Test Suggests Ion Thrusters Could Power Crewed Interplanetary Missions

A form of electric propulsion known as Hall thrusters — a type of ion thruster — may actually pack more bang for the buck than expected.

Hall thrusters have conventionally been used to adjust the orbit of satellites. But according to a new study, they could also be scaled up for interplanetary commutes like a crewed mission to Mars, something that was considered unlikely until now.

The belief so far has been that Hall thrusters — which work by accelerating ionized particles of gas like xenon using a magnetic field — can't drive enough propellant atoms at smaller sizes. In other words, they're fairly weak, and getting more power out of them would require a larger Hall thruster too impractical to fit on crewed spacecraft.

"People had previously thought that you could only push a certain amount of current through a thruster area, which in turn translates directly into how much force or thrust you can generate per unit area," explained study author Benjamin Jorns, an associate professor of aerospace engineering at the University of Michigan, in a statement.

The bottleneck arises from a function that Jorns calls a "buzz saw" surrounding the channel that the propellant atoms are driven through. That saw is needed to turn those atoms into positively charged ions that produce thrust.

But anything more than the small amounts currently used — and the buzz saw falls apart, leaving you with a useless neutral gas, in addition to overheating the engine.

"It's like trying to bite off more than you can chew," Jorns elaborated. "The buzz saw can't work its way through that much material."

Jorns didn't accept that common thinking, however. He and his team simply souped up a xenon-powered Hall thruster by about a hundred times and tried cooling it with water. Surprisingly, they found that it still operated at 49 percent efficiency and outputted up to 37.5 kilowatts, compared to its original efficiency of 62 percent when operating at only a measly nine kilowatts.

This time, using the lighter noble gas krypton as a propellant, they were able to reach 45 kilowatts, with an even greater efficiency of 51 percent, while producing 1.8 Newtons of thrust — not far off from the most powerful Hall thruster in the world, the X3, which is far larger. That's certainly punching above its weight.

"This is kind of a crazy result because typically, krypton performs a lot worse than xenon on Hall thrusters," said Leanne Su, an aerospace engineer at the University of Michigan, in the statement.

"So it's very cool and an interesting path forward to see that we can actually improve krypton's performance relative to xenon by increasing the thruster current density."

Their findings show that it may be possible to use smaller Hall thrusters for crewed spacecraft in the future, as large ones don't leave much room for their passengers. According to Jorns, crews could reach Mars or even the far side of the Sun using an array of thrusters that produce about a megawatt's worth of thrust.

But the next hurdle, Jorns said, is figuring out how to cool them in space — which is a lot harder, given the lack of atmosphere for exhausting generated heat.

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