12
$\begingroup$

I am writing a short story about an overworked planet whose civilization has achieved space elevators and orbital rings.

The planet, which is earth-sized, has three complete orbital rings in stable orbit, each one 200 meters in width, which is about an eighth of a mile.

The rings are used for heavy construction and other industrial uses.

Where do you have to put the rings relative to each other for them not to have any issues or break, and should the rings be fixed onto the ground or allowed to orbit freely?

$\endgroup$
  • 1
    $\begingroup$ Welcome to Worldbuilding! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. (a) You've provided detail that's not relevant to your question but will distract people (ring materials, etc.). I'd remove anything not 100% pertinent to your question. (b) If each ring is equal to the circumference of the earth, those rings have big, unstable gaps. The bigger the radius, the bigger the circumference. Remove that, too. $\endgroup$ – JBH Jul 3 at 21:22
  • $\begingroup$ Thanks JBH for the advice! $\endgroup$ – LegionOfAvatars Jul 3 at 21:26
  • 2
    $\begingroup$ Bingo. We have several savvy orbital mechanics folks on the site who will love sinking their teeth into this. We recommend giving a question at least 24 hours before selecting a "best answer" because we have users all over the planet. Time zones and what not. $\endgroup$ – JBH Jul 3 at 21:28
  • $\begingroup$ The only answer worth anything is the one by Algebraist, as he was the only one who understood what an orbital ring is. I'm assuming you wanna build one of these orbital ring and description in my answer here. If that is the case, you should ignore everything the other people said and provide a more detailed description of the ring or a link to a website with one to avoid confusion. That said, cool first question. $\endgroup$ – TheDyingOfLight Jul 7 at 7:32
  • $\begingroup$ >The sole purpose of the rings is to allow spaceships to enter orbit. Unfortunately this doesn't hold up to some analysis. A space elevator is required to lift spaceships up and down from orbit. Adding a ring serves no extra benefit in this regard. $\endgroup$ – Innovine Jul 7 at 8:52
3
$\begingroup$

I am writing a short story about an overworked planet whose civilization has achieved space elevators and orbital rings.

Note that there is no known material strong enough to build a space elevator. People will talk about carbon nanotubes, but even with the best manufacturing capabilities imaginable small imperfections in the material will reduce its breaking length to a point too short for elevators. With orbital rings you picked the best launch assist megastructure there is, since the ring is unchangeable regarding scalability, general utility, throughput and possible take of velocity.

The planet, which is earth-sized, has three complete orbital rings in stable orbit, each one 200 meters in width, which is about an eighth of a mile.

Sounds perfectly reasonable dimension wise. Note that orbital rings have tremendous energy requirements to be started up, a skeleton ring will need half our annual energy production to get going as noted is this paper, which will give you more information on orbital rings, but the civilisations building them should have mastered fusion or off world solar or at least mass fission, so it will be fine.

The sole purpose of the rings is to allow spaceships to enter orbit.

As Algebraist already noted is his answer, rings can be used for so much more. Definitely check out the Isaac Arthur video he linked, the guy is amazing. Concerning other things to do with the ring: hyper fast on planet transport, shellworlds, supermundane planets or megaearths, chandelier (hanging) cities (or gardens), colonising ice giants, gas giants and even suns and much more.

Where do you have to put the rings relative to each other for them not to have any issues or break, and should the rings be fixed onto the ground or allowed to orbit freely?

The simple answers are wherever and whatever you like. As long as the rotor sits in an orbital path around the planet and two rings rotors do not intersect everything is possible. Maybe having one equatorial ring for transfer to the planets habitat swarm and moon, one aligned with the systems plain of the ecliptic for interplanetary launches and one polar ring for power beaming receivers (Atlas towers will do the same) on the poles and more exotic launch trajectories is an optimal three ring setup.

I would suggest anchoring them to the ground, because one would lose so much of the rings utility if they were free floating. The only times you want to use free floating rings are when you are colonizing objects with no solid surfaces like ice-giants, gas-giants and suns.

