Can docking ships help regulate rotation in a spinning space habitat?

[dmac257]

I am writing a science fiction book and doing a little research about habitats in space to make sure my understanding of the physics is at least close to real.

I envision construction mostly in space from materials delivered by returnable cargo vessels. (either shuttle-like or space-X style .. the method isn't important for my understanding)

the hub would be first with a docking ring on either end of a 4 meter diameter cylinder (two ships at a time could dock) and three hatches which will eventually lead to 15 meter long "spokes" for travel from the hub to a toroidal ring.

Initially no spin until the toroidal ring is complete and the "station" is spun up by one of the docking ships.

Once the spin starts there will be "gravity" inside however the rotation will be kind of high.
radius of hub 2meter + 15meter spoke + 4meter toroid +thickness of hatches etc between ~1.5meter would total about 22meter and rotation for "earth like gravity" would be around 7 RPM. Setting aside what would be probably slower rotation during construction so it would be easier.

My question would be that during docking to deliver materials initially all the materials will be on the transport and the transport will match rotational speed to dock and everything is now spinning same speed right? As the materials are moved from the center to the outside that mass will slow the whole thing down? Before each undocking or during the unload/stow phase the transport will need to impart more spin to the station to put back what was lost?

And finally .. long term .. additional spokes lead out to a second toroid and again spokes out to the final diameter toroid, as the diameter of the station gets larger and the mass of the station increases it will become closer to Earth gravity at slower rotation? And the center would be microgravity? and each "ring" would be different "gravity"?

also anyone know if there is a space construction methods/materials thread or forum?

dmac257

 

[sophiecentaur]

You might want to reconsider those dimensions. Someone standing on the rim of the wheel would experience a significant difference in simulated g force between head and feet and the angular velocity would introduce a noticeable Coriolis effect. Merely moving your arms or head up and down would produce some disconcerting results. Why not consider a much larger diameter wheel and avoid that sort of problem? The sort of rotation rate you are suggesting is much higher than you get when a car or boat turns into a long curve and coriolis is quite noticeable, particularly when trying to move around on a boat during a sharpish 180° turn. People could possibly get used to it, of course.
Your comments about needing to maintain the angular velocity when more load is added are quite right but the adjustment could be quite gradual and a warning could be given if and when a heavy load is taken to the periphery. "Hold onto your hats folks"

 

[dmac257]

You might want to reconsider those dimensions. Someone standing on the rim of the wheel would experience a significant difference in simulated g force between head and feet and the angular velocity would introduce a noticeable Coriolis effect. Merely moving your arms or head up and down would produce some disconcerting results. Why not consider a much larger diameter wheel and avoid that sort of problem? The sort of rotation rate you are suggesting is much higher than you get when a car or boat turns into a long curve and coriolis is quite noticeable, particularly when trying to move around on a boat during a sharpish 180° turn. People could possibly get used to it, of course.
Your comments about needing to maintain the angular velocity when more load is added are quite right but the adjustment could be quite gradual and a warning could be given if and when a heavy load is taken to the periphery. "Hold onto your hats folks"

As I stated the initial spin probably will be lower, maybe 2 RPM, as full gravity will not be needed during construction. First ring will be for power distribution and storage of raw materials used in the next part of the construction. After around 1200 square meters of solar panels are installed between the three hubs there will be power to charge the batteries of the robots that will be doing the construction tasks. Much gravity will not be needed till construction of the second ring would be complete and I envision that ring to be 150meters. Spin that at 3 RPM would be slightly less than Earth normal. Construction continues with the extension to the final dimension of 1500meters which is about where the book begins. I still need to work out lots of stuff but it sounds like I am in the right spot for help.

dmac257

 

[sophiecentaur]

@dmac257 These topics have been discussed already, quite a bit on PF (not surprisingly). Search and you will find many opinions. It is an interesting discussion topic because it involved empathising with a totally unknown lifestyle. The engineering consequences of scale are so different from what they are on Earth. The rotational energy of the wheel is not a major factor because it just goes on and on . . . .. Slight changes in speed would be easily coped with (on the full sized wheel) by an experienced crew. It would only be necessary for the human payload to be on the rim. All the massive stuff could be right near the centre, which would make it easy for transport to and from the wheel. Also, the supporting structure for the heavy stuff would be much less stressed near the centre; most things would only weigh a fraction of their weight on Earth.
Just a thought - and it could spoil your scenario a bit - but would there need to be a massive human population on board? Experiments and manufacturing could easily be mostly robot based and the few live crew members could occupy pods on spokes, rather than a fully populated wheel. That could save a vast amount of cost and materials.

