Almost all satellites have some kind of thrusters aboard, but they tend to use them as little as possible to conserve chemical fuel. Reaction wheels are one way to make orientation adjustments without running the thrusters, and [Zachary Tong]’s liquid metal reaction wheel greatly simplifies the conventional design.
Reaction wheels are basically flywheels. When a spacecraft spins one, conservation of angular momentum means that the wheel applies an equal and opposite torque to the spacecraft, letting the spacecraft orient itself. The liquid-metal reaction wheel uses this same principle, but uses a loop of liquid metal instead of a wheel, and uses a magnetohydrodynamic drive to propel the metal around the loop.
[Zach] built two reaction wheels using Galinstan as their liquid metal, which avoided the toxicity of a more obvious liquid metal. Unfortunately, the oxide skin that Galinstan forms did make it harder to visualize the metal’s motion. He managed to get some good video, but a clearer test was their ability to produce torque. Both iterations produced a noticeable response when hung from a string and activated, and achieved somewhat better results when mounted on a 3D-printed air bearing.
Currently, efficiency is the main limitation of [Zach]’s motors: he estimates that the second model produced 6.2 milli-newton meters of torque, but at the cost of drawing 22 watts. The liquid metal is highly conductive, so the magnetohydrodynamic drive takes high current at low voltage, which is inconvenient for a spacecraft to supply. Nevertheless, considering how hard it is to create reliable, long-lasting reaction wheels the conventional way, the greatly improved resilience of liquid-metal reaction wheels might eventually be worthwhile.
If you’re curious for a deeper look at magnetohydrodynamic drives, we’ve covered them before. We’ve also seen [Zach]’s earlier experiments with Galinstan.
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