# Hard sci-fi question: rotational artificial gravity space station



## Quickleaf (Apr 17, 2016)

Taking a Bernal sphere for example, what would conditions on a space station be like in regards to gravity?







From NASA and Wikipedia sites, I've read that some areas of the station would spin (have artifical G) and others would be stationary (zero-G). For example, the "residential sphere" would spin, and centrifugal force would simulate gravity so buildings would be constructed on the inner walls of the sphere.

Would centrifugal force artificial G effectively operate like the gravity we experience? Or would there be some sort of weird curve that falling objects would experience?

Within the spinning sections, there'd 1/3rd to Earth-normal G (depending on specifics) along the "equator" of the section, with diminishing G the closer one gets to the axes. A lot of the Bernal sphere designs mention Zero-G recreation areas near the axes, though it probably wouldn't be true zero-G but more like low-G, right?

And my big question: What would it be like to be in the center of the spinning sphere? Imagine taking the sphere and dropping a tube from one axis to the other. Along that tube, what kind of "gravity" would exist? Since centrifugal force is strongest away from the center of the spinning object, would it essentially be zero-G?


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## Morrus (Apr 17, 2016)

Assuming it's the right rotation rate, on the inner surface it'll be 1G and indistinguishable from Earth. Depending on the size of the sphere, gravity might decrease rapidly so that the second floor of a building might be noticeably lower G. Climbing ladders could be odd! If it's small enough, your head might be in perceivable lower G.  With a larger sphere (like the one illustrated above), that effect would not be noticeable unless you climbed several floors at least.

The centre of the sphere would indeed be zero G.

Actually, we have a couple of pretty scientific types on the boards here, so @_*Umbran*_ or @_*freyar*_ might have more to add. I'm sure there's an equation or something you can use to determine the gravity at a given distance from the 'wall' for sizes of sphere/speeds of rotation.


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## RangerWickett (Apr 17, 2016)

My understanding is that if you entered the sphere at its axis, you'd just float, and the world would spin around you. 

If you went to where the equatorial plane crossed the axis, then pushed yourself toward the 'ground,' you still wouldn't feel any gravity. You'd just travel at whatever speed you accelerated to (and would slow down due to air resistance). However, the ground beneath you would be spinning, possibly quite fast. If you drifted to the ground, at some point you'd probably move into the path of something like a tree or building, and it would hit you and then drag you along the path of rotation. Only then would you start to experience gravity, as the rotational movement tries to move you away from the center, and the ground keeps you from flying away.

Check out this calculator: http://www.artificial-gravity.com/sw/SpinCalc/SpinCalc.htm

As an example, a 38 meter radius ring (239 meter circumference) rotating at 3 revolutions per minute produces an effective 1/3 G at 'ground level.' From a stationary observer it's moving at 43 kph, or 27 miles per hour. So even if there were no obstacles, if you drifted into it'd be like bouncing into a wall while traveling at 27 mph. Probably not fatal, but pretty unpleasant.

If you have the same ring and spin it almost 5 times per minute, you get a velocity of 44 mph. You probably don't want to fly into a house at that speed. So a well designed station would have ladders or elevators down from the central spur that would slowly increase your tangential velocity.


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## Morrus (Apr 17, 2016)

That calculator is fantastic!  Really useful tool for this sort of thing.


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## MarkB (Apr 17, 2016)

It would feel effectively like normal gravity at ground level, but thrown objects might not follow exactly the path you expect, due to the rotating frame of reference. It's a tricky thing to visualise.


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## Quickleaf (Apr 17, 2016)

Awesome feedback  Thanks!

My example is a 1 km diameter (500 m radius) "squashed" sphere.

  [MENTION=1]Morrus[/MENTION] That's exactly one of the questions I'm trying to determine. How high up do you have to climb a building for there to be noticeable change in gravity. I think it would affect how high-rise type buildings were constructed, since the shearing forces (might be using the wrong term) between regular G and lower-G would require stronger building materials. Plus it might suggest activities happening at the upper levels of high-rise buildings would be substantially different...for example moving construction activities to the lower-G zones for increased efficiency.

  [MENTION=63]RangerWickett[/MENTION] Really helpful on how to visualize entering at the zero-G "fixed" axis and seeing the entire station spin around you. I suspected some kind of shuttle or elevator would be necessary, but hadn't conceived of exactly why...

I plugged a 500 m radius in and got a Tangential Velocity (or "rim speed") of 156 mph, which would be "splat your dead" for anyone moving or falling from the zero-G axis to the ground...in scientific terms 

  [MENTION=40176]MarkB[/MENTION] That's another one of my questions. I mean, nothing we throw on Earth actually travels straight, technically. But in the rotational artificial G environment I'm wondering if it would be more obvious...or would it basically be a case of "throwing a baseball while in a moving car"? In other words, if everything/everyone is rotating at the same rate in relation to each other, there doesn't appear to be any change from Earth-standard gravity (assuming 1 g centripetal acceleration).

But what happens if I punt a football down a field or fire a railgun at the elevator/shuttle tube along the central axis when the station is rotating at 1.3 rpms and the rim is spinning at 156 mph?


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## Umbran (Apr 18, 2016)

Quickleaf said:


> My example is a 1 km diameter (500 m radius) "squashed" sphere.




How do you want it "squashed"?  Here's a note:  the sphere is in many ways a bad idea.  A cylinder is a better bet.  The centrifugal force felt depends upon the distance from the axis of rotation.  So, if you spin the thing so you feel a full G at the equator, anywhere "north" or "south) of the equator has lower force.  If you want more surface to be at 1 G, use a cylinder.




> [MENTION=1]Morrus[/MENTION] That's exactly one of the questions I'm trying to determine. How high up do you have to climb a building for there to be noticeable change in gravity.




It depends upon the size of the sphere.  If your change in elevation is negligible compared to the radius, you won't feel much difference.  If you are talking about a 100 meter tall building in a 1 KM radius sphere, you'll notice it.



> I think it would affect how high-rise type buildings were constructed, since the shearing forces (might be using the wrong term) between regular G and lower-G would require stronger building materials.




If you can build the sphere in the first place, building materials are probably not your biggest problem.  Remember that we *are* talking about cfreating a very large structure *in space* already.



> [MENTION=40176]MarkB[/MENTION] That's another one of my questions. I mean, nothing we throw on Earth actually travels straight, technically.




Well, be careful there.  When we are talking about deflection due to spin, it is actually that the object is moving straight, and the Earth is spinning under it!  



> But in the rotational artificial G environment I'm wondering if it would be more obvious...or would it basically be a case of "throwing a baseball while in a moving car"? In other words, if everything/everyone is rotating at the same rate in relation to each other, there doesn't appear to be any change from Earth-standard gravity (assuming 1 g centripetal acceleration).




