A little misleasing, as technically, this "motor" will not do anything to the satellite course - but just change its orientation. It's "only" a new spin (SCNR) on gyroscopes. Interesting nonetheless, even though magnetic bearings aren't that new either as a concept. Wish they'd write what's actually new about them.
Satellites (and other spacecraft for that matter) need to change orientation before they fire thrusters, so the original title was not really inaccurate.
There's also Passive Magnetic Attitude Control System (PMACS) that uses magnets, and does away with motors and reaction wheels. I guess PMACS is good enough for most of the nano/pico/femto satellites. It was recently used in an Indian student satellite "Swayam"[0].
Don't think you need one. The motor isn't actually driving anything directly - it's using conservation of angular momentum to cause the spacecraft to spin in the opposite direction to the motor. No output shaft required.
I can't quite tell, I think this is describing a miniaturised momentum wheel [1], but I had the impression that control moment gyroscopes [2] where generally preferred.
You aren't going to have much agility with torque rods. A small satellite can move a few degrees per second with reaction wheels. A large spacecraft needs CMGs to get the same slew rate.
There is a growing market for small spacecraft to be able to point very accuratley (as in 10 arcseconds or so). Blue Canyon Technologies XACT is attitude control in a box for a Cube Sat - star tracker, reaction wheels, torque rods, magnetometer, and gyro. This enables lots of imaging applications.
Correct me if im wrong, but I would assume that the motor must still be 'attached' somewhere, else the momentum would not translate into moving the satellite. (Wouldnt it be akin to having a gyroscope just floating inside of a satellite- Suddenly stopping it would have no effect on the satellite since it would not be actually attached- ala, the fly in a car scenario) - So, with that, if it is indeed attached somehow, wouldn't the friction component still be at play?
But how have you stopped that gyroscope? By providing a force (through electromagnetism). The force exerted to slow the gyroscope also speeds up the satellite, due to conservation of angular momentum.
The angular momentum can be transferred through the magnetic coupling.
What I'm curious about is how this system deals with 'saturation' (at some point, you can't spin it any faster...). "Real" satellites do use scaled up versions of this, but they use it to save fuel, not volume, as you still need attitude control thrusters to "desaturate" the reaction wheels from time to time. Having both systems works fine on a large satellite bus, but I don't see it scaling down all that well.
Magnetotorquers are used for desaturation. They are not good for fast maneuvers, so you need the reaction wheels for that. Most small satellites are in low earth orbit with a magnetic field.
It requires a complex, expensive, sensitive, tuned control system with more parts and a decent amount of cpu power. Also, they're subject to axial translation. And it saves friction, not energy, so the benefit is maintenance, si it's only ptavtical in some apps. Still as the systems get more predictable they're finding their way into more applications...
Even if the rotor is held by magnetic fields, the centrifugal forces exerted on the rotor must be huge. I guess it is small enough, but I'm curious if it was a problem or not (CDs will crack if they spin too fast).
I worked in a company that makes pumps with magnetic bearings for high purity applications, i.e. semiconductor manufacturing. The impeller is a magnet completely coated in a really inert material, e.g. PTFE, so no metallic (bearings, shafts) parts touch the liquids.
Efficiency is pretty low with these types of bearings, we got like 60-80%. Not sure if this is ideal for a satellite.
It's always better to have more options. Now they can weigh the tradeoff of taking more power from the solar panels in exchange for size+weight reductions of the craft and possibly an increased service life. Another tool in the box. Electricity is the only thing a spacecraft can get more of once it's up there.
For those curious how a spinning motor can be used to orient a satellite, I'd suggest Scott Manley's video on reaction wheels in Kerbal Space Program (don't worry if you don't play KSP, the bulk of the video is about the real world applications).
I always assumed satellites and probes used small jets of hydrazine to orient themselves. I know fule capacity is limited but I just figured they used it sparingly.
It interesting to see how they actually do it. There is a video of a walking cube on the Web I suppose it's the same principal, it seems like magic.
I wonder if the magnets will interfere with sensors?
Typically, a spacecraft will have reaction wheels or CMGs to generate internal torques which let you change attitude by the conservation of angular momentum.
However, there are also external torques in the system. In low Earth orbit, you get atmospheric drag. Solar radiation pressure also affects spacecraft. To overcome this saturation or buildup of momentum in wheels, you need to generate an external torque. This can be done with torque rods that work against Earth's magnetic field. But such a strategy does not work in deep space with no magnetic field, so you would need to use thrusters instead.
You typically try to keep your nagnetometer far from torque rods and wheels. Voyager had a huge boom to keep the magnetometer far from the spacecraft I believe. Not sure how you get good separation at femtosat distances.
> I always assumed satellites and probes used small jets of hydrazine to orient themselves.
It's important to distinguish between a change in orbital velocity (how high the orbit is, how fast it is, and the plane it's orbiting in) and a change in orientation/attitude (where the satellite is pointing).
