Astronaut in Space With a Spinning Gyroscope

[aeroseek]

One particular experiment that I remember from a visit to a Science Center is the one involving a revolving platform and a gyroscope.

The user spins the gyroscope - a heavy wheel on an axle and stands on the platform which is free to rotate about its vertical axis.

As I distinctly remember, suddenly moving the gyroscope, rotating it while holding the ends of the axles, caused me, the gyroscope and the platform to rotate left or right.

If an astronaut on a spacewalk holds a spinning gyroscope twists it right or left or in any direction, will it not cause him to rotate?

Could such a device be used to orient the astronaut without using external jets or rockets?

 

[Nugatory]

If an astronaut on a spacewalk holds a spinning gyroscope twists it right or left or in any direction, will it not cause him to rotate?

it will.

Could such a device be used to orient the astronaut without using external jets or rockets?

Yes, but jets or rockets also allow the astronaut to move from one place to another. The gyroscope won't do that.

 

[voko]

https://en.wikipedia.org/wiki/Control_moment_gyroscope

 

[aeroseek]

Do we have a 'reactionless' propulsion device here then?

I am a little surprised by the answers, since Professor Laithwaite and Sandy Kidd both attracted much criticism by suggesting something along these lines.

 

[aeroseek]

Wow there actually is such a device;

Thanks to "Voko" for the link. Once again I am shown something in Wikipedia I did not know exists:

A control momentum gyroscope (CMG) is an attitude control device generally used in spacecraft attitude control systems. A CMG consists of a spinning rotor and one or more motorized gimbals that tilt the rotor’s angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic torque that rotates the spacecraft .

https://en.wikipedia.org/wiki/Control_moment_gyroscope

 

[Nugatory]

Do we have a 'reactionless' propulsion device here then?

We do not, because there's no propulsion happening. All the astronaut can do is remain in one spot while changing the direction he's looking. There's a perfectly good action-reaction pair between the astronaut and the gyroscope.

 

[A.T.]

Do we have a 'reactionless' propulsion device here then?

The center of mass is not accelerated by this. The total angular momentum is constant. See also cat righting reflex:

https://www.youtube.com/watch?v=yGusK69XVlk

 

[aeroseek]

Back to the platform, though, if a moving platform that I am standing on can be made to rotate using a gyroscope, would it be possible to translate that rotation to a forward motion of the platform through gearing the platform to wheels?

 

[A.T.]

Back to the platform, though, if a moving platform that I am standing on can be made to rotate using a gyroscope, would it be possible to translate that rotation to a forward motion of the platform through gearing the platform to wheels?

Why so complicated? Gear the gyroscope directly to the wheels, and use it as a flywheel.

 

[Nugatory]

Back to the platform, though, if a moving platform that I am standing on can be made to rotate using a gyroscope, would it be possible to translate that rotation to a forward motion of the platform through gearing the platform to wheels?

Yes, but that's no more a reactionless drive than is the motor of my car, which is connected by suitable gearing to the wheels. You won't get any forward motion unless there's something for the wheels to push against - in space the car would just sit there with its wheels spinning.

If we're on the ground so that wheels do have something to push against, we'd be better off using the electricity that's spinning the gyroscope and operating the gimbal to run an electric motor connected directly to the wheels.

 

[aeroseek]

If we're on the ground so that wheels do have something to push against, we'd be better off using the electricity that's spinning the gyroscope and operating the gimbal to run an electric motor connected directly to the wheels.

But you have to admit it is curious, though, propelling a vehicle totally through internal means - only mechanical input that is transmitted through gyroscopic forces - still not reaction-less?

 

[Nugatory]

But you have to admit it is curious, though, propelling a vehicle totally through internal means - only mechanical input that is transmitted through gyroscopic forces - still not reaction-less?

No more curious than the way that my car propels itself "totally by internal means" until the fuel tank runs dry. It took power to spin up that gyroscope, and it takes power to operate the gimbals, and that power came from somewhere - either an onboard electrical generator or charged batteries that will have to recharged or replaced when they are exhausted.