Again, the difference will depend in general on the size of the sphere - the larger the sphere, the less noticeable the effect will be.  

In general, if you are throwing directly along or against the spin (so "east" or "west" - or what you'd probably want to call "spinward" and "anti-spinward"), what you'll see is your throw fall short or go farther than you'd expect.  If you throw north or south, you'll see it go wide to the left or right some.  



> But what happens if I punt a football down a field or fire a railgun at the elevator/shuttle tube along the central axis when the station is rotating at 1.3 rpms and the rim is spinning at 156 mph?




Not really in a position to work out the exact numbers at the moment, I'm afraid.  Perhaps later.


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## freyar (Apr 18, 2016)

There's a lot of good stuff here already, so I'm not sure exactly what to add.  As people have already said, if you're not already rotating with the station, you won't feel any "gravity."  Also, the Coriolis effect will be a lot more pronounced.  So, if you throw a ball "straight up," it will come back down but will not come back to you.  From your perspective (standing at one spot on the inner surface of the station), the ball will curve off as it flies up and down.


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## Quickleaf (Apr 18, 2016)

Umbran said:


> How do you want it "squashed"?  Here's a note:  the sphere is in many ways a bad idea.  A cylinder is a better bet.  The centrifugal force felt depends upon the distance from the axis of rotation.  So, if you spin the thing so you feel a full G at the equator, anywhere "north" or "south) of the equator has lower force.  If you want more surface to be at 1 G, use a cylinder.



Thanks for the feedback [MENTION=177]Umbran[/MENTION]! I'd looked at the O'Neill cylinder early on. My understanding, however, was that it might be advantageous (in terms of medical health, crystal production, maybe other things I'm unaware of) to have areas of low-G or micro-G in addition to 1 G. Is that wrong?



> In general, if you are throwing directly along or against the spin (so "east" or "west" - or what you'd probably want to call "spinward" and "anti-spinward"), what you'll see is your throw fall short or go farther than you'd expect.  If you throw north or south, you'll see it go wide to the left or right some.



Is there an easy way (or perhaps a site) I could approximate how severe the range increase/decrease or divergence would be? My hunch is that we're talking something relatively slight, maybe 10% difference at 100 m throw/shot, and not a severe change?


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## Umbran (Apr 18, 2016)

Quickleaf said:


> My understanding, however, was that it might be advantageous (in terms of medical health, crystal production, maybe other things I'm unaware of) to have areas of low-G or micro-G in addition to 1 G. Is that wrong?




Medical health?  Well, older folks might like *slightly* lower G, but not really low-G or micro-G.  We have to spend a lot of effort mitigating and repairing the effects of extended low-G, rather than wanting to use it much.

There are certainly industrial uses, but all you need to do then is put those facilities *on the axis*, rather than down on the very, very valuable habitation real estate.



> Is there an easy way (or perhaps a site) I could approximate how severe the range increase/decrease or divergence would be? My hunch is that we're talking something relatively slight, maybe 10% difference at 100 m throw/shot, and not a severe change?




There are a whole bunch of variables - size of the sphere, direction you throw, speed of the projectile, and so on.  Even the geometry may matter if you expect the projectile goes far enough that you can't consider the ground to be flat under it.  I don't know of any calculator made for it.  It is all soluble stuff, but there's lots of details.

I'd just say... I'n not want to play baseball in it.


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## Quickleaf (Apr 18, 2016)

freyar said:


> There's a lot of good stuff here already, so I'm not sure exactly what to add.  As people have already said, if you're not already rotating with the station, you won't feel any "gravity."  Also, the Coriolis effect will be a lot more pronounced.  So, if you throw a ball "straight up," it will come back down but will not come back to you.  From your perspective (standing at one spot on the inner surface of the station), the ball will curve off as it flies up and down.




I found one Coriolis effect calculator online, but it was asking for Latitude and wind speed and didn't make sense.

I also found something interesting written by Larry Bogan  about the seeming increase/decrease in astronaut weight (and therefor their speed) depending on whether running in the direction or rotation or against it. Just like a people mover at the airport.


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## Eltab (Apr 18, 2016)

Somebody (forget my original source) figured out that a 2001-like spinning station is too small: people get dizzy and disoriented from the rotation.  If you want to use a huge (km+ between 1G "ground" level and rotational axis) structure, that problem should fade away.

Larry Niven introduced a location called _Farmer's Asteroid_ in his Known Space series, which more-or-less is what you are looking at here.  He does not develop it extensively (characters visit, they don't stay) but you might be able to use the fragments for inspiration or to fill in parts you don't want to craft yourself.


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## Eltab (Apr 18, 2016)

Umbran said:


> I'd just say... I'd not want to play baseball in it.



Oh, I don't know about that.  (I'm horrible at baseball anyway; can't throw straight, never caught a fly ball in my life but have had some bounce off, slow-to-average run speed.)
Several astronauts on the Moon did 'basic science' experiments - drop a feather and a rock; gravity pulls all things equally - that were pretty cool, and watching the differences between 'centrifugal baseball' and 'gravity baseball' could be a learning opportunity.


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## Quickleaf (Apr 18, 2016)

Eltab said:


> Somebody (forget my original source) figured out that a 2001-like spinning station is too small: people get dizzy and disoriented from the rotation.  If you want to use a huge (km+ between 1G "ground" level and rotational axis) structure, that problem should fade away.



That's exactly why I went with a larger sized station, to avoid the "gravity disorientation/nausea" effect.

It would be interesting if there was a zero-G tram running the central axis of the sphere, which then connected to various elevators leading to the "ground." The view out that tram window would be of a 2.36 km circumference spinning at roughly 1.5 RPM. That's a pretty fast spin! 

Even if the numbers were smaller for a more efficient cylinder design, you'd still see the ENTIRE interior spin around the tram a couple times en route to your elevator.



Eltab said:


> Oh, I don't know about that.  (I'm horrible at baseball anyway; can't throw straight, never caught a fly ball in my life but have had some bounce off, slow-to-average run speed.)
> Several astronauts on the Moon did 'basic science' experiments - drop a feather and a rock; gravity pulls all things equally - that were pretty cool, and watching the differences between 'centrifugal baseball' and 'gravity baseball' could be a learning opportunity.




I was thinking that — if you had a reflective surface you could see another person in — you could, with practice throw a baseball so that it curved around an intervening obstacle to reach their catcher's mitt.


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## freyar (Apr 18, 2016)

Quickleaf said:


> I found one Coriolis effect calculator online, but it was asking for Latitude and wind speed and didn't make sense.




