Two astronauts stepped outside the International Space Station (ISS) on Wednesday performing a spacewalk aimed at general maintenance and enhancements of the station. The Extra Vehicular Activity (EVA) – designated US EVA-32 – began at 12:03 PM GMT and lasted just over seven hours in duration.
The main purpose of EVA-32 was to clear a number of outstanding tasks that have become back-logged over the past year due to delays and schedule changes associated with the failures of the Orbital ATK Cygnus and SpaceX Dragon vehicles and their subsequent stand-downs.
Many of these tasks – planned for previous EVAs – were postponed after the loss of International Docking Adapter-1 (IDA-1) aboard the SpX-7 flight in June, which in turn delayed the long planned ISS reconfiguration effort in support of future commercial crew vehicles.
Tasks conducted during Wednesday’s EVA included the removal of a Multi Layer Insulation (MLI) thermal cover from a Main Bus Switching Unit (MBSU) stored on ExPrESS Logistics Carrier-2 (ELC-2) on the outboard Starboard Truss of the ISS.
The MBSU is a component of the station’s power distribution system, and serves to distribute the power generated by the station’s solar arrays to user loads. The MBSU in question failed in August 2012, whereupon it was replaced with a spare unit.
The ISS program would like to attempt to recover the failed MBSU via opening it up and replacing the failed circuitry, however since this task is too fiddly to be conducted by astronauts on EVA, it must be brought back inside the ISS in order for the repair to be completed.
The repair is not slated to be attempted in the immediate future, so instead of bringing the failed MBSU back inside themselves, the spacewalkers instead removed the MLI blanket covering the MBSU in order to expose the robotic grasping interfaces beneath it.
Called Micro Square Fixtures (MSFs), these interfaces allow the station’s Special Purpose Dextrous Manipulator (SPDM) “Dextre” robotic hand to grasp the unit, and drive bolts to detach it from its stowage location. When required, the SPDM can then bring the MBSU back inside the ISS by transferring it to the Japanese airlock, via a piece of hardware called the JEM ORU Transfer Interface (JOTI).
While performing the MBSU MLI task on the outboard end of the Starboard Truss, the spacewalkers also took the opportunity to install a thermal cover on the Alpha Magnetic Spectrometer (AMS) experiment.
This was required in order to provide thermal shielding to pumps inside AMS which are showing some signs of degradation, as AMS was not originally designed to be on-orbit long term.
Currently, during high beta periods, where the Sun shines on the ISS side-on, AMS requires shading via either the SPDM being positioned above it, or by rotating the station’s own radiators in order to cast a shadow on AMS.
The new thermal cover will, however, provide adequate thermal shielding to AMS in order to prolong the lifetime of its pumps.
A big task for the EVA was the lubrication of the Space Station Remote Manipulator System (SSRMS) Latching End Effector (LEE). The LEE is the part of the station’s robotic arm which actually grapples payloads, or serves as the attachment interface between the arm and the station.
The LEE has many moving parts such as a central ballscrew, and latches which extend in order to make a secure connection between the arm and the grapple fixture. These moving parts, which have now been in the harsh environment of space for nearly 15 years, occasionally need to be lubricated in order to preserve their smooth operation.
Although there are two LEEs on the SSRMS (one on each end), one was already lubricated on an EVA back in February this year, and so now the remaining end was lubricated in the same manner.
Another big task for this EVA was the routing and pre-positioning of cables in order to support future commercial cargo vehicle dockings at the ISS. NASA wish to have two docking ports on the ISS able to support future crew vehicles, in order for one port to serve as a primary port, and the other as a back-up port.
Under this arrangement, one crew vehicle carrying four astronauts would arrive at the ISS, stay for six months, and then return home with its crew, prior to another vehicle arriving with another four crewmembers for another six-month expedition.
This “indirect handover” model would however require one Russian to fly on the US commercial crew vehicles, so that one US crewmember could fly on the three-seat Soyuz spacecraft, in order to maintain a US presence on the ISS during the periods when one US crew vehicle has returned to Earth prior to another one arriving in its place.
While this method has been the plan for the past few years, NASASpaceflight sources indicate that attitudes within NASA are now shifting in favour of the “direct handover” model, whereby one US crew vehicle arrives at the ISS with a new crew, before the old crew leave the ISS in their vehicle.
This would maintain a constant US presence on the ISS without the need to fly a US crewmember on the Soyuz, and thus eliminate the need to fly a Russian crewmember on the US crew vehicles. This is desirable since – in line with the worsening political situation between the two countries – issues have reportedly been raised regarding training Russian cosmonauts on US vehicles, which contain proprietary technology.
Regardless of which model is used, however, two docking ports on the US segment of the station are required in order to support future commercial crew vehicles.
