Question. (Conservation of angular momentum)

Thoughts on the following space station / space craft design with a view to propulsion.

Central hub with rotating outer rung. Outer rung rotates using magnetism, powered electrically from solar or nuclear.

Let’s assume this is large enough that the outer ring houses the living quarters, stores, flight deck and so on. The hub contains the drive to the “motor” that rotates the station / ship.

I would suggest this is a design theme often described in books and so on, specifically due to the centrifugal force that produces the effect of gravity at the correct rotational rate.

Now let’s say we attach 20 ton “weights” to the centre hub, with cables running out through the ring.

What would happen if, when the ship/station was rotating, one of the blocks was released and allowed to drift away using the centrifugal force, until the tether/cable stops its movement.

Wind it back in.

Repeat with the next weight and so on.

Obviously the craft would need to be built to withstand the “tug” each time the tether/cable reached its maximum extent, perhaps a clutch.

Anyway.

Just putting it out there.

a reply to: aoi3610

The pat answer is that the total energy in the system would remain the same, however, if you think of someone on a swing, you can achieve fairly significant synchronous harmonic motion within that constraint.

originally posted by: aoi3610
Thoughts on the following space station / space craft design with a view to propulsion.

Central hub with rotating outer rung. Outer rung rotates using magnetism, powered electrically from solar or nuclear.

Let’s assume this is large enough that the outer ring houses the living quarters, stores, flight deck and so on. The hub contains the drive to the “motor” that rotates the station / ship.

I would suggest this is a design theme often described in books and so on, specifically due to the centrifugal force that produces the effect of gravity at the correct rotational rate.

Now let’s say we attach 20 ton “weights” to the centre hub, with cables running out through the ring.

What would happen if, when the ship/station was rotating, one of the blocks was released and allowed to drift away using the centrifugal force, until the tether/cable stops its movement.

Wind it back in.

Repeat with the next weight and so on.

Obviously the craft would need to be built to withstand the “tug” each time the tether/cable reached its maximum extent, perhaps a clutch.

Anyway.

Just putting it out there.

The simple answer is that when the cable is let out the station rotation will slow down. When the cable is pulled back in, the station rotation will speed up.

This will have no effect on propulsion. Changes in angular momentum have no effect on linear momentum.

By the way, stations like you describe are not spun up and down by a motor between the hub and the ring. If you put a motor between the hub and the ring and spun the motor up, the ring would rotate in one direction and the hub would rotate in the other. However, the moment of inertia of the ring would be probably 100 times or more larger than the moment of inertia of the hub, so the hub would have to rotate 100 times faster than the ring in order to conserve angular momentum. That would probably be impossibly fast. Large objects like a station would be spun up and down by using rocket thrusters.

I’m an aerospace engineer and I design spacecraft for a living. That’s why I know this stuff.

a reply to: aoi3610

As you describe it, the whole thing would wobble like h3ll, first in one direction then the next. No net gain, even if you threw the mass directly in front of your craft: net gravity pull is still zero as they would cancel each other out.

And in space, how much something weighs is negligible to the overall craft’s momentum. That is why “slow and steady”, lke ion propulsion, works.

Now if you can reduce mass (or I guess, the effects of the universe upon said mass), then you would be wondering what the heck that weirdo is doing with those masses and throwing them out in front of the ship!!

originally posted by: aoi3610
Thoughts on the following space station / space craft design with a view to propulsion.

Central hub with rotating outer rung. Outer rung rotates using magnetism, powered electrically from solar or nuclear.

Let’s assume this is large enough that the outer ring houses the living quarters, stores, flight deck and so on. The hub contains the drive to the “motor” that rotates the station / ship.

I agree with 1947boomer that this doesn't sound like such a good idea. I found some guys on youtube designing space stations using Kerbal space program where the outer ring rotated and the core didn't, but one guy even pointed out how stupid that design was by saying you would have to do an EVA to get from the ring to the core.

The professional space station designs I've seen have the entire station rotate, so the Core and the ring are attached and all rotate together and you don't have to do any EVA to get from one to the other. Werner VonBraun came up with a design like this back in the 1950's:


The way you would get that to rotate is to put 2 or more rocket motors on the ring equally spaced, fire them to get the ring moving, and once it's at the rotation rate you want, it will keep rotating at that rate as long as the mass of the station stays the same and is distributed the same. If you start adding mass, like in the more modern design where modules can be added, the angular momentum would be conserved so more mass from adding modules would mean slower rotation rate, so you might need to fire the rockets for a bit to get the rotation rate back up to where you want it. This is a screen capture of the modernized modular design:

I would suggest this is a design theme often described in books and so on, specifically due to the centrifugal force that produces the effect of gravity at the correct rotational rate.