That calculator is set up for the Coriolis effect on earth and apparently for the effects on winds (hence the wind speed entry).  In case you're not familiar with the Coriolis effect, imagine you're sitting on a (fairly rapid) merry-go-round.  If you toss a tennis ball, it will not only appear to move outward but also curve away from you in a direction that depends on the direction you initially threw it.


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## Morrus (Apr 18, 2016)

Is there a reason for the empty space in the middle? It seems it would be a bit disorienting seeing ground above you, plus pretty wasteful. Would there not be a second wall, maybe a couple of hundred feet in from the exterior one, so that folks looking up just saw a ceiling? You could even use that second wall as a second floor; maybe "layer 1" is normal gravity, and "layer 2" is low gravity. Industrial stuff.  Some low-G sports fields. That sort of thing.


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## Umbran (Apr 18, 2016)

Morrus said:


> Is there a reason for the empty space in the middle? It seems it would be a bit disorienting seeing ground above you, plus pretty wasteful.




It wastes space.  But, dude, you're in *space*.  It isn't like you're strapped for room.  The only space limitations is in how much it costs to get the building materials up there.  And, on that score, the second layer costs only marginally less than the outer one.  So, if you are going to spend that money, why not make the living area just that much bigger?


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## tomBitonti (Apr 18, 2016)

Hi,

As above.  There are references for a maximum RPM: 1-3 RPM is taken as _probably_ necessary:

See: https://www.quora.com/What-is-the-minimum-size-for-a-ring-shaped-rotating-space-station



> About 2 rpm is considered safe for an astronaut.  So, if we use our equation: (Omitted)
> and we put in 2 for Omega and 9.8 for g, we end up with an r of 223.49 meters.
> 
> Now, that is the safe number, it would still be a distraction.
> ...




Also:



> In brief, at 1.0 rpm even highly susceptible subjects were symptom-free, or nearly so.  At 3.0 rpm subjects experienced symptoms but were not significantly handicapped.  At 5.4 rpm, only subjects with low susceptibility performed well and by the second day were almost free from symptoms.  At 10 rpm, however, adaptation presented a challenging but interesting problem.  Even pilots without a history of air sickness did not fully adapt in a period of twelve days.




A very definite problem in low-G environments (or at least in zero-G / micogravity environments) is the loss of bone strength.
See, for example http://www.space.com/6354-space-station-astronauts-lose-bone-strength-fast.html.



> Astronauts that spend long months aboard the International Space Station lose bone strength faster than previously thought and have a higher risk of breaking their hips later in life, a new study reports.
> 
> A survey of 13 space station astronauts found that their bone strength dipped by at least 14 percent on the average during their half-year stays aboard the orbiting laboratory.
> 
> Three of the astronauts lost up to 30 percent of their bone strength during their long-duration spaceflights, putting them on par with the bone strength of older women with osteoporosis on Earth, the study reported.




Thx!
TomB


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## Quickleaf (Apr 20, 2016)

Morrus said:


> Is there a reason for the empty space in the middle? It seems it would be a bit disorienting seeing ground above you, plus pretty wasteful. Would there not be a second wall, maybe a couple of hundred feet in from the exterior one, so that folks looking up just saw a ceiling? You could even use that second wall as a second floor; maybe "layer 1" is normal gravity, and "layer 2" is low gravity. Industrial stuff.  Some low-G sports fields. That sort of thing.




I've been thinking the same thing. A lot of the NASA designs are based on these open floor plan idyllic space colonies. I think the scale of these things is hard to imagine, since they're basically arcologies with their own weather systems and agricultural fields.

I agree that a more realistic take of the rotating sections is to have at least 2 layers set up with a ceiling/wall separating the strictly residential from the other uses that take advantage of low-gravity. Can't see any practical reasons not to do that, and it's how the Babylon 5 and Deep Space Nine stations were set up IIRC.


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## Umbran (Apr 20, 2016)

Quickleaf said:


> I agree that a more realistic take of the rotating sections is to have at least 2 layers set up with a ceiling/wall separating the strictly residential from the other uses that take advantage of low-gravity. Can't see any practical reasons not to do that, and it's how the Babylon 5 and Deep Space Nine stations were set up IIRC.




Again - you are thinking like someone who has to use compact architecture for reasons of limitations of real estate area, and the structural strength of materials that have to support their own weight under compression.  Neither of these hold for a structure Out There.  The stresses on the structure resemble those on a suspension bridge more than those on a skyscraper - specifically, most of a terrestrial building's forces are "compression", while this spinning sphere or cylinder instead has lots of tension.  And you don't have to worry about restricting your building to fit in a small ground footprint.

The structures we are considering were intended to maximize "normal" living area and psychological impact.  The idea is to have high open spaces, because humans are designed psychologically to walk under the open sky on a regular basis.  If you put in a ceiling, you negate that.

By the way, DS9 didn't have any major open areas - it was built like a starship that didn't move, and the biggest open areas we saw were large concourse hallways ("The Promenade"), at best a couple stories high.  But DS9 assumed artificial gravity generation, not using spin to generate gravity.


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## Umbran (Apr 20, 2016)

Quickleaf said:


> I agree that a more realistic take of the rotating sections is to have at least 2 layers set up with a ceiling/wall separating the strictly residential from the other uses that take advantage of low-gravity. Can't see any practical reasons not to do that, and it's how the Babylon 5 and Deep Space Nine stations were set up IIRC.




Again - you are thinking like someone who has to use compact architecture for reasons of limitations of real estate area, and the structural strength of materials that have to support their own weight under compression.  Neither of these hold for a structure Out There.  The stresses on the structure resemble those on a suspension bridge more than those on a skyscraper - specifically, most of a terrestrial building's forces are "compression", while this spinning sphere or cylinder instead has lots of tension.  And you don't have to worry about restricting your building to fit in a small ground footprint.

The structures we are considering were intended to maximize "normal" living area and psychological impact.  The idea is to have high open spaces, because humans are designed psychologically to walk under the open sky on a regular basis.  If you put in a ceiling, you negate that.

By the way, DS9 didn't have any major open areas - it was built like a starship that didn't move, and the biggest open areas we saw were large concourse hallways ("The Promenade"), at best a couple stories high.  But DS9 assumed artificial gravity generation, not using spin to generate gravity.


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## MechaPilot (Apr 20, 2016)

Since the image in the OP shows homes and the OP mentions a "residential" area, I'm going to have to assume that we're thinking that people will be having families there.  So far as I know, there really isn't any good information about the effects of low or zero gravity exposure to the development of fetuses and children.  If low or zero G exposure is potentially harmful or deleterious to the development of fetuses and children, then it's possible that the low or zero gravity sections might have warnings specifically for children and pregnant women.