 

[A.T.]

still not reaction-less?

You need a ground reaction force to propel a car via wheels. How is that "reaction-less"?

 

[Dale]

But you have to admit it is curious, though, propelling a vehicle totally through internal means

Since when is the ground "totally internal" to a vehicle?

 

[aeroseek]

Well OK, maybe it is 'reaction less power transmission'. We could use the rotation of the platform to charge a battery and run the vehicle with that.

I am just asking if Newton's Laws will be violated - it seems hard to say.

 

[A.T.]

I am just asking if Newton's Laws will be violated

Where exactly do you see a potential violation of which law?

 

[Bandersnatch]

Perhaps you're missing one important point here - once you connect the gyro to some transmission system that actually does work(by accelerating the centre of mass of the system, or maintaining motion against friction, or charging some batteries), the gyro will slow down and stop. You'll only transfer the energy that had been put into spinning the gyro to some other form.

 

[Nugatory]

Well OK, maybe it is 'reaction less power transmission'. We could use the rotation of the platform to charge a battery and run the vehicle with that.

If by "run the vehicle" you mean propelling it forward, you can't do that without the action-reaction pair you get from the wheels pushing on the ground.

If by "run the vehicle" you mean "operate its internal electrical systems, or even make the wheels spin idly", then many posts back I pointed out that there is a perfectly good action-reaction pair between the astronaut and the gyroscope so the rotation of the astronaut in place is not reactionless.

I am just asking if Newton's Laws will be violated - it seems hard to say.

It's easy to say: No.
If you think see a violation of anyone of them, ask about it.

 

[Dale]

Well OK, maybe it is 'reaction less power transmission'. We could use the rotation of the platform to charge a battery and run the vehicle with that.

I am just asking if Newton's Laws will be violated - it seems hard to say.

Nothing that you have described even remotely implies a violation of Newton's laws. I am not even sure what makes you think that it would.

If you have wheels pushing on the ground you do not have a reactionless anything, you have a reaction with the ground. If you are in space using gyros to alter your orientation then you do not have a reactionless drive because you are not being driven anywhere.

In space both the linear and angular momentum stay constant. On the ground the linear and angular momentum change according to the external force and torque provided by the ground.

 

[aeroseek]

First we had an astronaut in space. We know he can change his spatial orientation by using a gyroscope.

Next we have a person on a revolving platform on the ground - a platform rotates, but there is not motion in any direction, that is, the center of gravity does not get translated.

Now the person on the platform on the ground move horizontally by rotating the gyroscope in a particular direction? No, this cannot be done.

If a vertical gyroscope is moved to horizontal position, as I recall the force will tend to rotate the entire system.

Now the question is, can the forces generated by moving a gyroscope be used to create forward motion, without any movement of mass.

This can be done through electricity, that is, using the rotation of the platform to generate a current that then drives an electric motor.

Basically you are using the forces generated within an entirely self contained unit (think of a man in a box, on a platform ) to create motion

 

[A.T.]

First we had an astronaut in space. We know he can change his spatial orientation by using a gyroscope.

He doesn't need a gyroscope to change his orientation. See this video at 25:00min:
http://techtv.mit.edu/collections/l...cle-smarts-stability-translation-and-rotation

 

[Dale]

Now the question is, can the forces generated by moving a gyroscope be used to create forward motion, without any movement of mass.

I have no idea what you mean by this.

Basically you are using the forces generated within an entirely self contained unit (think of a man in a box, on a platform ) to create motion

The man-box-platform system is not entirely self contained. There are external forces acting on the man, on the box, and on the platform. Any restrictions imposed on the motion of isolated systems do NOT apply to the man-box-platform since it is not an isolated system.

 

[russ_watters]

This can be done through electricity, that is, using the rotation of the platform to generate a current that then drives an electric motor.

In order to "generate a current", via "rotation of the platform" you have to turn a generator shaft, while the body of the generator remains stationary. So the entire device will need to be connected to the ground to make that happen. Hence: not reactionless. And not even on point anyway: that isn't propulsion.