I don't think there is any specific place for space construction methods on PF. Something for the future, perhaps; we so often discuss engineering aspects of space travel. A subset of the Engineering section, perhaps. After a few k contributions (lol - but it does happen) perhaps you could initiate one.

 

[dmac257]

@dmac257 These topics have been discussed already, quite a bit on PF (not surprisingly). Search and you will find many opinions. It is an interesting discussion topic because it involved empathising with a totally unknown lifestyle. The engineering consequences of scale are so different from what they are on Earth. The rotational energy of the wheel is not a major factor because it just goes on and on . . . .. Slight changes in speed would be easily coped with (on the full sized wheel) by an experienced crew. It would only be necessary for the human payload to be on the rim. All the massive stuff could be right near the centre, which would make it easy for transport to and from the wheel. Also, the supporting structure for the heavy stuff would be much less stressed near the centre; most things would only weigh a fraction of their weight on Earth.
Just a thought - and it could spoil your scenario a bit - but would there need to be a massive human population on board? Experiments and manufacturing could easily be mostly robot based and the few live crew members could occupy pods on spokes, rather than a fully populated wheel. That could save a vast amount of cost and materials.

I don't think there is any specific place for space construction methods on PF. Something for the future, perhaps; we so often discuss engineering aspects of space travel. A subset of the Engineering section, perhaps. After a few k contributions (lol - but it does happen) perhaps you could initiate one.

Your suggestion isn't a change in scenario. The station is almost entirely built and maintained by robots. Not much experimentation but everything is manufactured in space. Imagine a 3D printer in space, but instead of a flat build plate, the printer moves. Need a 15000liter water tank? It would be expensive to make it and send it whole into space. Print it in space and it becomes economical .. need 70 of them .. even more economical if made inside the station. This is one of the reasons I wondered about a construction methods site. Imagine all this with no human man hours involve. Vacuum Vapor Deposition is hard on Earth because the chamber is sized for the process, the vacuum in space is the perfect place. Other methods might work easier in space, this is just a couple examples. The big question is how long this construction would go on before the first humans could work in atmosphere. This was why I was asking about methods discussion thread.

dmac257

 

[stefan r]

I am writing a science fiction book and doing a little research about habitats in space to make sure my understanding of the physics is at least close to real...

also anyone know if there is a space construction methods/materials thread or forum?
...

Video by Isaac Arthur on spaceports and an earlier one on rotating habitats. Also see the rest of his episodes for trove of information science fiction writers should be aware off.

Project Rho is designed for science fiction writers. It does have some references. Has lots of diagrams and charts and it includes pictures when possible. If you stay within project rho's guidelines readers will never know that you do not know the physics behind something. Here is artificial gravity and space stations.

You might want to reconsider those dimensions. ..

...
My question would be that during docking to deliver materials initially all the materials will be on the transport and the transport will match rotational speed to dock and everything is now spinning same speed right? As the materials are moved from the center to the outside that mass will slow the whole thing down? Before each undocking or during the unload/stow phase the transport will need to impart more spin to the station to put back what was lost?...

Use 2 (or more) rings and counter rotate.

You can pump fuel, air, or water around similar to how ships use ballast.

 

[dmac257]

Video by Isaac Arthur on spaceports and an earlier one on rotating habitats. Also see the rest of his episodes for trove of information science fiction writers should be aware off.

Project Rho is designed for science fiction writers. It does have some references. Has lots of diagrams and charts and it includes pictures when possible. If you stay within project rho's guidelines readers will never know that you do not know the physics behind something. Here is artificial gravity and space stations.

View attachment 220392
Use 2 (or more) rings and counter rotate.

You can pump fuel, air, or water around similar to how ships use ballast.