Of course, it would make sense that such areas would have warning signs anyway (I'm imagining a diamond-shaped yellow and black sign, sort of like a pedestrian crossing road sign, but with a stick figure floating horizontally in zero g instead of crossing a street), and the people who live there might already be aware of any child or pregnancy issues low or no gravity might cause.


Note: I am using zero g and no gravity despite knowing that there really is no such thing as no gravity: it's just a convenient shorthand.  I do know that every object with mass exerts some kind of gravitational influence on other matter, it's just usually so sleight that it's imperceptible in daily life.


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## MarkB (Apr 20, 2016)

Quickleaf said:


> I agree that a more realistic take of the rotating sections is to have at least 2 layers set up with a ceiling/wall separating the strictly residential from the other uses that take advantage of low-gravity. Can't see any practical reasons not to do that, and it's how the Babylon 5 and Deep Space Nine stations were set up IIRC.




Actually, Babylon 5 had both. Its working and accommodation areas were mostly layered and enclosed, but a large portion of its interior was entirely open and landscaped, with a train running along the central spine.


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## Morrus (Apr 20, 2016)

Umbran said:


> Again - you are thinking like someone who has to use compact architecture for reasons of limitations of real estate area, and the structural strength of materials that have to support their own weight under compression.




More because building a Jupiter-sized space station is a lot more work than building a football field sized one. Same reason the ISS isn't the size of the moon, despite being in space. And once you have your X-sized space station, you then optimize usage of the interior of it. There might be a lot of space, but you can't live outside the thing.


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## tomBitonti (Apr 20, 2016)

MechaPilot said:


> Since the image in the OP shows homes and the OP mentions a "residential" area, I'm going to have to assume that we're thinking that people will be having families there.  So far as I know, there really isn't any good information about the effects of low or zero gravity exposure to the development of fetuses and children.  If low or zero G exposure is potentially harmful or deleterious to the development of fetuses and children, then it's possible that the low or zero gravity sections might have warnings specifically for children and pregnant women.
> 
> Of course, it would make sense that such areas would have warning signs anyway (I'm imagining a diamond-shaped yellow and black sign, sort of like a pedestrian crossing road sign, but with a stick figure floating horizontally in zero g instead of crossing a street), and the people who live there might already be aware of any child or pregnancy issues low or no gravity might cause.
> 
> ...




The term "micro-gravity" is used instead of "zero gravity".  Gravity still has an influence.  One particular one is tidal forces.

There are probably studies on micro-gravity fetal development in small animals.  Something to search for.  Not something that seems advisable for people.  Edit: There are a lot of matches.  Not only is development to be considered, but the physical mechanics of fertilization and implantation must be considered as well.

Moments of low gravity are all around us.  Any fall has a moment of no gravity.  A tall drop in a roller coaster has a longish moment.  So short exposures are not harmful.  Long term exposure is a problem for everyone -- see my prior link.  I have no idea after how much time there starts to be a problem.

Thx!
TomB


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## Umbran (Apr 20, 2016)

Morrus said:


> More because building a Jupiter-sized space station is a lot more work than building a football field sized one.




You're being hyperbolic, and that obscures the point.

On Earth, when you want to build a structure, one of the first constraints is the size of the plot of land you have to work on.  If you want to maximize how much value and/or use you can get out of that plot of land, you need to build up or down, because your reach sideways is limited.  You are driven by constraints to build layers to get the most out of a particular footprint.

In space, you are not limited in footprint.  



> Same reason the ISS isn't the size of the moon, despite being in space.




No.  The ISS is the physical size it is because of limits of *mass* (specifically, the cost of lifting that mass), not limits of available space to put it in.    The ISS isn't the size of the moon because we can't lift that much stuff into orbit!

But, consider - The ISS has 32,333 cubic feet of pressurized space.  If being compact were really the driving factor, then that could be fit in a cube about 32 feet on a side.  Or in a sphere of about 20 foot radius.  But, instead, it's in a string of modules over 160 feel long.  Compared to what it could have been, it is a sprawling structure.  Having the pressurized space be compacted into minimal external dimensions wasn't driving the design.  It was driven instead by weight considerations, and being able to build segments on the ground, and merely attach them to each other once in orbit.  For this huge station, we can't build functional segments on the ground at all, so that's not a concern.  Mass is the issue.



> And once you have your X-sized space station, you then optimize usage of the interior of it.




On Earth, yes.  However, when considering these structures, there's reason to consider making two cylinders, or making one twice as long, rather than make one multi-level cylinder.  And it is related to what I mentioned about the forces involved - tension.

We are going to make a spinning cylinder.  On Earth, where most structures of any size don't spin, a building has to support itself under it's own weight - the controlling engineering issue is whether the foundation will support the pressing weight of the building.  And, with our terrestrial building materials, we can build a foundation that will support two stories for just about the same cost as a foundation that will support one story.  We have the option of simply throwing more reinforced concrete at most building designs.

With the station, as we spin this cylinder, it has to hold together not against forces that are going to crush it, but against forces that are trying to fling it apart.  The result is basically that, in terms of engineering, this cylinder is really like a suspension bridge - take a length of bridge, and pull the ends up until they meet - the cables become like spokes on a wheel.  Spin that wheel, and the cables hold the thing together.  Stack these wheels side by side, and put caps on the end, and you have a cylinder.

The controlling issue is the strength of those cables.  How much tension can they support?  Since you *only* lift to space the amount of materials you need, with as little waste as possible, your cables are not terribly over-engineered - they are as light as you can get them.  They're only as strong as you need them to be.  So, you don't get a whole new layer for free - that layer must be supported by more cable.  

So, if you need to effectively build a separate "foundation" of cables to support that second layer, you don't get much cost advantage to making the layered form.  If you want a low-G area, it is just about as cost effective as to build a separate cylinder of smaller size.

Which is all to point out, in the markedly different environment, what counts as "efficient" may not be the same as it is on the ground.


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## Quickleaf (Apr 20, 2016)

Umbran said:


> Again - you are thinking like someone who has to use compact architecture for reasons of limitations of real estate area, and the structural strength of materials that have to support their own weight under compression.  Neither of these hold for a structure Out There.  The stresses on the structure resemble those on a suspension bridge more than those on a skyscraper - specifically, most of a terrestrial building's forces are "compression", while this spinning sphere or cylinder instead has lots of tension.  And you don't have to worry about restricting your building to fit in a small ground footprint.
> 
> The structures we are considering were intended to maximize "normal" living area and psychological impact.  The idea is to have high open spaces, because humans are designed psychologically to walk under the open sky on a regular basis.  If you put in a ceiling, you negate that.
> 
> By the way, DS9 didn't have any major open areas - it was built like a starship that didn't move, and the biggest open areas we saw were large concourse hallways ("The Promenade"), at best a couple stories high.  But DS9 assumed artificial gravity generation, not using spin to generate gravity.




