STS-135: Enabling a new era of robotic satellite refuelling in space
As Space Shuttle Atlantis prepares to blast off to the International Space Station (ISS) for her STS-135 mission on Friday, signalling the end of the Space Shuttle era, a new era in robotic satellite refuelling will just be beginning, as Atlantis will transport to the ISS a payload designed to demonstrate and evaluate the technologies and techniques needed for robotic satellite refuelling.
The benefits of robotic satellite refuelling:
Almost all of the thousands of satellites currently orbiting the Earth carry their own supply of fuel to control their attitude and orbit. A significant drawback of this, however, is that eventually this fuel runs out, meaning that satellites are no longer able to control their attitude and orbit. This means that many perfectly serviceable satellites are rendered defunct simply because their fuel tanks have run dry, causing satellite operators to build and launch replacement satellites at great cost.
If, however, it were possible to refuel those satellites, at a fraction of the cost of building and launching a new satellite, then a potentially lucrative business model exists for a mobile orbital gas station.
This commercial concept is currently being pursued by Canada’s MacDonald, Dettwiler and Associates (MDA), in the form of the Space Infrastructure Servicing (SIS) program. The basic concept is to place a robotically-controlled spacecraft in geosynchronous orbit, which would feature fuel tanks and tools to transfer that fuel to a satellite.
Thus, once a satellite runs out of fuel, its operators could contract MDA to refuel the satellite, whereupon the robotic refuelling spacecraft would rendezvous with and attach to the satellite, and transfer some new fuel to its tanks, thereby extending its operational life.
The robotic refuelling spacecraft would be capable of flying between different satellites in different geosynchronous orbits, making it useful for more than just once refuelling job. Once the robotic refuelling spacecraft itself runs out of fuel, it would be resupplied with a new shipment of fuel launched from Earth.
Scheduled for launch in 2015, the SIS program’s first customer will be Intelsat, who operate the world’s biggest fleet of commercial satellites.
Using ISS to develop technologies and techniques for robotic satellite refuelling:
The problem with in-space robotic satellite refuelling, a delicate and intricate operation at best, is that it has never been tried before, and thus the experience necessary to develop a robotic satellite refuelling spacecraft does not exist.
Further complicating matters is the fact that, whereas all future satellites can be designed with in-space refuelling in mind, past satellites were not, making the refuelling process for these satellites even more tricky.
In order to gain the much needed knowledge in technologies and techniques for in-space satellite refuelling, NASA decided to form the Satellite Servicing Capabilities Project (SSCP).
The SSCP team members, based at NASA’s Goddard Space Flight Center (GSFC), previously worked on the highly intricate Space Shuttle-based Hubble Space Telescope (HST) Servicing Missions, during which they amassed a huge amount of knowledge in performing intricate tasks in space, most notably during the highly complicated and successful STS-125 mission in May 2009.
Thus, the SSCP project was formed at GSFC to leverage this knowledge for the purposes of robotic satellite refuelling.
The SSCP team quickly realised the benefits of using the ISS to demonstrate the technologies and techniques needed for in-space satellite refuelling, due to the ISS’ outstanding robotics capabilities in the form of Canada’s Special Purpose Dextrous Manipulator (SPDM), or “Dextre” – which coincidentally was designed and built by MDA, the same company now looking to get into the business of robotic satellite refuelling.
This allows satellite refuelling demonstration payloads to be designed and flown without their own robotic control systems, since they can use those already present on the ISS – which reduces the time and cost to develop satellite refuelling demonstration payloads. The ISS’ SPDM is extremely useful for in-space satellite refuelling demonstrations due to its outstanding fine dextrous control capabilities.
The SPDM has already highlighted the need to thoroughly test robotic satellite refuelling techniques in space before certifying them for use. In July last year, the SPDM attempted to Remove & Replace (R&R) a Remote Power Control Module (RPCM) on the ISS, a very delicate and intricate operation never before attempted, but ran into trouble since the RPCM required more force to remove than anticipated.
This is because it is impossible to accurately simulate how robotics systems will behave in microgravity environments without first testing them in space.
