Throttling Up Ion Thruster Technologies

Although I want to start the week by looking at hybrid propulsion technologies, let’s start by considering developments in ion propulsion before going on to see how they can be adapted for future deep space missions. Hall thrusters are a type of ion engine that uses electric and magnetic fields to manipulate inert gas propellants like xenon. The electric field turns the propellant into a charged plasma which is then accelerated by means of a magnetic field [see reader ‘Supernaut’s correction to this in the comments below].

We’ve seen the benefits of ion engines in missions like Dawn, which has recently received a second mission extension to continue its work around the asteroid Ceres. Now we learn that a Hall thruster called X3, developed at the University of Michigan by Alec Gallimore, has received continuing design modification by researchers at NASA Glenn and the US Air Force, scaling up the low thrust levels produced by conventional ion engines. A series of tests have demonstrated results that have implications for future manned space missions.

In fact, the X3 breaks all previous Hall thruster records, producing 5.4 newtons of force compared with the earlier 3.3 newtons. As noted in this University of Michigan news release, the X3 design also doubles the operating current record, reaching 250 amperes vs. 112 amperes, while running at slightly higher power levels than previous designs. A Hall thruster with higher power and improved thrust levels could shorten travel times, a significant factor as we work to mitigate radiation problems for human crews on long interplanetary missions.

At Glenn Research Center, doctoral student Scott Hall (University of Michigan) worked with NASA’s Hani Kamhawi on experiments to test the improved thruster, using the only vacuum chamber in the United States large enough to cope with the X3’s exhaust, even though the sheer amount of xenon can still cause some of it to drift back into the plasma plume, affecting the results. An upgraded vacuum chamber at the University of Michigan should be ready in early 2018.

Image: A side shot of the X3 firing at 50 kilowatts. Credit: NASA.

The work at Glenn involved four weeks to set up the thrust stand, mount and connect the thruster with propellant and power supplies, deploying a custom thrust stand to bear the X3’s weight. 25 days of testing then produced the results above in a project funded by NASA’s Next Space Technologies for Exploration Partnership. The next step here will be to integrate the X3 with power supplies now being developed by Aerojet Rocketdyne. By the spring of next year, new tests at NASA Glenn using the Aerojet Rocketdyne power processing system are expected.

Image: The X3 nested-channel Hall thruster with all three channels firing at 30 kW total discharge power. Credit: University of Michigan.

These developments in current Hall thruster technology are exciting in themselves and have implications for the near-term in missions to destinations like Mars. But I’m also interested in pursuing how we might move ion technologies in new directions by creating hybrid designs, with Kuiper Belt objects and the gravitational focus at 550 AU as potential destinations. With laser methods now in the spotlight as Breakthrough Starshot continues its analysis of a mission to Proxima Centauri, hybrid ion engine designs boosted by laser power are coming into consideration. I’ll take a look at the possibilities in tomorrow’s post.