Space centerfuges. How big should we make them?

[Kerbart]

4 hours ago, kerbiloid said:

And the cable elasticity turns it into yoyo.

That is an excellent thought. On earth we don’t have to deal with that (gravity, atmospheric drag) but in space that will likely be an issue. It should dampen out under continuous spin though, eventually... right?

[linuxgurugamer]

Let me see if I have this right.

Let's assume that each vessel weighs about the same as Slylab.

That's about 85 tons in each vessel.  Total  of 170 tons at 1g.

So you now need a cable capable not only of supporting 170 tons, but being able to do thai in space, experiencing extreme high and low temperatures.

You also want to be an able to wind it up which will require spools at each end in order to keep the masses even.

Now you have to be able to distribute that 170 tons of force evenly on each end.

I could go on. But it's not as simple as string a cable

[Wjolcz]

6 hours ago, linuxgurugamer said:

Well, to start, the stresses may not be the same.  Then you have the issue of spinning up and back down.  Midcourse corrections will be difficult. To say the least.  Finally, if the cable breaks. The tanker and ship will be on diverging courses, both of which will be sure to miss the target.

This isn't that big of a deal if both ships are identical and can perform the correction burns on their own. One just has fuel where crew would normally be. Or:

1 hour ago, tomf said:

For midcourse correction you would just cancel the spin, wind the cable in and reconnect the halves, perform the burn and then spin up again.

 

1 hour ago, linuxgurugamer said:

That's about 85 tons in each vessel.  Total  of 170 tons at 1g.

What about the mass being distributed unevenly? Have a heavier tanker on one end and a crew ship on the other?

1 hour ago, linuxgurugamer said:

But it's not as simple as string a cable

Still simpler than an air donut in space.

Those donut centrifuges are a waste of living space and building material. They cost more, are harder to build and use and a pain to get into space.

Edited July 2, 2019 by Wjolcz

[kerbiloid]

Every time when:

• humans walk along the cabin or stand up/sit down;
• humans drink/undrink water;
• RCS engines spend fuel;
• hull extends/shrinks due to temperature variations;
• some equipment detail moves  inside;
• some antenna rotates outside, following the Earth,

the system CoM moves.

In a hard centrifuge you can compensate this by unstopabble relocation of counterweights.

In the cable system this makes the cable tension grow/decrease.
This means waves, oscillations, and sudden tugs and jumps.
At last this will either break the cable or make the cabins get chaotically jumping and breaking each other.

43 minutes ago, Wjolcz said:

Those donut centrifuges are a waste of living space and building material.

+1
Donuts are not true. A bunch of parallel cylinders are.

Edited July 2, 2019 by kerbiloid

[linuxgurugamer]

44 minutes ago, Wjolcz said:

This isn't that big of a deal if both ships are identical and can perform the correction burns on their own. One just has fuel where crew would normally be. Or:

The suggested use was a crew ship and a supply ship.  Big difference in design, mass, etc.

 

45 minutes ago, Wjolcz said:

Still simpler than an air donut in space.

Those donut centrifuges are a waste of living space and building material. They cost more, are harder to build and use and a pain to get into space

Really?  Has anyone ever tried to do either of these, or is it still just a talking point?

[Wjolcz]

1 hour ago, linuxgurugamer said:

Really?  Has anyone ever tried to do either of these, or is it still just a talking point?

How much cheaper/simpler is a cable with a winch and some sort of dampening system vs a whole rotating mechanism that forces the designers to put two of those on a ship to counter the rotation?

Ok, so let's go with an O'Neill cylinder if spinning sections on otherwise rigid ships are too problematic. Are they easier to build? It depends how much money you have to throw it at all of the problems of building one of these.

None have been built and tested in space. But at least one of those technologies is being used here on Earth all the time in pretty much any major sea port or construction site. It's a no-brainer IMO. The details can be worked out with a bit of testing.

[Kerbart]

3 hours ago, linuxgurugamer said:

Let me see if I have this right.

Let's assume that each vessel weighs about the same as Slylab.

That's about 85 tons in each vessel.  Total  of 170 tons at 1g.

85 tons. It’s a common gotcha in physics exams.

Imagine a 1 ton weight hanging from a rope attached to the ceiling. Obviously the force pulling on the rope is one ton, right (yeah bring it on, pedantic 9,815 N whiners; the units are not the problem we’re dealing with here)?

Now replace the ceiling with a giant hand pulling the string.

Has anything changed? But now there’s an upward force of one ton at the end of the string! Yes, there always was. In an equilibrium there’s always two equal (but opposite) forces at work.

Still, for two 85 ton vessels keeping each other in a stable spin there’s 85 tons pulling, not 170. If you’re using plain FE360 (not that anyone would, in space), you probably want some safety margins (based on a max elastic deformation limit of 300 N/mm², so let’s settle for 150 N/mm², that gets us a cable area of 5,560 mm², or a cable diameter of 84 mm. That is substantial—about 3³/₈” for the non-metric speakers here, and likely adds to the weight, though it’s likely we’d be using a light-weight, high tensile material in reality. Good point.