Oh, I totally get what you're saying about tensive vs. "compressive" forces & psychology of "livability." 

I was thinking economics. I have a station that's supposed to be one of the older models of its kind, now outclassed by newer tech with improved materials, construction techniques, and an entirely greater magnitude of size.

My understanding of the costs of building in space is that, besides human labor, the main cost would mostly be for fuel required to get the building materials into space.

I was assuming because of this we would maximize use of space — not out of any structural limitations / space being at a premium — but out of a need to build as economically as possible. I mean, the ISS isn't exactly roomy.

But maybe that's false?

Maybe in the future it will be as economical to build larger as it would be to build smaller in space?


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## Umbran (Apr 20, 2016)

Quickleaf said:


> But maybe that's false?




What may be false is that you can add that second layer at dramatically reduced cost (and weight), as outlined above - unless/until you have super-lightweight materials of arbitrarily high strength, making two separate modules may be more efficient than making one module be heftier.


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## tomBitonti (Apr 20, 2016)

For big cylindrical O'Neal type structures, two cylinders are preferential to one because the pair can be rotated in opposite directions, giving the whole structure zero rotational momentum.

Favor drifted from cylinders to tori when the minimum size constraint was realized.  That is, when it was realized how big across a cylinder needed to be to provide adequate gravity while avoiding motion sickness, a torus was looked to as more practical.  A torus is, after all, a section of a cylinder: Building a torus is the same as building a small part of a big cylinder.  Then, it's not hard to see that putting a ceiling on a torus is less material than two big flat walls reaching to the center.  To create more living space in a torus, multiple floors seem practical.

If a long cylinder is being built, what prevents there being multiple layers on the surface of the cylinder?  Choosing torus or cylinder seems to be independent of deciding how many floors to have.

That tori can be more easily divided into sections seems to be a big advantage: Safer (decompression is limited to one section), and easier to build and put into use a section at a time.

IMO, for aesthetics, a big O'Neal cylinder wins hands down over a torus.

Thx!
TomB


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## tomBitonti (Apr 20, 2016)

Side note:

The plural of Torus is Tori, not torii, which is a kind of Japanese gate:

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



> A torii (鳥居 ?, literally bird abode, /ˈtɔəri.iː/) is a traditional Japanese gate most commonly found at the entrance of or within a Shinto shrine, where it symbolically marks the transition from the profane to the sacred (see sacred-profane dichotomy).




For some reason I had the doubled "i" stuck in my head as the pluralization, and had to fix that.

Thx!

TomB


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## Quickleaf (Apr 20, 2016)

[MENTION=13107]tomBitonti[/MENTION] Are you talking about two separate but connected O'Neal Cylinders like this?


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## Umbran (Apr 20, 2016)

tomBitonti said:


> F A torus is, after all, a section of a cylinder: Building a torus is the same as building a small part of a big cylinder.  Then, it's not hard to see that putting a ceiling on a torus is less material than two big flat walls reaching to the center.  To create more living space in a torus, multiple floors seem practical.




But, then, why not multiple tori?  That gets you back to them being counter-rotating, so that the overall structure doesn't have angular momentum to worry about.



> If a long cylinder is being built, what prevents there being multiple layers on the surface of the cylinder?




Not much prevents it.  I'm merely pointing out that it isn't necessarily cheaper or easier to build, that the ground-bound intuition on how to build things may not always apply.



> Choosing torus or cylinder seems to be independent of deciding how many floors to have.




I'd largely agree with you.  The difference is mostly a question of scale (and thus cost in lifting mass).



> That tori can be more easily divided into sections seems to be a big advantage: Safer (decompression is limited to one section), and easier to build and put into use a section at a time.[/.quote]
> 
> Well, opposing pairs of sections, at least.  If it is lopsided, and you try to spin it up, I believe it will start to tumble, and that's *bad*.


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## tomBitonti (Apr 20, 2016)

Quickleaf said:


> [MENTION=13107]tomBitonti[/MENTION] Are you talking about two separate but connected O'Neal Cylinders like this?




Hi,

Yes, exactly.  Just one rotating cylinder by itself is a big gyroscope -- hard to turn.  With two cylinders:

https://en.wikipedia.org/wiki/O'Neill_cylinder



> The cylinders would rotate in opposite directions in order to cancel out any gyroscopic effects that would otherwise make it difficult to keep them aimed toward the Sun.




Two cylinders also give direction control which does not require rockets.  From the same article:



> Attitude control
> 
> The habitat and its mirrors must be perpetually aimed at the Sun to collect solar energy and light the habitat's interior. O'Neill and his students carefully worked out a method of continuously turning the colony 360 degrees per orbit without using rockets (which would shed reaction mass).[1] First, the pair of habitats can be rolled by operating the cylinders as momentum wheels. If one habitat's rotation is slightly off, the two cylinders will rotate about each other. Once the plane formed by the two axes of rotation is perpendicular in the roll axis to the orbit, then the pair of cylinders can be yawed to aim at the Sun by exerting a force between the two sunward bearings. Pushing the cylinders away from each other will cause both cylinders to gyroscopically precess, and the system will yaw in one direction, while pushing them towards each other will cause yaw in the other direction. The counter-rotating habitats have no net gyroscopic effect, and so this slight precession can continue throughout the habitat's orbit, keeping it aimed at the Sun.




Thx!

Thx!
TomB


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## Morrus (Apr 20, 2016)

Umbran said:


> You're being hyperbolic, and that obscures the point.




Does my use of the word "Jupiter" really obscure my point to you, Umbran? I don't think it does.



> No.  The ISS is the physical size it is because of limits of *mass* (specifically, the cost of lifting that mass), not limits of available space to put it in.    The ISS isn't the size of the moon because we can't lift that much stuff into orbit!




Yes. That's my point. Building small things is easier than building big things. This is one of the main reasons.  

Honestly, did my use of the word  "Jupiter" really obscure that? Replace it with "Empire State Building" (or any other word of your choosing) if the word's really causing that much of a blockage. Hyperbole is a valid debating tactic, y'know! It's usage doesn't diminish the underlying point.


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## Umbran (Apr 20, 2016)

Morrus said:


> Does my use of the word "Jupiter" really obscure my point to you, Umbran? I don't think it does.




It attempted t obscured the point, by way of bait-and-switch.  By blowing it way out of proportion, you blew past the fact that "size" actually has a few different meanings here.  We were really talking about one, when you brought in another under the same name.

