Two astronauts aboard the International Space Station (ISS) have completed a spacewalk that successfully stowed a now unused spare radiator and also installed the first two hardware elements of a new system of external HD television cameras. The spacewalk (EVA-37) got underway at 11:53 PM UTC, and lasted 6 hours and 48 minutes.
The main aim of the Extra Vehicular Activity (EVA) was to perform the long-awaited task of retracting and stowing an unused thermal radiator on the P6 Truss called the Trailing Thermal Control Radiator (TTCR), pronounced “ticker”.
The TTCR was originally a component of the Early External Thermal Control System (EETCS), which was used to cool the ISS during the early years of its construction when the station’s full cooling system was not yet constructed.
The EETCS featured two Thermal Control Radiators (TCRs), one of which is the TTCR, on the P6 Truss, which at the time was mounted atop the Z1 Truss. These two TCRs provided cooling for the entire US segment of the station during its initial phase of construction.
However, the EETCS did not have the capacity to provide cooling to the fully constructed ISS, and so once the station’s main Truss structure was in place, which featured a separate, more capable cooling system known as the External Thermal Control System (ETCS), the EETCS ceased to be used.
In contrast to the EETCS’s two radiators, the ETCS features six radiators, called Heat Rejection Subsystem Radiators (HRSRs). The six HRSRs are separated into two “loops”, each with three radiators, with one loop located on the P1 Truss, and the other located on the S1 Truss.
When the main ETCS loop was brought online and all six of its HRSRs were deployed, the two TCRs of the EETCS 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.
While the ETCS is responsible for dissipating the heat generated from the internal and external electrical equipment located on the US segment, the cooling architecture is complicated by the fact that the station’s solar arrays and associated power generation and distribution equipment generate their own heat load.
Since the solar arrays must rotate in order to face the sun via the Solar Alpha Rotary Joints (SARJs), it would be extremely difficult to transfer ammonia coolant across the rotating joints in order for the heat to be dissipated via the ETCS.
As such, each one of the four power generation Truss segments has its own individual cooling system, called the Photo Voltaic Thermal Control System (PVTCS), with each system serviced by its own dedicated radiator, called a Photovoltaic Radiator (PVR).
The PVRs of the PVTCS differ from the HRSRs of the ETCS 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 (of which only one exists on the ISS) cannot be used as a PVR spare, and vice versa.
However, although the unused TCRs of the old EETCS are no longer needed to provide cooling to the ISS, they can be used as spares for the PVTCS PVRs should such a situation ever be required, since the TCRs are of the same construction type as the PVRs.
Since 2006, ISS flight controllers were tracking a slow ammonia leak in the channel 2B cooling loop in the P6 Truss PVTCS PVR, and in June 2012 flight controllers noticed a sharp increase in the leakage rate, although in relative terms the leak was still very small.
However, the leak was still large enough for the ammonia in the channel 2B cooling loop to hit its minimum quantity limit within around six months, whereupon channel 2B would have shut itself down as a protective measure.
At the time, flight controllers believed the leak was likely a second leak, rather than a worsening of the original leak, caused by a debris strike to the P6 PVR. As such, in November 2012 an EVA was conducted to isolate the suspected leaking 2B coolant loop in the P6 PVR, and instead deploy and utilize an unused TCR for channel 2B cooling. Thus, the TTCR was deployed.
It was always known that this measure may not work, as it could not be confirmed that the leak was in the PVR – flight controllers also believed that the leak may have been in an external cooling line, or in the 2B Pump Flow Control Subassembly (PFCS).
Unfortunately, just six months after the EVA to attempt to stop the leak, in May 2013 a large increase in the leakage rate from channel 2B occurred. This led to a contingency EVA being performed to replace the channel 2B PFCS.
This did the trick, and finally stopped the leak in the 2B power channel. Since then, however, channel 2B’s cooling continued to be provided by the TTCR, despite the PVR being proven to not be the source of the leak.
