Greetings and a Question

[srjskam]

Hello, one and all!

I registered here because this seemed like the most likely place to find people who know about megastructures. I recently wrote about heliocentric toroidal worlds and wanted some feedback from people. Worldbuilding stack exchange didn't seem right because I don't have a specific question, so maybe here?

Frankly I don't have the intention of writing a OA articles, just to throw this idea at people who have a better sense of physics and such, and see if any of it sticks. The question is: should I just go ahead and post it in some forum (which?) or would it be frowned upon and I should take it elsewhere (where?)?

(The world: http://srjskam.blogspot.fi/2018/04/helio...world.html .)

[Drashner1]

Hi There, Welcome to OA

Re your questions:

1) No problem with posting a question(s) to the forum, even if you don't plan to contribute an article. We don't mind helping out other projects from time to time as long as they aren't doing wholesale copying of OA or OA ideas without attribution or the like. If it turns out we are able to help with your idea, we wouldn't object if you wanted to give us a positive mention or shout out in your end product somewhere, but it isn't something we require

2) Feel free to post your questions to this thread for now. If the discussion turns into a major thread that looks like it's going to inspire new content for OA or the like, the Admins can always move it to another sub-forum or just request that people continue the discussion in a new location. Or people wanting to create OA articles inspired by this discussion can create a fresh thread in the relevant sub-forum.

3) Regarding both of the points above, and having looked at the link you sent, OA does have a mention of a smaller version of the Hoopworld, but it looks like you've done more work on the concept than we have actually. So we might end up creating/expanding an article using some of the math and stuff that you've figured out. In which case we will make sure to give you proper attribution in the article.

4) Depending on how things shake out, we might also eventually discuss a link-share between our two sites - but that's something we can talk about later if/when the issue becomes relevant.

Hope this all helps, looking forward to your questions, and once again - Welcome to OA!

Todd

[stevebowers]

This is an intriguing concept. There would be a certain amount of stretching in a hoopworld of this kind, assuming we wanted to have it rotating; but I'm not sure that the value of the Earth's radius would be very relevant when considering the amount of stretching. After all, the stretching would always be along the length of the hoop, and the radius would remain the same. This stretching would represent 5.7 centimetres per kilometre - more stretching than I would expect a solid crust to absorb, actually, so the idea of having a series of flexible joints at intervals makes sense. Maybe section the planet's surface up into kilometre-sized rings or plates.

Note that most planets can be modeled as a fluid, so the hoopworld may as well be mostly made of fluid- how about making it a huge toroidal balloon full of water? Kilometre-wide landscape hoops or plates could float on top of this water, each connected by centimetre-scale flexible junctions. If the hoop is only filled with water, it could be much less dense, and would be bigger (but this makes the stretching problem worse). The centre of the hoop would be a ring of solid, high pressure ice, which would melt significantly because of the deformation (but the ice core would not melt completely, I think, instead becoming a kind of semiplastic solid).

Note also that the constant stretching and compression of the spinning hoop would gradually slow the hoop's spin, so you'd need to constantly pump energy into the hoop to maintain a constant diurnal rotation. This would warm the hoop - it would be a warm place, heated from below by friction, so you wouldn't want to put it too close to the local star- once again this would make the hoop even bigger than a simple model would predict. The ground would be warm - but the local star would appear dimmer than one might expect, because it would need to be further away to avoid overheating the surface.

Where would you get the material for this hoop from? If I were making such a hoop,, I'd either build it from an accretion disc around a T-Tauri star, which would include a lot more dust and water than a mature solar system; or I'd extract oxygen, hydrogen and other materials straight from the star, using star-mining tech as suggested by Paul Birch.

So this megastructure is doable, but it would require a constant input of energy, and could be much bigger than you suggest; the structure has some problems in common with Cableville, which also needs a flexible infrastructure and a constant input of energy to maintain rotation. If I were to put one in OA I'd put it around a red dwarf, or even a cool, old white dwarf, making it much more managable in scale.

[srjskam]

The mention of Earth's radius was only to put the number into some sort of a scale. Cm per km would have been better in retrospect. Water hoop is a nice idea, it would solve both the stretching and acquiring mass, basically the biggest problems (seems to me) of this design. Getting hydrogen and oxygen would require much less hassle than metals and silicon and such.

Losing energy to deformation did cross my mind, but I failed to give it a second thought. I don't have any clue on the magnitude of these forces, and what kind of counter forces you'd need, nor how to administer them. Another new question.

The questions I had in mind for the simple case:

• The problem with acquiring the building material / mass
  
• Is it sufficient to model the thing as an infinitely long cylinder, given that it is pretty massive (ie. would you have to account for the gravity of the far side of the world when looking at surface gravity)?
  
• The problem of stretching
  
• Since the torus turns along the 'circular axis' as well as around the sun, would gyroscopic precession be an issue? I have trouble a) getting gyroscopes in general, and b) thinking of the structure in terms of rotation to begin with: if you thought about it as distinct cylinders rotating, what would it mean to have each cylinder's axis push at the next one's at whatever angle would result? How about thin disks? Would the forces manifest as shear forces or just cancel each other out when the disk's size approaches zero? Simply put, would the torus disintegrate into a string of individual planets?
  