I can conceive of a system that utilizes counter rotating flywheels to store and retrieve energy without causing the device to rotate, but the actual propulsion is still just wheels on the ground.

 

[A.T.]

... to create forward motion, without any movement of mass.

Motion without motion?

 

[rcgldr]

I could imagine that an astronaut in space drifting away from the space station he wanted to return to, thinking that the gyro would help solve his dilemma, after a few minutes of frustation of failed attempts to achieve linear acceleration from a gyro, simply threw it away out of anger only to realize he solved the problem. Now if only he had thrown it away from the spaceship he was trying to return to instead of at the space ship

 

[aeroseek]

I think the astronaut in space with a gyroscope problem has been answered quite completely.

The "Man in the box" scenario is different - can a man in a box, on a trolley do anything to make the trolley move - eg - pushing against the box jumping up and down etc. The answer would be no. This relates to mechanically transferred energy only.

Thinking out of the box(!) - it is possible however for the man in the box to use other types of energy - heat, electrical energy etc to move the trolley, if it was suitable set up with a generator, motors, etc.

Actually a simple railway trolley arrangement will suffice.

So the question is can the man in the box exert a force anywhere on the box to make it move - the answer would be no then, right?

 

[Dale]

The "Man in the box" scenario is different - can a man in a box, on a trolley do anything to make the trolley move - eg - pushing against the box jumping up and down etc. The answer would be no.

The answer would be yes. The man box trolley is not an isolated system.

 

[georgir]

The center of mass is not accelerated by this. The total angular momentum is constant. See also cat righting reflex:

https://www.youtube.com/watch?v=yGusK69XVlk

This is in fact completely wrong, the cat would not be able to turn in a vacuum, unless it rotated its internal organs or something.
And in reality it does so with the help of air resistance.

 

[Bandersnatch]

This is in fact completely wrong, the cat would not be able to turn in a vacuum, unless it rotated its internal organs or something.
And in reality it does so with the help of air resistance.

What? No. You can yourself turn in space by spinning your arms. You don't need air resistance - this is about conservation of angular momentum, not about swimming in air.

 

[rcgldr]

What? No. You can yourself turn in space by spinning your arms. You don't need air resistance - this is about conservation of angular momentum, not about swimming in air.

Or similar to the cat, bending at the waist, and swiveling your legs in a circular motion.

 

[georgir]

Conservation of angular momentum, exactly. You can only spin some part of you while spinning another part the other way. The cat can flop around all it wants but all it will achieve is look silly.
Just like you spinning your arms, unless you got a loose joint at your shoulder and can do a complete 360 revolution (actually it will require a completely cut off and detached skin and flesh, yuck), you'll end up in exactly the same position and orientation you started.
[actually, the blood flowing through us meatbags makes my statement wrong, and maybe that can be abused for turning in space, i don't know. but the math for a simple solid (flexible, but nowhere detached) object is easy and categorical, no net turning is possible]

 

[A.T.]

This is in fact completely wrong, the cat would not be able to turn in a vacuum,

Because it would be dead in vacuum, right?

But seriously, you are in fact completely wrong. Air is not needed for this maneuver. It's all about conservation of angular momentum. That's why they investigated this as part of the space program, and trained astronauts wearing space suits for the space walks.

See this paper:
http://pentagono.uniandes.edu.co/~j...inicursoJK-Uniandes/robotic examples/kane.pdf

More pictures from the study:
http://www.theatlantic.com/video/index/244829/can-an-astronaut-move-like-a-falling-cat/

Related lecture at MIT (at 20:30):
http://techtv.mit.edu/collections/l...cle-smarts-stability-translation-and-rotation

unless it rotated its internal organs or something.

Because only internal organs have rotational inertia, or what?