Thanks for the links. As I said in the OP this research is for clarification about my understanding of the physics of what happens during the construction of the station. there would only be ONE ring and during the construction of the larger diameter ring mass will be moving from the center to the outer ring and slowing down the ring. After the outer ring is built, delivered materials would be moving from the center hub to locations farther from the hub slowing down the station. Initially the station would not have any form of compensation for this. I like the idea of moving water from tanks on the rim towards tanks inward as ballast. This sparks another question. Assume two tanks equal in size with one "higher"(inward) and other "lower"(outward) each filled 1/2 full. If I open a valve between the BOTTOMS of each tank the fluid will move outward till the air on top of the bottom tank compresses enough to stop the flow. If I also open a valve to equalize the air volumes nothing would stop the fluid's flow to the bottom. If I pressurize the air above the lower tank and vent the air in the upper tank it would move the fluid from the lower tank to the upper tank. How do I calculate the pressure needed to move the fluid? I am trying to do this without using electricity but it might just be easier to have a pump. Also how do I calculate the mass of water to move to change the rotational speed so I can design the tank sizes?

<edit> it occurs to me that without knowing the mass of other materials on the outside of the ring at this point there isn't enough information to calculate it anyway. Might be easier to simply measure the rotation and move fluids to maintain rotation .. once the materials are on board and there physically tanks in the outer and inner rings. mass of new materials moving outward would be replaced by mass of fluids moved inward with adjustments being made when there is a supply ship attached.<end edit>
dmac257

 

[256bits]

https://en.wikipedia.org/wiki/Moment_of_inertia
Similar to the figure skater twirling and moving his/her arms in and out changing their rotational speed.
( another convoluted Wiki article - you can probably find better )
https://www.engineeringtoolbox.com/moment-inertia-torque-d_913.html
http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

 

[sophiecentaur]

The actual amount of Energy involved in shifting masses about is very small, compared with the energy needed to get them up there in the first place. The cheapest solution could well be to avoid too much of the in - out - counterbalance arrangement and just use the output from an extra few m³ of PV cells. Time is on your side and no batteries need be involved.

 

[dmac257]

The actual amount of Energy involved in shifting masses about is very small, compared with the energy needed to get them up there in the first place. The cheapest solution could well be to avoid too much of the in - out - counterbalance arrangement and just use the output from an extra few m³ of PV cells. Time is on your side and no batteries need be involved.

How does the electricity of PV cells translate into speeding up the wheel? I agree that nothing need be corrected quickly. My concern is during the construction before completion of the design. What problems will I encounter? It might be possible to change the order of the build to put some kind of system in place, I just don't know what would be needed.
dmac257

 

[sophiecentaur]

How does the electricity of PV cells translate into speeding up the wheel?

You made me re-think that one. The problem of conservation of angular momentum means that you would either need a flywheel reservoir of momentum or an electrical 'gun' with high speed ejecta. (High speed enough to be sure it wouldn't turn up again at some point.) I haven't done the sums but a 'Catharine Wheel' ion drive system could produce some torque - probably not enough for that application.

 

[256bits]

I just don't know what would be needed.

If you want to use the mass movement method, you will have to do the calculations from the post I previously gave.
First off start at the beginning, rather than jumping ahead too much.
First off, your fluid will start out in the centre.
First off there will be NO rotation to your system.
You can move all your fluid to the outer tank, but then to initiate a rotation you still will have to provide a torque.
How will that be provided for?

Secondly, after cargo has been dropped off, and moved to the outer ring(s), you will eventually run out of fluid to pump from the outer tank to the inner.
The outer tank will have to re-filled. That process necessitates a compensating torque on the system, the same one used initially.

You will have to decide if two systems, a compensating mass transfer in-out, along with a compensating torque system, or only the compensating torque system is necessary and/or sufficient. Or perhaps, another mesh of compensating systems so as to achieve constant angular momentum, such as flywheel, as mentioned by @sophiecentaur. In any case, you cannot get away with expending energy, either to re-charge the flywheel, or the outer fluid tank.

How do I calculate the pressure needed to move the fluid?

Same way you would do it pumping one tank to a higher tank on earth.

it occurs to me that without knowing the mass of other materials on the outside of the ring

Why is that important?
The angular momentum of that material is not changing.
The calculation is to be done on the cargo mass moving out, its rate of movement, so that the torque system can compensate, or the flywheel, or the mass transfer system, or combination thereof. Play around with some numbers to get a feel.