This was an undesirable configuration to be in, as it left both the PVR and TTCR deployed and exposed to debris strikes, thus needlessly exposing the TTCR – which serves as a spare for any of the four PVRs – to risk of damage.
In November 2015, an EVA was performed to transfer channel 2B’s cooling from the TTCR back to the PVR, and disconnect and stow the TTCR. However, during the EVA in question, timeline issues meant that that TTCR stow was not able to be completed, although channel 2B’s cooling was successfully transferred back to the PVR, with the TTCR being disconnected from channel 2B’s cooling.
As a result, the now disconnected TTCR was still deployed and thus was at risk of debris strikes, despite not being used. Hence, NASA wished to stow the TTCR in order to protect it as a spare for the future.
The other main task of the spacewalk was the installation of the first of four new External High Definition Camera Assemblies (EHDCAs), which are new HD TV cameras for the exterior of the ISS.
The EHDCAs will augment, rather than replace, the existing Standard Definition (SD) TV cameras located outside the ISS, which date back to the 1990s, before the advent of HD technology.
The new HD external cameras will be used to provide better quality views of external activities, including inspections of Soyuz spacecraft for damage prior to their return to Earth.
The EHDCAs consist of commercial Nikon D4 DSLRs with a 28-300mm lens and 2X teleconverter mounted inside a sealed enclosure containing dry nitrogen at atmospheric pressure.
They will be capable of filming in 720p HD, and producing 16.2 megapixel still images.
An external motor will drive the telephoto lens to provide a zoom capability, while control of the cameras will be enabled via external connections to the camera and custom firmware.
Using commercial off the shelf cameras inside an atmospherically sealed container enabled NASA to minimize costs and development times of the new cameras, an increasingly common practice aboard the ISS.
US EVA-37 is being performed by astronauts Jeff Williams as EV-1 wearing the suit with the red stripes, and Kate Rubins as EV-2 wearing the all-white suit.
Upon egressing the Quest airlock, Williams and Rubins began making their way to the P6 Truss, setting tethers along the way. Once at the worksite, a brief inspection was performed, followed by the first task of the day – the TTCR retraction.
The TTCR folds in accordion-like fashion via the driving of a single bolt using a Pistol Grip Tool (PGT), with no manual folding required. Once the TTCR was folded, however, the two spacewalkers needed to install four cinches to ensure that the radiator is firmly held down.
Once the four cinches were installed, a protective thermal/debris cover was installed over the stowed TTCR in order to protect it from the harsh environment of space and ensure its future availability as a spare.
Once complete, Williams headed to the station’s robot arm, where he installed and ingress a foot restraint, meanwhile, Rubins moved to the airlock to retrieve the new HD camera, and passed it on to Williams.
Williams then proceeded to install the new EHDCA onto the side of the existing SD camera on the nadir side of the P1 Truss, at the P1 Lower Outboard (LOOB) position, also known as Camera Point-9 (CP-9). Williams also installed a new camera light at the same position, to replace a failed unit.
Thanks to getting ahead of the timeline, a get-ahead task of a second camera installation also took place.
Prior to the EVA, the station’s Dextre robotic hand successfully performed the replacement of a failed external Remote Power Control Module (RPCM) circuit breaker box on the P1 Truss, which will provide the power for the new camera, and the heater power for the new light.
Meanwhile, Rubins tightened some bolts on the Port Solar Alpha Rotary Joint (SARJ) and performed a visual check of the SARJ lubrication, in an attempt to reduce some vibration signatures seen on the SARJ by ground teams.
Rubins also removed a thermal cover from the aforementioned failed PFCS stowed on the outside of the P6 Truss, in order to allow access for Dextre to remove the failed PFCS and replace it with a future spare unit launched on SpaceX’s Dragon vehicle.
Get-ahead tasks included tying down some brake handles on a Crew Equipment Translation Aid (CETA) cart and photographing the Alpha Magnetic Spectrometer (AMS) – although the latter task was deleted from the timeline.
At this point, both spacewalkers cleaned up their respective worksites and headed back inside the airlock to conclude the EVA.
(Images via NASA and L2).