No to your motor idea, but yes to the artificial gravity idea, in fact this is a youtube promo video for a project to make such an artificial gravity space station, as seen in the second image above:

SpaceX Starship and The Von Braun Rotating Space Station

Now let’s say we attach 20 ton “weights” to the centre hub, with cables running out through the ring.

I had trouble interpreting this. You mention 20 ton weight so I drew that, you mention cable so I drew that with some slack in it, and you mention ring, so one end of the cable is attached to the weight and the other attached to the ring.

What would happen if, when the ship/station was rotating, one of the blocks was released and allowed to drift away using the centrifugal force, until the tether/cable stops its movement.

I tried to sketch that, here's what I came up with. When the cable stops the movement, whether the cable is aligned with the axis of the station depends on a number of things, where the attachment point for the cable is on the station when the weight is released, length of cable, etc, so the alignment is just coincidental in this first illustration (I left the spokes out of the drawing to avoid clutter but it would have spokes like the images above):



Is this what you had in mind? Note that if the timing, length of cable etc are different the cable could also become taut in either of these positions:



So what do you think would happen? You didn't say. The engineers and physicists here have had training to analyze problems like these.

Why are you asking anyway, is this some "free energy" idea or something? Once the rotation is started, you no longer need any motor to keep the rotation going, so you don't have any energy requirements to maintain rotation, like you would with say a car which has lots of friction. The friction on a rotating space station is pretty small, though it depends on altitude. If it's in orbit around the Earth, the higher the altitude, the lower the friction since the atmosphere is thinner the higher you go.

If you were worried about maintaining rotation due to friction of the thin atmosphere, you might be able to offset that somewhat using ion drives powered by solar panels, but I think they would be too weak to get it moving initially in a reasonable time. They might be enough to counteract the small amount of friction in low earth orbit, again depending on a number of factors like altitude.

The inertial mass of the weight from its rotation with the spin axis of the space station would displace the spin axis with respect to the new center of mass of the weight , space station system. If the inertial mass of the weight exceeds the mass of the space station then the space station would rotate around the weight.

Please correct if I am wrong.

originally posted by: eManym
If the inertial mass of the weight exceeds the mass of the space station then the space station would rotate around the weight.

Please correct if I am wrong.

Not quite right. If the weight is attached to the space station, then the rotational axis will be through the center of mass of the combined mass of the weight and the space station.

When one object is much more massive you could get the idea the axis of rotation is for that object, but it is a little different.

For example, when the moon orbits the Earth, it's not orbiting the center of the Earth. What actually happens is the Earth and the moon both orbit a common barycenter, which happens to be inside the earth.



Similarly if you attach a large mass and a small mass and get them rotating, the rotation axis will probably be in the large mass (barring some unusual configuration), but not at the center of the large mass, rather at the center of the combined masses.

In the OP the weight mass was given as 20 tons. The space station mass was not given but probably a rotating space station would have a mass as great as the ISS or greater, which is about 420 tons. So I wouldn't expect an artificial gravity rotating space station to have a mass less than 20 tons. Von Braun's proposal was for a 76m or about 250 feet diameter rotating station, which is not small. I don't know if he ever estimated a mass for it or if the Gateway project has estimated a mass for their upgraded version.

Re: the first part of your post, yes if you added a single weight as I drew, again the rotation axis would be at the combined center of mass, however the OP said "Now let’s say we attach 20 ton “weights” to the centre hub" so if adding more than one weight, they would probably be added in positions of symmetry to maintain the central rotation axis.

a reply to: Arbitrageur

Free energy, no.

Propulsion using no propellant.

Lets assume a 300 ton station, 20 ton blocks, continually powered by the motor in the centre.
Let’s assume the motor spindle is spinning fast as pointed out above re mass of the main ring and the hub. It matters not, all that matters is it’s possible.

The question is:

Would the stations velocity in the direction the block is stopped by the tether change.

a reply to: Arbitrageur

Your diagrams show the principal perfectly.

So what would happen to the space station?

Plus, what would happen if the same principal were repeated at the same rotational point as the station rotates.
(Assume multiple masses attached to the station ring, each detached at the correct time to be come taught at the same point and same direction)

Addendum:

I get the idea sounds crazy, and there are other problems for example the shock on the station each time the mass reaches the tether end and so on.

All I’m trying to determine is if it’s possible to convert rotational energy into acceleration in any particular direction using tethered masses on a rotating object in a vacuum.

