Aboard the International Space Station (ISS) over the past two weeks, Dextre, the Canadian “robotic hand”, and NASA’s Robotic Refuelling Mission (RRM) payload have together made history in proving a vital capability for future space exploration and utilisation – the ability to robotically refuel a satellite in the space environment.
The successful demo opens new doors for future missions to extend the lifetime of satellites, with it creating an entire new industry dedicated to satellite refuelling,
Dextre and RRM background:
Dextre, also known as the Special Purpose Dextrous Manipulator (SPDM), is a Canadian Space Agency (CSA) robotic “hand” that fits onto the end of the Space Station Remote Manipulator System (SSRMS) “Canadarm2”, designed to enable much more intricate and dextrous robotic tasks to be performed outside the ISS.
Controlled entirely from the ground, Dextre features two arms, both with shoulder, elbow and wrist joints, although only one arm can be used at any one time.
Both arms are terminated with ORU Tool Changeout Mechanisms (OTCMs), which include “jaws” to grasp objects, a camera and light, and an umbilical connector to provide and receive power/data to and from a gripped object. The OTCMs also include Force/Moment Sensor (FMS) technology, giving the arms a “sense of touch”.
The arms are connected to a central body which features a Power & Data Grapple Fixture (PDGF) at one end, which enables Dextre to be grappled and controlled by the SSRMS, and a Latching End Effector (LEE) at the other end, which enables the SPDM to grapple and attach to other PDGFs on the ISS.
RRM is an ISS payload developed by the Satellite Servicing Capabilities Office (SSCO) at NASA’s Goddard Space Flight Center (GSFC) – the same team that managed the highly intricate Hubble Space Telescope (HST) Servicing Missions via the Space Shuttle. It is designed to test procedures for refuelling satellites in space.
The RRM payload itself – launched on STS-135 in July 2011 – is box-like in appearance, and is attached to the ISS’ ExPrESS Logistics Carrier-4 (ELC-4) via the Flight Releasable Attachment Mechanism (FRAM) interface.
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It features replaceable task boards containing a selection of interfaces, such as valves that are common to most satellites, and four tools designed to interact with those interfaces.
Those four tools, each of which can be gripped by one of Dextre’s OTCMs, are the Wire Cutter Tool (WCT), which cuts safety wires attached to fuel caps, the Safety Cap Tool (SCT), which performs capture, removal and stowage of the safety cap from the fill/drain valve, and the EVR Nozzle Tool (ENT), which connects to the satellite’s fuel valve using a Quick Disconnect (QD) fitting and is capable of opening and closing the valve.
The fourth and final tool is the Multifunction Tool (MFT), which locks onto four tool adapters, thus enabling one tool to perform four functions – the Tertiary Cap Adapter (TCA), T-Valve Adapter (TVA), Ambient Cap Adapter (ACA), and Plug Manipulator Adapter (PMA) – which then perform four separate tasks – either capture, removal or stowage of three different caps, and capture and removal of a gas plug.
Dextre and RRM’s big task – Satellite refuelling demo:
Ever since launch of the RRM payload in July 2011, the RRM and Dextre teams have been progressively increasing their capabilities with a series of tasks, which started in September 2011 with the Launch Lock Removal and Vision Task, which provided data to help the SSCO teams develop machine vision algorithms based on the actual space environment.
Next to be performed was the Gas Fittings Removal Task, which was split into two parts, with the first part being performed in March 2012, and the second part being performed in June 2012.
These tasks demonstrated the capability to perform intricate robotics tasks in space, by removing fittings found on the fuel valves of satellites that would need to be removed to gain access to the fuelling port.
With these tasks successfully accomplished, the way was clear for the main task of the RRM mission: Actual demonstration of satellite refuelling in the environment of space. The difficult task had been long in the planning by the RRM and Dextre teams, and would present a number of challenges to both teams, requiring skill and precision to overcome.
