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Old 03-31-2019, 11:09 PM   #11
Agemegos
 
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Default Re: [Spaceships] SM+35 Toroid Station?

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Originally Posted by DataPacRat View Post
I'm planning on doing some plots within a rotating space station based on John Varley's "Gaea": radius 650 km, width 250 km, centrifugal gravity 0.2G, air pressure 2 bars
I calculate that you'll need a structural material about twelve times as strong as, and one-quarter the density of, high-tensile steel for that structure to resist the tension generated by its own rotation. You could build it out of a polymer reinforced with carbon nanotubes. But not stone.

And that's not counting the 15km of stone as a passive load, that's leaving it out altogether. With 33,000 tones per square metre of load you'd need an 8-km-thick band of nanotube-reinforced polymer, or 1.5-1.7 km of flawless diamond or continuous nanotubes above and below to support the stone.
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Last edited by Agemegos; 03-31-2019 at 11:14 PM.
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Old 04-01-2019, 05:24 AM   #12
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Default Re: [Spaceships] SM+35 Toroid Station?

15 km thick armor seems...excessive for a habitat, as is any production of power points for industrial purposes (just use vast amounts of minifacs). In addition, such a habitat is a massive waste of resources, as you could use the same resources to make 1,000 habitats one-tenth the dimensions each that would have a total surface area ten times as large, allowing them to support ten times as many people. Such a complex of habitats could be connected through static structures that could support solar panels and radiators, and could support any industry that truly needed power points.
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Old 04-01-2019, 06:27 AM   #13
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Default Re: [Spaceships] SM+35 Toroid Station?

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Spaceships assumes that the armor functions as structural support.
Not really. A ship can be built with as little armor as you want - including no armor at all - and still be assumed to have structural supports. What Spaceships assumes is that each component has enough structural support for itself, and the ship as a whole has enough structural support for all of it's components.
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Old 04-01-2019, 07:01 AM   #14
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Default Re: [Spaceships] SM+35 Toroid Station?

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Try SM+22 (you have about 1/3,000,000 the area you need).
Can do. :)


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Originally Posted by Agemegos View Post
With 33,000 tones per square metre of load you'd need an 8-km-thick band of nanotube-reinforced polymer, or 1.5-1.7 km of flawless diamond or continuous nanotubes above and below to support the stone.
If it's that important for verisimilitude, then I can always make the 13th system of armor Nanocomposite instead of another Stone. (Looks like it'll add 700k dDR, and $150 quintillion; though I'm slashing the budget by a lot more than that by dropping the SM+35 factory.)


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Originally Posted by AlexanderHowl View Post
15 km thick armor seems...excessive for a habitat, as is any production of power points for industrial purposes (just use vast amounts of minifacs). In addition, such a habitat is a massive waste of resources, as you could use the same resources to make 1,000 habitats one-tenth the dimensions each that would have a total surface area ten times as large, allowing them to support ten times as many people. Such a complex of habitats could be connected through static structures that could support solar panels and radiators, and could support any industry that truly needed power points.
And that brings us into the station's actual design goals.

The original builders' purpose was to create what's effectively an offline backup system for their civilization, by placing a selection of stations in various out-of-the-way points in the galaxy; each of which could maintain itself, independently, for x million years, and maintain a population of baseline humans (or near-human equivalent) for the duration. I'm assuming that simply keeping an offline set of records of DNA and some vatfacs prepped doesn't meet their criteria. I'm trying to take some of the lessons of Biosphere 2 into account, and that keeping an active biosphere working for a long time requires more redundancies than is immediately obvious to account for various complex interactions. I'm also assuming that there are issues in most forms of active management of any such collection of ecosystems, so it all has to mostly run itself. (Plus or minus the occasional automated nudge, such as vatfaccing up some individuals of a species that's undergoing too much genetic drift; and with at least one release valve for any human-equivalents who are too curious to avoid poking their fingers into dangerous pies; and with a set of systems to rapidly increase the inhabitants' tech-level if galactic society vanishes for too long.)

