Monday, March 26
Bugger.
I knew that Niven-style Ringworlds required materials with impossible tensile strengths (beyond anything known by quite a few orders of magnitude), but I wasn't sure about Banks-style orbitals. Orbitals, as well as being much smaller, have the attraction that they are in orbit about their sun and don't exhibit the dynamic instability of ringworlds.
So how big can we build an orbital? The answer is, not very.
Micro-scale samples of carbon nanotubes have apparently been tested with a tensile strength of 62GPa. That means they can withstand a tensile force of 62GN/m2* before breaking.
The density of said carbon nanotubes is 1.34g/cm3. Assuming we want an acceleration of 1G - let's round it up to 10m/s2 for simplicity - this gives us a maximum circumference of 62GPa / 13.4kN/m3 = 4600km.**
The theoretical maximum tensile strength for carbon nanotubes is 300GPa, which would give us 23,000km. But I'd want to have a safety factor of 5, which would eat that gain, or reduce our circumference to about 900km for current materials.
That last figure also means that you couldn't rely on gravity to hold your atmosphere in; for any reasonable atomosphere retention you'd need walls rising pretty much the radius of the structure. So it would have to be a sealed environment.
An orbital that size*** with a population density of, say, The Netherlands, would support 18 million people. Its mass is kind of arbitrary, but if we make it 100m thick, we get about 6 trillion tons. The asteroid 10 Hygiea†would provide enough materials to build 10,000 of these, giving us room for 180 billion people.****
Just in case you were looking for an investment property...
* Hey, I have super/subscript in the comments editor, but not in the post editor. I really need to fix that!
** I think I got the units right there. I'm pretty sure I at least got the scalar quantities right.
*** Let's give it a width of 50km for the sake of getting a number here.
**** If we had theoretical-limit materials and scaled everything up linearly - including the thickness - we'd get 80 habitats capable of holding 450 million people each.
†A test of my new Wikipedia linky tags, [wp] and [wikipedia]. It's a generalised bit of code that will allow me to extend it to making easy links to any site. It doesn't have the smarts I added for the [youtube] tag, though; that one can take the video ID itself, but given a full Youtube URL it will extract the ID before putting it into the template.
Posted by: Pixy Misa at
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Posted by: TallDave at Tuesday, March 27 2007 04:17 PM (odS+4)
So, the circumference would be about 540 miles, or almost a round trip from Chicago to Minneapolis.
Seems like it'd be a rather... how to put this?... well, dull place to live. Particularly with 18 million people living in it. Maybe I've not got a good handle on the overall scope of the thing, but it seems like it may just be a tad crowded.
Posted by: Wonderduck at Tuesday, March 27 2007 08:20 PM (135bi)
And I imagine, for the sake of simplicity, the topography is similar to that of... oh... Nebraska. Anything else could throw things out of balance.
Posted by: Will at Tuesday, March 27 2007 10:06 PM (SOx9v)
Until you get a meteor puncture, anyway.
Maybe I've not got a good handle on the overall scope of the thing, but it seems like it may just be a tad crowded.
Weeeell... It would have the size, population, and approximate topography of the Netherlands.
Posted by: Pixy Misa at Tuesday, March 27 2007 10:39 PM (PiXy!)
It would have the size, population, and approximate topography of the Netherlands.
And this is supposed to be something in it's favor, is it?
Why not just 'glue' thirty or forty of these things together, make a tube out of them (and use the materials of another 20 to provide structural strength to the outside)? Though it'd still be a boring place, more or less, it'd be much bigger.
Posted by: Wonderduck at Tuesday, March 27 2007 11:41 PM (VdgKc)
Who says there can only be one?
HELP Pixy! You know how your RSS feed opens a page with comments that = New Comment Thingy? Ace's don't, and it makes it painful to read, knowing I can't put a snarky comment on it. And the jawas are even worse, since it lets you comment and then just deletes it.
Please fix their RSS feeds for them. If you do, I promise to send you 1 dollar. Yes, I'm that serious.
Posted by: Kevin at Wednesday, March 28 2007 12:12 AM (/ndDU)
Weeeell...
Why not just 'glue' thirty or forty of these things together, make a tube out of them (and use the materials of another 20 to provide structural strength to the outside)? Though it'd still be a boring place, more or less, it'd be much bigger.
The idea of an orbital is that being a narrow ring, you can get plenty of sunlight in, albeit at an oblique angle. Now I just need to calculate how fast the thing would need to rotate... Hmm. This scribble says it would rotate every 7.5 minutes. That makes for a pretty short day...
Posted by: Pixy Misa at Wednesday, March 28 2007 02:02 AM (PiXy!)
I'm beta-testing the new mu.nu blogging system here. Ace and the Jawas will be moving to it soon, and then all those problems will be fixed.
(To be replaced by a whole new set of problems...)
Posted by: Pixy Misa at Wednesday, March 28 2007 02:03 AM (PiXy!)
Posted by: Kevin at Wednesday, March 28 2007 10:26 AM (/ndDU)
Though it'd still be a boring place, more or less, it'd be much bigger.
That depends. Would it also have the Netherlands' laws?
Anyways, topography need not be dull, at least according to this:
The inside of the hoop can be formed to any type of planetary environment, from desert to ocean to jungle to glacier. The hoop is usually divided into individual 'plates', similar to continents, though there is usually no directly visible indication for the transition from one plate to another. Some plates mimic natural environments very closely, other are wild exaggerations possible only by advanced matter forming and intricate (but usually hidden) machinery - such as a gigantic river circumventing the whole orbital, which in some reaches travels on immense, kilometre-high bridge- or mountain-range-like constructions, and in other regions might act as an immense 'waterslide' for a floating event stadium.
http://en.wikipedia.org/wiki/Orbital_(The_Culture)
Posted by: TallDave at Thursday, March 29 2007 07:42 PM (odS+4)
Wiki article wrote:That's a whole lot bigger than what we could theoretically build today.The Culture's orbitals are approximately ten million kilometres in circumference and have widths varying between one thousand and six thousand kilometres
Posted by: Pixy Misa at Thursday, March 29 2007 08:22 PM (PiXy!)
"...approximately ten million kilometres in circumference and have widths varying between one thousand and six thousand kilometres..."
Now THAT'S what I'm talkin' about!
Posted by: Wonderduck at Friday, March 30 2007 01:07 AM (135bi)
Posted by: Pixy Misa at Friday, March 30 2007 01:25 AM (+7YmF)
The Ringworld rotated slower than that; it had the shadow squares to generate day an night.
Posted by: Pixy Misa at Friday, March 30 2007 01:34 AM (+7YmF)
Posted by: TallDave at Wednesday, April 04 2007 02:49 AM (odS+4)
Oops thats the circumference. Why did I think it was a square lol?
So 900 long, 50 wide, about 45000 sq km (can you treat the surface of a ringworld as a rectangle?). Hmm, not that big, but you could still have an awesome water park or two.
Posted by: TallDave at Wednesday, April 04 2007 02:54 AM (odS+4)
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