[linuxgurugamer]

17 minutes ago, Kerbart said:

Still, for two 85 ton vessels keeping each other in a stable spin there’s 85 tons pulling, not 170. If you’re using plain FE360 (not that anyone would, in space), you probably want some safety margins (based on a max elastic deformation limit of 300 N/mm², so let’s settle for 150 N/mm², that gets us a cable area of 5,560 mm², or a cable diameter of 84 mm. That is substantial—about 3³/₈” for the non-metric speakers here, and likely adds to the weight, though it’s likely we’d be using a light-weight, high tensile material in reality. Good point.

Ummm, I disagree.  think about it, if you had something at the centerpoint.  There would be 85 tons in each direction.  Adding those together would be 170 tons total.

each vessel would be exerting 85 tons to the imaginary axle in the center

17 minutes ago, Kerbart said:

Imagine a 1 ton weight hanging from a rope attached to the ceiling. Obviously the force pulling on the rope is one ton, right (yeah bring it on, pedantic 9,815 N whiners; the units are not the problem we’re dealing with here)?

Now replace the ceiling with a giant hand pulling the string.

The giant hand isn't also moving and generating additional centrifugal force in the opposite direction

Here is another example.

You have a vertical axis.  Attached to it is a balance scale, and then your 1 ton weight.  The axis spins at a speed so that the weight pulls outward with a force of 1 ton.

Now, add another weight on the other side.  Now when they spin, each pulls outward with a force of 1 ton, the combined force on the center axis is 2 tons.

 

Edited July 2, 2019 by linuxgurugamer

[kerbiloid]

A sign change point.

[Nuke]

7 hours ago, linuxgurugamer said:

Let me see if I have this right.

Let's assume that each vessel weighs about the same as Slylab.

That's about 85 tons in each vessel.  Total  of 170 tons at 1g.

So you now need a cable capable not only of supporting 170 tons, but being able to do thai in space, experiencing extreme high and low temperatures.

You also want to be an able to wind it up which will require spools at each end in order to keep the masses even.

Now you have to be able to distribute that 170 tons of force evenly on each end.

I could go on. But it's not as simple as string a cable

i dont think id use a cable. how about we take our capsule, strip it down to make it as light as possible and move everything to the service module. things like batteries, life support, consumables, if you have a nuclear reactor or a separate descent stage, etc. rather than the service module be mounted behind the capsule apollo style, it connects to the front with a long narrow cylindrical boom, and the service module at the other end. the point is to put the cg as far from the crew module as possible, close to the service module. also if its close to the service module then you dont need a whole lot of extra structure there as there is less spin gravity acting on those components. the boom would also be fairly rigid since there will need to be power and life support linkages running through it, possibly fuel as well (for rcs) and crew access (your snacks are over there). since the whole structure is ridgid you could make on-spin course corrections. spin axis would be aligned with the burn vector and thrusters on cm/sm would perform the burn. or perhaps a mid boom thruster pack that hangs out around the cg (which being variable might require the engine pod traverse the boom). 

 

obviously the design gets better the larger you build it. if you can keep your full life support and supplies in the crew area and eliminate the need for crew access to the service module that would add safety to the design. the boom could just be an open truss with few or no linkages and making it longer increases the gravity quality. you could fix the central engine pod and transfer fuel between ballast tanks to keep the cg lined up with the engines reducing complexity (no transversal required). would be great design for mars and outer planets long haul manned missions. i want to demonstrate this in ksp but i have things to do at the moment, maybe later.

Edited July 2, 2019 by Nuke

[tater]

As I said in the other version of this thread/post, it's also a matter of how big CAN you build them. For carbon (nanotubes and graphine), the answer is a circumference of at most ~6500km. So the largest possible radius is likely ~1000km.

[Bill Phil]

5 hours ago, linuxgurugamer said:

Ummm, I disagree.  think about it, if you had something at the centerpoint.  There would be 85 tons in each direction.  Adding those together would be 170 tons total.

each vessel would be exerting 85 tons to the imaginary axle in the center

Exactly, in either direction.

The vectors for the centripetal (centrifugal in a different frame) acceleration of either would be in opposite directions. If we decided one was in the positive direction (in the rotating frame of reference) the other must be in the negative direction. The sum of the two is thus zero.

This makes intuitive sense - the only net force that should be acting upon the center of gravity is - well, gravity. If the total force was 170 tons it would veer off course.

Basically, Newton's Third Law means that the sum of forces on the whole system is zero, but on individual pieces it can be non zero. For example, in a rocket-exhaust system the net force will be zero save for gravity. But the rocket experiences force, as does the exhaust. But in the combined system the forces must sum to zero. If we added a more detailed gravitational model and accounted for the CG of the entire planet-rocket-exhaust system, the same would apply even for gravity.

Essentially we can take a cable that can support 85 tons on Earth, build a second one and then put the two together in space. After spinning them up the net tension is zero but the tension in each half has a magnitude of 85 tons times the acceleration.