In my defence the idea of exploding a nuclear device behind a craft to “ride the wave” and push a craft forward is equally crazy.

originally posted by: aoi3610
a reply to: Arbitrageur

Free energy, no.

Propulsion using no propellant.

Lets assume a 300 ton station, 20 ton blocks, continually powered by the motor in the centre.

Let’s assume the motor spindle is spinning fast as pointed out above re mass of the main ring and the hub. It matters not, all that matters is it’s possible.

Let's re-read 1947boomer's post which you don't yet seem to understand. Until you understand it I don't think further discussion will be put in proper perspective:

originally posted by: 1947boomer
The simple answer is that when the cable is let out the station rotation will slow down. When the cable is pulled back in, the station rotation will speed up.

This will have no effect on propulsion. Changes in angular momentum have no effect on linear momentum.

1947 boomer, do you want to reconsider that after reviewing the sketch I made? I suspect you visualized cables under constant tension for your assessment, but that's not what the OP described nor what my sketch shows.

By the way, stations like you describe are not spun up and down by a motor between the hub and the ring. If you put a motor between the hub and the ring and spun the motor up, the ring would rotate in one direction and the hub would rotate in the other. However, the moment of inertia of the ring would be probably 100 times or more larger than the moment of inertia of the hub, so the hub would have to rotate 100 times faster than the ring in order to conserve angular momentum. That would probably be impossibly fast. Large objects like a station would be spun up and down by using rocket thrusters.

I’m an aerospace engineer and I design spacecraft for a living. That’s why I know this stuff.

1947 boomer is right about that part, and now that he can see the sketch he may revise his opinion of momentum transfer based on the sketch. So re-read this to understand what the motor would do. If you're still stuck on this motor, concept, unless you can explain why it doesn't do what 1947boomer said, I'm lost, because I don't think you understand what he's saying.

originally posted by: aoi3610
All I’m trying to determine is if it’s possible to convert rotational energy into acceleration in any particular direction using tethered masses on a rotating object in a vacuum.

OK but it would be nice if you could learn why your motor idea is wrong. 1947boomer's post explains why unless you can clarify what's wrong with his explanation of what the motor would do and why it wouldn't be a good idea.

The way we would rotate those stations is to put 2 or more rocket motors on the outside of the outer ring, symmetrically spaced, firing tangentially to the ring, as boomer suggested. You can get a glimpse of that concept if you watch the Gateway Project video I posted showing their idea for a rotating space station. If you wanted linear motion instead of rotation, you could use rocket thrusters to move the station linearly in the direction you want, instead of using them to rotate the station, then apply all that stress in the station where the cable attached to the weight attaches to the station. So I don't see the point in using the weight/cable system to do what a rocket thruster can do just as well without all that stress.

As for converting rotational energy to create propulsion, isn't that sort of what cars do, at least the internal combustion engine types? But let's see if 1947 boomer will update his assessment of momentum transfer now that he's seen the sketch I made of what you described, since he claims to be the expert. Then I can give my opinion whether I agree or disagree.

For his prior assessment, I don't think 1947boomer was thinking of the same idea as you because I don't think he read your post carefully enough. He didn't seem to consider the cable stopping the moving weight, he seemed to assume constant tension on the cable, which is a different case.

In any case once you correct your fundamental misunderstanding of how to get the station rotating, I suspect that may kill your idea, once you figure that out.

In my defence the idea of exploding a nuclear device behind a craft to “ride the wave” and push a craft forward is equally crazy.

That is actually the best idea I've seen so far for traveling to other stars, but the problem is how to do it safely. Project Orion was scrapped after the nuclear test ban treaty, since it would have used nuclear explosions in the atmosphere for propulsion until it escaped earth's gravity. But there's still the idea to use conventional rockets to get outside earth's atmosphere, and then start using the nuclear propulsion once it's far enough away to not irradiate Earth or the space around the Earth.

Thank you for a comprehensive reply.

Let's re-read 1947boomer's post which you don't yet seem to understand. Until you understand it I don't think further discussion will be put in proper perspective:

Agreed.

I suspect you visualized cables under constant tension for your assessment, but that's not what the OP described nor what my sketch shows.

Yes.

By the way, stations like you describe are not spun up and down by a motor between the hub and the ring. If you put a motor between the hub and the ring and spun the motor up, the ring would rotate in one direction and the hub would rotate in the other. However, the moment of inertia of the ring would be probably 100 times or more larger than the moment of inertia of the hub, so the hub would have to rotate 100 times faster than the ring in order to conserve angular momentum. That would probably be impossibly fast. Large objects like a station would be spun up and down by using rocket thrusters.

