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I'm trying to create something like a generation ship, but it has no destination, and is travelling endlessly, so is more like a deep space non-orbital habitat for humans. One helpful and appealing suggestion was a fusion reactor like a small star at the centre of a rotating ring, providing energy and light. Could this be plausible in a softish SF context? The one question: 1. How is the reactor held (in some sort of force field)?

(My previous question was put on hold as too broad. I understand why, and with gratitude and respect to the people who responded previously, and in the light of their responses, I'm trying to break it down into a small number of more specific questions...)

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    $\begingroup$ Please 1 question per post. Anything more than 1 makes the question too broad. $\endgroup$ – L.Dutch - Reinstate Monica Nov 12 '19 at 17:48
  • $\begingroup$ Engineers have been designing solutions to hold things where they have to be held for literally thousands and thousands of years. What's wrong with the usuall trusses? Why would a force field be needed or desirable? $\endgroup$ – AlexP Nov 12 '19 at 17:58
  • $\begingroup$ @L.Dutch-ReinstateMonica Argh, OK, thanks. I'm trying to do this right, because the answers and comments I'm getting are really helpful. Part of the problem is that as a non-scientist I don't know where the practical problems might be with ideas that are artistically appealing to me as a writer, so I keep asking around them. But I'm grateful for the guidance. $\endgroup$ – TheSpidermonkey Nov 12 '19 at 18:04
  • $\begingroup$ How big is the habitat? If the reactor is "like a small star", do you need to "hold" it at all? There are several examples in SF of rings orbiting actual stars, where the star-habitat system is being moved. (As terrible as it was Bowl of Heaven is the first example that comes to mind.) $\endgroup$ – Matthew Nov 12 '19 at 18:05
  • $\begingroup$ @Matthew it's small. One of the few story-critical factors is that the "crew" has a fixed population of 150 (the means of keeping the population at a fixed number being the meat of the story...) $\endgroup$ – TheSpidermonkey Nov 12 '19 at 18:08
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Your characters do not know how it works.

The ship is much older than anyone living on it. It is not known by your characters if it was designed by their ancestors or possibly found by their ancestors or maybe something else. It is not clear to them if the star is an artificial construct or a found thing, or possibly something supernatural (which was a longtime favorite explanation for our own star). There is no official documentation, and as with our own star that aspect of the ship operates without maintenance or intervention by the humans.

This offers narrative potential: they describe what they see, and speculate on the mysteries of what they don't know according to their inclination and worldviews. There may be several theories prevalent among the current inhabitants. Over the course of the story your characters may learn other attributes of their ship and its sun, as you need to advance your story. If the star powers down for some reason they might see that it is a construct of some sort. You could have struts leading to it but it is not clear if they support it or are supplying materials / removing materials.

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    $\begingroup$ Or, all the documentation is written in a language that died 100K "years" ago. $\endgroup$ – Green Nov 12 '19 at 18:03
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    $\begingroup$ Ha, thank you, that works fine with my story. My worry was more about readers rejecting the story because the world is scientifically impossible... $\endgroup$ – TheSpidermonkey Nov 12 '19 at 18:07
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    $\begingroup$ This is an absolutely brilliant answer! +1 $\endgroup$ – user20762 Nov 12 '19 at 19:24
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Positioning a small sun (or at least, a kinda sunlike object) in the middle of a habitat is great if you want to provide heat and light to a large area or volume. Given the scale of your ship (150 people) this kind of setup seems somewhat wasteful, and waste is very much not something you want or can even afford if you're flying between stars. Space is big, as the good book says, and you want to still be alive (or at least, have your descendants be alive) at the end of the trip.

So, the main problem here is colour temperature. Our sun is a toasty 5800K, though our daylight colour temperatures on earth are more like 4800K. In either case, the temperatures are quite above the melting point of our most refractory (and theoretical) materials like hafnium-carbon-nitrogen (melting point somewhere above 4400K). That makes building your sun a bit tricky, if it is just something that's heated up by a nuclear fire til it produces a cheery glow then it will be liquid. And you certainly don't want to expose people directly to the distinctly uncheery glow of a nuclear reaction, which is likely to involve unhealthy amounts of UV, xrays, gamma rays and fast neutrons which are bad even for just dumb matter, let alone living things or electronics. Of course, having a liquid or plasma light source could work given enough scifi wizardry... careful confinement with electromagnetic fields, perhaps, and careful deployment of vortices of cool buffering gas and so on. Seems a bit of a hazardous thing to have near a habitat though, to be honest.

What you probably want, then, are giant light and heat emitters... big arrays of incandescent or fluorescent lights, basically. Maybe they're pumped directly by the radiation of a nuclear reactor underneath, or maybe secondarily by high energy electron beams making use of the massive voltages a fusion reactor might output, or maybe just boring old electricity. As elements wear out, they can be replaced or refilled or repaired or recycled as necessary, possibly by automated systems.

Given that you're illuminating a ring, one neat design for a light-emitting structure is a solid of revolution formed from a compound parabolic concentrator. It might look a little like this in cross section:

CPC of revolution

The coloured cylinder in the middle is the light source, the rest is the reflector. This ensures that light is efficiently cast upon the ring, but there's no wastage. There's more information on this idea here, also the source for the above image. The edges of the reflector can be tethered to the edges of the ring, like spokes on a bicycle wheel. These spokes might be more or less invisible from the ring itself, given that the structure doesn't need to be accelerated, they don't need to be super substantial.

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If your fusion reactor is anything like the ideas we have for building fusion reactors in the real world, then having it be 'open' to radiate heat and light on the interior of the habitat is likely both impractical and dangerous. Even if you could build an open-sided reactor, the radiation it emits is more likely to be dangerous than benign, and will almost certainly not be the combination of heat and visible light that humans appreciate.

A more practical approach would be to build the reactor at some other (more secure) location, and use the energy it produces to power giant lamps carefully tailored to give off the correct amounts and types of heat and light.

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