The six day refuelling task, spread over a period of almost two weeks and controlled entirely from the ground, began on the January 11 with the SSRMS “Canadarm2”, with Dextre attached, being moved into position to support the RRM ops. However, as detailed in L2’s rolling ISS updates, the maneuver did not go entirely as planned, as the SSRMS did not move in the way it was expected to, requiring the SSRMS to be safed.
This left the SSRMS and Dextre in a position that was too close to the ISS structure to conduct a reboost (such as a Debris Avoidance Maneuver) should it have been necessary, as the acceleration from the reboost could have caused contact between the ISS structure and the SSRMS/Dextre.
It was soon determined that the problem was due to a Frame Of Reference (FOR) issue caused by the fact that, once Dextre is latched in place on the end of the SSRMS, the capture snares on the SSRMS Latching End Effector (LEE) are released in order to reduce fatigue.
This caused the software to believe that Dextre was released, which caused the SSRMS to move as though it was not carrying any load. The LEE snares were re-closed, and the SSRMS begin responding nominally, allowing RRM ops to begin.
After a weekend break, day one of RRM operations got underway on January 14, with the first order of business being for one of Dextre’s arm’s – terminated by its ORU Tool Changeout Mechanism (OTCM) – to grip one of RRM’s four tools, namely the Wire Cutter Tool (WCT) and remove it from the RRM module.
Dextre then used the WCT to cut a wire on a Tertiary Cap on a mock Plumbed Fill/Drain Valve (FDV) – called Plumbed Valve 1 (PV1) – on the RRM module, a task that would be required in order to gain access to the fuelling valve on a real satellite.
This operation was successful, however the teams encountered a surprise when they had to unexpectedly use the WCT to remove the newly cut wire, as its position could have prevented later removal of the Tertiary Cap.
Using Dextre’s second arm, teams then gripped and removed a second tool from the RRM module – this time the Multi Function Tool (MFT) which had attached to it a Tertiary Cap Adapter (TCA) left over from the RRM operations in June 2012. The MFT with TCA was then used to remove the Tertiary Cap from PV1, concluding day one of operations.
Day two of operations, conducted on January 15, consisted of Dextre using the MFT to stow the TCA, with Tertiary Cap attached, into the Tertiary Cap Receptacle (TCR) on the RRM module. However, the next planned tasks were cut short due to another issue with the SSRMS – this time with rate limiting software, which caused the CSA to request a temporary halt to RRM operations.
L2’s exclusive ISS updates section provided more details on the issue: “The concern from CSA is that the SSRMS could move at unloaded rates while it’s loaded. The SPDM was moved far enough from the structure to allow for reboost and then stopped.
“It was found recently that there were several instances over the past few months where the SSRMS maneuvering speeds did not match the expected rate limit”.
The issue was a particular concern for the RRM ops as the arm is required to work in close proximity to ISS structures – meaning any unexpected rate of movement could result in a contact and potential damage.
According to the L2 notes, a workaround was developed which involved ensuring that proper payload parameters in the SSRMS Arm Computer Software and Dexterous Manipulator Computer Software were correctly loaded.
With the SSRMS issue resolved, RRM was cleared for day three of operations, which occurred on January 17-18. The ops involved two tasks delayed from day two – which were using the WCT to cut two wires on a Safety Cap and Actuation Nut on PV1 – both of which are more hardware that would need to be removed to access a satellite’s fuelling port – and then Dextre stowing the WCT held in one arm, and MFT held in the other arm, back into the RRM module.
The tasks originally planned for day three were then conducted, which involved Dextre gripping the Safety Cap removal Tool (SCT) and removing it from the RRM module.
However, an issue was noted (L2) with this process, involving Dextre’s umbilical extender mechanism (which mates power/data/video connectors between Dextre’s OTCM and a payload, in this case the SCT) having a “stick-slip behaviour” as it pushed past the SCT umbilical connector doors, requiring teams to retract Dextre’s gripper and exercise the umbilical mechanism, which solved the problem.
Once the SCT was removed from the RRM module, Dextre conducted a check out on the never before used tool, following which the SCT was re-stowed into the RRM module in preparation for a break from RRM activities for the inauguration weekend.