$\endgroup$
  • 1
    $\begingroup$ Anchoring them to the ground will cause extreme stress on the structure, so they'd need to be build of some ridiculous handwavium. Any ring not in geostationary orbit will move with respect to the ground, so when you put anchors down the entire ring will always be trying to violently twist itself off its supports, OR, the weight of the whole ring needs to be held by these supports, or some value in between. This is why free floating works, it'll be in orbit, and weightless. $\endgroup$ – Innovine Jul 7 at 8:57
  • 1
    $\begingroup$ @Innovine Check out the paper I linked. The rings are active support structures and these must by definition be held up by active forces. The rotor keeps the stator stable. The rotor is inside the ring and moves at faster than orbital velocity to keep the stator sitting over a fixed position. Magnets force both to interact. I explain the concept in great detail here. While you are right that there will be huge forces involved, which increase withe the rings inclenation, those can be conterbalenced by the magnetic accelerators. $\endgroup$ – TheDyingOfLight Jul 7 at 9:14
  • 1
    $\begingroup$ @Innovine Magnetic deflection is the name of the game for non equatorial rings. $\endgroup$ – TheDyingOfLight Jul 7 at 9:15
11
$\begingroup$

There is only one orbit for a space-ring consistent with space-elevators - that is an equatorial orbit at an altitude above the earth's surface of approximately 35,800 km. At that altitude the ring will be in geo-stationary orbit - i.e. the ring will rotate at the same angular velocity as the earth-s surface so tethers stretching from the earths surface to the ring (i.e. space elevators) will not stretch or bend (significantly).

If you don't want to connect your space elevators directly to the ring (i.e. you have both, but they are independent from each other) then in principle you can have the ring in any orbit. That is to say at any altitude (outside the atmosphere) and at any orientation to the earth's rotational axis. So they could be equatorial, trans-polar, or at any angle you propose.

I say in principle, as:

  • any orbital ring will be in an unstable equilibrium, so would need continual correction to remain in one piece;

  • no materials are known to exist that could be used to construct such a ring;

And in regards to your comment

The sole purpose of the rings is to allow spaceships to enter orbit.

A single space-elevator-tethered-geostationary space-ring has obvious benefits for getting materials into space in an energy-efficient manner (if you ignore the enormous sunk energy cost of building it in the first place). But I can see no energy, resource, or logistics benefit in multiple rings, so it is unlikely that this would be their "sole purpose".

$\endgroup$
  • $\begingroup$ Thanks Penguino, I did not realize that. I guess the civilization has more advanced materials. The planet is a trading hub, but I guess it would make more sense to have one orbital ring and multiple stations instead of multiple orbital rings. $\endgroup$ – LegionOfAvatars Jul 3 at 21:52
  • 1
    $\begingroup$ Three comments: 1) Many known materials are strong enough to build the ring, since it will be in zero gravity. 2) The energy cost for putting the material into orbit can be greatly reduced by building it of stuff harvested from asteorids. 3) The ring will need to be slightly outside geostationary orbit to create an outward pull to counteract the pull of the weight of the space elevator. This will also act to stabilize the ring. $\endgroup$ – Klaus Æ. Mogensen Jul 4 at 7:49
  • 1
    $\begingroup$ @Cadence The thing is, space is big. The ring would have to be ~250 000 km in circumference, which is rather impractical for transportation. Individual space stations would probably be better even if you could build a structure this big (we sure as hell can't). In the end, most of the ring would be materials wasted just for the purpose of connecting the actually useful parts - something much more easier done even with simple rocket ships. Even for storage - a toroid of mere 1 meter width would easily store the entire world's current yearly production of iron ore. You don't need that. $\endgroup$ – Luaan Jul 4 at 8:05
  • 1
    $\begingroup$ @KlausÆ.Mogensen You're talking about a ring around the planet. No matter how you build it, it's not in "zero gravity" (free fall). It will need to support its own weight. Needless to say, we have no materials that could do that. Millions of space stations in geostationary orbit wouldn't need support - a single solid ring definitely would, and support well beyond any material we have or theoretise, if you only want a static system (there are tricks we could use, but...). Yet another reason why it's hard to imagine why we would ever want to build such a thing. $\endgroup$ – Luaan Jul 4 at 8:22
  • 5
    $\begingroup$ @Luaan: You are wrong. The idea with geostationary orbit is precisely that anything orbiting there (in free fall) stays above the same point on Earth. An orbital ring will no more need to support its own weight than the International Space Station does. $\endgroup$ – Klaus Æ. Mogensen Jul 4 at 10:11
3
$\begingroup$

Once you have a single orbital ring, your best bet is to make it wider and thicker. The places you bean stalks (space elevators) have extensions anyway for the counter weight to balance the weight of the stalk. Usually this is a small asteroid at around 100K from the earth.

