However, if the SSRMS needs to reach either end of the Truss structure, it can move, or “change its base”, to the MBS, which will then, along with the MT to which the MBS is attached, move along the rails located on the Truss, thus allowing the SSRMS to reach the outer edges of the Truss structure.
The MT can only move at a maximum rate of one inch per second, and is propelled by a piece of hardware called the Linear Drive Unit (LDU). However, the MT cannot stop at any given location along the Truss rails.
Instead, it must stop at any of the ten designated “worksites” located along the Truss rails. These worksites feature mechanical and electrical connections to allow the MT to secure itself to the Truss, and to provide power to the MBS and its attached robotic hardware.
When the MT moves along the Truss, or is stationary in between any of the worksites, it is only lightly attached to the Truss rail structure via wheels known as Roller Suspension Units (RSUs).
Electrical power for MT translation via the LDU is available through the Trailing Umbilical System (TUS), which is a cable reel that extends and retracts as the MT moves. The MBS and attached hardware are unpowered during MT motion or when the MT is not located at a worksite.
However, when located at any one of the ten worksites, the MT securely clamps itself to thicker portions of the Truss rails via Load Transfer Units (LTUs), which take over from the RSUs. Electrical power is available via the Umbilical Mechanism Assembly (UMA), which provides power to the MBS and attached hardware.
Two Crew & Equipment Translation Aid (CETA) carts are also attached to either side of the MT. The CETA carts are transportation aids for spacewalkers, to which EVA hardware can be attached, with the CETA carts then being manually moved along the Truss rails by spacewalkers.
The CETA carts feature two Dynamic Brake handles, two Parking Brake handles, and two Parking Brake Release handles, all of which can be manually operated by spacewalkers in order to arrest the CETA cart’s motion and secure it to a desired location along the rails.
The CETA carts are attached to the Truss rails via four Wheel/Brake Assemblies, with one being located in each of the four corners of their respective CETA cart. The two CETA carts are attached to the MT when not in use, which allows them to translate along the rails along with the MT whenever it moves.
Problem & concerns:
On December 17, while the MT was in the ground-controlled process of translating from Worksite-4 (WS-4) to WS-2, the Translation Drive (TD) Integrated Motor Controller Assembly-A (IMCA-A) experienced an error, resulting in MT motion being stopped shortly after the MT had left WS-4.
Another error was then experienced when the MT attempted to translate back to WS-4.
The MT was effectively “stuck” in this position just short of WS-4, which is located just to the Starboard side of the S0 Truss, very close to the center of the Truss.
As such, the MT was unable to engage the LTUs or the UMA, and as such was only lightly attached to the Truss rail via its RSUs (which are braked).
Electrical power to the MBS is being provided via the Mobile Transporter Relay Assembly (MTRA), which was only recently installed on the ISS in October 2014, with the purpose of providing power to the MSS when the MT has stopped motion between worksites, in order to provide “keep alive” power to the MSS in the event that the MT gets stuck.
After analysis, ground teams believed that the stuck MT was likely caused by a brake being engaged on the Starboard CETA cart, which was preventing the MT from moving as the CETA cart is attached to the MT.
Ahead of the EVA it wasn’t fully known whether the brake handles were responsible for the engaged brake, or whether one of the Wheel/Brake Assemblies had somehow failed.
The Starboard CETA cart brake handles were only recently tied-down by astronaut Scott Kelly during US EVA-33 back in early November, leading to the possibility that they could have somehow inadvertently been engaged at that time.
2015-12-20-235742The concerns with having the MT remain unsecured to a worksite, aside from the fact that robotic operations are greatly impacted as only keep-alive power is available to the MBS and attached hardware, are that the ISS cannot conduct “dynamic” operations, such as attitude changes, as the potential exists for the MT to move around and become damaged as it is not securely latched to the ISS.
This in turn affects dockings of Soyuz and Progress vehicles, which require ISS attitude changes.
Following the successful departure of Progress M-28M (19 December), Progress MS-1 launched (21 December), for a docking two days later. Hence, there was a desire to resolve the stuck CETA cart/MT issue before then.
Following a meeting (20 December), the ISS Mission Management Team (IMMT) gave a GO for an EVA to attempt to free the stuck CETA cart, to occur Monday (21 December).
This EVA was planned to last around 3.5 hours, conducted by NASA astronauts Scott Kelly and Tim Kopra.
The plan was to have the spacewalkers check that all the brake handles on the starboard CETA cart are not engaged, and if they are engaged, to release them.
That proved to be the case, as Kelly worked to put one of the brake handles back into a nominal configuration – allowing the MT to return to action.
The spacewalkers had to go about their work carefully, ensuring the CETA cart did not suddenly “jolt” when the brakes were disengaged, due to stored mechanical energy.
Once the spacewalkers verified that none of the brake handles were no longer engaged, ground controllers moved the MT back to WS-4, which indicated that the problem has been resolved. The MT is now back in a nominal configuration, with electrical power confirmed by the ground controllers.
Alternative options – no longer required – would have included the removal of the CETA cart from the Truss rails completely.
It was also possible to remove and replace each of the four Wheel/Brake Assemblies on the CETA cart.
NASA Astronaut Tim Kopra on Dec. 21 Spacewalk
Expedition 46 Flight Engineer Tim Kopra on a Dec. 21, 2015 spacewalk, in which Kopra and Expedition 46 Commander Scott Kelly successfully moved the International Space Station's mobile transporter rail car ahead of Wednesday's docking of a Russian cargo supply spacecraft. After quickly completing their primary objective for the spacewalk, Kelly and Kopra tackled several additional "get-ahead" tasks. Kelly routed a second pair of cables in preparation for International Docking Adapter installment work to support U.S. commercial crew vehicles. Kopra routed an Ethernet cable that ultimately will connect to a Russian laboratory module. They also retrieved tools that had been in a toolbox on the outside of the station, so they can be used for future work.
The three-hour and 16-minute spacewalk was the second for Kopra, who arrived to the station on Dec. 15, and the third for Kelly, who is nine months into a yearlong mission.
Expedition 46 Commander Scott Kelly on a Dec. 21, 2015 spacewalk, in which Kelly and Flight Engineer Tim Kopra successfully moved the International Space Station's mobile transporter rail car ahead of Wednesday's docking of a Russian cargo supply spacecraft. After quickly completing their primary objective for the spacewalk, Kelly and Kopra tackled several additional tasks. Kelly routed a second pair of cables in preparation for International Docking Adapter installment work to support U.S. commercial crew vehicles. Kopra routed an Ethernet cable that ultimately will connect to a Russian laboratory module. They also retrieved tools that had been in a toolbox on the outside of the station, so they can be used for future work.
The three-hour and 16-minute spacewalk was the third for Kelly, who is nine months into a yearlong mission, and the second for Kopra, who arrived to the station on Dec. 15. Kelly shared this picture with his social media followers after the conclusion of the spacewalk, writing "Fix was pretty easy, but the rest always a challenge. With great team on earth we got it done safely. #YearInSpace"
Tim Peake :
My ringside-seat view of yesterday’s #spacewalk – taken from the cupola