Two astronauts aboard the International Space Station (ISS) stepped outside the orbiting outpost on Thursday to successfully perform an unexpected, US segment-based spacewalk to fix an ammonia leak in the cooling system of a power distribution channel. The Extra Vehicular Activity (EVA) was 20th to be performed on the US Segment by an ISS crew, giving it the designation US EVA-20.
Solar array cooling architecture:
The reason for EVA-20 was to isolate a leak in the ammonia cooling system of power channel 2B, which is located on the Port 6 (P6) Truss of the station. Channel 2B is fed power from Solar Array Wing (SAW)-2B, also located on the P6 Truss. In addition, SAW-4B is also located on the P6 Truss, which supplies channel 4B, meaning the P6 Truss features two SAWs and thus two power channels in total.
Since solar power generation creates a heat load, the P6 Truss needs its own cooling system to dissipate this heat, called the Photo Voltaic Thermal Control System (PVTCS). This cooling system is separate from the main ISS cooling system, which is called the External Thermal Control System (ETCS).
The purpose of this is to keep the cooing associated with solar power generation separate from the main ISS cooling loop, to both reduce the operating temperature of the main loop, and also avoid having to transfer cooling fluid over the rotating Solar Alpha Rotary Joint (SARJ).
The P6 PVTCS has its own dedicated radiator, called a Photo Voltaic Radiator (PVR), which uses ammonia to transport heat away from the electronic equipment and radiate it into space. The two power channels on the P6 Truss – 2B and 4B – both share the same PVR for cooling.
The PVTCS PVR is different to the ETCS radiators, which are called Heat Rejection Subsystem Radiators (HRSRs), in that the PVRs are smaller and have less cooling loops than the HRSRs, which are designed to dissipate a larger heat load than the PVRs. Thus, an HRSR spare cannot be used as a PVR spare, and vice versa.
The exact same arrangement as the one described above is also present on the P4, S4, and S6 Trusses, which also each support two SAWs and thus two power channels, making for eight power channels and four separate PVTCS loops on the ISS in total.
However, the P6 Truss differs from the P4, S4, and S6 Trusses, in that the P6 Truss also contains two now unused radiators that were previously part of the Early ETCS (EECTS) cooing system, which cooled the ISS via the Early Ammonia Servicer (EAS) tank during the early years of ISS construction, when the main ETCS loop was not yet constructed and fully online.
The EETCS featured two Thermal Control Radiators (TCRs) that were both of the same construction type as the PVRs, with the two TCRs being denoted as the Trailing TCR (TTCR), pronounced “ticker”, and Starboard TCR (STCR), pronounced “sticker”.
When the main ETCS loop was finally brought online however, and all HRSRs were deployed, the TTCR and STCR were retracted as they were no longer needed, prior to the P6 Truss being relocated from its previous home atop the Z1 Truss, to its current home on the end of the P5 Truss.
Thus, since the retracted TTCR and STCR are now unneeded, and are of the same construction type as the PVRs, they can now be used as spares for the PVRs, a duty that one – the TTCR – will assume during the EVA.
Since 2006, ISS flight controllers have been tracking a slow ammonia leak in the channel 2B cooling loop in the P6 PVTCS PVR, which, due to its very low leakage rate, was not considered a major concern, with the only action required being a periodic ammonia re-fill of the leaking loop, a task that was successfully accomplished during an EVA on the STS-134 Space Shuttle mission in May 2011.
At that time, based on the known leakage rate, it was projected that another re-fill would not be needed until 2015.
In June 2012 however, ISS flight controllers noted a sharp increase in the leakage rate, although in relative terms the leak is still very small – the leak itself is, according to ISS Program Manager Mike “Suff” Suffredini, likely no bigger than the diameter of a human hair.
However, the leak is still large enough to lead flight controllers to believe that the ammonia in the channel 2B cooling loop will hit its minimum quantity limit in around late December or early January, whereupon channel 2B will shut itself down as a protective measure. This would be bad news for the ISS, since channel 2B carries many “critical” loads for the station.
Flight controllers believe the leak is likely a second leak, rather than a worsening of the original leak, with the most likely cause at this time being a Micro Meteoroid Orbital Debris (MMOD) strike to the P6 PVR itself. Thus, it was decided to conduct an EVA to isolate the suspected leaking 2B coolant loop in the P6 PVR, and instead use the unused TTCR for channel 2B cooling. Channel 4B would continue to use the PVR for cooling, as the leak is not present in the 4B cooling lines.
