It was thought that running more propellants through a Hall thruster would destroy its efficiency, but new experiments suggest it could power a manned mission to Mars – ScienceDaily

He thought that Hall thrusters, an efficient type of electric propulsion widely used in orbit, had to be large to produce a lot of thrust. Now, a new study from the University of Michigan indicates that smaller Hall thrusters can generate much more thrust — making them candidates for interplanetary missions.

“People previously believed that you could only push a certain amount of current through the area of ​​the thruster, which in turn translates directly to the amount of force or thrust you could generate per unit area,” said Benjamin Jorns, associate professor of aerospace engineering at UM. Hall led a new study that will be presented at the AIAA SciTech Forum in National Harbor, Md., today.

His team defied this limit by running a 9-kW motor at up to 45 kW, while maintaining nearly 80% of its nominal efficiency. This increased the amount of force generated per unit area by a factor of approximately 10.

Whether we call it a plasma propellant or an ion drive, electric propulsion is our best bet for interplanetary travel — but science is at a crossroads. While Hall thrusters are a well-proven technology, an alternative concept, known as the Magnetoplasmadynamic thruster, promises to pack more power into smaller thrusters. However, it has not yet been proven in many ways, including for life.

He believes that hall pushers are not competitive because of the way they work. The propellant, a noble gas such as xenon, moves through a cylindrical channel where it is accelerated by a strong electric field. Thrust is generated in the forward direction as it exits from behind. But before the propellant can be accelerated, it needs to lose some electrons to give it a positive charge.

The electrons are accelerated by a magnetic field to run in a loop around the channel – which Jorns described as a “buzzsaw” – that knocks electrons off the propellant atoms and turns them into positively charged ions. However, calculations indicated that if the Hall impulse tried to force more fuel through the engine, the electrons scurrying around in a loop would break out of the configuration, breaking down the “buzzsaw” function.

“It’s like trying to bite off more than you can chew,” Yourns said. “A buzz saw couldn’t force its way through that much material.”

In addition, the engine will get very hot. The Jorns team put these beliefs to the test.

“We named our engine the H9 MUSCLE because basically, we took the H9 engine and made a muscle car out of it by turning it into 11 — really up to a hundred, if we’re going by exact analogy,” said Lian Su, a doctoral student in aerospace engineering who will present the study. .

They addressed the heat problem by cooling it with water, which allowed them to see how big of a problem a buzzsaw breakdown would be. Turns out nothing was too much trouble. The H9 MUSCLE was powered by xenon, conventional fuel, up to 37.5 kW, with an overall efficiency of about 49%, not far from the 62% efficiency at its 9 kW design power.

Using krypton, a lighter gas, they increased their power supply at 45 kilowatts. With an overall efficiency of 51%, they achieve a maximum thrust of around 1.8 N, on par with the larger X3 Hall engine in the 100 kW class.

“This is a rather crazy result because typically krypton performs much worse than xenon on Hall engines. 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 impulse current density,” Su said.

Telescopic thrusters such as the X3 — developed in part by UM — have been explored for interplanetary cargo transport, but they are much larger and heavier, making it difficult for them to transport humans. Now, regular Hall engines are back on the table for manned flights.

Jorns says the cooling problem will need a space-worthy solution if Hall thrusters can operate at such high powers. However, he is optimistic that individual thrusters can run from 100 to 200 kilowatts, arranged in arrays that provide megawatts of thrust. This could enable manned missions to reach Mars even on the far side of the sun, traveling 250 million miles.

The team hopes to pursue the cooling issue as well as the challenges in developing both Hall thrusters and magnetodynamic thrusters on Earth, since few facilities can test thrusters at the scale of a Mars mission. The amount of spent fuel comes from the propellant too quickly for the vacuum pumps to maintain conditions inside the test chamber like space.

The research was supported in part by the Joint Propulsion Institute.

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