Day four of operations got underway on January 22-23, and consisted of Dextre once again grasping the SCT and removing it from the RRM module, and using it to prepare a Safety Cap Receptacle (SCR) on RRM for future use.
This, according to L2 notes, consisted of breaking the SCR’s launch torque with a two turn rotation, followed by exercising SCR function through 30 turns unfastening (which is the action to store a Safety Cap) and then 30 turns fastening (which is the action to restow the SCR).
Dextre then used the SCT to remove the Safety Cap on PV1, following which the SCT with Safety Cap attached was stowed back into the RRM module. With both the Tertiary Cap and Safety Caps removed from PV1, thus exposing the fuelling port, the stage was then clear for the big task: Transferring fuel.
Day five of operations occurred on January 23-24, and involved Dextre using one of its arms to grasp onto a micro fixture square interface on ELC-4 (to which RRM is attached) for the purposes of stabilisation during the delicate refuelling task.
Dextre’s other arm then grasped the EVR (Extra Vehicular Robotics) Nozzle Tool (ENT) and removed it from the RRM module, with L2 notes stating that a “maximum force of 80 Newton’s was required to extract the ENT from the tool bay due to minor misalignments in the yaw orientation”.
A check-out of the never before used ENT was then performed, prior to Dextre maneuvering the ENT into alignment with PV1 in an overnight park position, in preparation for the big refuelling task the following day.
Day six – the final day of operations – occurred on January 24-25, and started out with Dextre threading the ENT onto PV1 in order to create a tight seal for transferring fuel, a task that was especially tricky since the ENT has a hose attached to it (which the simulated fuel travels down), which had a tendency to try and pull Dextre’s hands away from their desired target.
Once the ENT was threaded onto PV1, the moment that the RRM and Dextre teams had waited so long for had finally arrived: Time to finally demonstrate the ability to refuel a satellite in space.
As commands were issued from the ground, ethanol (used as a mock satellite fuel) began to be transferred from one tank in the RRM module, down the hose to the ENT, through PV1, and into another tank in the RRM module. The task was a complete success. Satellite refuelling in space, using valves never designed to be accessed on orbit, had been proven.
With the refuelling demo complete, the ENT unthreaded itself from PV1, with residual ethanol flying out in all directions as the connection was broken, boiling off in the vacuum of space. The ENT left behind a Quick Disconnect (QD) fitting over PV1 in order to facilitate a simpler connection to PV1 for future refuelling – a capability which means the complex process of wire cutting a cap removals would only need to be done once for each satellite that is refuelled.
The final task for Dextre was to stow the ENT back into the RRM module, and then ungrasp its other arm from ELC-4.
While the RRM mission is far from over, with many more tasks still remaining – namely Thermal Blanket Manipulation, Screw/Fastener Removal, and Electrical Cap Removal – the main purpose of the mission has been successfully proven: It is possible to refuel satellites in space that were never designed for such a purpose.
The technologies demonstrated by the RRM teams, using the very capable robotics facilities on the ISS, could go on to save billions of dollars for satellite operators by eliminating the requirement to purchase new satellites once existing satellite’s fuel tanks run dry, and preventing many perfectly functional satellites from becoming space junk simply because they are out of fuel.
Ed Rezac, NASA SSCO Robotics Facility Manager and former EVA guru for four Space Shuttle Hubble Servicing Missions, had this to say about the innovative and now highly successful RRM mission and support teams:
“I am so very proud of the Satellite Servicing Capabilities Office team and the extended team that includes our partners at the Canadian Space Agency, Johnson Space Center, Marshal Space Flight Center and all the International Space Station support members – to the individual.
“Every precaution taken, every concern addressed, every anomaly solved, every success realized so far is to be savored by the people responsible. Well done – I can hardly wait to see where they bring us today and tomorrow, and tomorrow and tomorrow!”
(Images: NASA, NASA TV, L2’s ISS Section and Ron Smith, NASASpaceFlight.com)
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