Test of electromagnetic formation flight shows the value of two-way ISS communication
The first of at least three long sessions experimenting with electromagnetic spacecraft control on the International Space Station (ISS) validates both the target technology and the value of keeping researchers on the ground in the loop when astronauts conduct their science.
Engineering faculty and students from the University of Maryland and the Massachusetts Institute of Technology worked with ISS flight engineer Mike Hopkins Nov. 4 on a 3-hr. experiment with the Resonant Inductive Near-field Generation System (Rings) delivered to the station in August. Mounted on two of the ISS's cold-gas Synchronized Position Hold Engage and Reorient (Spheres) minisatellites, the Rings hardware generates electromagnetic fields that in theory can substitute for thrusters to control platforms in orbit.
As would be the case in a laboratory on the ground, the researchers were able to adjust the experiment as it progressed, tweaking it in light of observed results. Space station managers have realized the importance of giving scientists on the ground more control over experiments in the orbiting laboratory (AW&ST July 22, p. 28), and the Maryland/MIT group took advantage of it.
“I can't overemphasize the value there,” says Ray Sedwick, an aerospace engineering professor at Maryland and Rings principal investigator.
During the experiment, Allison Porter, a Ph.D. candidate at Maryland who helped design and build the Rings hardware as a master's-degree project, guided Hopkins through 30 tests with the two 75-cm-dia. (28-in.) test articles, watching his progress with the rest of the research team on a large flat-screen television (see photo, page 36). Alvar Saenz-Otero, the Spheres lead scientist at MIT, supervised the control room, encouraging the young engineers to anticipate problems and speak up as the experiment progressed.
The utility of real-time, two-way communication first appeared as Hopkins was setting up the experiment in the Japanese Experiment Module (JEM), the normal location for work with Spheres because of its large interior volume. Preparations required Hopkins to install Rings hardware on the Spheres minisatellites, juggle settings on the two Spheres, the two Rings surrounding them, laptop programs running the tests and infrared and ultrasound beacons in the JEM that collected data and sent it down viascience links. At one point, Saenz-Otero asked controllers to be “enabled” to speak directly to Hopkins so he could expedite a Spheres procedure that was outside the training Porter and the astronaut had received. Things went smoothly, and Porter was soon back on the com link.
That relative procedural informality is new to NASA human spaceflight culture, which until recently, required that “capsule communicators” on the ground be trained astronauts. Once the Rings testing started, the advantages for conducting science were quickly apparent. Hopkins would check the procedure for a given run on his laptop, position the two test articles as required in the center of the module and use the laptop to start the run. Depending on the configuration, power levels and other factors, the two minispacecraft would move eerily toward or away from each other while Hopkins watched closely, his feet hooked under a rail to keep him from floating out of position. Occasionally, the team would request a rerun or an adjustment, after discussing what they had seen on the screen. Hopkins could ask questions about changes, and the research team could change its mind.
The Spheres use carbon-dioxide cartridges to drive their thrusters, which came into play to restrict the Rings' electromagnetic effects to the desired axis for a given experiment. When the CO2 was spent, work stopped while Hopkins replaced a cartridge. The team also got breaks during “LOS” (loss of signal), when the ISS orbit took it out of position for continuous com, and during the crew lunch break. In total, the work was allotted 3 hr. of Hopkins's time, and the group at MIT used all of it—and all the tests preloaded in Hopkins's laptop.
Just as the researchers benefitted from the real-time interaction with Hopkins, they will be able to adjust their research in detail over a longer time frame. They will have two more chances to collect data with sessions on the ISS, the first as early as next month and another next spring. Sedwick says the team will spend about two weeks analyzing detailed data from the first tests and modifying protocols to take into account what they have learned. In addition, they need to address the failure of the Spheres thrusters to control completely the electromagnetic force, either by lowering the power in the coils or increasing the thrusters' output.
Software simulations based on the real data will help them decide if it is worth the extra CO2 consumption to keep the power levels in the coils the same. The fact that the electromagnetic force was stronger than expected probably bodes well for the ultimate application the team is developing: electromagnetic flight formation (EMFF) in space.
“We have actually demonstrated this technology on the ground using superconducting wire, and that presents the ability to operate at much longer separation distances,” Sedwick says. “If you're thinking of kind of typical spacecraft thrust levels for formation-keeping against disturbances, we could do that probably out to maybe 70 to 100 meters [230-328 ft.]. The real application, if you imagine the things you might want to formation-control for, . . . is to generate a sparse-aperture array in space.”
An orbiting formation of sensors arrayed across those kinds of distances could dramatically improve Earth-observation and space-astronomy capabilities—and without breaking the bank. In a checkout test, the Rings hardware also demonstrated that it can transfer power across open space via resonant inductive coupling. That could lead to constellations built around a primary spacecraft supplying power to a swarm of smaller and simpler satellites held in formation by EMFF.
The Maryland/MIT team is studying what it would take to move the experiments outside the space station. An experiment such as that probably would be teleoperated from the ground and would certainly be more expensive. But it would be driven by the same principle NASA and its partners are making available inside the station.
“If we think something anomalous might have happened, we can always rerun the experiment,” says Sedwick of the work last week. “We sent up what we thought were more experiments than we could actually get through, and we had a decision tree. That's something you couldn't do if you just had a preprogrammed sequence.”