 

[rcgldr]

Conservation of angular momentum, exactly. You can only spin some part of you while spinning another part the other way.

but that is good enough to end up facing some other orientation after you stop spinning one part of your body. As mentioned, imagine spinning your arms in near vertical circles so when viewed from the side, your arms rotate clockwise, and your body rotates counter clockwise, when you stop whirling your arms, you end up facing in a different direction. You can bend at the waist and swivel your legs to rotate about your body's main vertical axis. So absent any external forces, you can change your orientation, but you can not move your center of mass in any linear direction.

 

[georgir]

you're all not getting it. your arm can not do a complete 360 degree turn unless it is detached or it twists and winds up more and more at your shoulder. you do a rotation, but you undo it as you untwist your shoulder.

a simpler case is an astronaut holding a heavy object. he can stretch his arms further away or pull them closer, changing the distance of that object from the center of gravity and thus axis of rotation. a layman can think that rotating the object with stretched arms and unrotating it with arms pulled close nets an overall rotation but in reality, the rest of your body is also pulled and pushed farther and closer to the axis of rotation, so it balances out - every time you unrotate the object, you unrotate the rest of your body and return to exactly the original orientation, no matter how stretched your arms are. the only way to beat that is to let go of the object and grab it in a new place, so it has done a full rotation that does not need to be undone.

 

[Dale]

you're all not getting it. your arm can not do a complete 360 degree turn unless it is detached or it twists and winds up more and more at your shoulder. you do a rotation, but you undo it as you untwist your shoulder.

No, you are not getting it. This is well established in the space program and there is even training and studies on executing these maneuvers in emergency situations:
http://mvl.mit.edu/MVLpubs/MVL_09.03_StirlingNewmanWillcox.pdf

First, there are many 360º rotations that you can do. For example, stick your arms straight out along the axis from left shoulder to right shoulder, then rotate them along a conical surface inclined say 20º from the shoulder axis. You can do that motion. Second, in many cases you do not even need to do a complete 360º rotation because you can change your moment of inertia. If you do 180º rotation of some joint with a high moment of inertia posture and then change to a low moment of inertia posture and do a -180º rotation rotation of that same joint then you will change your angular position.

 

[A.T.]

your arm can not do a complete 360 degree turn unless it is detached or it twists and winds up more and more at your shoulder. you do a rotation, but you undo it as you untwist your shoulder.

If that's how your shoulder joints work then you should have them checked. Most people can certainly swing their arms 360° in the sagittal plane continuously, without introducing torsion that needs untwisting. If you do this while floating in space, the rest of the body will counter rotate in the sagittal plane. For other rotation axes see the video of the astronaut here at 25:00:
http://techtv.mit.edu/collections/l...cle-smarts-stability-translation-and-rotation

Here is a video of a cat flipping around in only ~0.3sec. It should be obvious that this cannot be achieved with aerodynamic forces, unless you have vary large surface area limbs (like a bird wing).

https://www.youtube.com/watch?v=RHhXbOhK_hs

 

[aeroseek]

The flipping cat example is considered solved: before the internet Physicists would argue for hours over the concept - but it is clear to me the rule is: you can twist anyway you want but you can't move your centre of mass.

About the man in the box or let's put the cat in a box, and the box on a trolley with frictionless wheels (magnetic bearings?)

Then what?

 

[A.T.]

you're all not getting it. your arm can not do a complete 360 degree turn unless it is detached or it twists and winds up more and more at your shoulder. you do a rotation, but you undo it as you untwist your shoulder.

I think your confusion comes from the wrong idea that angular momentum requires rotation. That is not the case. Even a linearly moving object has angular momentum w.r.t. to any point, that is not on the objects path. If an object moves on a circular path, then it has angular momentum, even if it doesn't change it's own orientation.

If you swing your arms around, you don't twist them, and your thumb always points where your head is. But each part of the arm moves in circles around the left-right-axis, so the arms have angular momentum. And the rest of the body has equal but opposite angular momentum.

 

[jbriggs444]

The flipping cat example is considered solved: before the internet Physicists would argue for hours over the concept - but it is clear to me the rule is: you can twist anyway you want but you can't move your centre of mass.

