Space Mega-structure Feasability

Question

I was reading a lot about the hypothetical orbital mega-structures that could potentially exist in the future if technology and manpower advances enough. A few of the more notable ones are the following (please post ones that aren't listed if you know about others):

Trans-orbital Structures

• Skyhooks - Orbital Transport Method
• Space Elevators - Orbital Transport Method
• Space Fountaions - Orbital Transport Method
• Launch Loops - Launching spacecraft into orbit
• Space Tethers - Orbital Propulsion

Orbital Structures

• Orbital Cable - Interstellar Launching after Geostationary Orbit
• Bernal Sphere - Stationary Orbital Habitat
  • Rotating Wheel Space Station - Space Station with Artificial Gravity
  • Bishop Ring - Same Basic Idea.

In addition to these seemingly non-fictional devices, there's tons of fictional mega structures like Dyson Spheres, Ring Worlds, Terraforming and other more "fictional", "long-term" or "planetary-scale" solutions are out of scope for this question.

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The Question: Are any of these methodologies implemented currently at a "micro" scale for certain functions onboard existing spacecraft and are there any planned mega-structures (public or private) being built at the moment?

Answer 1

Many of the ideas/proposals you've listed falls into the category of requiring some sort of powerful tether such as the elevator, launch loop, space tethers, skyhook, and possibly the orbital cable idea. This is currently limited by materials science/our ability to produce the required material in mass quantities.

There are many companies working on stronger tethers (mostly carbon fiber/graphene type stuff). In fact, there's lots of research institutions (such as MIT) working on getting graphene to leave the lab and become industrially produced. This might "accidentally" unlock the technology needed for all these tether-related technologies.

As for the megastructures requiring active supports such as the fountain or the loop, I couldn't find any company or research project that has left the hypothetically-physically-possible-drawing-board phase.

There are a couple thinktank-companies and old space agency plans for orbital rotating stations (or parts of stations), although not usually in the scale of a full blown Bernal sphere. There's lots of discussion and speculation on forums and the consensus seems to be that making a rotating space station today would be feasible--just expensive and counterproductive: The main advantage orbital space currently has over earth is primarily the micro-gravity environment and maybe secondarily as an isolated environment. For a rotating habitat to make sense, it would house scientists and tourists who want to sleep in gravity but work in micro-gravity or preform experiments at lower gravity levels.

The furthest any space-related megastructure has gotten is probably the StarTram concept which proposes a cargo and a human space launch system which uses a really long evacuated tube (130km for cargo, ~1300km for humans). They claim that the technology needed to build it is here today and there have been cost-estimates citing a construction cost of ~15 billion for the cargo version and another ~60 billion for the human version if construction began in 2010. This launch method probably has the most serious artist's impressions out of all the space-megastructures.

If I had to guess the order at which these will be developed (if!) Id wager that we'll see an orbital station with artificial gravity within the next 10-40 years. If the space industry is worth it, I expect a non-rocket space launch system will come after that, probably a tether-based design.

Answer 2

Most of the structures that you listed are "active structures". That is, they rely on active components that must be reliable, and without which they would not function as intended. Placing one's trust in an active structure might seem like a bad idea at first - until you consider that aircraft might be considered "active structures" and they are trusted to support roughly half a million people in the air at any given time.

On a "micro" scale, there are plenty of other examples of "active structures".

Some skyscrapers and bridges use Active Mass Dampers

as a means of mitigating the undesirable effects of dynamic loading on structures including alleviation of floor vibration induced by human activities as well as wind and seismic responses of buildings, towers, and bridges.

This video (narrated by Steve Taylor of Kurzgesagt) shows off an active system used to control an adaptive shell structure.

CERN's Large Hadron Collider (LHC) confines a circulating mass stream and that mass stream exerts a respectable 3 N of outward centripetal force per meter on its beam pipes. It also maintains a vacuum in the order of $10^{-10}$ to $10^{-11}$ mbar inside each of its two 26.7 km long (in circumference) superconducting storage rings.

While technically the LHC is more of a research tool than an active structure, it does help us to buy down some of the risk associated with confining a fast-moving mass stream - a technology that will likely be used in inertially supported active structures in the future.