That's the point of talking about volumes.  The ISS can be compact, or spread out, for the same useful volume.  They chose a spread out design - for the same useful volume it was built with a larger effective "size".


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## Morrus (Apr 20, 2016)

Umbran said:


> It attempted t obscured the point, by way of bait-and-switch.  By blowing it way out of proportion, you blew past the fact that "size" actually has a few different meanings here.  We were really talking about one, when you brought in another under the same name.
> 
> That's the point of talking about volumes.  The ISS can be compact, or spread out, for the same useful volume.  They chose a spread out design - for the same useful volume it was built with a larger effective "size".




But we're talking about spheres. A larger volume sphere is a larger mass by definition. There's no getting around that. A big space station like described is harder to build than a small space station like described.


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## MarkB (Apr 20, 2016)

Morrus said:


> But we're talking about spheres. A larger volume sphere is a larger mass by definition. There's no getting around that.




A larger-volume single-layered sphere is not necessarily more massive than a smaller-volume multi-layered sphere. And aren't we mostly talking about cylinders?


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## Morrus (Apr 20, 2016)

MarkB said:


> A larger-volume single-layered sphere is not necessarily more massive than a smaller-volume multi-layered sphere. And aren't we mostly talking about cylinders?




Sphere/cylinder makes no difference to this particular point of discussion. 

OK, let me try to explain what I mean. You build your sphere (or cylinder). It has X mass and Y interior surface area. It's lovely, but the population grows.  You can then increase the interior surface area by (a) replacing the entire thing with another, larger spherical space station or (b) putting an interior floor in. Which do you do?


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## MarkB (Apr 20, 2016)

Morrus said:


> Sphere/cylinder makes no difference to this particular point of discussion.
> 
> OK, let me try to explain what I mean. You build your sphere (or cylinder). It has X mass and Y interior surface area. It's lovely, but the population grows.  You can then increase the interior surface area by (a) replacing the entire thing with another, larger spherical space station or (b) putting an interior floor in. Which do you do?




(c) Build a second station of equal size. That way, I don't have to mess with the existing, populated station, and I'm building a tried-and-tested design. Also, I now have multiple redundancy.

And that's the whole point, really. If you're going to build a second floor, it doesn't cost much more in materials to build that second floor adjacent to your existing station instead of inside it.


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## Morrus (Apr 20, 2016)

MarkB said:


> (c) Build a second station of equal size. That way, I don't have to mess with the existing, populated station, and I'm building a tried-and-tested design. Also, I now have multiple redundancy.
> 
> And that's the whole point, really. If you're going to build a second floor, it doesn't cost much more in materials to build that second floor adjacent to your existing station instead of inside it.




Sure it does. You have to provide air and power and stuff to your second station. You have to replicate the entire infrastructure, rather than just building a new floor. There's no way that building an entire new station is easier than adding a floor to an existing one.


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## MarkB (Apr 21, 2016)

Morrus said:


> Sure it does. You have to provide air and power and stuff to your second station. You have to replicate the entire infrastructure, rather than just building a new floor. There's no way that building an entire new station is easier than adding a floor to an existing one.




Modifying an existing, working structure can be very costly. But we're drifting off point here.

The discussion was never about adding to an existing design - that was just part of your example. It was about how to build the initial structure more efficiently. And for that, there's no particular advantage in building two concentric cylinders compared to using the same quantity of material to build a single larger cylinder.


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## tomBitonti (Apr 21, 2016)

I'd think that whether an existing structure could be extended would depend a lot on how the initial structure was build.  If the initial structure wasn't built to be extended, adding to it might be very hard.  I'm considering, for example, adding a whole new layer of roads to an existing bridge.  I'm thinking that for most bridges, that would be very hard, because the bridge design was done as a whole, with the design fit very carefully to having just one roadbed.  Similarly a space station might be hard to extend.  But it would really depend on the initial engineering.

This seems to be a bit of a quibble.  There seems to be no reason a-priori that you couldn't have multiple layers, or not have multiple layers.  I would prefer a structure with several layers, if not a dozen or more, if the material physics allows it, simply because the structure seems more robust, and you get a lot more living space that way.

In any case, any modifications would need to preserve the mass distribution so the entire structure didn't wobble or have undue stress in any area.  But that's no different in principle than adjusting weights in a plane or on a boat to adjust the center of gravity of the vehicle, and can mostly go unstated.

I don't know if three or four element structures would work out better or worse than two.  I just know that one by itself has problems.  I can imagine that the physics that applies to two cylinders could be applied to an ensemble of three or more, except I don't know how well balancing the rotations would work with an odd number of cylinders.

To keep this straightforward: Both a cylinder and a torus can be made to work, with the caveat that there is a minimum size which means the cylinder might have to be very big, and that a single spinning structure is hard to turn.  (I wonder how that is fixed for a single torus, or if these would be designed to have two parallel parts spinning in opposite directions.)  And there are a lot of options for the particulars of the structure, including how many layers and whether there is a corridor running down the middle, or if there are round or flat end caps, or if a torus is built with multiple parallel rings, some smaller and with lesser gravity and others larger and with more gravity.

A problem which we have not discussed is protection from radiation.  That either limits where the structure is put (inside the magnetic belt of the earth) or requires quite a bit of shielding on the outside to provide protection (I've read that a structure could be built from material sent from the moon via linear accelerator, with unused/waste material put on the outside as a protective layer.)

What I've read as a way to bootstrap all of this is to build a base on the moon which would be a combination mine/solar power array/linear accelerator, to send materials to one of the Lagrange points, then build big O'Neal cylinders there as habitats and factories, with the eventual goal, in part, to manufacture power satellites which would transmit sunlight converted to microwaves to the surface.

Anyways,
TomB


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## Quickleaf (Apr 21, 2016)

Umbran said:


> What may be false is that you can add that second layer at dramatically reduced cost (and weight), as outlined above - unless/until you have super-lightweight materials of arbitrarily high strength, making two separate modules may be more efficient than making one module be heftier.




I see, so the cost question can be simplified to: (1) Does it cost less to launch the materials for an entirely new station into space? (2) Or does it cost less to strengthen the "suspension bridge cables" holding the station together under the centrifugal force?

My assumption was that #2 would be cheaper because in scenario #1 not only are you launching suspension bridge cables but you're also launching life support tanks, exterior structure, water reclamation systems, etc, etc.

But maybe I oversimplified what strengthening the "suspension bridge cables" would involve? Even though I'm the architectural field, this sort of engineering is something I have absolutely no familiarity with. 

Anyone have anecdotes or insight into which would be cheaper, #1 or #2 and why?