You're summing magnitudes, not vectors.

5 hours ago, linuxgurugamer said:

The giant hand isn't also moving and generating additional centrifugal force in the opposite direction

Here is another example.

You have a vertical axis.  Attached to it is a balance scale, and then your 1 ton weight.  The axis spins at a speed so that the weight pulls outward with a force of 1 ton.

Now, add another weight on the other side.  Now when they spin, each pulls outward with a force of 1 ton, the combined force on the center axis is 2 tons.

 

I don't think that's how it would add up. Though you are only measuring magnitudes, the directions are opposite, and thus the net force is zero.

Don't get me wrong, you need strong cable, but your design requirement is not supporting 170 tons, but 85 on either side. Like a pulley system that has a cable running around it with two equal masses on either side - assume it's in equilibrium. Each side only has to support the attached mass, not the total mass.

1 hour ago, tater said:

As I said in the other version of this thread/post, it's also a matter of how big CAN you build them. For carbon (nanotubes and graphine), the answer is a circumference of at most ~6500km. So the largest possible radius is likely ~1000km.

As far as my understanding goes this can be extended - the breaking length of a material is applicable only if it is supporting itself, however it should be possible to have additional support columns stemming from the center of rotation. These can even be tapered to optimize strength. That said, there still is a maximum size, but I suspect it is larger than 1000 km radius. 

Basically the breaking length of a material is how far it can go when supporting itself, but I don't think it has to only support itself.

Though I doubt any such structures will be much larger than 1000 km radius, only that these support columns would add a degree of safety. The design may be similar to a suspension bridge, with the idea of transferring loads to the tensile supports radiating away from the center.

[Kerbart]

6 hours ago, linuxgurugamer said:

You have a vertical axis.  Attached to it is a balance scale, and then your 1 ton weight.  The axis spins at a speed so that the weight pulls outward with a force of 1 ton.

Now, add another weight on the other side.  Now when they spin, each pulls outward with a force of 1 ton, the combined force on the center axis is 2 tons.

The problem is, that if you're only pulling on one side, the scale is going to show 0 as the whole ensemble is going to accelerate with 1g in the direction that you're pulling. If you want the scale to remain in the center, you have to zero that force out. That's what happens on earth. When Yo Mamma steps on the scale, it's not just being pushed on from the top - it would sink through the surface of the earth. The earth is pushing back as well.

 

Edited July 3, 2019 by Kerbart
In the second diagram, the scale shows 85 tons. Doh.

[tater]

14 minutes ago, Bill Phil said:

As far as my understanding goes this can be extended - the breaking length of a material is applicable only if it is supporting itself, however it should be possible to have additional support columns stemming from the center of rotation. These can even be tapered to optimize strength. That said, there still is a maximum size, but I suspect it is larger than 1000 km radius. 

True, usually the huge "Orbital" type (the Banks name for them) are not shown with spokes, but yeah, that's presumably possible, with each section between spokes limited by braking strength.

[Bill Phil]

2 hours ago, tater said:

True, usually the huge "Orbital" type (the Banks name for them) are not shown with spokes, but yeah, that's presumably possible, with each section between spokes limited by braking strength.

I’m not sure if an Orbital is possible.

The acceleration goes down with 1/r though and not 1/r^2.

Could be interesting to study loads in such a system and maybe simulate it, potentially finding the maximum theoretical size.

Millions of kilometers in radius doesn’t seem possible but maybe tens of thousands...

[tater]

10 minutes ago, Bill Phil said:

Millions of kilometers in radius doesn’t seem possible but maybe tens of thousands...

Yeah, the huge ones with a 24 hour rotation period won't work, obviously (that's 1.8 million km radius).

The 1000km radius version 500km wide has a name apparently, a Bishop Ring. Anyway, it has about the area of India, so there's plenty of room.

[kerbiloid]

On 7/3/2019 at 2:39 AM, Nuke said:

we take our capsule, strip it down to make it as light as possible and move everything to the service module. things like batteries, life support, consumables

There is a small problem with this minimalistic design.

By definition , the artificial gravity is required for a long flight.
And in a long trip, the crew requires ~28 m³ of free cabin space per person.

The Harmony ISS module is 15 t heavy and ~78m³ empty.
So, it's a habitat for 2..3 humans.

So, the crew requires ~8 t of habitat per person.

A human spends ~20 kg of resources per day (drinking water, washing water, cleaning water,the breathing water, food, napkins, clothes), including ~15 kg of water.
So, if the flight lasts for 3 years, every human spends ~6 t of expendables + 16 t of water.

Thus, even without any water recycling, the service module would weight nearly the same as the habitat.
As a crew of 6 needs ~100 t of water for the Martian flight, they anyway should use a water recycler, so the habitat will weight more or nearly same as the service module.
This means that the arms of such centrifuge will have nearly same length, and here we return to a typical symmetric centrifuge.

Edited July 4, 2019 by kerbiloid