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.
I’m going to preface this comment with the fact that this is neat experimentation and all science starts with someone just asking a question.
This metal is ill suited for a reaction wheel because it freezes at -19C. Temperatures in LEO can get as low as -65C. MEO is worse. The wattage budget will be higher than 22 just keeping the metal liquid.
Still super cool stuff. (Pardon the unintended pun.)
Also, friction is going to be high
Agreed. Maybe not friction strictly speaking, but the high viscosity is going to result in low efficiency in an application where you probably don’t want to waste energy.
Agreed! It seem only CsNaK might be suited for the job with a melting point of −78.2 °C but I have no idea if it’s ferrous. It also has a nasty problem of being reactive with both air and water. Humanity will need to find a much better metal alloy before this concept can seriously be considered.
“Agreed! It seem only CsNaK might be suited for the job with a melting point of −78.2 °C but I have no idea if it’s ferrous.”
Then chemistry might not be the field for you. I hear the Post Office has a special program for people who aren’t sure about the “ferrousness” of CsNaK. Some people, less tolerant than me might take you to task for not detecting the absence of the letter “F”. This is why we can’t have nice things.
and why would you need ferrousness? it only needs to be conductive for MHD… there’s not a whole lot of F in sea waters for exemple…
I understand some people have difficulty understanding how another person might be thinking. It’s okay, I’ve got you covered: they likely meant paramagnetic when they said ferrous.
Often, if you’re confronted by something that seems obviously incredibly silly, it’s down to a less silly underlying misunderstanding. By considering before you react rashly, you can be a better, more helpful person. :)
Why? Because “ferrous” and “paramagnetic” are commonly confused? No sale.
“By considering before you react rashly, you can be a better, more helpful person. :)” Why? Because Billy Mumy is gonna materialize from the Twilight Zone and wish me into the corn?
“Often, if you’re confronted by something that seems obviously incredibly silly, it’s down to a less silly
underlying misunderstanding.” Nope, no soap. It’s developmentally disabled turtles all the way down.
I agree, and that’s actually how I read it initially. Jumping to conclusions on the net and then trying to pretend you had a good point really hurts though, so they aren’t going to accept it.
After all, the only reason they posted was to gotcha another user to make themselves feel good. Even a pointed question would have been more reasonable.
Alone, I would also point out that ferromagnetic materials (those able to be magnetised, in contrast to paramagnetic, those attracted to a magnetic field) need not contain iron. That seemed less apt in this context but it seems like you might be unaware.
S O, thank you.
I suspect that the energy budget for a mechanical reaction wheel will always be about half of what the liquid metal device would be, all other things being equal, because of the momentum storage in the spinning wheel when you are done changing your attitude.
You can move angular momentum into and out of a solid wheel, but moving liquid metal cannot energy with rotation because of turbulence, unless it is a superfluid liquid metal. When you are removing angular momentum from a reaction wheel, you can still use it for controlling orientation, only in the opposite sense.
Reaction wheel longevity has been greatly improved after the discovery of the effect of geomagnetic storms on metal balls and races, but it’s not like you can replace a the average reaction wheel once it has been deployed…
Could a ferrofluid be used instead of a liquid metal?
It would need to be conductive but if iron particles could be suspended in a conductive fluid, that could improve the density of the moving mass.
The key misconception is that the working fluid in MHD needs to be paramagnetic. It’s the flow of current that generates a magnetic field in the fluid.
Ah yes, good old magic motors!
Consider how momentum is transferred: EMF is used to spin up the wheel, moving it from the stator, connected to the satellite, to the free-spinning wheel. Friction (hopefully slowly) then reverses the process, transferring momentum, in the opposite direction, to the satellite. Fluid friction, not turbulence, makes the MHD a poor choice for a reaction wheel. However, turbulence does reduce efficiency, as well.
Space-tolerant bearings for reaction wheels have been studied for decades now, and AFAIK they are pretty good and their lifetime can be predicted well.
It seems to me that magnetohydrodynamic motors always need the electrode contact, which will necessarily have friction and lower the efficiency.
Magnetic bearing(even electronically controlled) for solid flywheel… People, it is old tech.
Impressive the motor, and even more impressive coming up with an application for a motor with a rotor you cannot really attach anything to.