For in system flights by matching speed with one of these exterior spoke (cable to a docking engine) you can save substantial amounts of fuel. Repeated encounters have to balance momentum in the long run.

See Arthur C. Clarke "The Fountains of Paradise" Charles Sheffield "The Web Between the Worlds.

$\endgroup$
  • $\begingroup$ Downvoting because you did not borther to google what an orbital ring is. If you had done so you would have realised that it is an active support megastructure and can be placed anywhere around a planet, not only at geostationary orbit. Most ring proposals aim for 80 to 500 km orbits. Read the Wikipedia article and my answer here for more information on orbital rings. $\endgroup$ – TheDyingOfLight Jul 7 at 7:36
  • 1
    $\begingroup$ What do you mean by active support? $\endgroup$ – Innovine Jul 7 at 8:45
3
$\begingroup$

Any way you want.

It depends on the energy you have available.

To not tether them requires that you have thrusters of some kind to enable them to maintain position (ie. not crash into the planet).

The most energy efficient way to orient them would seem to be in line with the ecliptic plane, because this will require little energy to maintain.

Another possibility is that one could be as stated above requiring low energy to maintain, another could be in an orbit (since there are three) at 60 degrees to the first, another at 120 degrees. This arrangement would potentially require no energy to sustain it's orientation either. The reason why would be gyroscopic stability, It would mean that your planet would revolve beneath these layers, and the layers would stay on their own planes, relative to the stars (ie. not relative to the planet or sun). This would mean that the latter two rings would precess around the planet once a year, but since the planet rotates once a day, it would allow you to select which ring at which time you need to land on to be closest to any destination on the surface of the planet.

However: You can have them in any orientation you wish if you have the energy to counter the forces that would prevent their precession (Which would be quite prodigious BTW).

Ultimately, the whole thing would be a trade-off between the energy required to keep them from crashing into the planet versus the energy (if tethered) to stop them from deforming because of gravitational forces caused by the planet's rotational axis being not tidally-locked to the sun, not to mention the moon's pull on their bodies. It depends on the materials, the flexibility and the energy available to make them behave.

$\endgroup$
  • $\begingroup$ Hm. Interesting, and you're saying the orbital rings are not attached to the planet, which makes sense if you need to maintain multiple rings. Thanks! $\endgroup$ – LegionOfAvatars Jul 3 at 21:54
  • $\begingroup$ @LegionOfAvatars Finished now. $\endgroup$ – Measure of despare. Jul 3 at 22:13
2
$\begingroup$

Using the orbital ring itself for habitation is a poor use of its potential on a utility/unit mass basis.

To keep building up an orbital ring, you need to spin up the rotor even faster, or add mass to the rotor. Both are non-trivial requirements.

A more elegant solution would be to have a band or orbital habitats, connected together by physical supports but traveling at orbital velocity, with each habitat spinning to generate artificial gravity on the inside.

People planetside can hop on the elevators (they're not true space elevators, they don't bring you up to orbital velocity at the end), transfer to a maglev carriage on the orbital ring that accelerates them to match orbital speed with the habitat ring, then disembark unto whatever habitat they want to travel to.

You might even have a second maglev system for inter-habitat travel, which does not require the enormous rotor of the orbital ring.

Isaac Arthur did a good video on Orbital rings on YouTube: https://www.youtube.com/watch?v=LMbI6sk-62E

$\endgroup$
  • $\begingroup$ Thanks for the video! This guy seems pretty good at space stuff. $\endgroup$ – LegionOfAvatars Jul 5 at 0:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.