To this end, the SSCP team designed the Robotic Refuelling Mission (RRM) payload, formerly known as the Robotic Refuelling Dextrous Demonstration (R2D2). Originally targeting a launch on the third operational flight of SpaceX’s Dragon capsule to the ISS, the GSFC team switched their focus to the STS-135 mission in late 2010.
The late addition of STS-135 to the Space Shuttle manifest, carrying the Lightweight MPESS Carrier (LMC) able to transport external payload to the ISS, provided an opportunity for the RRM payload to fly to the ISS much sooner than originally planned, but this also led to a highly accelerated design, build and testing schedule.
As Benjamin Reed, Deputy Project Manager for the SSCP at GSFC recalled in an E-mail interview with NASASpaceflight.com: “The development schedule for RRM was approximately 18 months from conception to launch, a testament to the efficient ‘skunk works’ team at GSFC and their close counterparts at CSA, JSC, KSC and MSFC. The aggressive schedule was indeed challenging but the team knew that missing the opportunity to catch a ride on the last shuttle flight was too good an opportunity to pass up”.
The RRM payload features four tools which can be grappled and used by the SPDM’s two arms, which are each terminated by an ORU Tool Changeout Mechanism (OTCM). It is possible for two OTCMs to grapple two tools simultaneously, but only one can be used at any one time. Once the appropriate tool(s) have been grappled, they are removed from their stowage location(s) on RRM, whereupon their work to demonstrate robotic refuelling techniques begins.
The four tools on the RRM payload are each designed to perform different functions, such as interfacing with a variety of valves and Multi Layer Insulation (MLI) panels on RRM, to demonstrate the refuelling of a satellite. Mr Reed provided a detailed overview of all four tools and their function to NASASpaceflight.com:
“[The] Wire Cutter and blanket manipulation Tool (WCT) cuts through thermal blanket tape and severs safety wires attached to the fuel cap and other gas caps to allow [fuel or gas cap] removal.”
“[The] Multifunction Tool (MFT) does not interface/touch any satellite hardware directly, rather it locks onto one of four tool adapters, which then perform four separate tasks: capture/removal/stowage of three distinctly different caps, and capture and removal of a gas ‘plug’.
“These adapters are the Tertiary Cap Adapter (TCA), [which] accommodate the Tertiary Cap removal for the refuelling task. [The] T-Valve Adapter (TVA), Ambient Cap Adapter (ACA) and Plug Manipulator Adapter (PMA) are involved in the coolant valve panel tasks. These adapters are stowed in the MFT Adapter Receptacles (MARs for the ACA, PMA, TCA) or the T-Valve Receptacle (TVR for the TVA).”
“[The] Safety Cap Removal Tool (SCT) [is for] capture/removal/stowage of the safety cap, and its crushable seal, from the fill/drain valve. In addition, the SCT uses two adapters, the SMA Adapter (SMA-A) perform the SMA Cap removal, and the Torque Set Adapter (TSA) interfaces with the #10 fasteners. These last two are essentially [a] robotic nut driver and screwdriver respectively.”
“[The] EVR Nozzle Tool (ENT) connects to the satellite’s fuel valve using a Quick Disconnect [QD] fitting and is capable of opening and closing the valve. After refuelling, the Quick Disconnect fitting is left behind, giving operators ease of Multifunction Tool access for future refuelling. The spare QDs are stowed in the Quick Disconnect Receptacles (QDRs).”
Mr Reed also provided NASASpaceflight.com with an overview of the valves and MLI panels found on RRM:
“There are two models of fill drain valve on RRM and are presented in a variety of disassembly [states] to assure all robotic demonstrations are feasible.
“All of the RRM tools perform the actions to disassemble, cut, open or take apart components to gain access to the basic fill/drain valve. Only the ENT QD is threaded onto a fill/drain valve to allow fluid to pass across the mated interface.
“Other devices stow disassembled valve parts: The Tertiary Cap Receptacle (TCR) stows the Tertiary Cap and the Safety Cap Receptacle (SCR) stows the Safety Cap. The TPP [is] a mock Test Port Panel that simulates an MLI flap that needs to be cut and folded open (via WCT) in order to gain access to a plate underneath.”
