Aboard the International Space Station (ISS) this past week, Canada’s Dextre robot and NASA’s Robotic Refuelling Mission (RRM) experiment have completed their second period of joint operations designed to demonstrate the technologies for robotic satellite servicing and refuelling in space. Meanwhile, the station’s US Lab-based Carbon Dioxide Removal Assembly (CDRA) was successfully re-activated following a failure the previous week.
Following on from the Gas Fittings Removal (GFR) Part One tasks performed in March this year – which successfully demonstrated the ability to sever a fuelling valve’s lock wire in space – the latest operations saw Canada’s Special Purpose Dextrous Manipulator (SPDM), more commonly known as “Dextre”, and the NASA Goddard Space Flight Center Satellite Servicing Capabilities Office (GSFC SSCO) developed RRM team up for the second time to complete Part Two of the GFR tasks.
The two-part GFR tasks centered around the Coolant Valve Panel (CVP) on the RRM module, which is one of many panels and task boards on RRM which contains mock-ups of many of the different types of valves commonly found on most satellites.
In addition to the valve mock-up panels, the RRM also features four tools that attach to the SPDM’s ORU Tool Changeout Mechanism (OTCM) “grippers” which terminate both of its two arms. Designed to perform multiple satellite servicing/refuelling functions, these four tools are the Safety Cap Tool (SCT), Wire Cutter Tool (WCT), EVR Nozzle Tool (ENT), and Multi-Function Tool (MFT).
The MFT is designed to be an all-purpose tool that performs specific functions via attaching to four adapter tools – the T-Valve Adapter (TVA), Tertiary Cap Adapter (TCA), Ambient Cap Adapter (ACA), and Plug Manipulator Adapter (PMA) – in the same way that different tips can be attached to a screwdriver. Thus, by having one tool that performs four functions, the MFT is “designed to be compact and multipurpose so that it can deliver smart and cost-effective servicing options in space”.
With the SPDM positioned near RRM on the end of the Space Station Remote Manipulator System (SSRMS) “Canadarm2”, which was in turn positioned on the Mobile Base System (MBS), the joint SPDM/RRM operations, controlled entirely from the ground with no ISS crew involvement, got underway on 20 June.
The first order of business, performed overnight from 19-20 June, was for the SPDM Arm 1 to grasp hold of the MFT (Multi-Function Tool) and remove it from its stowage bay on RRM.
Following a repositioning of the SSRMS/SPDM, the MFT then in turn grasped its T-Valve Adapter (TVA) tool (a task which was successfully completed on the second attempt) and removed it from its MFT Adapter Receptacle (MAR) on RRM – making for a long “chain” of progressively smaller robots linked together, starting with the MBS, to which the SSRMS was attached, to which the SPDM was attached, to which the MFT was attached, to which finally the TVA was attached.
The MFT with TVA attached then proceeded to begin actual valve removal demo operations, with the TVA attaching to a mock T-Valve, followed by unscrewing and removing it from the CVP.
The TVA, with removed T-Valve attached, were then stowed in a receptacle on the RRM on the second attempt, thus completing the T-Valve removal operations. Closing out the first day of RRM operations, the MFT then grasped the Ambient Cap Adaptor (ACA) and removed it from its stowage receptacle on RRM.
Day two of operations, conducted overnight from 20-21 June, began with the ACA, grasped by the MFT the previous day, attaching to mock-up ambient cap #2 on the RRM – a piece of hardware that would cover important fluid and gas lines on a real satellite – followed by the ambient cap being unscrewed from the RRM, and then being stowed, along with the still attached ACA, in a receptacle on RRM, thus demonstrating ambient cap removal capabilities.
Day two’s operations were closed out with the MFT, now free of the ACA, grasping the Plug Manipulator Adaptor (PMA) in preparation for day three’s activities.
The third and final day of joint SPDM/RRM operations, conducted from 21-22 June, consisted of the MFT using the PMA to raise a plug beneath the ambient cap removed the previous day, including using a plunger inside the PMA to simulate breaking the seal on a fuel tank.
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According to a set of RRM ops summary notes acquired by L2 – L2 Link, following plug manipulation tasks, “at the RRM team’s request, the SPDM Arm 1 was maneuvered back over to the AC2 (Ambient Cap 2) connection so that the RRM team could verify that the plug had indeed been raised”.
With the plug tasks verified, the MFT stowed the PMA back in its receptacle on RRM, following which the MFT went onto grasp its fourth and final adaptor – the Tertiary Cap Adaptor (TCA). Once the TCA was removed from its receptacle on RRM, touch and push tests were performed between the TCA and the Tertiary Cap Receptacle (TCR), which included pushing the TCA over a tertiary cap. At completion of this task, all GFR CVP tasks were complete.
The SPDM then stowed the MFT – with TCA still attached – back into its tool bay on the RRM, with the TCA being left attached to the MFT as a get-ahead for the RRM refuelling demo later this summer, which will require removal of a tertiary cap by the MFT/TCA combo.