For changes in orbital velocity, some form of thrust is required. This can be as simple as pressurized gas being ejected from a nozzle without being burned. The thrust vector of this propulsion needs to pass through the center of mass of the spacecraft to avoid making it tumble. These propulsion events are quite rare, and many smaller satellites don't even carry the hardware necessary for them. They're launched into one orbit and stay there, with the exception of the tiny amount of atmospheric drag eventually causing the orbit to decay.
Changes in attitude are much more frequent. If the spacecraft is otherwise unoccupied, it might want to orient itself so its solar panels get maximum sun exposure and the batteries stay charged. If it's over a ground station for a communication pass, it will want to orient its antenna for maximum efficiency. And if it has cameras or other sensor packages, it might need to point them in a particular direction to achieve a mission objective.
Since those attitude changes are so frequent (several of them might take place in a single ~90 minute orbit), using up propellant to achieve them isn't really an option. Also, because the goal is rotation around the center of mass, it wouldn't be feasible to use the same thruster you use for orbit changes. You'd need 3 thrusters, orthogonal to each other and offset from the center of mass, plus all the accompanying plumbing.
So, it's much more common to have reaction wheels [0] and/or torque rods [1], which are electrically powered instead of consuming fuel. You typically need 3 of them, oriented in the X, Y, and Z axes, for full control. As spacecraft get smaller, this consumes a larger portion of its mass budget, which is why the small size of the reaction wheel described in the article is so important.
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[ 3.6 ms ] story [ 66.7 ms ] threadThe point is you dont need to waste fuel changing the orientation when the engines can only power/accelerate you "forward".
Im struck by two things. Firstly the gyroscopic means to change orientation is cool.
Secondly. I'm amazed they aren't using magnetic bearings already.
[0] http://www.coep.org.in/csat/subsystems/acs/
[1] https://en.wikipedia.org/wiki/Reaction_wheel
[2] https://en.wikipedia.org/wiki/Control_moment_gyroscope
What I'm curious about is how this system deals with 'saturation' (at some point, you can't spin it any faster...). "Real" satellites do use scaled up versions of this, but they use it to save fuel, not volume, as you still need attitude control thrusters to "desaturate" the reaction wheels from time to time. Having both systems works fine on a large satellite bus, but I don't see it scaling down all that well.
http://www.treehugger.com/renewable-energy/the-worlds-first-...
https://www.youtube.com/watch?v=7Js5x4NhUxU
It interesting to see how they actually do it. There is a video of a walking cube on the Web I suppose it's the same principal, it seems like magic.
I wonder if the magnets will interfere with sensors?
However, there are also external torques in the system. In low Earth orbit, you get atmospheric drag. Solar radiation pressure also affects spacecraft. To overcome this saturation or buildup of momentum in wheels, you need to generate an external torque. This can be done with torque rods that work against Earth's magnetic field. But such a strategy does not work in deep space with no magnetic field, so you would need to use thrusters instead.
You typically try to keep your nagnetometer far from torque rods and wheels. Voyager had a huge boom to keep the magnetometer far from the spacecraft I believe. Not sure how you get good separation at femtosat distances.
It's important to distinguish between a change in orbital velocity (how high the orbit is, how fast it is, and the plane it's orbiting in) and a change in orientation/attitude (where the satellite is pointing).
For changes in orbital velocity, some form of thrust is required. This can be as simple as pressurized gas being ejected from a nozzle without being burned. The thrust vector of this propulsion needs to pass through the center of mass of the spacecraft to avoid making it tumble. These propulsion events are quite rare, and many smaller satellites don't even carry the hardware necessary for them. They're launched into one orbit and stay there, with the exception of the tiny amount of atmospheric drag eventually causing the orbit to decay.
Changes in attitude are much more frequent. If the spacecraft is otherwise unoccupied, it might want to orient itself so its solar panels get maximum sun exposure and the batteries stay charged. If it's over a ground station for a communication pass, it will want to orient its antenna for maximum efficiency. And if it has cameras or other sensor packages, it might need to point them in a particular direction to achieve a mission objective.
Since those attitude changes are so frequent (several of them might take place in a single ~90 minute orbit), using up propellant to achieve them isn't really an option. Also, because the goal is rotation around the center of mass, it wouldn't be feasible to use the same thruster you use for orbit changes. You'd need 3 thrusters, orthogonal to each other and offset from the center of mass, plus all the accompanying plumbing.
So, it's much more common to have reaction wheels [0] and/or torque rods [1], which are electrically powered instead of consuming fuel. You typically need 3 of them, oriented in the X, Y, and Z axes, for full control. As spacecraft get smaller, this consumes a larger portion of its mass budget, which is why the small size of the reaction wheel described in the article is so important.
0: https://en.wikipedia.org/wiki/Reaction_wheel
1: https://en.wikipedia.org/wiki/Magnetorquer