Hall thrusters are a form of electronic propulsion known as ion thrusters being developed for satellites and spacecraft. Ion thrusters work by accelerating ionized particles of gas like xenon using a magnetic field. They have conventionally been used to adjust the orbit of satellites in Earth orbit. A new study has just been published that claims they could also be scaled up for interplanetary travel like a crewed mission to Mars. Until now, the use of ion thrusters for such missions has been considered unlikely.
The belief has been that Hall thrusters can not drive enough propellant atoms at smaller sizes. In other works, ion thrusters are fairly weak. Getter more power out of them would require a larger Hall thruster too big to fit on crewed spacecraft.
Benjamin Jorns is an associate professor of aerospace engineering at the University of Michigan and the author of the new study. He issued a statement that said, "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.”
The thruster bottleneck arises from a function that Jorns calls a buzz saw surrounding the channel that the propellant atoms are driven through. That saw turns those atoms into positively charged ions that produce thrust. However, if anything more than a small amount of current is used, the buzz saw falls apart, leaving a useless neutral gas. It also overheats the engine. Jorns said that, “It's like trying to bite off more than you can chew. The buzz saw can't work its way through that much material.”
However, Jorns refused to accept that common assumption. He and his team simply souped up a xenon-powered Hall thruster by about a hundred times. They attempted to cool it with water. They were surprised to find that the thruster still operated at forty nine percent efficiency and generated up to thirty seven kilowatts. This can be compared to the original efficiency of sixty two percent when operating at only a paltry nine kilowatts.
When went to the lighter noble gas krypton as a propellant, they were able to generate forty five kilowatts with an even greater efficiency of fifty one percent. This configuration produced one and eight-tenths Newtons of thrust. This is not far below the most power Hall thruster in the world, the X3, which is much larger than the Jorns test rig.
Leanne Su is an aerospace engineer at the University of Michigan. She said in the statement from Jorns’ team, “This is kind of a crazy result because, typically, krypton performs a lot worse than xenon in Hall thrusters. 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.”
The findings of the Jorns team show that it may be possible to use smaller Hall thrusters for crewed spacecraft in the future. According to Jorns, crews could reach Mars or even carry out missions to the far side of the Sun using a array of ion thrusters that produce about a megawatt’s worth of thrust.
The next hurdle, Jorns said, was to figure out how to cool ion thrusters in space. This is difficult because the lack of atmosphere makes it difficult to exhaust generated heat.