While there is no certainty that the leak actually is in the PVR – it could also potentially be in one of the 2B ammonia coolant routing lines, or in the 2B Pump Flow Control Subassembly (PFCS) – isolating the PVR will allow flight controllers to determine where the leak really is, even if it does not stop the leak itself.
An additional benefit of using the TTCR for channel 2B cooling is that the TTCR received a “backfill” of ammonia during the 2B PVTCS re-fill activities in May 2011, as it was “in the pathway” to the PVTCS. This means that the TTCR will effectively give channel 2B a free ammonia re-fill, which will allow it to run until around fall 2013, even if the leak still persists, before the coolant would hit its minimum quantity limit.
This would give ISS flight controllers ample time to determine the source of the leak, and conduct another EVA to isolate it. If however the 2B PVTCS leak does disappear once the TTCR is used, thus proving that the leak is in the 2B PVR cooling lines, then the ISS could operate with both the PVR and TTCR deployed indefinitely.
The EV crew for EVA-20 were ISS commander Suni Williams and Japanese astronaut Aki Hoshide. Williams was wearing the Extravehicular Mobility Unit (EMU) spacesuit with the red stripes, and was making her seventh EVA. Hoshide was wearing the EMU with the white stripes, and was making his third EVA.
Utilising Williams and Hoshide for this EVA was highly desirable to the NASA team, since they both have very recent experience conducting EVAs together during US EVAs 18 and 19 in late August/early September, to remove & replace the troublesome Main Bus Switching Unit-1 (MBSU-1). This also meant the EMUs on the ISS were already sized for the pair of them.
Additionally, Williams, who already holds the record for the most amount of time spent spacewalking by a female, participated in US EVA-7 to stow the TTCR in February 2007, and so she already had actual EVA experience of the worksite that was the center of attention on Thursday.
In addition to that, both Williams and Hoshide must return to Earth aboard their Soyuz TMA-05M spacecraft on November 19, which will leave insufficient numbers of USOS crewmembers aboard the station to perform an EVA until the December 21 docking of Soyuz TMA-07M, which would be getting very close to the channel 2B shutdown limit due to lack of adequate ammonia.
Inside the ISS, newly arrived crewmember Kevin Ford served as Intra Vehicular (IV) crewmember, although he did not provide Space Station Remote Manipulator System (SSRMS) support for the EVA, and on the ground in Mission Control Center-Houston (MCC-H), astronaut Mike Fincke served as Capcom, as he performed the 2B PVTCS re-fill EVA during STS-134 in May 2011, and so has personal experience of the worksites in use.
For the duration of the EVA, the ISS flew in a different attitude than normal, in order to protect the spacewalkers from plasma build-up on the exterior of the ISS. Normally, the ISS flies in the +XVV attitude, meaning the +X axis faces the Velocity Vector, which places PMA-2 facing forward along the ISS’ orbital track.
However, for Thursday’s EVA, the ISS flew in the +YVV attitude, which will effectively make the S6 Truss the most forward part of the ISS, meaning the P6 Truss worksite will be the part of the ISS which is furthest aft.
Throughout the EVA, the two Solar Alpha Rotary Joints (SARJs) were both parked, as were both of the Thermal Radiator Rotary Joints (TRRJs). All Beta Gimbal Assemblies (BGAs) apart from the 2B BGA were left in auto track, with the 2B BGA parked due to its proximity to the EVA worksite.
Two cameras were taken on the EVA, one with a 28mm lens, and one with a 50mm lens – meaning the 10mm wide-angle lens that has been a feature of recent US EVAs did not feature on EVA-20.
For this EVA, the In Suit Light Exercise (ISLE) protocol was used, which saw the EV crew conduct some light exercise while at a lower pressure inside their EMUs (10.2 psi instead of the 14.7 psi of the ISS). This purges the nitrogen from their bloodstreams, while also eliminating the need for a campout in the Quest Airlock (A/L) the night before the EVA, which was previously used to accomplish this goal.
Following the completion of ISLE, both Williams and Hoshide were placed in the Quest A/L crew lock, and the hatch to the equipment lock was closed, whereupon depressurisation began. External hatch opening followed, after which the EMUs were placed on internal battery power, making the start of the 6.5 hour EVA at 12:29 PM GMT.