The station currently has one usable docking port in the form of Pressurised Mating Adapter-2 (PMA-2) on the Forward end of Node 2, and a second docking port will be created by relocating PMA-3 from its current storage location on the Port side of Node 3, to the Zenith side of Node 2.
Two International Docking Adapters (IDAs) will then be installed onto the ends of the two PMAs in order to convert their legacy docking systems to the new International Docking System Standard (IDSS)-compatible Soft Impact Mating Attenuation Concept (SIMAC) ports.
The PMA-3 relocation and the installation of both IDAs were planned to be completed this year. However, the loss of IDA-1 on the SpX-7 Dragon mission in June delayed these plans. The new plan is for IDA-2 to launch on SpX-9 early next year, and be installed onto PMA-2.
IDA-3, which is currently under construction to replace the lost IDA-1, will then be launched to the ISS and installed onto PMA-3 in the Spring 2017 timeframe. The date of the PMA-3 relocation to the Node 2 Zenith port is still unclear.
Wednesday’s EVA did, however, route two power and data cables from Node 1 to the Forward end of Node 2, and connected to PMA-3/IDA-3 where they will be installed on the Node 2 Zenith position during a future EVA. Provision also exists to route a third 1553 cable.
The final task that was to be completed during the spacewalk was the installation of a Non Propulsive Vent (NPV) on the Forward side of Node 3. An NPV is designed to vent ISS atmosphere and gases into space if required, without creating a thrust which could lead to the need to perform a desaturation of the station’s Control Moment Gyroscopes (CMGs).
The NPV was removed from the exterior of Node 3 earlier this year since it would have caused minimum clearance issues during the robotic operation to install the Permanent Multipurpose Module (PMM) onto the Node 3 Forward port during its relocation in May.
Now that the PMM has been relocated. However, there are no longer any robotic clearance issues associated with the NPV, and thus it will be reinstalled. However, this was deleted from the timeline during the EVA.
EVA procedures – *Follow Along Live*:
The 6.5-hour EVA began with EV-1 Scott Kelly, wearing the suit with the red stripes, exiting the Quest airlock first, followed by EV-2 Kjell Lindgren, wearing the plain white suit. For both astronauts, it was their first spacewalk.
Once all bags were removed from the airlock and stowed outside, Kelly took the bag containing the AMS MLI cover and headed out to the outboard end of the Starboard Truss, while Lindgren removed an Articulating Portable Foot Restraint (APFR) from External Stowage Platform-2 (ESP-2) and followed Kelly to the worksite.
Once at the worksite, Kelly peeled back the MLI cover on the MBSU, removed a tie-down skirt, and then drove two bolts to detach the MLI cover from the MBSU.
Lindgren meanwhile installed the APFR onto the AMS, ingressed it, and took pictures of the AMS worksite.
Lindgren deployed an MLI “wedge” between the AMS radiators, which popped out like a tent in order to protect the radiators. He installed and secured the new MLI cover over the side of the AMS, attached four straps, and an electrical grounding pin.
The pair then translated back to the airlock, carrying with them the unneeded MBSU MLI cover, and also the APFR, which Kelly installed back onto ESP-2, and ingressed it at that location.
Lindgren then took the cable bag and translated to the Port side of the US Destiny Laboratory, and began to secure the forward coil of data cable about halfway along the Port side of the Lab. He then routed the aft coil to the Nadir side of Node 1, and connect it to a socket at that location.
He went back to the forward coil previously secured halfway along the Lab, and routed that to the Zenith side of Node 2, where it was secured ready to be connected to PMA-3/IDA-3 on a future EVA.
Lindgren then followed the same procedure for the second cable, this one a power cable, but will not route the forward coil to Node 2. Instead, Kelly will complete this later in the EVA, while
Lindgren was to head to Node 3 to begin the NPV task. However, this was deleted from the EVA due to time constraints.
Kelly meanwhile was hard at work performing the SSRMS LEE lubrication, first applying grease to the LEE’s centralising ballscrew using a special tool called the Ballscrew Lubricating Tool (BLT).
Next, with latches extended, equalisation brackets and rollers were lubricated, and with latches retracted, the linear bearing tracks on each side of the latch were lubricated with a grease gun, as well as the ballscrew behind the latches. This process was repeated for all four latches. However, one element of the task was deleted from the timeline due to schedule constraints.
The duo then cleaned up their worksites and stowed their tool bags into the Quest airlock. They then concluded the EVA by entering the airlock and closing the hatch.
The EVA ended with a duration of seven hours and 16 minutes.
(Images: via NASA and L2 Special Sections – including L2 artist Nathan Koga – The full gallery of Nathan’s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*).
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