I understand, so for the purpose of this example we will assume the hub and station are the same mass. Now the motor will rotate at the same speed as the station, but in the opposite direction.

The way we would rotate those stations is to put 2 or more rocket motors on the outside of the outer ring, symmetrically spaced, firing tangentially to the ring, as boomer suggested. You can get a glimpse of that concept if you watch the Gateway Project video I posted showing their idea for a rotating space station. If you wanted linear motion instead of rotation, you could use rocket thrusters to move the station linearly in the direction you want, instead of using them to rotate the station, then apply all that stress in the station where the cable attached to the weight attaches to the station. So I don't see the point in using the weight/cable system to do what a rocket thruster can do just as well without all that stress.

The point being propulsion without propellant.

As for converting rotational energy to create propulsion, isn't that sort of what cars do, at least the internal combustion engine types? But let's see if 1947 boomer will update his assessment of momentum transfer now that he's seen the sketch I made of what you described, since he claims to be the expert. Then I can give my opinion whether I agree or disagree.

However the tyre uses friction against the earth the change acceleration.

For his prior assessment, I don't think 1947boomer was thinking of the same idea as you because I don't think he read your post carefully enough. He didn't seem to consider the cable stopping the moving weight, he seemed to assume constant tension on the cable, which is a different case.

Agreed. I am assuming, perhaps incorrectly that there will be an equal and opposite reaction when the mass is stopped by the tether.

In any case once you correct your fundamental misunderstanding of how to get the station rotating, I suspect that may kill your idea, once you figure that out.

Fundamentally wrong?

Why is the principal fundamentally wrong please?

originally posted by: aoi3610
Fundamentally wrong?

Why is the principal fundamentally wrong please?

Because you still don't understand angular momentum:

"I understand, so for the purpose of this example we will assume the hub and station are the same mass. Now the motor will rotate at the same speed as the station, but in the opposite direction. "

Let's tweak this to say hub, and outer ring are the same mass. So hub is let's call it 150 tons, and outer ring is 150 tons.

The ring is let's say Von Braun ring which is 250 feet diameter, 125 foot radius. How big is the hub, shall we say 30 feet diameter, 15 feet radius? Now, how do you calculate angular momentum? It's a function of distance from the axis.

So let's model the ring angular momentum using a simplified model, of two 75 ton masses on two spokes 125 feet long.
Let's model the hub angular momentum using a simplified model, of two 75 ton masses on two spokes that are 10 feet long.

Put a motor between these simplified models, and you can calculate how much faster the hub will rotate than the ring. They will have equal but opposite angular momentum, so the hub will have to spin much faster to get as much angular momentum, since the spokes are only 10 feet long. This is sort of what 1947boomer was trying to explain.

Angular momentum is a function of moment of inertia.
let's call the moment of Inertia "I"
let's call the mass "m"
let's call the radius "r"

The moment of inertia for each weight in the models above is:

I=mr²

for the hub, I= 75T x 10'² + 75T x 10'² = 15,000 tons*ft²

for the ring, I= 75T x 125'² + 75T x 125'² = 2,343,750 tons*ft²

For them to rotate at the same rate using your proposed motor idea, they would need the same moment of inertia. But with the same mass, the moment of inertia of the ring is 156 times greater than the moment of inertia of the hub, so the hub would rotate 156 times faster, using a motor, with the above model.

In other words rotating the hub 156 times faster (in rpm or whatever angular motion units you choose) gives it equal but opposite angular momentum to the ring, such that the total angular momentum of the hub plus the ring is still zero, which is all you can get with the type of motor you propose.

a reply to: aoi3610

What would happen if, when the ship/station was rotating, one of the blocks was released and allowed to drift away using the centrifugal force,

The station and the block will drift away from each other with same but opposite momentum. The centrifugal force acts on both.

The rotation axis will also move away from station center, remain at center of mass of the block-station system.

until the tether/cable stops its movement.

Station and block would stop moving away from each other.

Wind it back in.

Station and block will move towards each other returning to the initial position.

a reply to: moebius

Sounds annoyingly correct.

Central hub with rotating outer ring. The hub contains the drive to the “motor” that rotates the station / ship.

Impossible. If the power for rotating the outer ring is derived from the core, then the core will counter-rotate. Newton’s Third Law.

Besides, what is the hub supposed to be stationary relative to?

a reply to: Astyanax

Besides, what is the hub supposed to be stationary relative to?

Reminds me of a joke.

How many airline captains does it take to change a lightbulb?

a reply to: Phage

Does the punch line involve paperwork?

a reply to: Astyanax

No.
It involves the captain being the center of the Universe.