Dextre and RRM complete record breaking week of robotics on ISS

by Pete Harding

Canada’s Dextre robotic space helper, working with NASA’s Robotic Refuelling Mission (RRM) experiment, have together completed a record breaking week of robotics operations on the International Space Station (ISS), a week which saw the first ever attempt at satellite servicing tasks successfully performed in space.

Dextre background:

Dextre, also known by its technical name of the Special Purpose Dextrous Manipulator (SPDM), was designed and built by MacDonald Dettwiler and Associates, (MDA), and financed by the Canadian Space Agency (CSA).

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It is designed as a “hand” for the Space Station Remote Manipulator System (SSRMS), and gives the ISS robotic Mobile Servicing System (MSS) the capability to perform fine dextrous tasks. The SPDM was launched to the ISS on STS-123 in March 2008, and is controlled entirely from the ground.

The SPDM 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 retractable socket drive, a camera and light, and an umbilical connector to provide and receive power, data and video to and from a gripped object.

The OTCMs also include Force/Moment Sensor (FMS) technology, giving the arms a “sense of touch”.

Both arms are connected to a central body which features a Power & Data Grapple Fixture (PDGF) at one end, which enables the SPDM 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 background:

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 last year – 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. It contains a selection of interfaces, such as valves that are typically found on most satellites, and four tools designed to interact with those interfaces.

Those four tools, each grappled by one of the SPDM’s OTCMs, are the Wire Cutter and blanket manipulation Tool (WCT), which cuts safety wires attached to fuel caps, and the Multifunction Tool (MFT), which locks onto four tool adapters – 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.

The third tool is the Safety Cap removal Tool (SCT), which performs capture, removal and stowage of the safety cap from the fill/drain valve, and finally 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.

It was these tools that were the focus of this week’s operations.

Initial RRM operations:

Since being installed onto ELC-4 by the SPDM in September last year, RRM has been awaiting an SPDM software update to be able to begin operations. With that update recently performed, starting on Wednesday 7th March and continuing through Friday 9th March, the SPDM in conjunction with RRM began the first ever operations to checkout and test satellite refuelling equipment in space.

Officially known as Part 1 of the Gas Fittings Removal task, beginning on Wednesday 7th March, the SPDM on the end of the SSRMS maneuvered close to RRM on ELC-4, whereupon one of the SPDM’s OTCMs grappled and removed three of the four tools from the RRM module in turn, via their micro fixture grapple targets.

Once the tools – the SCT (Safety Cap removal Tool), WCT (Wire Cutter Tool) and MFT (Multi Function Tool) – were removed from RRM, a full functional electrical and mechanical check-out was performed on them, whereupon they were re-inserted into RRM.

On the second day of operations, Thursday 8th March, the SPDM used the MFT to remove seven launch locks that held the MFT’s four tool adapters – the TCA (Tertiary Cap Adapter), TVA (T-Valve Adapter), ACA (Ambient Cap Adapter), and PMA (Plug Manipulator Adapter) – in place during launch.

Once this task was successfully performed, the SPDM re-inserted the MFT into the RRM module, and then removed the WCT in order to perform the first true servicing test task using RRM – the cutting of two lock wires, of 20 thousandths of an inch (0.5 millimeters) in diameter, located on both the Ambient Cap and T-Valve on the Coolant Valve Panel (CVP) task board on RRM.

On the third and final day of operations, Friday 9th March, all tools were re-stowed into RRM.

Despite challenges such as dynamic lighting and jitter, the monumental wire cutting task, a pre-requisite to removing satellite parts on any servicing missions, was successfully performed, using only the visual input from the camera on the WCT and the force-feedback of the SPDM.

Setting aside the technical challenges and success of the wire cutting task, many historical firsts were performed during the operations – including the first ever use of the SPDM for Research & Development (R&D), tasks for which it was never designed, the first ever attempt at satellite servicing in space, and the most intricate and complex robotics activities ever performed in space.

As noted by Frank Cepollina, Associate Director of the SSCO at GSFC, “RRM showcases the best of what the International Space Station can offer as a test bed for state-of-the-art space technologies. With the established infrastructure that the space station provides, our RRM team had support as we conceived, designed, built, and flew the RRM demo to space station in 18 months — a timeline that many declared impossible.”

“The impact of the ISS as a useful technology test bed cannot be overstated. Fresh satellite-servicing technologies will be demonstrated in a real space environment within months instead of years. This is huge. This represents real progress in space technology advancement”, continued Cepollina.

Future RRM operations:

The next scheduled operations using RRM will not occur until May 2012, where Part 2 of the Gas Fittings Removal task will be performed, followed in the summer timeframe by the main task for which RRM was designed – the actual demonstration of a removal of a fuel cap and the transfer of a simulated liquid fuel.

RRM operations are scheduled to continue through to the end of 2013, with tasks such as thermal blanket manipulation, screw/fastener removal, and electrical cap removal being performed. The first release of data/lessons learned from RRM is scheduled for May this year at the upcoming Second International Workshop on On-Orbit Satellite Servicing

Looking further into the future, the dynamic duo that is the SPDM and RRM will provide a wealth of data on satellite servicing, as the ISS demonstrates its value in using its advanced and capable infrastructure to demonstrate new capabilities in space, capabilities that could not only advance the cause of space exploration, but lead to new markets and business sectors here on Earth.

(Images: L2 Content, CSA, and NASA)

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