Megastructures

[Simon_Jester]

Thing is, that only works if you put solar satellites in very close orbit around the star- it's not a habitable structure.

And Starglider- good point about why the 'pancake' idea requires supermaterials. I feel silly having missed that. Even if the ring is in free fall, the local structure of the Dyson sphere would still have to withstand the gravity of its own pancake.

There's basically NO way to build a megastructure around a star that doesn't involve weightlessness unless you have magitech.

[Zeropoint]

I don't know if you could make one that would keep the air in with nothing but spin gravity, but a much more plausible "ring world" would be a smaller ring that orbits a star not quite on edge, so that the side away from the star gets sunlight. Size it right and you could have a 24-hour day and 9.807 m/s^2 apparent gravity.

Hang on, I should be able to find the diameter from that information. a = w^2 * r, so r = a / w^2. Acceleration a is given above, angular velocity w (I don't have a lower-case omega key on my laptop) is one revolution in 24 hours = 2 * pi / (24 * 60 * 60) = 7.2722e-5 radians per second. r = 9.807 ms^-2 / (7.2722e-5 s^-1)^2 = 1.854e9 meters, or 1.854 million kilometers. For comparison, the radius of the Earth is about 6,371 km.

The question of "how tall do the walls need to be to keep the air in?" is . . . not entirely clear to me. Wikipedia says that the top traces of the Earth's atmosphere, where you can't tell it apart from space any more, go up to about 10,000 km, so I guess that's one estimate. It puts us in the super-material realm, though.

[Simon_Jester]

Hm.

An object that large would be subject to some very significant tidal stresses on top of everything else. The side of it closer to the sun wouldn't be in the same stable orbit as the side farther from the sun.

Also, if you're rotating 72 microradians per second and the radius is 1.8 million kilometers... that is 72 times 1.8 kilometers is somewhere in the neighborhood of 120 to 125 kilometers per second as the velocity of the rotating habitat surface.

[Zeropoint]

Ooh, I hadn't thought about the tidal forces. Good catch. Unless you had thrusters to boost the spin, it would experience tidal slowing and eventually become uninhabitable as it lost spin gravity.

Regarding the air retention walls: it occurred to me today that they don't HAVE to be the sheer vertical structures we normally think of as "walls". You could have gently sloping walls, like a titanic mountain range. It's kind of a moot point, though, if it's going to slow down. Maybe O'Neil cylinders really are the best way to build a space habitat.

[SMJB]

I'm surprised no one's brought up supramundate worlds yet. Build a shell (with mass stream technology) over a large planet or small star at the level where the effective gravity is 1 gee, cover it with atmosphere, and use the star at your heart for power.

Orion's Arm builds a fuckton of shit out of mass stream technology, and does limit themselves to what's at least theoretically possible, though I imagine that they wouldn't always limit themselves to the most mainstream theory available.

[Me2005]

The question of "how tall do the walls need to be to keep the air in?" is . . . not entirely clear to me. Wikipedia says that the top traces of the Earth's atmosphere, where you can't tell it apart from space any more, go up to about 10,000 km, so I guess that's one estimate. It puts us in the super-material realm, though.

Kind of surprised no one has answered this. I don't know the figure, but it's got to be less than 10,000 km: Earth keeps it's atmosphere in by gravity, you'll get to use walls & centripetal force. Also, atmosphere at 10,000 km is atmosphere that no human can actually use, really the atmosphere on top of Mt. Everest (8.8 km) is only useful for breathing for brief periods without supplemental oxygen.

[Zeropoint]

Yes, but the pressure wouldn't even be THAT high without all the air that's sitting on top of that air. If you build your air-retention walls only Everest-high, then air will spill out over the top until the pressure at the top is zero, which will have undesirable effects on the air pressure at the bottom.

[Sea Skimmer]

A big problem with air spilling over the top solar heating. The top will keep heating up more then the bottom and trying to spill over the top even as it looses density. This is already a major factor in why the earth's atmosphere is so deep in the first place. I don't see a way to negate this without having walls in the 100-200km kind of height, or else simply constantly generating new air, or some kind of grid of superconducting supercooled mesh that chills air trying to escape and causes it to constantly fall back down. A sort of ceiling as it were, but maybe 1% more plausible then a glass roof?

[SMJB]

100 km walls is what these sorts of megastructures have in the Orion's Arm universe.

Not seeing the problem, really--if you're even building something like a Ringworld or a Banks Orbital in the first place, you've clearly got some of making structures which should collapse or tear themselves to shreds from their own acceleration not do that. What difference does a little wall make?

[bilateralrope]

I'm surprised no one's brought up supramundate worlds yet. Build a shell (with mass stream technology) over a large planet or small star at the level where the effective gravity is 1 gee, cover it with atmosphere, and use the star at your heart for power.

Orion's Arm builds a fuckton of shit out of mass stream technology, and does limit themselves to what's at least theoretically possible, though I imagine that they wouldn't always limit themselves to the most mainstream theory available.

What exactly is "mass stream technology" ?

Because I can't find any reference to it outside of Orion's Arm and, when the first page of Google results includes a page titled Pope Francis Mass Live Stream 2015: Here's How to Watch Papal Canonization Online I don't think tweaking my search terms will help me find whatever you are talking about.

[SMJB]

You know how a space fountain works? It's that.

[Esquire]

... Okay, I'll bite. What's a space fountain? And why do you think Orion's Arm, source of homo jihadi among other questionable ideas, is an authority on the possible, practicle, or feasible?

[Starglider]

What exactly is "mass stream technology" ?

Using linear accelerator tubes containing streams of ball bearings / iron filings / ions as structural members. Instead of static transfer of force via tension & compression, force is applied by changing the momentum of the matter stream, using powerful (preferably superconducting) electromagnets. The continuous acceleration and deceleration of the matter stream by different bits of the structure apply the desired forces. Despite attempts by Orion's Arm writers to the handwave the inherent issues, this is exactly as dangerous to build and operate as you would expect; any failure in matter stream containment or even just alignment tends to cause rapid auto-disassembly of the whole structure. The engineering difficulties of even a launch loop are formidable; sensible engineers have looked at it and pointed out several hard design issues we don't have a solution to, and noted that if and when we do solve them, the thing should be built over an ocean to minimise casualties when it inevitably collapses.

[SMJB]

What exactly is "mass stream technology" ?

Using linear accelerator tubes containing streams of ball bearings / iron filings / ions as structural members. Instead of static transfer of force via tension & compression, force is applied by changing the momentum of the matter stream, using powerful (preferably superconducting) electromagnets. The continuous acceleration and deceleration of the matter stream by different bits of the structure apply the desired forces. Despite attempts by Orion's Arm writers to the handwave the inherent issues, this is exactly as dangerous to build and operate as you would expect; any failure in matter stream containment or even just alignment tends to cause rapid auto-disassembly of the whole structure. The engineering difficulties of even a launch loop are formidable; sensible engineers have looked at it and pointed out several hard design issues we don't have a solution to, and noted that if and when we do solve them, the thing should be built over an ocean to minimise casualties when it inevitably collapses.

Kinda figured there'd be a hitch.

[Corvus 501]

Why not mix a Dyson Swarm with a Ringworld? Build the Ringworld using "scrith" and use parts of the Dyson Swarm to shade over the Ringworld to provide a day/night cycle, and use the Swarm to provide power and to defend the Ring from pesky little issues like rouge planets trying to hit the orbital ring, or wandering fleets attempting a Dreadnought Grand Assault.