 

[dmac257]

If you want to use the mass movement method, you will have to do the calculations from the post I previously gave.
First off start at the beginning, rather than jumping ahead too much.
First off, your fluid will start out in the centre.
First off there will be NO rotation to your system.
You can move all your fluid to the outer tank, but then to initiate a rotation you still will have to provide a torque.
How will that be provided for?

each time the cargo vessel docks to deliver building materials the materials will be off loaded from the center (docking hub) to a staging area. last thing before undocking the cargo will initiate a torque burn.

Secondly, after cargo has been dropped off, and moved to the outer ring(s), you will eventually run out of fluid to pump from the outer tank to the inner.
The outer tank will have to re-filled. That process necessitates a compensating torque on the system, the same one used initially.

agreed, during the delivery docking, the water will be moved from inner tank to outer tank along with the cargo being moved from the center to the staging area, then the torque burn and undocking.

You will have to decide if two systems, a compensating mass transfer in-out, along with a compensating torque system, or only the compensating torque system is necessary and/or sufficient. Or perhaps, another mesh of compensating systems so as to achieve constant angular momentum, such as flywheel, as mentioned by @sophiecentaur. In any case, you cannot get away with expending energy, either to re-charge the flywheel, or the outer fluid tank.

When I said?

it occurs to me that without knowing the mass of other materials on the outside of the ring at this point there isn't enough information to calculate it anyway.

I meant that I don't have a proper feel for the TOTAL mass that WILL be in the outside ring.

Why is that important?
The angular momentum of that material is not changing.
The calculation is to be done on the cargo mass moving out, its rate of movement, so that the torque system can compensate, or the flywheel, or the mass transfer system, or combination thereof. Play around with some numbers to get a feel.

If I understand the numbers, the existing mass doesn't matter. Only the shifting mass, so new materials moving outward will slow the station. Just that if the total mass is 4 billion tons, shifting 16 tons won't change rotation much. But in my case during the construction if the station mass is only 100 tons, shifting 16 tons would change the rotation more. In both cases, I would need to move water from outside inward equal in mass to what construction materials move outward. Kilogram for kilogram and total rotation would stay the same.

Initially the size of the tanks would only need to hold enough mass to compensate for the size of the payload of the cargo vessel. So if 16 tons is delivered would need 16 cubic meters total tank volume in the outer ring and another 16 cubic meters total tank volume in the inner ring. Again on delivery I would need to move the water out and spin up before undocking.

dmac257

 

[sophiecentaur]

Same way you would do it pumping one tank to a higher tank on earth.

I'm not sure that a liquid 'ballast' would be the best solution. A solid mass would be just as manageable and wouldn't require a leakproof container. That would be one less thing to worry about up there. We have to assume that the vast area of the disc would allow plenty of room for a couple of symmetrical elevators to do the job. Exactly the same energy considerations would apply and the masses could be assembled into any shape that was required.

 

[sophiecentaur]

If you want to use the mass movement method, you will have to do the calculations from the post I previously gave.

Firstly , the calculations are not a big deal and, secondly, a servo loop would produce the right result with no formal calculation as long as the available correcting torque is available.
It is important to develop an awareness of the difference between things down here and things up there. There's a whole different lot of criteria to be satisfied up there.
Those graphs of the effect of different g environments on the body should be taken with a pinch of salt as there is hardly any serious long term data available (is there actually any?

 

[stefan r]

You made me re-think that one. The problem of conservation of angular momentum means that you would either need a flywheel reservoir of momentum or an electrical 'gun' with high speed ejecta. (High speed enough to be sure it wouldn't turn up again at some point.) I haven't done the sums but a 'Catharine Wheel' ion drive system could produce some torque - probably not enough for that application.

A drive loses mass.

You can use solar. Light has momentum. Mirrors are cheap. Rotate the mirror so that it knife edges when the mirror orbits toward the sun, 45 degrees when orbiting either parallel, and perpendicular when away from the sun. When the station is rotating perpendicular the sun you can keep the mirrors at 45 degrees all the way around.

You can also aim the radiator panels if you go nuclear. Photovoltaic panels could also provide torque if you are willing to give up the electric power for part of each rotation.

