NASA Glenn tests solar electric propulsion thruster for journey to metal world
NASA is preparing to travel to a world unlike any other it has visited before. The agency has sent spacecraft to terrestrial planets, gaseous planets, icy moons, and rocky asteroids. Now, following its approval as a Discovery mission in February of this year, a spacecraft set for launch in 2022 will visit the main-belt asteroid Psyche, a metal world that scientists think is made almost entirely of nickel and iron.
The nickel-iron makeup of Psyche suggests that it may be the exposed core of an early protoplanet, torn apart by hit-and-run collisions during the early history of the Solar System.
“Psyche is a unique body because it is, by far, the largest metal asteroid out there; it’s about the size of Massachusetts,” said David Oh, the mission’s lead project systems engineer at the Jet Propulsion Laboratory (JPL) via a news release. “By exploring Psyche, we’ll learn about the formation of the planets, how planetary cores are formed and, just as important, we’ll be exploring a new type of world.”
Psyche will be explored by a spacecraft of the same name. The Psyche spacecraft is currently scheduled for launch in summer of 2022 with a planned arrival at the asteroid in 2026. Getting there and staying in orbit around the asteroid long enough to achieve the targeted science goals of the mission won’t be easy. It will require the use of solar electric propulsion (SEP) and the ability to operate that technology in some new ways.
Those requirements led the mission team of JPL scientists and engineers as well as researchers at Arizona State University to turn to the NASA Glenn Research Center, leaders in solar electric propulsion for decades.
NASA Glenn, in Cleveland, Ohio, has been developing and testing solar electric propulsion technology since 1964. Solar electric propulsion uses electricity generated from solar arrays to ionize atoms of the propellant xenon. These ions are then expelled by a strong electric field out the back of the spacecraft, producing thrust. So, in short, SEP is a propulsion system that is a combination, or coupling, of solar array technology and ion thruster technology.
The SEP thruster planned to be used by Psyche is the SPT-140 Hall-effect thruster, produced by SSL (formerly Space Systems/Loral, LLC), the subcontractor with JPL for the mission. To achieve the science objectives once in orbit around the asteroid, the SEP thruster must perform in a new way that required a new round of testing that could only be conducted at NASA Glenn.
“They’ve tested that thruster before, but they had not throttled that thruster to lower levels,” Carol Tolbert, project manager at NASA Glenn’s Space Science Project Office, told Spaceflight Insider. “They had used it at a higher level, but they wanted to know how we can expect it to behave at a lower throttling. So that is what we were testing. We were able to simulate it to give them enough data to understand that.”
The SPT-140 Hall-effect thruster was put into the large thermal vacuum chamber at NASA Glenn’s Electric Propulsion Laboratory (EPL). It is the largest and best-equipped facility of its kind in the world. The testing began on September 5 and was completed on September 18.
“That thruster is nominally rated at 4.5 kW operating power. For Psyche, the trajectory is such that they would like to throttle down or go to a lower gear for large parts of their trajectory,” Jason Frieman, Electric Propulsion Systems engineer at NASA Glenn, told Spaceflight Insider. “There are a bunch of reasons they want to do that. They want to save some power, they don’t need as much thrust, they want to operate in a little different mode when they are at the asteroid. Because it’s a 4.5 kW thruster, most of the previous testing was done at that throttle point. For Psyche, they want to go all the way down to 900 watts. So part of this test was figuring out how we run this thruster down there at that throttle condition. How do we run this bigger thruster with less power stably.”
The testing at NASA Glenn was critical to understanding how the throttled-down thruster would perform. The large vacuum chambers would provide the test with the most space-like environment available, in order to inform mission planners how the thruster would perform at the new throttle point in the vacuum and extreme temperatures of space.
“What we’ve observed, unfortunately, is that Hall thrusters are sensitive in terms of how they perform to that ambient pressure condition,” Frieman said. “Other places don’t have a chamber like this. They have smaller chambers. So they will test and the pressure will be higher and we’ll see the performance change if the pressure is moved up or down. And so in order for the trajectory modelers to really pinpoint the details of their thrust and efficiency and specific impulse, they need to build a model that shows how the performance of the thruster changes as a function of pressure. Then they can extrapolate that and figure out what it is going to be on orbit.”
Because other vacuum chambers aren’t as big as the one at NASA Glenn, they can only test for a certain range of pressures. Mission planners cannot calculate their trajectory models down at the low-pressure end, which is what they are most interested in because it gives them the condition closest to what space is.
“The significance of this test is that because we are able to use the much larger tank, we’re able to achieve a much lower pressure when we’re running, to get these points that we can’t get anywhere else in how this thruster performs and how its performance changes as we get to those lower pressures,” Frieman said.
The testing yielded crucial data for mission planners about how the thruster will perform during the important on-orbit phase of the mission.
“The de-throttle is a function of the science they want to do around the asteroid,” Frieman said. “It gives them more time for their science instruments and imagers to do what they need to do.”
The instruments planned for the Psyche spacecraft include a multispectral imager, gamma ray and neutron spectrometer, magnetometer, and an x-band gravity science experiment.
“I think it’s exciting to go out and look at that kind of an asteroid,” Tolbert said. “I think it may give us clues about the beginning of our Solar System. Anything that can give us those kinds of answers is wonderful.”
NASA Glenn will continue to partner with the Psyche team at JPL and Arizona State University, as further testing for the mission may become necessary in the progress toward the 2022 launch date.
“At first glance, the test results confirm our predictions regarding how the thruster will perform,” Oh said, “and it looks like everything is working as expected. But, we will continue to refine our models by doing more analysis.”
Video courtesy of NASA Glenn
Michael Cole is a life-long space flight enthusiast and author of some 36 educational books on space flight and astronomy for Enslow Publishers. He lives in Findlay, Ohio, not far from Neil Armstrong’s birthplace of Wapakoneta. His interest in space, and his background in journalism and public relations suit him for his focus on research and development activities at NASA Glenn Research Center, and its Plum Brook Station testing facility, both in northeastern Ohio. Cole reached out to SpaceFlight Insider and asked to join SFI as the first member of the organization’s “Team Glenn.”