Put another way, the purpose of the station isn't maximizing industry, or energy, or even population; it's maximizing absurdly-long-term ecological stability. There are plenty of arguments that could be had about the trade-offs between structural strength and biosphere acreage, but a Varleyian Gaea seems to both be possible without superscience and to provide a plausibly large area, so seems a good starting point.


For example, if I wanted to get a similar amount of acreage with a SM+34 station, I'd need at least 10 of the systems to be Open Spaces (and with fewer armour systems, the radiation protection would be roughly halved); and couldn't do it at all with a SM+33 station, even with all twenty systems being Open Space.


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Not really. A ship can be built with as little armor as you want - including no armor at all - and still be assumed to have structural supports. What Spaceships assumes is that each component has enough structural support for itself, and the ship as a whole has enough structural support for all of it's components.
:thumbs-up:
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Old 04-01-2019, 07:54 AM   #15
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Default Re: [Spaceships] SM+35 Toroid Station?

You only need 10 metric tons of shielding per square meter to provide shielding similar to the Earth, so 1 km of air at 1 atm would provide sufficient shielding. A shell of carbon nanotubes 1 km thick would provide more than enough additional protection to stop asteroid impacts. Even a TL 12 civilization would balk at the cost of a SM+35 ark and would probably rather go for 100,000 SM+27 ark.

If we assume an overall density (including static structural elements and static armor) of 1 metric ton per 14 cubic meters, an SM+27 ark will have a volume of 42 trillion cubic meters. If we assume a cylinder with a length 4 times the radius (and nonrotating armor and structural elements), you would end up with a length of 220 km, a rotational radius of 55 km, and a projected area of 2 million square kilometers. With 100,000 of them, you end up with a total projected area of 200 billion square kilometers. Even if only 50% of the projected area is habitat support, that gives you a total of 250 Earths in surface area.
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Old 04-01-2019, 08:01 AM   #16
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Default Re: [Spaceships] SM+35 Toroid Station?

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If we assume a cylinder with a length 4 times the radius (and nonrotating armor and structural elements), you would end up with a length of 220 km, a rotational radius of 55 km, and a projected area of 2 million square kilometers.
Are you sure that's right? If the radius is 55 km, the circumference is about 345 km; and the cylinder's surface area would be 220 * 345 = 76,000 km^2. Am I missing something?
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Old 04-01-2019, 08:05 AM   #17
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Default Re: [Spaceships] SM+35 Toroid Station?

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You only need 10 metric tons of shielding per square meter to provide shielding similar to the Earth, so 1 km of air at 1 atm would provide sufficient shielding.
As Earth has a lot more than 1km of air, that doesn't follow.
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Old 04-01-2019, 08:11 AM   #18
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Default Re: [Spaceships] SM+35 Toroid Station?

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As Earth has a lot more than 1km of air, that doesn't follow.
I've seen other estimates for the 10 tons per square meter for radiation shielding. If air is 1.225 kg / m^3, then to get 10 tons worth, you need a depth of 7.4 km of such air. (This doesn't include air getting less dense at higher altitudes.)
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Old 04-01-2019, 10:20 AM   #19
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Default Re: [Spaceships] SM+35 Toroid Station?

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As Earth has a lot more than 1km of air, that doesn't follow.
The scale height of the atmosphere is a bit above 8 km. Remember, it gets thinner as altitude increases, so the total mass isn't linear in the height of the atmosphere.

A fairly simple proof for this: air pressure at sea level is 101 kPa. The source of pressure is the weight of the atmosphere above you. The weight of atmosphere is mass * G, so the mass is 101 kPa/9.8N/kg = 10,300 kg/m^2.
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Old 04-01-2019, 10:30 AM   #20
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Default Re: [Spaceships] SM+35 Toroid Station?

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Are you sure that's right? If the radius is 55 km, the circumference is about 345 km; and the cylinder's surface area would be 220 * 345 = 76,000 km^2. Am I missing something?
Sorry, I tend to layer my habitat designs and went back to an old design that had fifty-five habitat shells, each with 1 km of thickness It ends up being a effective projected area of 2 million square kilometers.
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