Plasma Thrusters Ran at 500% Beyond Old Power Limits

Benjamin Jorns, U-M associate professor of aerospace engineering led a new Hall thruster study that operated a Hall Plasma thruster at five times old power limits.

His team challenged ran a 9 kilowatt-rated Hall thruster at up to 45 kilowatts while maintaining roughly 80% of its nominal efficiency. This increased the amount of force generated per unit area by almost a factor of 10.

Hall thrusters are a well-proven technology. Previously, making larger and more powerful Hall thrusters would lose any higher power benefits from the higher mass. It turns out the old limit was not physics-based limits but engineering issues.

Now super-powered Hall Thrusters have a path to be made light and powerful enough to power high ISP megawatt propulsion.

Hall thrusters are able to accelerate their exhaust to speeds between 10 and 80 km/s (1,000–8,000 s specific impulse), with most models operating between 15 and 30 km/s. Old devices operated at 1.35 kW produce about 83 mN of thrust. High-power models have demonstrated up to 5.4 N in the laboratory. Power levels up to 100 kW have been demonstrated for xenon Hall thrusters. The new systems could achieve 40-100 newtons of power at megawatt power levels with up to 8000 ISP (80 km/second).

There will still be a need to create megawatts of light solar panels or megawatt-class space vehicle-class nuclear power sources.

As of 2009, Hall-effect thrusters ranged in input power levels from 1.35 to 10 kilowatts and had exhaust velocities of 10–50 kilometers per second, with thrust of 40–600 millinewtons and efficiency in the range of 45–60 percent.

Magnetoplasmadynamic (MPD) thrusters were expected to have much more power into smaller engines. However, MPD are unproven in many ways, including lifetime. Hall thrusters were believed to be unable to compete because of 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. It generates thrust in the forward direction as it departs out the back. But before the propellant can be accelerated, it needs to lose some electrons to give it a positive charge.

Running with xenon, the conventional propellant, the H9 MUSCLE ran up to 37.5 kilowatts, with an overall efficiency of about 49%, not far off the 62% efficiency at its design power of 9 kilowatts.

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, on par with the much larger 100-kilowatt-class X3 Hall thruster.

They will need to make the cooling system work in space at these high powers. They are optimistic that individual thrusters could run at 100 to 200 kilowatts, arranged into arrays that will 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.

Mission Tables for the Solar System

Projectrho has mission times for high power short duration acceleration rockets (like chemical rockets) which are in the impulse columns and mission times for more constant acceleration.

Megawatt hall thrusters could have mission profiles looking more like the 0.01g more constant acceleration profiles.

There will still be challenges to keep all systems on vehicles powerful and lightweight for maximum performance. Travel times to the closest Mars approach could be in the 30-50 day one-way trip range. A close Mars approach is about 50 million miles from Earth. Going to Mars at any orbital alignment would be about 250 million miles.

Aerospace Research Central – Operation and Performance of a Magnetically Shielded Hall Thruster at Ultrahigh Current Densities.

Abstract:

The performance of a 9-kW class magnetically shielded Hall thruster is characterized at 300 V discharge voltage for a channel current density a factor of ten greater than the nominal current density associated with its 300 V and 4.5 kW condition. An inverted pendulum thrust stand and a far-field probe suite are employed to measure the global performance and efficiency modes respectively. It is found that when operating on xenon, thruster anode efficiencies range from 52.9 ±0.8% to 62.2 ±2.2% over the power range of 4.5–37.5 kW at 300 V. Anode efficiencies for krypton span from 48.5 ±7.0% to 56.4 ±1.0% at 4.5–45 kW at 300 V. The thrust and specific impulse are found to be 1650 ±10 mN and 2309 ±17 s respectively at 37.5 kW for xenon and 1839 ±10 mN and 2567 ±16 s for 45 kW on krypton. The thrust density at the maximum power setting of 45 kW is shown to be ∼7× higher than its nominal 4.5 kW condition. It is also demonstrated that the thruster can achieve thrust densities and thrust-to-power ratios on par with or even greater than applied-field magnetoplasmadynamic thrusters in the sub-50 kW power range. These results are discussed in the context of Hall thruster theory, conventional scaling laws for the maximum achievable current density in these devices, and challenges with high power thruster testing. The implications of the demonstrated ability to achieve atypically high current densities with minimal performance reduction for high power electric propulsion development are also examined.

16 thoughts on “Plasma Thrusters Ran at 500% Beyond Old Power Limits”

  1. If laser beams have a physical or kinetic PUSH, an optical laser phenomenon called “Repercussion “, which was discovered by DARPA in the 1960s, when they experimented with chemically powered laser rifles, which would knock the laser rifle out of the soldier’s hands, when the laser hit a mirror or reflective surface, then, would it be technically possible to retrofit some kind of optical laser device around the business end of this ionized Xenon gas rocket, that contains the blast inside a mile-long cone of ‘Repercussion’ laser light, so that, instead of the Xenon gas’s push being dispersed sideways by the vacuum of outer space, its in a very long, straight line action/reaction tapering cone formation, to make this ‘rocket’ have greater speed and fuel mileage?