As a final task, SPDM Arm 1 grasped another tool – the EVR Nozzle Tool (ENT) – and then proceeded to check out the ENT’s cameras via power and video supplied through the SPDM, also in preparation for the RRM refuelling tasks. The ENT stayed secured inside its tool stowage bay throughout this checkout, and at successful completion, the SPDM Arm 1 ungrasped the ENT, which concluded the joint SPDM/RRM ops.
The SSRMS/SPDM were then manouvered to an overnight park position, where they will remain until the week of 25-29 June, when the SSRMS will stow the SPDM on the MBS Power & Data Grapple Fixture-2 (PDGF-2), whereupon the SSRMS will be moved to the Node 2 PDGF in preparation for capturing Japan’s HTV-3 spacecraft on 27 July.
The next set of SPDM/RRM ops will be their biggest task yet – the actual demonstration of a satellite refuelling in space, using a mock fuel as opposed to a real fuel. According to GSFC SSCO employee and RRM project engineer Ed Rezac, these operations are scheduled for this August.
Although RRM has been in space for less than one year, the results of the experiment are already starting to bear fruit, with a GSFC SSCO presentation from the Second International Workshop on On-Orbit Satellite Servicing, held on May 30 this year, noting that “RRM operations have been extremely successful to date.
“The SSCO at NASA/GSFC is confident that the robotic technologies that exist today are mature enough to effectively service spacecraft robotically in Low Earth Orbit and beyond“.
Lab CDRA failure and recovery:
Another item of interest on the ISS this past week was the failure of the US Lab-based Carbon Dioxide Removal Assembly (CDRA), as outlined on L2 ISS Status notes – L2 Link.
As its name suggests, the CDRA is used to keep the ISS atmosphere scrubbed of carbon dioxide (CO2), and thus is a vital part of the ISS Environmental Control & Life Support System (ECLSS).
There are two rack-sized CDRAs on the US Operating Segment (USOS) of the ISS – one in the Node 3 module, and the other in the US Lab “Destiny”, although only one is required to be operational at any one time (thus the second one provides vital redundancy).
The Russian Segment (RS) of the ISS also has its own CO2 scrubbing capability in the form of the Vozdukh CO2 scrubber, however this device is unable to provide CO2 scrubbing for the entire station.
During nominal ISS flight, the Lab CDRA is operational, with the Node 3 CDRA powered off due to ongoing issues with its Air Selector Valves (ASVs) sticking – however it is able to step in as a backup CDRA if necessary (but is not the preferred CDRA due to the ASV issues).
The failure was soon traced to erratic operation of temperature sensor C on CDRA’s absorbent bed 202. Although the Lab CDRA was recovered on this occasion, temperature sensor C was believed by ISS engineers to be suspect – which presented a problem of its own.
Lab CDRA absorbent bed 202 was manufactured with three temperature sensors – A, B and C. However, sensor A was deactivated in 2011 after showing erratic data readings (which led to CDRA shutdowns), during which sensor C was also inadvertently disabled, leaving only sensor B operational.
However, on 11 June this year, sensor B began outputting erratic data, causing two CDRA shutdowns, leading ISS engineers to instruct the on-board station crew to access the connector pins that carry sensor data, disconnect the erratic sensor B, and reconnect the still good but previously disconnected sensor C.
This meant however that the Lab CDRA was operating with only one good temperature sensor remaining (sensor C) – which then failed on 16 June.
As such, ISS program managers and engineers decided to begin development of Pre-Positioned Load (PPL) software to enable the Lab CDRA to operate without any bed 202 temperature sensors. With the software not immediately available however, the Lab CDRA was re-activated.
On 20 June however, the Lab CDRA once again failed, again due to erratic data from temperature sensor C. Since this was the second Lab CDRA failure in as many days, ISS managers and engineers made the decision to put the Lab CDRA into standby and instead activate the Node 3 CDRA, despite its known ASV sticking issue.
The PPL software delivery date was also advanced from the 22 to the 21 June, in order to allow the Lab CDRA to sooner operate with no temperature sensor data (thus bypassing any erratic readings and preventing further shutdowns).
The PPL software update was uplinked to the ISS on the morning of 22 June, following which the Lab CDRA was successfully re-activated, following which the Node 3 CDRA was deactivated. The Lab CDRA has been operating successfully ever since, meaning the temperature sensor issue is likely resolved, at least for the interim.
The swift resolution to the CDRA issue demonstrated the skills and knowledge of the ISS program managers, flight controllers, engineers, and on-board crews in resolving failure situations, while simultaneous science activities in the form of RRM, and the crew’s usual busy schedule of experiments, went unaffected by the recovery process, allowing ISS to sail into another week of seldom noticed, but nonetheless important, scientific and technology demonstration work in space.
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