 

[sophiecentaur]

A drive loses mass.

That may be relevant for a starship but not a particular problem for a space station. After all, the visiting ships will be using basic reaction engines at the speeds they would be using.
The solar panels would be pretty vast and would be subject to significant light pressure forces. Normally they are rotated to face the Sun and they would be 'balanced'. But they could be used to 'sail' the wheel round at the right speed. It would not be necessary to give up electric power as they could be still pointing at the Sun - just offset a bit to provide torque for a motor to bear against. Maximum torque would actually be at the same angle as maximum PF output.

 

[dmac257]

That may be relevant for a starship but not a particular problem for a space station. After all, the visiting ships will be using basic reaction engines at the speeds they would be using.
The solar panels would be pretty vast and would be subject to significant light pressure forces. Normally they are rotated to face the Sun and they would be 'balanced'. But they could be used to 'sail' the wheel round at the right speed. It would not be necessary to give up electric power as they could be still pointing at the Sun - just offset a bit to provide torque for a motor to bear against. Maximum torque would actually be at the same angle as maximum PF output.

So, if I understand it correctly, you are saying the solar panels would be on its own counter-rotating gantry with a motor supply torque between the station and the panels gantry system?
dmac257

 

[stefan r]

So, if I understand it correctly, you are saying the solar panels would be on its own counter-rotating gantry with a motor supply torque between the station and the panels gantry system?
dmac257

Yes, something like a weed wacker or unicycle. Same way a helicopter uses a tail rotor but a mirror instead.

Lots of possible arrangements. Here they just need to move one panel over and they start building torque.

This one should be able to work.

In this one you only need 1 of the 4 rotating habits.

 

[sophiecentaur]

So, if I understand it correctly, you are saying the solar panels would be on its own counter-rotating gantry with a motor supply torque between the station and the panels gantry system?
dmac257

Same with all satellites; the panels are rotated to face the Sun at all times. Standard practice is to have a balanced pair of arrays because radiation pressure would make the craft rotate. The Olympus TV satellite lost steerage on one panel and its life was shortened as propellant had to be used to keep it pointing. Caveat - I have done no sums about the quantities involved and the available torque may be far too little for my suggestion to work.
But the energy needed for wheel rotation could easily be factored into the cost of supply launches. Not much of a problem in the overall scale of things.

 

[stefan r]

... Caveat - I have done no sums about the quantities involved...

solar radiation pressure at 1 au is 4.54 μPa

...The total force exerted on an 800 by 800 meter solar sail, for example, is about 5 Newtons (1.1 lbf) at Earth's distance from the Sun, making it a low-thrust propulsion system, similar to spacecraft propelled by electric engines, but as it uses no propellant...

It is not a lot of umph but the momentum builds up and there is nothing to slow it down. It also varies by location. Around Mercury the same mirror gets more like 50 Newtons.

Should point out that you cannot move from the moving parts to the non-moving section without going through a vacuum. Sci-fi stories often miss that part. You could bury the entire centrifuge inside of a larger station/ship. When it is internal people could travel from one part to the other without EVA suits. You would need to draw power to compensate for drag.

 

[sophiecentaur]

It is not a lot of umph but the momentum builds up and there is nothing to slow it down.

The rotation speed of the panel mount is only the orbital rate and it would normally be rotating almost free relative to the ship and perfectly balanced. Some coupling between ship and (offset) panels could provide torque, in either direction.

Should point out that you cannot move from the moving parts to the non-moving section without going through a vacuum.

Why shouldn't the whole thing rotate? It's quite acceptable for the docking ship to rotate itself before docking. At the rate that the OP is proposing, no one is going to get giddy.
But the rotational control would only be needed due to movement of stuff to the rim. That mass would be mainly people and lightweight equipment. Air, food and water in and out would largely be balanced flow from life support at the hub. I think this problem is, (relative to the rest of it all) fairly trivial.

 

[Tiran]

When the docked ship is ready to leave, it's docking port could be electrically spun back to zero. The act of slowing the ship rotation would speed up the rotation of the station, just like using a counterweight, and would make it easier on the ship crew.

But the easiest solutions are going to involve using an electric motor to counterspin something rather than having huge solar reflectors or reaction mass. Flyweights are reaction mass that you can reuse.