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  2. How about a direct comparison to say, the VASIMR powerplant, in terms of power in, output, and estimated time savings to Mars (or any other location), and other interesting metrics.

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  3. Need to significantly ramp up Xe production in order to use these for interplanetary travel. Some quick googling shows NASA Dawn is using 0.4t of Xenon to propel a 0.7t vehicle to Ceres. Starship Heavy is 85t with a 150t payload (to LEO), using Hall thrusters to move an equivalant craft to higher orbits or out into the solar system will require alot more fuel. Sadly, I have neither the background or time to do the caluclations on exactly how much. And as of 2008 – the world was only producing about 60t of Xe/year.

    But it looks like it’s time for the engineers to get to work, the research boffins have demonstrated it’s possible.

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    • I note that 21% of fissions produce a stable xenon isotope and the unstable xenon isotopes have half-lives of no more than a few days. Xenon can easily be a by-product of processing used nuclear fuel to recover the fissionable material.

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        • You’d have to pump Xenon-135 into some kind of chamber with a neutron source to generate Xe-136, which isn’t “stable” but would work for these practices. Sounds like a nightmare to me, but I’m not a nuclear engineer.

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    • Tongue in cheek? Thought so!
      And I agree.

      Strictly though, it all depends on the exhaust velocity of the ion stream produced by the thruster. If say the plasma plume is zipping along at 100 km/s, each kilogram of it has ½MV² or 0.5 × 1 × (100×10³)² = 5×10⁹ joules of kinetic energy. Nothing is free, so at 50% efficiency, it will have taken 10×10⁹ joules of energy input. The propulsive force however is only (mV) or 1 × 100,000 = 100,000 newton-seconds. If this is accomplished at the ‘megawatt’ level, then all that become 10×10⁹ J ÷ 10⁶ W = 10,000 seconds … and in turn the impulse is 100,000 Ns ÷ 10,000 s = 10 newtons.

      So, 10 newtons per megawatt.

      One not unreasonably might assume that a 1 megawatt (continuous!) energy source, very likely a nuclear reactor with all its whizbang mechanics (but none! of the shielding) would weigh at LEAST 10 tons. Assuming (again reasonably) that 50% of the unpropelled mass is going to be propellant once fueled up, and that the whole thing weighs 100 tons fueled, instrumented, computers, reactor, and every bit of everything else, well … let’s see.

      Tsiolkovsky’s ‘rocket equation’ is ΔV = Ve ln( mi / me ) or the exhaust velocity times the logarithm of the initial-mass divided by the end-mass. That would be 100,000 m/s • ln( 100 t / (100 – 45) t ) = 147 km/s. Moreover, it takes 45,000 kg × 10,000 sec/kg = 450,000,000 sec or 14.3 YEARS to chuck up to that speed. And 147 km/s is 4.67×10¹² m/year = 30.9 AU/year. Fast! Highly attractive! Just have to have enough nuclear material on board to keep pushing for 14.3 years.

      As you can see, it’d take a LOT more power than 1 megawatt to get ‘up to speed’ faster. Like maybe 15x as much, to have the acceleration/deceleration phases last only ½ year each side. Now we’re talking a bigger reactor. More fraction of the overall mass to it. Lower (less helpful) ratio of final-to-initial masses. Lower overall speed. Maybe only 10 AU/year, assuming a thrust-drift-drift-drift-antithrust profile to get someplace juicy like Pluto or one of the larger Kuiper Belt Objects.

      Just saying… GoatGuy

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      • Need solid state power conversion system (like an RTG) to have it work for 15-years. What is the go-to thermoelectric junction for RTG? If you could get 5% thermoelectric efficiency then 10 tons of oxide fuel pins in NaK heat pipe(s) could be a reasonable solution to get 1MWe for 15 years. If you could be clever and confident about dealing with swelling/distortion maybe you could do it with half that fuel mass.

        Why does every space problem look like trying to get across the Atlantic in a canoe?

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      • Now I have been paying attention to phys.org a lot recently…and there is a new alloy that gets tougher as it gets colder:
        https://www.sciencealert.com/this-alloy-is-the-toughest-known-material-on-earth-and-it-gets-tougher-in-the-cold/amp

        Now…could you modify this to serve as a structural battery?

        It starts life as an NTR…tankage and frame acting as a battery—-charging up as the hydrogen empties…turbopump also a flywheel.

        That gives you a good thump out of Earth-Moon. The now lighter craft then stretches its legs as an NEP with RTGs that slide down by the hot reactor for supercharging as it were.

        Once the structural battery and NEP bits are used up—they stage…leaving a hot core that perhaps can become a fission fragment rocket at the last—and the core payload ahead of that…

        Nuclear staging….doable?

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