About the man in the box or let's put the cat in a box, and the box on a trolley with frictionless wheels (magnetic bearings?)

Then what?

One assumes that the trolley+cat+box starts out motionless and that not only are the wheels frictionless but that all other external net forces (wind resistance, etc) are also zero. One also assumes that the cat can't "jump" the box so that it moves from its starting position relative to the trolley. One assumes that the cat cannot get out of the box.

Let m be the mass of the cat. Let M be the mass of the trolley+box. Let w be the size of the box in the direction of the tracks.

Given the assumptions about frictionlessness, there are no external forces on the system. No net acceleration of the center of mass. Given the assumption about starting at rest, the center of mass of the cat+box+trolley system has a fixed position. It will not move.

Given the assumption about the box not moving relative to the trolley and the cat not being able to get out of the box, the center of mass of the box+cat+trolley system is constrained to a finite region of size $\frac{w m}{M}$ relative to the trolley.

Accordingly, the trolley cannot move more than $\frac{w m}{M}$ from its starting point in this scenario. It can move that far if the cat moves from one end of the box to the other.

 

[jbriggs444]

If the trolley is positioned on tracks that curve back and forth, a side to side "skating" method of propulsion might be possible.

edit: Not just back and forth curves. And "skating" is not the only mode that can work.

 

[A.T.]

One also assumes that the cat can't "jump" the box so that it moves from its starting position relative to the trolley.

And if you glue the box to the trolley, then you also have to rule out that the cat can jump the entire trolley, or even lift one axis to rotate it. Otherwise it might do something like this:

https://www.youtube.com/watch?v=m_6NGjXujxQ

 

[rcgldr]

If the trolley is positioned on tracks that curve back and forth, a side to side "skating" method of propulsion might be possible.

That would involve an external force. By moving the center of mass within the box, a side to side force is exerted onto the curved track, which would respond with both side and forward forces (assuming angled part of track), allowing the box to be propelled.

edit: Not just back and forth curves. And "skating" is not the only mode that can work.

And if you glue the box to the trolley, then you also have to rule out that the cat can jump the entire trolley, or even lift one axis to rotate it. Otherwise it might do something like this: video of tic tacs

Lifting one axis is only needed because the wheels can't pivot far enough. With split or free line or drift skates (different names for the same thing, like a skateboard cut into two and using inline skate wheels), or a snake board / street board (like a skateboard hinged in the middel or with both ends that can pivot), there is no need to lift the wheel(s). In the case of a skateboard once sufficient speed is achieved, the tic tac like method of propulsion can be peformed without lifting the wheels off the pavement. For the initial start, the rider leans to one side, exerting a side force onto the wheels, which exert a side force onto the pavement, and the pavement exerts an opposing side force onto the wheels, accelerating the rider and skateboard in the direction of lean. Note that the pavement is exerting an external force on the rider and skateboard, which allows them to accelerate. Then the skateboard is turned into the direction of velocity acquired by that acceleration, in the case of the video, lifting of the front wheels is done so the skate board can rotate more quickly and freely. (and technically the skateboard is turned a bit past the direction of velocity so that the next lean produces a component of acceleration in the desired (forward) direction). The process is then repeated, leaning to one side or the other (relative to the skateboards new orientation). At sufficient speed instead of leaning to generate side forces at the wheels, the rider can just twist side to side generating a torque onto the skateboard while weaving and steering the skateboard out of phase so that side forces on the wheels continue to propel the skateboard forward.

 

[Dale]

About the man in the box or let's put the cat in a box, and the box on a trolley with frictionless wheels (magnetic bearings?)

Look at the forces and torques that can can be supported. The trolley can support torques along all 3 axes and forces along the vertical and left-right axes, but not along the forward-backward axis.

 

[A.T.]

Lifting one axis is only needed because the wheels can't pivot far enough.

It's not clear what kind of wheel steering aeroseek's trolley allows, so I assumed none. I also assumed that "friction-less wheels" means that they still have lateral resistance, just no rolling resistance.