The final EVA to complete an upgrade to the Space Station’s robotic arm’s end effector has been completed. EVA-48 was conducted by Expedition 54 Flight Engineers Mark Vande Hei of NASA and Norishige Kanai of the Japan Aerospace Exploration Agency. This EVA was originally delayed after the previous spacewalk, EVA-47, included a software issue that was set to result in the need to reverse the work conducted on the EVA before a software patch was developed.
The main task of these spacewalks was to replace Latching End Effector-B (LEE-B) on the Space Station Remote Manipulator System (SSRMS), prior to returning one inside the Station for a return on a future SpaceX Dragon.
The LEE allows the SSRMS to grapple onto objects, and the SSRMS has one LEE on each of its ends, in order to enable the SSRMS to relocate itself around the ISS via so-called base changes.
The LEE features three snares which close around a grapple pin on target objects, in the same way that the Space Shuttle’s robotic arm used to work. This enables the LEE to grapple any object which features a Flight Releasable Grapple Fixture (FRGF) interface.
However, the LEE also has some more complex features due to the fact that it was designed for long-term use on the ISS. Specifically, it features latches, which allow it to securely clamp itself onto compatible Power & Data Grapple Fixture (PDGF) interfaces, and also electrical and data interfaces which can connect the arm to the object it has grappled.
This latching capability allows the arm to tightly hold onto the ISS wherever it is based, whilst reducing wear on the snares, and the electrical and data interfaces allow for command & control of the arm, whilst also enabling the arm to provide power to objects it has grappled.
ISS teams have been tracking some degradation of performance in both LEEs for several years now – specifically, increasingly high motor currents have been required in order to extend the latches. This in itself is not unexpected, and the LEEs were designed for an orbital lifetime of 10-15 years, and have currently been on-orbit for 16 years.
This LEE, although degraded, still works and will be used as a spare. The other LEE – removed last year – will return home on a SpaceX Dragon. pic.twitter.com/NGANPYO8k1
— Chris B – NSF (@NASASpaceflight) January 23, 2018
The LEE is a very complex electro-mechanical system which can be subject to significant forces, and since they are such critical components in enabling ISS operations (for example the capture of cargo vehicles), a change-out of the LEEs has been in planning by the ISS teams for several months now.
LEE-A was changed out back in October last year when it was swapped with the LEE on the Payload ORU Accommodation (POA), which is essentially a LEE which is attached to the Mobile Base System (MBS) in order to serve as a temporary stowage location for external hardware.
Since the POA LEE had been used much less than LEE-A, the decision was made to move the POA LEE to the SSRMS LEE-A position, and place the previous LEE-A on the POA.
Once the former POA LEE was installed on the SSRMS to become LEE-A, testing and checkouts were performed in the months that followed, with only a few minor issues identified, as detailed in L2 notes:
“SSRMS LEE-A Force Moment Sensor (FMS) Fine Characterization – Since FMS circuitry was never activated on LEE-A when originally installed on the POA, it is necessary to fully characterize the FMS system in its new role as SSMRS LEE-A.”
“During the FMS Fine Characterization test, a crosscheck error was observed during the roll axis loading. Ground teams developed a recovery plan to release, recalibrate FMS, and repeat the test, but the same crosscheck errors were observed during roll loading. The translational (x-, y-, z-axis) characterization was completed successfully and rotational objectives were deferred.”
Additionally, another issue was noted during the capture and berthing of the SpX-13 Dragon:
“During capture, tension on LEE-A rose slowly to 6823N, above the expected value of 5916N. Per chit 15986, the high tension was expected as the Configuration Data Files (CDFs) for the new LEE-A had not yet been updated with a new Rigidize Target Position.”
Finally, another minor issue was noted:
“During SSRMS support for EVA-45, teams experienced issues commanding LEE-A camera to focus both near and far. Power cycles are able to correct this issue intermittently.”
The first spacewalk, designated US EVA-47, was performed by NASA astronauts Mark Vande-Hei as EV-1 and Scott Tingle as EV-2.
The singular task of the spacewalk involved removing LEE-B from the SSRMS and replacing it with a brand new spare LEE which resides on a storage platform outside the ISS.