• How to stabilize the whole inherently unstable thing? Active masses moving around?
  

Questions about the complex case (double helix tori):

• If the loops were thick enough or near enough to become Roche-like, what shape would they be, thanks to the different falloff of long cylinders vs. point masses?
  
• Would there be a magnetic field, or more generally, is there a net charge on an planet (more detail in the post)?
  
• What can be said about the weather, wind directions and such?
  
• Is the math sound?
  

[selden]

In a quick glance at the blog, I didn't see any mention of Larry Niven's Ringworld. Did I overlook it?

After that book was written, many people analyzed the celestial mechanics of hoops around suns and their associated problems. One problem in particular is that a hoop (or torroidal) world doesn't have a stable orbit with reference to a central sun: there's no net radial restoring force. A relatively recent discussion of this is at https://physics.stackexchange.com/questi...able/41257

[Drashner1]

A few quick thoughts on this:

1) Re day and night - Is it really necessary to spin the hoop to get day and night? If you've got the tech to build it in the first place, you've likely got the whole local solar system industrialized anyway. Depending on the available tech level, you might use self-replicating machines to create a 'mechosystem' of machines that, as a side effect of their existence, create and maintain arrays of active shades and mirrors to generate a day night cycle. The arrays would be in a series of orbits that would act to periodically block sunlight from hitting the hoop or reflect light to the backside to give that light.

An alternative option would be to use Weather Machine technology to essentially roof over the entire habitat and generate day and night from the top of the atmosphere. A somewhat less advanced version might use a literal worldhouse roof.

The upshot of this is that trying to spin a solid body for day night might be overly complex and that simpler options that just use the mass of the construct for gravity might work better.

2) Re issues with orbiting a star and stability - If you can build something like this in the first place, is it really necessary to put it in orbit around a star? Even if we assume just fusion technology (rather than OA style total mass-energy conversion), you could place the hoop in deep space and use fusion powered light sources (either orbiting around it or on tall poles or whatever) to provide light and heat, probably for vastly longer than the lifetime of a natural star. If water is used to make up the bulk of the structure, you could have it double as a fuel source or ship in ice from the local Oort Cloud/Kuiper Belt or bring in fusion fuel from a relatively nearby gas giant or the like.

Hope this helps,

Todd

[srjskam]

In a quick glance at the blog, I didn't see any mention of Larry Niven's Ringworld. Did I overlook it?

[...]One problem in particular is that a hoop (or torroidal) world doesn't have a stable orbit with reference to a central sun: there's no net radial restoring force.[...]

No, no mention of pretty much anything similar. I do know about Niven's ring, but thought it was different enough concept not to merit mentioning, relying on spin gravity and materials of great tensile strength. (Well, actually I recall erasing a sentence along the lines of "an unholy union of a torus planet and Niven's ring", but probably trying just to tone down wise-assery.)

The thing about orbital transverse instability is news to me. I've only thought of this as analogous to a long string of separate planets. Does it make any difference that the structure is not rigid but flexible/fluid-like, or is it just a question of the parts of the hoop attracting each other?

1) Re day and night - Is it really necessary to spin the hoop to get day and night?
[...]
2) Re issues with orbiting a star and stability - If you can build something like this in the first place, is it really necessary to put it in orbit around a star?

"No" and "no", but then it would be a traditional hoopworld or whatever and I wouldn't have had a reason to make a blog post . Same goes with the unnecessary and problematic helix arrangement. Rule of cool.

A minimum amount of active technology needed to make it work would be preferable to me personally, but at least you'd need some sort of active stabilization.

[Rynn]

Orbital instability was a problem with Niven’s ringworld because it wasn’t really in orbit, it span for artificial gravity at a rate much faster than orbital speed for its altitude. A hoopworld around a sun wouldn’t have the same problem, just like a topopolis gravity isn’t coming from rotation along the long axis:

http://www.orionsarm.com/eg-article/4cea908020d1e

Just for the record I’m not really happy with the fact you have no intention of contributing anything to OA but thought you’d come along and use us as a resource. This constitutes soft spam and discussing personal settings is a privilege that should be given to long time members and fans.

[srjskam]

Rynn: Discussing an idea works always both ways. As much as this is a request for comments, it's also an offering of futuristic ideas to people who specifically enjoy futuristic ideas. Isn't it a contribution in itself? As Drashner1 said, this might be something that can be infused into OA in a way or another, not necessary as text from me.

[Rynn]

If you had been polite and asked first rather than posting a link to direct traffic and attention to your setting that would be one thing. You didn’t, you announced you’re only here for help on your project. At the very best this is rude, at worst it’s the type of self promoting spam that has previously been stopped.

It’s been approved for now but if you frankly announce that you don’t have an interest beyond getting help for your own project you shouldn’t be surprised by frank feedback.