Plasma Thrusters: Smaller Hall Thrusters in Interplanetary Missions
It was previously thought that Hall thrusters, which are a widely used and efficient form of electric propulsion in orbit, required large sizes in order to produce a significant amount of thrust. However, a new study from the University of Michigan suggests that smaller Hall thrusters may be able to generate even more thrust, making them a viable option for interplanetary missions.
According to Benjamin Jorns, an associate professor of aerospace engineering at the University of Michigan, it was believed that the amount of current that can be pushed through a thruster is directly related to the amount of thrust that can be generated per unit area. However, his team challenged this limit by increasing the current running through a 9 kilowatt Hall thruster to 45 kilowatts while maintaining a high level of efficiency. This resulted in an increase of the force generated per unit area by almost a factor of 10.
Electric propulsion, whether referred to as a plasma thruster or an ion drive, is considered the best option for interplanetary travel. However, there is an alternative concept known as a magnetoplasmadynamic thruster that has the potential to produce more power in smaller engines. However, this concept is not yet proven in many ways, including its lifetime.
Hall thrusters were previously believed to be less competitive due to the way they operate. The propellant, typically a noble gas like xenon, moves through a cylindrical channel where it is accelerated by a powerful electric field, generating thrust in the forward direction as it departs out the back. However, before the propellant can be accelerated, it needs to lose some electrons to give it a positive charge.
The fifth paragraph explains that in order for Hall thrusters to work, electrons are accelerated by a magnetic field to run in a ring around the channel, which is described as a “buzz saw” by Jorns. This process knocks electrons off the propellant atoms and turns them into positively charged ions. However, it was believed that if a Hall thruster tried to drive more propellant through the engine, the electrons running in the ring would get knocked out of formation and the “buzz saw” function would break down.
Jorns compares this problem to “trying to bite off more than you can chew” and states that the “buzz saw” would not be able to work through that much material. Additionally, the engine would become extremely hot.
Jorns’ team put these beliefs to the test by creating a thruster named the H9 MUSCLE, which they describe as taking the H9 thruster and making a muscle car out of it by turning it up to a much higher power. They cooled it with water in order to test if the “buzz saw” breakdown would be a problem.
They found that running the H9 MUSCLE with xenon, the conventional propellant, it could run up to 37.5 kilowatts with an overall efficiency of about 49%. This is not far off from the 62% efficiency at its design power of 9 kilowatts. When running with krypton, a lighter gas, they maxed out their power supply at 45 kilowatts. At an overall efficiency of 51%, they achieved their maximum thrust of about 1.8 Newtons, which is on par with the much larger 100-kilowatt-class X3 Hall thruster.
The ninth paragraph states that the team found that krypton performed better than expected when the thruster current density was increased, which is a surprising and interesting result.
The tenth paragraph notes that while nested Hall thrusters like the X3 have been considered for interplanetary cargo transport, they are much larger and heavier, making them difficult to use for transporting humans. However, the new findings suggest that ordinary Hall thrusters may now be a viable option for crewed journeys.
Jorns points out that the cooling problem would need to be addressed before Hall thrusters could be used for space travel. However, he believes that individual thrusters could run at 100 to 200 kilowatts and arranged into arrays that provide a megawatt’s worth of thrust. This could enable crewed missions to reach Mars even on the far side of the sun, traveling a distance of 250 million miles.
The final paragraph mentions that the team hopes to address the cooling problem and also work on developing both Hall thrusters and magnetoplasmadynamic thrusters on Earth, but they face challenges due to the lack of facilities that can test thrusters at the level required for Mars missions. The exhaust of propellant from the thruster is too fast for the vacuum pumps to keep the testing conditions space-like.
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