
Running at 150,000 RPM, a tiny motor could help satellites - kartD
https://techcrunch.com/2016/07/22/running-at-150000-rpm-this-tiny-motor-could-help-satellites-keep-on-course/
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JshWright
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).

[https://www.youtube.com/watch?v=7Js5x4NhUxU](https://www.youtube.com/watch?v=7Js5x4NhUxU)

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swatkat
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].

[0]
[http://www.coep.org.in/csat/subsystems/acs/](http://www.coep.org.in/csat/subsystems/acs/)

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iamgopal
How to launch such femto satellites??

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donquichotte
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.

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johnm1019
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.

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dghughes
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?

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planteen
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.

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NickNameNick
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.

[1]
[https://en.wikipedia.org/wiki/Reaction_wheel](https://en.wikipedia.org/wiki/Reaction_wheel)

[2]
[https://en.wikipedia.org/wiki/Control_moment_gyroscope](https://en.wikipedia.org/wiki/Control_moment_gyroscope)

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dnautics
cmgs are preferred for larger satellites. momentum wheels for smaller sats.

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chris_va
For really small satellites, magnetorquers seem to be a better tradeoff.

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planteen
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.

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chris_va
Nope, but mosts small satellites don't need to do much except keep pointing at
the same ground angle.

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planteen
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.

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endymi0n
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.

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dang
Ok, we've taken "keep on course" out of the title above to try to make it less
misleading. Presumably it "helps satellites" somehow or other.

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mSparks
Dont see what's wrong with "keep on course".

The 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.

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jokoon
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).

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DanBlake
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?

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JshWright
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.

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Gravityloss
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.

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SeanDav
This technology has been proposed for some time for use on wind turbines as
frictionless bearings. I am not sure why they are not in common use yet.

[http://www.treehugger.com/renewable-energy/the-worlds-
first-...](http://www.treehugger.com/renewable-energy/the-worlds-first-
magnetic-levitation-wind-turbines-unveiled-in-china.html)

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chillingeffect
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...

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Qantourisc
Am I the only one thinking about the 150k RPM wear-and-tear on the reduction
box ?

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makomk
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.

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kekusvult
Can anyone do the math on the access speed of a 150,000 RPM disk drive?