Morrus said:


> Sure it does. You have to provide air and power and stuff to your second station. You have to replicate the entire infrastructure, rather than just building a new floor. There's no way that building an entire new station is easier than adding a floor to an existing one.




Yes, that was my thinking too. 

I knew that strengthening "suspension bridge cables" wouldn't be quite as simple as I was imagining it, but I figured it couldn't be so much more involved that it would be more costly than making a new station from scratch.



MarkB said:


> Actually, Babylon 5 had both. Its working and accommodation areas were mostly layered and enclosed, but a large portion of its interior was entirely open and landscaped, with a train running along the central spine.




That's kind of how I was envisioning it would work. You wouldn't want to completely enclose a residential section for psychological / livability reasons. But having a separate low-grav section for industry, sports, and the tram makes sense.



MarkB said:


> Modifying an existing, working structure can be very costly. But we're drifting off point here.
> 
> The discussion was never about adding to an existing design - that was just part of your example. It was about how to build the initial structure more efficiently. And for that, there's no particular advantage in building two concentric cylinders compared to using the same quantity of material to build a single larger cylinder.




Sorry!  That's my fault, since it's a precept of the scenario I'm working on that an old space station built around mining has grown overcrowded due to being relatively isolated and classism issues.


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## Quickleaf (Apr 21, 2016)

tomBitonti said:


> I don't know if three or four element structures would work out better or worse than two.  I just know that one by itself has problems.  I can imagine that the physics that applies to two cylinders could be applied to an ensemble of three or more, except I don't know how well balancing the rotations would work with an odd number of cylinders.
> 
> To keep this straightforward: Both a cylinder and a torus can be made to work, with the caveat that there is a minimum size which means the cylinder might have to be very big, and that a single spinning structure is hard to turn.  (I wonder how that is fixed for a single torus, or if these would be designed to have two parallel parts spinning in opposite directions.)  And there are a lot of options for the particulars of the structure, including how many layers and whether there is a corridor running down the middle, or if there are round or flat end caps, or if a torus is built with multiple parallel rings, some smaller and with lesser gravity and others larger and with more gravity.




So, if you have two rotating cylinders, how does that work? They spin in opposite directions to counteract one another's forces? 

Wouldn't you still need a power source to keep them spinning just like you would with one cylinder?

And in the case of one cylinder _within_ another, how the heck would one move between cylinders spinning in opposite directions?



> A problem which we have not discussed is protection from radiation.  That either limits where the structure is put (inside the magnetic belt of the earth) or requires quite a bit of shielding on the outside to provide protection (I've read that a structure could be built from material sent from the moon via linear accelerator, with unused/waste material put on the outside as a protective layer.)
> 
> What I've read as a way to bootstrap all of this is to build a base on the moon which would be a combination mine/solar power array/linear accelerator, to send materials to one of the Lagrange points, then build big O'Neal cylinders there as habitats and factories, with the eventual goal, in part, to manufacture power satellites which would transmit sunlight converted to microwaves to the surface.




You're describing layering, which is passive protection. As I understand it, there are broadly two other categories of protection: active (using human-created magnetic or electrostatic fields) & planning (taking advantage of areas in space that require less shielding than others).

After reading over the 2015 Mars Mission radiation challenge it seemed like none of the theoretical designs were getting the radiation shielding results NASA was looking for, so maybe some kind of "silver buckshot" approach combining passive, active, and planning protection will be used in the future?


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## tomBitonti (Apr 21, 2016)

Quickleaf said:


> So, if you have two rotating cylinders, how does that work? They spin in opposite directions to counteract one another's forces?
> 
> Wouldn't you still need a power source to keep them spinning just like you would with one cylinder?
> 
> ...




For multiple tori, I was imagining them side by side.  I agree, putting them one inside the other is problematic.

If left alone, there would be no forces to cancel out.  There is a force when you try to re-orient (turn) the spinning cylinder.  The forces from two spinning cylinders can be made to cancel.  (The force is perpendicular to how you are trying to turn the cylinder.  Two of the ends will push in to each other; the other two wifi pull apart.)

Each cylinder will keep rotating forever, unless acted on by external forces.  The two could act on each other, but presumable that would be prevented by keeping the cylinders apart.  The connection struts would need a low friction coupling.  I suppose there would be small losses, say, due to induced current from spinning in the earth's magnetic field.  (Induced currents might be a huge problem, but I don't know hardly enough about that to say more.)

I don't think I'm qualified to say much about active shielding either, except that it only works for charged particles (I think).  I don't think it works for high speed neutrons or high energy photons. In any case, I have no idea whether passive or active shielding is better for big space stations.

Thx!
TomB


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## tomBitonti (Apr 21, 2016)

Being able to turn the cylinders is important because they need to be turned so to keep them pointed at the sun.  That is presuming big fixed mirrors are used, as seems to be typical for the big cylinder design.

Thx!
TomB


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## tomBitonti (Apr 21, 2016)

double post


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## MarkB (Apr 21, 2016)

tomBitonti said:


> Being able to turn the cylinders is important because they need to be turned so to keep them pointed at the sun.  That is presuming big fixed mirrors are used, as seems to be typical for the big cylinder design.
> 
> Thx!
> TomB




It seems like each option would have its challenges. If you have a single spinning cylinder, then once you get it up to speed it will essentially keep spinning indefinitely under its own momentum with only occasional corrective input required - whereas if you have two linked, counter-rotating cylinders there will inevitably be friction at the point of connection, which will require motors to maintain the spin and periodic maintenance for wear-and-tear.

I wonder if there would be an advantage to using a single rotating cylinder (or torus) with a compact, rotating counterweight to cancel out the angular momentum. It feels like that would be easier to maintain, but maybe I'm just not visualising it correctly.


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## Mustrum_Ridcully (Apr 22, 2016)

I believe a problem with the idea of extending one station at a later point is that the assumption that you could just "reuse" the existing stations energy supply and air supply and what not might rely too much on the idea that the original station would have any "spare" supplies for that purpose. The primary problem is getting stuff into space in the first place. You will try to avoid bringing more than you need, since it's really costly.


What I wonder however about materials. Do we need a cylinder or torus? COuld we take a torus and take only, say 2 12th of it, put at opposing ends, so we have something like this: (-----) and have that rotate?
You would need to go through micro-gravity to get to the other half of the station, but you would be able to achieve a bigger radius while needing less material. But I have no idea if such a construction would really rotate as well as a cylinder or torus.


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## happyhermit (Apr 23, 2016)

A sphere would be strange in many ways, for instance not only would things be "pulled downwards" but they would also be pulled towards the "equator". I am not wording that correctly but basically, the further something is from the "equator" the less aligned the "gravity" would be with the "floor". There are also some air circulation things that would happen.

