[Spaceships] SM+35 Toroid Station?

[Anthony]

It's debatable whether a ring configuration makes sense. The general virtue of a ring type configuration is that the atmospheric pressure can be supported the inside of the ring rather than along the entire radius of the spin compartment, reducing the amount of mass you need for atmospheric containment, but this effect is reduced if there is a pressure difference between top and bottom of the chamber.

Judging from the diagram, the outer torus has a height of around 150 km. At 0.2G and 20C, the scale height of the atmosphere is roughly 40 km, so the pressure at the top is 2-3% of the pressure at the base, and you're barely saving anything.

A cylinder with length 650 km and radius 250 km (i.e. just reversing the dimensions) has the same projected area but lower structural mass. It would rotate a bit faster (period about 26m instead of 42m) but has lower velocity, and is probably simpler to construct.

[DataPacRat]

Originally Posted by Rupert

The effective gravity was given as 0.2g, and the diameter 650km, so the rotational period is about 2530 seconds, or ~0.024 rpm. The rotational velocity of the rim would be about 800 m/s.

Radius 650 km, not diameter; the canonical rotation period for the original Gaea is 61 minutes and 3.5 seconds, and with a circumference of a hair over 4,000 km, a rotation speed of 1,114 m/s.

Originally Posted by Anthony

It's debatable whether a ring configuration makes sense. The general virtue of a ring type configuration is that the atmospheric pressure can be supported the inside of the ring rather than along the entire radius of the spin compartment, reducing the amount of mass you need for atmospheric containment

I thought the main virtue of a ring was that it's much simpler to set up passive sun-mirrors to light up the interior, without needing to install a large electrically-powered lighting system.

Originally Posted by Anthony

, but this effect is reduced if there is a pressure difference between top and bottom of the chamber.

Judging from the diagram, the outer torus has a height of around 150 km. At 0.2G and 20C, the scale height of the atmosphere is roughly 40 km, so the pressure at the top is 2-3% of the pressure at the base, and you're barely saving anything.

In Varley's source material, the spokes have valves at top and bottom, and have a regular cycle to pump air up three spokes at a time and down the other three.

Edit: With a ground-level atmospheric pressure of 2 bars, wouldn't that mean the atmospheric scale height is 80 km, so that at ~160 km, the "natural" pressure without pumping would be about 13% (or a quarter of a bar)?

Originally Posted by Anthony

A cylinder with length 650 km and radius 250 km (i.e. just reversing the dimensions) has the same projected area but lower structural mass. It would rotate a bit faster (period about 26m instead of 42m) but has lower velocity, and is probably simpler to construct.

In Spaceships terms, I think this would count as being built as SM+34 with 10 Open Space systems, instead of SM+35 with 5.

Thinking about it, it... has potential. The first choice I'd have to make is lighting, since the first options I can think of lead to different shapes. Eg, doubling the cylinder's surface area, making three sections transparent with long mirrors angled to shine down; or an electric sun-line down the middle; or go umbrella-style, with a large mirror focusing light down along a similar sun-line using mumble-mumble optics to distribute it down the whole length.

I can think of several reasons for this approach, and the only reasons I can think up against it are backfilled justifications involving handwaved long-term effects. (Well, plus the "reason" of the pure style of having a toroid the stars can be seen from. Maybe the sun-facing endcap of the cylinder could be a transparent hemisphere?)

[Anthony]

Originally Posted by DataPacRat

I thought the main virtue of a ring was that it's much simpler to set up passive sun-mirrors to light up the interior, without needing to install a large electrically-powered lighting system.

That wasn't mentioned in the design, but I have some doubts about the passiveness.

Originally Posted by DataPacRat

Edit: With a ground-level atmospheric pressure of 2 bars, wouldn't that mean the atmospheric scale height is 80 km

Scale height doesn't depend on pressure, though the pressure at the top is proportional to the pressure at the bottom.

Originally Posted by DataPacRat

I can think of several reasons for this approach, and the only reasons I can think up against it are backfilled justifications involving handwaved long-term effects. (Well, plus the "reason" of the pure style of having a toroid the stars can be seen from. Maybe the sun-facing endcap of the cylinder could be a transparent hemisphere?)

The rotational axis of the station, whatever configuration you use, is going to be parallel to the orbital axis, so neither end will be sun-facing.

