they both performed US EVAs 18 and 19 in late August/early September, and thus their recent experience with EVAs is valued over using newly arrived crewmembers to perform the EVA(s).
ISS program managers had been hoping to reduce the amount of US EVAs to only one per year in the post-Shuttle era, in order to increase crew time available for scientific utilisation, however 2012, only the first full post-Shuttle year, already looks set to see at least three, and maybe four US EVAs performed.
While the increased number of EVAs can be attributed to unexpected failures, it highlights the challenges of maintaining the station in the post-Shuttle era, where there are no Space Shuttle EVA crews to perform regular external maintenance tasks on the station.
Looking ahead, the ISS program wish to decrease the amount of EVAs performed by increasing the use of robotics, both in the form of the Special Purpose Dextrous Manipulator (SPDM) “Dextre”, and, in future years, the humanoid Robonaut 2.
Currently however, robotic technologies and techniques are not at the point where robotics can completely replace spacewalks, although the ISS will undoubtedly serve as a pathfinder for increasing robotics capabilities in space, which will go on to have high value in future Beyond Earth Orbit (BEO) missions and spacecraft.
(Images: L2’s ISS Section. Soyuz, plus Aki and Suni image via NASA.gov)
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