Plasma thrusters used on satellites could be much more powerful than previously believed

Plasma thrusters used on satellites could be much more powerful than previously believed

Plasma thrusters used on satellites can be much more powerful

The glow of the H9 MUSCLE Hall thruster’s plasma during a krypton propellant test. Credit: Plasmadynamic and Electric Propulsion Laboratory

It was believed that Hall thrusters, an efficient type of electrical propulsion commonly used in orbit, must be large to produce much thrust. Now, a new study from the University of Michigan suggests that smaller Hall thrusters could generate much more thrust, potentially making them candidates for interplanetary missions.

“People previously thought you could only push a certain amount of power through a thruster area, which in turn directly translates into how much force or thrust you can generate per unit area,” said Benjamin Jorns, an associate professor of aerospace engineering at the UM. led the new Hall thruster study being presented today at the AIAA SciTech Forum in National Harbor, Maryland.

His team challenged this limit by running a Hall thruster from 9 kilowatts to 45 kilowatts, maintaining about 80% of its rated efficiency. This increased the amount of force generated per unit area by nearly a factor of 10.

Whether we call it a plasma thruster or an ion propulsion, electric propulsion is our top choice for interplanetary travel, but the science is at a crossroads. While Hall thrusters are a proven technology, an alternative concept known as a magnetoplasma thruster promises to pack much more power into smaller engines. However, they are unproven in many respects, including longevity.

It was believed that Hall thrusters could not compete because of the way they operate. The propellant, usually a noble gas such as xenon, moves through a cylindrical channel where it is accelerated by a powerful electric field. It generates thrust in the forward direction as it departs aft. But before the propellant can be accelerated, it must lose some electrons to give it a positive charge.

Electrons accelerated by a magnetic field to ring around that channel — described as a “buzz saw” by Jorns — knock electrons off the propellant atoms, turning them into positively charged ions. However, calculations suggested that if a Hall thruster tried to force more propellant through the engine, the electrons whizzing into a ring would be knocked out of formation, breaking down that “buzz saw” function.

“It’s like trying to bite off more than you can chew,” Jorns said. “The circular saw can’t cut its way through that much material.”

In addition, the engine would become extremely hot. Jorns’ team put these beliefs to the test.

“We named our thruster the H9 MUSCLE because we essentially took the H9 thruster and turned it into a muscle car by cranking it to ’11’ – really up to a hundred, if we scaled accurately,” said Leanne Su, a PhD student in aerospace engineering who will present the study.

They tackled the heat problem by cooling it with water, which allowed them to see how big of a problem the circular saw would break. It turned out not to be much trouble. Running with xenon, the conventional propellant, the H9 MUSCLE ran up to 37.5 kilowatts, with an overall efficiency of about 49%, not far off the 62% efficiency at the 9-kilowatt design power.

Running with krypton, a lighter gas, they reached their maximum power supply at 45 kilowatts. With an overall efficiency of 51%, they reached their maximum thrust of about 1.8 Newtons, comparable to the much larger 100-kilowatt X3 Hall thruster.

Plasma thrusters used on satellites can be much more powerful

Ph.D student Will Hurley leaves the room where the new Hall Plasma Thruster is being tested in the PEPL lab. Credit: Marcin Szczepanski/Michigan Engineering

“This is a bit of a crazy result because krypton typically underperforms xenon on Hall thrusters. So it’s very cool and an interesting path to see that we can actually improve the performance of krypton over xenon by increasing the current density of the thruster,” Su said.

Nested Hall thrusters like the X3, also partly developed at UM, have been studied for interplanetary cargo transport, but they are much larger and heavier, making them difficult to transport people. Now regular Hall thrusters are back on the table for crewed travel.

Jorns says the cooling problem needs a space-worthy solution if Hall thrusters are to operate at these high powers. Still, he’s optimistic that individual thrusters could run at 100 to 200 kilowatts, arranged in arrays that deliver megawatts of thrust. This could enable crewed missions to reach Mars, even on the far side of the sun, over a distance of 250 million miles.

The team hopes to pursue both the cooling problem and the challenges in developing both Hall thrusters and magnetoplasma dynamic thrusters on Earth, where few facilities can test Mars-mission-level thrusters. The amount of propellant coming out of the thruster comes too fast for the vacuum pumps to keep the test chamber conditions spacious.

More information:
Leanne L. Su et al, Operation and Performance of a Magnetically Shielded Hall Thruster at Ultra-High Current Densities, AIAA SCITECH 2023 Forum (2023). DOI: 10.2514/6.2023-0842

Offered by the University of Michigan

Quote: Plasma thrusters used on satellites may be much more powerful than previously believed (2023, January 24) Retrieved January 25, 2023 from powerful-previously.html

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