All of the SPDM’s robotics operations will be ground controlled from GSFC, the Johnson Space Center (JSC) in Houston, Texas, the Marshall Space Flight Center (MSFC) in Huntsville, Alabama, and the Canadian Space Agency (CSA) control center in St. Hubert, Quebec. This means that no ISS crew time will be required to support RRM operations.
RRM deployment and utilisation schedule:
Following Shuttle Atlantis’ arrival at the ISS on the STS-135 mission, an EVA (Extra Vehicular Activity) will be performed by ISS Expedition 28 astronauts Ron Garan and Mike Fossum to remove the RRM payload from the LMC in Atlantis’ payload bay, and install it onto its temporary home on the ISS, the SPDM’s Enhanced ORU Temporary Platform (EOTP).
RRM will remain attached to the EOTP for around a month, while the SPDM performs an R&R of an RPCM on the P1 Truss – the same task which, as aforementioned, was previously attempted last year.
In August, once the RPCM R&R is complete, the SPDM will install RRM onto ExPrESS Logistics Carrier-4 (ELC-4), along with a Cargo Transportation Container (CTC) payload that the SPDM has been holding onto since it was removed from Japan’s HTV-2 cargo vehicle in early February.
The reason for this is that the CTC will be utilised for the RPCM R&R task since it contains the new RPCM, and will be used to stow the old RPCM, and so it makes sense to install both RRM and the CTC onto ELC-4 at the same time.
According to Mr Reed, once installed onto ELC-4, “RRM will have its launch lock and opened a variety of machine vision tasks performed. Micro conical fixtures [grapple fixtures for the SPDM] allow the release of the ENT hose and the contingency tool bench launch lock”.
Following an SPDM software update scheduled for sometime this fall, NASA will start performing some basic operations with the RRM, which will lead to more complex operations being performed toward the end of this year.
RRM operations are expected to run through to March 2013, whereupon the payload will likely be disposed of via a SpaceX Dragon vehicle. It is possible, however, to R&R task boards on the RRM, so if more demonstration experience were needed, then new task boards could be added to RRM.
Uses of RRM data:
The engineering data gleaned from RRM will be openly transferred to commercial entities to help foster a vibrant robotic satellite servicing industry, thus quite possibly becoming the first major commercial application for ISS research, and one of the first examples of NASA’s new mantra to work in conjunction with commercial companies.
According to Mr. Reed: “RRM is designed to reduce technical risk for future satellite servicing missions. NASA is conducting this mission on the International Space Station to learn how feasible it will be to provide robotic refuelling. The results of this work will be shared with all interested parties.
“In recent days, NASA managers have met with officials from MDA and Intelsat, who understand that NASA plans to take the RRM hardware to the International Space Station to use as a technology test bed.
“The results of the RRM tests will be shared with everyone, including them. NASA is not doing this to compete with industry. In fact, by conducting these tests on the space station, NASA believes it will help reduce the eventual risk and cost to industry.
“The first information to the ground will be the vision data which should happen this fall. This data will allow for the development of more advanced, more accurate machine vision algorithms.”
Another robotic satellite servicing payload once planned for the SPDM was the Dextre Pointing Platform, but according to Mr Reed, that payload is now no longer on the manifest
“Autonomous rendezvous and capture of non-cooperative spacecraft is the other major area, besides dexterous robotic refuelling technology, where risk reduction is warranted. The Dextre Pointing Package or DPP was a concept for a flight demonstration on ISS which would have helped refine client spacecraft position estimation and machine vision algorithms needed for capture of spacecraft not designed with on-orbit servicing in mind.
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“NASA now believes that most of the risks for future satellite servicing missions can be reduced via ground based robotic demonstrations at Goddard Space Flight Center.”
As NASA moves into the post-Shuttle era of ISS utilisation in support of scientific research and Beyond Earth Orbit (BEO) exploration, the RRM payload will be the start of a long line of technology demonstrations that will continue on ISS throughout the next decade.
(Images: Via L2 content and NASA.gov. Further articles on STS-135′s status in work, driven by L2′s fast expanding STS-135 Special Section which is covering the live flow to launch, surrounded by a wealth of FRR/PRCB/MMT and SSP documentation/pressentations, videos, images and more.
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