The first order of business for the duo was to translate out to the P6 Truss worksite – with Hoshide stopping at the Z1 Truss to retrieve a tool along the way.
Once at the worksite, the first task of the EVA began – which was to remove four bolts of a metallic shroud covering the Fluid Quick Disconnect Coupling (FQDC).
Once the shroud was removed, Williams used a Pistol Grip Tool (PGT) to turn a bolt in order to close the supply and return valves of the channel 2B cooling system to the PVR. The bolt had to be turned at least seven times in order to ensure that the valves are fully closed and that the channel 2B PVR is isolated.
This was the first time that the FQDC has been manipulated on-orbit. Once the valves were closed, the FQDC shroud was re-installed and the four bolts re-tightened.
The next task for Williams was to reconfigure two Early Ammonia Services (EAS) jumpers, in order to connect the channel 2B cooling system to the TTCR. This task involved de-mating the first jumper, FH-01, from its M9 male connection, venting all the nitrogen from it with a nitrogen vent tool, and then connecting it to the M10 male connector. The FH-02 jumper was then connected to the M9 male connector.
Bails were then be opened on both of the jumpers, which will allow FH-01 to carry warm ammonia from the 2B electronics to the TTCR, and allow FH-02 to carry cooled ammonia from the TTCR to the channel 2B electronics.
However, as in past EVAs, operating QDs has typically proven to be difficult and time consuming. This time, the SPD’s – that are not designed to be placed on the QDs – proved to be a problem on one of the QDs at the half way point of the entire task. Suni secured the hardware to the required levels via wire ties.
The entire “Wet QD” ops was timelined in order to allow enough time for a “bake out” prior to ingress, should the EMUs become contaminated with any leaking ammonia from the QDs.
Had wet QD ops not been complete on time, the EV crew would have moved onto their next task – the TTCR shroud removal – but wouldn’t proceed to actually deploy the TTCR, as it cannot be left undeployed without a flow of ammonia – which cannot occur until QD ops are complete. In such a situation, the QD ops and TTCR deploy would need to be completed during a second EVA.
Given the EAS jumper reconfig work was completed as planned, Williams then joined Hoshide, who took photography of the 2B PVR and Integrated Equipment Assembly (IEA) – where the PFCS is located – and also removed and stowed the beta cloth shroud covering the retracted TTCR.
Once the shroud was removed, both Williams and Hoshide worked to release six cinches and two winch pip pins holding the TTCR in its stowed position.
Once this was completed, the EV crew gave a go for the ground to command the TTCR to deploy, which took several minutes to fully complete. Had the deploy stalled at any point, a manual drive bolt was available so that the TTCR could be deployed manually by the EV crew using their PGTs.
If for some reason there had been trouble with isolating the PVR, one jumper could have been used to transfer some ammonia from the TTCR to the PVR, in order to buy some more time for the ground to figure out what to do on a future EVA.
However, this operation all went to plan.
A small get-ahead task, in the form of a port SARJ inspection was carried out as the spacewalkers translated back to the airlock at the latter end of the EVA.
Both spacewalkers concluded the EVA by plugging their EMUs back into ISS utilities, closing the Quest A/L hatch, and beginning repressurisation activities. The duration of EVA-20 was 6 hours, 38 minutes.
With EVA-20 a success, it was the final spacewalk of 2012, as a previously planned spacewalk to remove & replace a Sequential Shunt Unit (SSU) on power channel 3A is off the cards, as that channel has now been successfully re-integrated back into the ISS following a trip in the channel 3A Direct Current Switching Unit (DCSU) in early September.
The root cause of the trip is now believed to be a short in one of 82 capacitors in the SSU, which is used to provide a steady 160 Volts Direct Current (VDC) to electrical components using ISS primary power, which can sometimes vary wildly depending on solar array generation.
The capacitors are used to smooth the 160 VDC output, and while the shorted capacitor is believed to have caused the trip, the short also likely caused the capacitor to burn out, meaning it is effectively no longer present in the SSU, and so is unable to cause any more trips.
As there are another 81 capacitors remaining in the SSU, its loss is not considered a critical one.
(Images: L2’s ISS Section. Soyuz, plus Aki and Suni image via NASA.gov)
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