[AlexanderHowl]

Passive lighting is an issue. The best way would be to have an array of shades that rotate slower than the habitat and the have the habitat perpendicular to the orbital plane (the Sun being always 'above' the habitat). With the rotating shades, you can block out the sunlight for an artificial night. If you want, you can have an array of rotating mirrors behind the habitat to give more light during the 'day'.

[DataPacRat]

Originally Posted by Anthony

The rotational axis of the station, whatever configuration you use, is going to be parallel to the orbital axis, so neither end will be sun-facing.

Anyone good at mechanical physics calculations, especially for gravity-gradient stabilization and tidal locking?

I can figure out that for a station with a mass of 10e15 tons, and a length of 650 km, then a long cylinder of a station pointing at the sun would experience about 467 giganewtons of tidal force; and pretending it's a simple rod, it would have a moment of inertia around 3.19e29 m^2*kg. But I'm having trouble googling up any comprehensible equations to see if that would allow for the station to be tidally-locked to the sun.


Edit:

Originally Posted by AlexanderHowl

Passive lighting is an issue. The best way would be to have an array of shades that rotate slower than the habitat and the have the habitat perpendicular to the orbital plane (the Sun being always 'above' the habitat). With the rotating shades, you can block out the sunlight for an artificial night. If you want, you can have an array of rotating mirrors behind the habitat to give more light during the 'day'.

I'm fine with 24-hour lighting, for this particular station. As soon as I figure out what direction it's pointing, I can decide whether to have one mirror, umbrella-style, or a large mirror at each end at 90degrees pointing to a smaller mirror at each end-cap.

[Anthony]

Originally Posted by DataPacRat

I can figure out that for a station with a mass of 10e15 tons, and a length of 650 km, then a long cylinder of a station pointing at the sun would experience about 467 giganewtons of tidal force; and pretending it's a simple rod, it would have a moment of inertia around 3.19e29 m^2*kg. But I'm having trouble googling up any comprehensible equations to see if that would allow for the station to be tidally-locked to the sun.

That's easy. No, by definition it can't be tidally locked. A tidally locked objects rotates once per year, and this object rotates a bit more than once per hour. There are no stable solutions for 'spinning around an axis that is pointed at the sun'.

Originally Posted by DataPacRat

I'm fine with 24-hour lighting, for this particular station. As soon as I figure out what direction it's pointing.

If you want it to not wobble a lot, it's pointing along the same axis as its orbiting.

[AlexanderHowl]

You could have counter rotating habitats connected by a stable structural element. You would need such an element anyway to anchor solar panels, shades, mirrors, etc.

On a more serious note, do we need a hollow object that is 95% trace atmosphere? If you constructed multiple levels, you could have g's ranging from .1 to 1.0 without difficulty. You could have each with 20 km separation and just have a rotational radius of 200 km. You could have four counter rotating habitats, each with a rotational radius of 200 km and a length of 1,000 km, each with ten levels connected, all connected by a static structural element.

In that type of setup, you would end up with an effective projected area of around 27 million square kilometers. The counter rotation would keep everything pointed in the right direction. Of course, that would be more complex than the first design.

[Anthony]

Originally Posted by AlexanderHowl

On a more serious note, do we need a hollow object that is 95% trace atmosphere? If you constructed multiple levels, you could have g's ranging from .1 to 1.0 without difficulty.

Yes, but it's unclear what benefit you get out of it, since your primary limit is that you can saturate your available energy and heat dissipation with a very small number of layers.

If you don't care about horizons, though, you can save a lot of weight by making the outer shell have a low ceiling. With no ceiling, that 2 bar surface pressure corresponds to an atmospheric mass of 100 tons per square meter, or 10^17 kg for the entire million square kilometers of habitat. Put a 1 km ceiling on the world and you reduce that to 2.4 tons.

Note that this requires a much different design than previously suggested, because the natural shape of an inflated torus is round, and with a 250 kilometer width, that's ridiculous. Fortunately, we already know how to make a flat inflated object: it's called an air mattress. Somewhat counter-intuitively, this also saves us on structural mass, because most of the atmospheric pressure is supported by internal struts (support length 1 km) instead of the belt of the torus (support length 650 km); the only stuff that has to be supported by the belt is now the static mass, which at 10 tons per square meter (plenty for a dirt belt) is still only around a tenth of the atmosphere.

The drawback is that you have considerably shortened sight lines and a bunch of pillars limiting line of sight (using the same sorts of materials as you'd need for the general case, you need something like a 3 meter pillar every kilometer).