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In my science fiction universe, there are two ultra-strong nanomaterials widely used in civilisation. These are carbon nanotubes, and “synthodiamond”; synthetic diamond created using nanotech. Both can be constructed with more or less equal efficiency and at a similarly low cost.

In this universe, too, many beings inhabit rotating space habitats. Nothing on the scale of a Niven ring or Banks Orbital, I might add; these are rings 4-20 km in circumference and 100-500 metres in width. They have spokes, transparent roofs, and use mirrors affixed on a module at the hub to reflect light onto the inner surface. Day and night is simulated by placing the habitat in a 24 hour orbit around a larger body.

My question: most thinkers say that carbon nanotube is your go-to material for building a space habitat, but does synthetic diamond have any advantages over it? Specifically for the construction of the ring itself; in this project’s habitats, synthodiamond is already used in the mirrors and transparent roof.

What are the advantages and disadvantages of synthetic diamond and carbon nanotube for building a space habitat?

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  • $\begingroup$ From a practical perspective, you're asking us to explain the benefits of two technobabble phrases. Why? (a) We can't make either in any quanitity necessary for building a space habitat and (b) We can't build a space habitat, either. So you might be straining at a gnat asking for material details for an application that's a guess. But, if you want an honest assessment for suspension of disbelief, diamonds stink for this application. You need more flexibility and less brittle than diamond can provide. $\endgroup$
    – JBH
    Mar 10, 2023 at 5:21
  • $\begingroup$ Carbon nanotubes are just one specific class of fullerene structure. If you can make diamondoid carbon and carbon nanotubes with equal ease, you should be able to make objects using a wide variety of structures. But why are they specifically trying to make everything out of carbon? Space habitats don't have exceptional material requirements, and other usable materials are much more available. $\endgroup$ Mar 10, 2023 at 14:45
  • $\begingroup$ Why only nanotubes and diamond? Carbon can be arranged into practically any kind of crystal lattice, tuned to have almost any desired properties. We currently have only simple shapes but that's only technological limitation. $\endgroup$
    – Juraj
    Mar 11, 2023 at 10:19

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Diamonds, synthetic or natural are too brittle for large scale construction. Carbon nanotubes should be fine, though. But the question is: what is wrong with good ol' steel? Or other metals? I would wager that space habitats would be constructed primarily with materials found nearby, so if you have metal-rich asteroids on hands, you make them out of metal.

Since you are not doing some insanely huge megastructures you don't actually need extremely high-performance materials. Hell, it is highly likely it is possible to make a space habitat primarily out of wood!

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  • $\begingroup$ Wood won't work (in any sense where it's still wood) as detailed in the answers to this question: worldbuilding.stackexchange.com/questions/242389/… $\endgroup$
    – jdunlop
    Mar 10, 2023 at 9:16
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    $\begingroup$ Nothing in those answers said that wood couldn't be used to construct space habitats (rockets would be a different story). Sure, you would need coating (which you would need for any material), and radiation shielding (again, you would need one for any material). There were quite high performance aircraft and ships build primarily out of wood, and in space you don't have issues with atmosphere/rapid acceleration. $\endgroup$
    – Negdo
    Mar 10, 2023 at 9:24
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Bulk diamond (if you could make it) is probably not useful, but very thin diamond-like coatings would be a useful finish for all sorts of surfaces. Do not put down lots of coatings and hope for big diamonds.

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We do not actually know what the properties of carbon nanotubes would be on this scale. They look very promising, but the nature of the material means that we mostly just have theory.

This is the real reason they are used so much. With mundane materials, there are a lot of people who know more or less what they are capable of, and will be taken out of the story if they see something used in a way that doesn't make sense given it's properties. This problem doesn't exist with materials who's properties are not well documented, so you can get away with a lot.

That's a long way of saying "I don't know, and that's ok".

Diamonds are diamonds though. Synthetic diamond will have similar properties to natural diamond, modified by impurities or lack there of. Diamonds are extremely scratched resistant, but also very brittle. Sorta like a more extreme version of glass. most of their perceived toughness comes from their small size, and the fact that we tend to use them for jobs they are very well suited for. They are not equally suited to all jobs though.

If synthetic diamond is really cheap in your universe, then that is probably why it is used instead of glass. The nature of glass is such that you cannot get around it being very energy intensive to make. It's up to you though.

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