Rendezvous with Rama is not a terrible read, if your into that sort of thing.


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## Eltab (Apr 25, 2016)

Quickleaf said:


> My understanding of the costs of building in space is that, besides human labor, the main cost would mostly be for fuel required to get the building materials into space.



Older book you may find helpful: Space Resources, Breaking the Bonds of Earth.

If a society has the industrial capability to build a 1 km radius cylinder, it will have some industrial capacity already in space / in orbit.  Use THAT as your primary source of parts.  Bring in a metallic asteroid for raw materials.


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## Eltab (Apr 25, 2016)

MarkB said:


> whereas if you have two linked, counter-rotating cylinders there will inevitably be friction at the point of connection



Use magnets to avoid physical friction.  (I'm thinking something that looks like a ball-and-cup joint).
Maybe you can generate electricity from them, too - or maybe you get a nasty short circuit.  Comments from electricians welcome.
You still would need a motor to keep up spin in case the counter-rotating magnetic fields cause resistance against each other.


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## Eltab (Apr 25, 2016)

If you want to put an "upper story" inside your rotating habitat, you could locate residences for the elderly there.  Old people are prone to falls and recover poorly from one, so being in lower-gravity would be a boon to them.


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## Quickleaf (Apr 25, 2016)

happyhermit said:


> A sphere would be strange in many ways, for instance not only would things be "pulled downwards" but they would also be pulled towards the "equator". I am not wording that correctly but basically, the further something is from the "equator" the less aligned the "gravity" would be with the "floor". There are also some air circulation things that would happen.
> 
> Rendezvous with Rama is not a terrible read, if your into that sort of thing.




I thought the cylindrical sea along the equator was interesting in that book. While Arthur C. Clarke's science was probably more accurate, as far as narrative goes I am more a fan Robert Heinlein.



Eltab said:


> Older book you may find helpful: Space Resources, Breaking the Bonds of Earth.
> 
> If a society has the industrial capability to build a 1 km radius cylinder, it will have some industrial capacity already in space / in orbit.  Use THAT as your primary source of parts.  Bring in a metallic asteroid for raw materials.




Very interesting premise. It makes sense of course, but making that connection between being able to build a 1 km radius cylinder and industrial capacity in space is a great insight. I wouldn't have assumed that.



Eltab said:


> If you want to put an "upper story" inside your rotating habitat, you could locate residences for the elderly there.  Old people are prone to falls and recover poorly from one, so being in lower-gravity would be a boon to them.




I was actually thinking of the upper story being more light industrial. Long-term low-G dwelling apparently could contribute to bone brittleness, so that's probably contraindicated for elderly at increased risk of osteoporosis.


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## Shayuri (Apr 25, 2016)

Just remember, if you have an 'open architecture' cylinder or torus, then the air pressure is going to drop towards the axis. Any habitable volume up there is going to need life support. I suspect it'd be fun to 'parachute' from the axis to the ground, but you'd better bring some oxygen up there when you do.


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## tomBitonti (Apr 25, 2016)

A sphere doesn't seem that bad, and would naturally provide a set of terraces with good views / access to the central open space.  The outermost section could be scaffolded to provide a big flat band near the equator (with lots of space beneath for offices, storage, industry, or whatever.  Having different gravity levels might be an advantage.  The design is one of the three top designs, and is called a Bernal Sphere.

Something I recently read: A problem with rotating cylinders is that they want to convert the rotation from along the long axis to along a short axis.  I'm sure there is a statistical / kinematic reason why that happens.  The tendency means that active spin management is needed, independent of hub friction problems.

Thx!
TomB


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## Shayuri (Apr 28, 2016)

I don't know the exact physics, but I wonder if, with a sphere, there might be problems with different bands of air rotation being in contact with one another. The air rotating at the equator of the sphere would be moving faster than the air rotating along north and south of the equator.

That happens on Earth too, and makes for a lot of entertaining weather patterns you probably wouldn't want to duplicate on a space station, even on a smaller scale.


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## Quickleaf (Apr 28, 2016)

Shayuri said:


> Just remember, if you have an 'open architecture' cylinder or torus, then the air pressure is going to drop towards the axis. Any habitable volume up there is going to need life support. I suspect it'd be fun to 'parachute' from the axis to the ground, but you'd better bring some oxygen up there when you do.



Totally. There was actually an episode of Babylon5 (maybe in season 4?) where Captain Sheridan had to leap out of the train running the axis due to a bomb threat. The way the emergency door blew off made it seem like there was a difference in air pressure between the interior of the train and the environment of the central axis of the cylindrical station. Of course, then he started falling, and IIRC there was something about him hitting the ground at 60mph once he landed.



tomBitonti said:


> A sphere doesn't seem that bad, and would naturally provide a set of terraces with good views / access to the central open space.  The outermost section could be scaffolded to provide a big flat band near the equator (with lots of space beneath for offices, storage, industry, or whatever.  Having different gravity levels might be an advantage.  The design is one of the three top designs, and is called a Bernal Sphere.
> 
> 
> 
> ...


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## tomBitonti (Apr 28, 2016)

Quickleaf said:


> Does "convert the rotation from along the long axis to along a short axis" mean that the cylinder would tend to wobble over time? (without active intervention)




Yes.

Here is an example:

https://en.m.wikipedia.org/wiki/Explorer_1



> Explorer 1 changed rotation axis after launch. The elongated body of the spacecraft had been designed to spin about its long (least-inertia) axis but refused to do so, and instead started precessing due to energy dissipation from flexible structural elements. Later it was understood that on general grounds, the body ends up in the spin state that minimizes the kinetic rotational energy for a fixed angular momentum (this being the maximal-inertia axis). This motivated the first further development of the Eulerian theory of rigid body dynamics after nearly 200 years—to address this kind of momentum-preserving energy dissipation.[18][19]




Lecture notes:

Lecture L27 - 3D Rigid Body Dynamics: Kinetic Energy; Instability; Equations of Motion

http://ocw.mit.edu/courses/aeronaut...fall-2009/lecture-notes/MIT16_07F09_Lec27.pdf

Thx!
TomB


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## Eltab (May 2, 2016)

Quickleaf said:


> Long-term low-G dwelling apparently could contribute to bone brittleness, so that's probably contraindicated for elderly at increased risk of osteoporosis.



Your point is well-taken.

Near-Zero-G is known from experience to cause problems.  I haven't heard of any experiments in say Half-G or 1/3-G or 2/3-G.  (Where would such experiments be held ?!)

If a sphere is built, a doctor could balance the health factors for each fragile elderly person, and place them in the gravity zone where they would be most comfortable / safe.


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