In-space satellite refueling system tests make progress on the ground and at ISS testbed
Spacecraft-servicing experts who cut their teeth on the five maintenance and repair missions flown to the Hubble Space Telescope are preparing for a new set of technology tests at the International Space Station (ISS) that may one day be applied on Mars or one of its moons.
Delivery of the second round of experiments to the servicing testbed on a European Automated Transfer Vehicle (ATV) is set for later this year. It will come as roboticists at's finish digesting the results of a just-completed teleoperations exercise that transferred corrosive nitrogen tetroxide (NTO) through a standard satellite-fueling valve at (KSC) with a robot controlled from Goddard.
That ground test, and the upcoming second phase of the Robotic Refueling Mission (RRM), continue pushing technologies that may allowto stretch the service lives of expensive science satellites in Earth orbit, as with the Hubble, and support deep-space human missions over the long haul. In-space repair is a key factor in life-support systems under development to keep crewmembers alive on those missions (see page 18).
“The ability to repair and maintain and take care of critical emergencies when you're going long, long distances is even more important than when you're going to fixed distances,” says Frank Cepollina, associate director of the Satellite Servicing Capabilities Office at Goddard, a 52-year NASA veteran who has been designing modular satellites and servicing them since the Solar Maximum Mission that was launched in 1980 and repaired by shuttle astronauts in 1984. “Things happen no matter how brilliant you are as far as reliability.”
The Remote Robotic Oxidizer Transfer Test (RROxiTT) completed last month is another step on the road to space-servicing. Goddard engineers set up an industrial robot equipped with custom software in the satellite-fueling facility at KSC and used it to transfer NTO, controlling the operation from a robotics workstation 800 mi. away at their Greenbelt, Md., NASA field center. They designed a tool to interface with the standard satellite servicing valve on the KSC testbed, while Kennedy engineers developed the hose management system and propellant transfer assembly attached to the back end of the robot.
For ISS crew safety, the on-orbit refueling tests have used ethanol as a simulated propellant rather than NTO or toxic hydrazine used in satellite propulsion systems. At Kennedy, technicians wore the Self-Contained Atmospheric Protection Ensemble (Scape) suits required for spacecraft fueling, while at Goddard Alex Janas, the lead RROxiTT roboticist, worked in shirtsleeves as he teleoperated the refueling test rig.
“The biggest takeaway here was the forced integration points in this test,” says Janas. “We've always been developing the software and the mechanisms and the propellant transfer all separate from each other. The fact that this test forced us to integrate with all the other teams, I think, was one of the biggest lessons. We gained a lot of information about each other's systems and how we interact.”
Someday, that overarching lesson and the detailed engineering results the team is assembling now, could inform the robotic transfer of liquid methane, processed on Mars or its moon Phobos by an in situ resource utilization system, into a waiting spacecraft bound back to Earth. The satellite-servicing office at Goddard eventually hopes to use the robotics testbed on the ISS to transfer cryogenic oxygen and hydrogen, and perhaps even the xenon used as propellant in solar electric propulsion (SEP) systems. Commercial geostationary communications satellites use SEP for stationkeeping, and NASA is pushing the technology as propulsion for the proposed asteroid-redirect mission and for generic space tugs.
New tools that the Canadian-built Special Purpose Dexterous Manipulator (Dextre) robot will employ in the next phase of the RRM work have been delivered to the space station, with additional task boards and other hardware slated to arrive as early as this summer in the ATV. Designed to be passed through the airlock in the Japanese Experiment Module Kibo, the new toolkit includes a “visual inspection poseable invertebrate robot tool” dubbed Vipir that will allow extremely close visual examination of in-space hardware to aid diagnosis of problems.
“One of the key functions of a robotic servicing vehicle is to be able to see what's wrong,” says Cepollina. “We have what's called a Vipir, which is an extensible articulated arm. It allows you to sit there on the ground and move this inspection camera 360 degrees around, lift the elbow, look up, look down, all these kinds of things.”
Overall, the second phase of RRM has work for about 12 months that could be stretched over a three-year service life, depending on ISS operational priorities. Other tasks include reassembly of cryogenic valves that were disconnected earlier, robotic work with electrical connectors linked to a solar-powered LED indicator to show when connections are made, and an experiment to test plugging techniques with an open aluminum tube (AW&ST July 29, 2013, p. 38). The hardware also includes four-junction gallium arsenide solar cells supplied by Glenn Research Center that will be tested in space on the RRM real estate.
The satellite-servicing office at Goddard is evaluating the results of a request for information issued to industry last year as part of its planning for an eventual public-private partnership that would develop a free-flying servicing mission in geostationary orbit in 2018-23. One of the office's stated goals is to help “position the U.S. to be the global leader in in-space repair, maintenance and satellite disposal.”
Cepollina says that work involves “the five Rs”—rendezvous and inspection, refueling, repositioning, repairing failed components and replacing those that cannot be fixed. He likens the spacecraft that can be developed from the ongoing technology work to a tow truck that goes out to fix spacecraft that have broken down, just as a terrestrial version would repair or tow an automobile. Round-trip space shuttle missions spelled the beginning of the end of throw-away spacecraft, he says, and in-space servicing started lowering costs.
“The strength of this whole concept of serviceability revolves around the word 'economics,'” he says. “What is so important about the commercial communications satellite market is there are so many of them, and there are so many that get into trouble each year. The return on investment can be rather significant. But NASA has very expensive assets in orbit, and so does the [Pentagon] world. So the end goal is to be able to . . . extend their useful life by putting new science on board and fixing the machinery that breaks.”