Specifically, the first order of business for the two spacewalkers upon egressing the Quest airlock was to make their way to External Stowage Platform-2 (ESP-2), where the spare LEE is located, attached to a stowage mount called the Flight Support Equipment (FSE).
The spare LEE was originally located on ExPrESS Logistics Carrier-1 (ELC-1) after being launched to the ISS on STS-129 in November 2009. However, ahead of the EVA it was robotically relocated from ELC-1 to ESP-2, as ESP-2 is located much closer to the airlock than ELC-1, which reduces the need for the spacewalkers to translate out to the end of the Truss where ELC-1 is located.
The spacewalkers removed the spare LEE from its FSE on ESP-2 via removing insulation blankets, launch restraints and six Expandable Diameter Fasteners (EDFs), which are the main bolts which attach the LEE to both the SSRMS and its FSE. As their name implies, the bolts increase in diameter as they are driven, to in order to ensure a very tight fit.
With the SSRMS maneuvered into position near the ESP-2 worksite, the Camera/Light Pan/tilt Assembly (CLPA) was removed from LEE-B and installed onto the spare LEE, which did not have a CLPA pre-installed.
Then, the SSRMS was powered down and LEE-B will be removed from the SSRMS via the driving of its six EDFs, and temp-stowed out of the way. The spare LEE will then be removed from its FSE on ESP-2 and installed onto the SSRMS to become LEE-B.
A problem with a mate/demate device resulted in a software issue. However, this was resolved with controllers recycled the device. At least that was the call during the spacewalk.
Later it was revealed the issue was still not fully resolved.
“LEE-B Checkout – Ground teams tried again to recovery the Primary string of the Space Station Remote Manipulator System (SSRMS) Latching End Effector B (LEE-B). Since it was installed on US Extravehicular Activity (EVA) 47, only the Redundant string has been fully functional,” noted L2 ISS notes last week. “The LEE Electronics Unit (LEU) software was rolled back to Mobile Servicing System (MSS) Software version 9.0. An attempt was then made to power up on the primary string but was unsuccessful.
“Teams are working toward a diagnostic software patch to be installed over the weekend to provide additional insight to the anomaly before executing EVA 48 on Monday. During EVA 48, the spare LEE that was installed on US EVA 47 will be removed and replaced by the original LEE with the degraded snare cables. ”
The old LEE-B was installed onto the empty FSE on ESP-2 for stowage and the initial plan was to swap the two LEEs – the one installed during the previous EVA with the spare – if a software solution wasn’t found.
Thankfully, teams managed to create a software patch that was successful in solving the issue.
Space flight is hard but sometimes with the right people and the drive to fix problems we can overcome those things that make it hard and grow stronger for having done it successfully. Congrats to @MDA_Robotics and all those tireless engineers who keep us going! https://t.co/9ih8i6Z62H
— Ed Birchnall 🇨🇦 (@ROBO_Birchnall) January 28, 2018
As a result, the final EVA of the sequence was moved to later in the month and followed the previously planned path.
“The decision was made after the Canadian Space Agency (CSA) and its robotics specialist team developed a diagnostics software patch confirming an anomaly noted in a primary communications string on the spare end effector installed during a prior spacewalk Jan. 23 was not hardware related, and can be corrected through the implementation of software. A confidence test verifying the software upgrade was successfully completed Saturday night,” added NASA.
The original spacewalk by Vande Hei and Kanai to bring an end effector inside the station removed from the arm during a spacewalk last October, and install the end effector on the mobile base system rail car on the station’s truss, has now taken place.
The two spacewalkers move the LEE from a payload attachment on the MSB rail car to the Quest airlock. They also moved an aging, but functional, LEE that was detached from the arm during the previous spacewalk and move it from its temporary storage outside the airlock to a long-term storage location. That LEE will be available as a spare part on the Mobile Base System, which is used to move the arm and astronauts along the station’s truss structure.
The tasks were achieved in double time, with the primary objectives completed ahead of the half way mark of the EVA. As such, several get-ahead tasks were conducted, including lubrication of the LEE latches.
The spacewalk was the 208th in support of space station assembly and maintenance and the third this year.
The spacewalk was the fourth in Vande Hei’s career and the first for Kanai, who became the fourth Japanese astronaut to walk in space. Vande Hei wore the suit bearing the red stripes and Kanai’s suit had no stripes.