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article imageNASA's version of a Hall Thruster breaks records in testing

By Karen Graham     Oct 24, 2017 in Science
A thruster prototype being developed for future missions to Mars has broken several records during recent tests, with project team members saying it is possible the technology could take us to the Red Planet within 20 years.
The X3 Nested-channel Hall Thruster (NHT), is a 100-kW class thruster developed jointly by the Plasmadynamics and Electric Propulsion Laboratory (PEPL) at the University of Michigan, led by Alec Gallimore, University of Michigan professor of aerospace engineering and the Robert J. Vlasic Dean of Engineering, in collaboration with NASA, and the Air Force Office of Scientific Research.
The Hall thruster is one of three Mars Mission prototypes in development today. The new system propels a spacecraft by using electric and magnetic fields to ionize gases, like xenon, expelling the ions to produce thrust. This technique is much cleaner, safer and produces greater fuel efficiency than conventional chemical rocket engines.
Head-on view of three-channel X3 ion thruster during testing.
Head-on view of three-channel X3 ion thruster during testing.
The X3 thruster offers efficient plasma-based propulsion by using very small amounts of propellant very quickly, achieving top speeds in a fraction of the time of conventional rockets.
"Mars missions are just on the horizon, and we already know that Hall thrusters work well in space," Gallimore said. "They can be optimized either for carrying equipment with minimal energy and propellant over the course of a year or so, or for speed—carrying the crew to Mars much more quickly."
Recent test records were broken by X3 thruster
In a recent demonstration conducted at NASA's Glenn Research Center in Ohio, the X3 broke records for maximum power output, thrust, and operating current achieved by a Hall thruster to date, according to the research team at the University of Michigan and representatives from NASA.
One of the problems researchers hoped to overcome was maximum velocity in long-distance space travel. Chemical rockets produce a maximum velocity of about 5 kilometers per second, whereas an ion thruster could get a spacecraft up to 40 kilometers per second. "You can think of electric propulsion as having 10 times the miles per gallon compared to chemical propulsion," Gallimore told
The X3 thruster being tested at the NASA Glenn Research Center  firing at 50 kilowatts.
The X3 thruster being tested at the NASA Glenn Research Center, firing at 50 kilowatts.
However, and this is interesting, ion thrusters have a very low thrust and must operate for a long time to accelerate to high speeds. Additionally, ion thrusters aren't powerful enough to overcome Earth's gravitational pull, so they can't be used to launch a spacecraft.
"Chemical propulsion systems can generate millions of kilowatts of power, while the existing electrical systems only achieve 3 to 4 kilowatts," Gallimore said, adding that commercially available Hall thrusters aren't powerful enough to propel a crewed spacecraft to Mars.
"What we would need for human exploration is a system that can process something like 500,000 watts (500 kW), or even a million watts or more," Gallimore said. "That's something like 20, 30 or even 40 times the power of conventional electric propulsion systems."
The nested-channel X3 thruster
To overcome the technical problem of an engine having enough power, the research team made the thruster bigger than the other systems and developed changes in the design of the thruster that they believe will address the power problem.
"We figured out that instead of having one channel of plasma, where the plasma generated is exhausted from the thruster and produces thrust, we would have multiple channels in the same thruster," Gallimore said. "We call it a nested channel."
Scott Hall  a doctoral student in aerospace engineering at U-M  carried out the tests at the NASA Gl...
Scott Hall, a doctoral student in aerospace engineering at U-M, carried out the tests at the NASA Glenn Research Center in Cleveland, along with Hani Kamhawi, a NASA Glenn research scientist.
Actually, using three channels allowed engineers to make the thruster smaller and more compact than a single channel Hall thruster. Scott Hall, a Ph.D. student at the University of Michigan who has worked on the X3 project for the past five years says the work has been challenging.
"It's heavy — 500 pounds [227 kilograms]. It's almost a meter in diameter," Hall said. "Most Hall thrusters are the kind of thing that one or two people can pick up and carry around the lab. We need a crane to move X3 around."
Looking ahead to the future, the X3 will soon be ready to be integrated with power supplies being developed by Aerojet Rocketdyne, a rocket and missile propulsion manufacturer. And in early 2018, Hall will head back to NASA Glenn to test the X3 with its added power supply. That test will last 100 hours.
More about ion thruster, NASA, X3 Thruster, electron bombardment, Hall Thruster
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