• March 21, 2019, 4:44 p.m.

    Imagine you lived on an Earth where orbital rings and towers give access to space, the outer planets have been transformed into supramundane worlds, and the Sun is encompassed by a Dyson Swarm. What would the night sky look like?

  • Moderator
    March 22, 2019, 9:14 a.m.

    For megastructures and artificial habitats, just imagine seeing the ISS as it is now orbiting overhead, but multiplied across the entire sky and in many different shapes and apparent magnitudes. Orbital rings and space-elevator structures would resemble bright, shining beams of light arcing across the sky. Mega-planets would probably look like any other planet visible to the naked eye, unless their overall size was significantly increased. If the latter is true, then perhaps you could make out some details on their surfaces if their albedo is not so high. This is all assuming that the engineers do not make efforts to reduce the reflectivity of these structures, perhaps even to the point where some are invisible to the naked eye (i.e. a perfect blackbody object).

    Orbital/Lagrange/Statite mirrors will probably reflect sunlight in many interesting ways, including onto the nightside of a planet if required, or on specific objects in space. It is unlikely that the sun itself would dim much from a vantage on Earth when it is surrounded by a Dyson Swarm - unless the Swarm is REALLY thick or some crazy megastructure is orbiting the star and has enough surface area to significantly cover a portion of Sol's face. Our sun certainly will be dimmer (or even invisible) in certain wavelengths at this time though, as some forms of light the mega-engineerers might find useful for any number of technological tasks like photosynthesis or solarcells.

  • March 22, 2019, 1:33 p.m.

    Given the reflecting surface area of an orbital ring, it would seem to me that it would be like a permanent full moon on Earth's night time surface.

    Jupiter has an albedo of 52 percent, while Earth's is 30. If you built a shell around it with atmosphere, land masses, oceans and enough focused sunlight to approximate Earth, it would be something like three times brighter and it's already a tremendously bright object.

    If you built your Dyson Swarm inside Earth's orbit, how would you keep it from shading out Earth? If you built it outside, wouldn't it blot out most of the stars?

  • Moderator
    March 23, 2019, 6:31 a.m.

    This really depends on the reflectivity of the material these structures are made of. If they are manufactured from a material with very low albedo OR were coated in a highly absorptive layer (which would be the case if they were solar-cells), then absolute magnitude could be significantly reduced.

    In plain terms, if there is a relatively low density of objects orbiting the Sun, then our star's overall luminosity will outshine the shading of those disruptive objects, rendering them invisible against the glare of Sol. As more and more objects are placed in orbit between the Sun and an observer, then certain wavelengths will be cut into until there are so many orbiting objects that the Sun disappears. I won't do the math here, but it would take many millions of sizable objects orbiting our star to completely dim it from an Earth perspective. Furthermore, I wouldn't worry about the Earth being completely shrouded in darkness from spatial infrastructure, because orbital mirrors reflecting light where it is needed would be a ready solution.

    A similar scenario would play out here. Depending on the distance of the objects from Earth, even objects that are many hundreds or thousands of km in size would disappear among a backdrop of stars, at a rate inverse to their proportional distance from Earth and Sol. Most would be invisible but for their reflective surfaces, if they have any. These farther objects would form many new artificial constellations, and I'm sure amateur astronomers would love seeking out and observing these new nightsky objects.
    It would need to be a very thick Dyson Swarm indeed to block out all starlight. Artificial structures that orbit closer then Mars I could see as posing a more immediate concern to our nightsky visibility, but perhaps clever use of orbital mirrors or artificial-skies projected by satellites can remedy that. By the time such megastructures reach this level of pervasiveness in our solar-system, Earth itself may be a Shell-world of some sort, so the ceiling of each layer where people live could support a fully accurate artificial sky.

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  • May 9, 2019, 5:21 p.m.

    The solar system has a zodiacal light. If astronomical seeing is good where you live you can see the zodiacal light before sunrise and after sunset. The energy emission corresponds roughly to a K1.3 civilization. Up to K1.2 in the solar system there will not be any visible change. At K1.4 it would not be noticeable to most people without someone pointing to it. Also the Dyson swarm will be adsorbing sunlight. The natural dust particles should have a different albedo. If the swarm used a polar orbit the zodiacal light would look very different. I am not sure why a K1.3 civilization would do that. The opposite may be more likely. The swarm will show up as a denser line along the ecliptic. There may also be feint rings around Venus and Mercury.
    A Kardeshev 1.3 civilizations would use around 1 million times the energy consumed by humans today.

    There is also currently a gegenschein in the opposite direction from the Sun. It is harder to see. You would get something like the gegenshien at Earth-Moon Lagrange 3, 4 and 5. The natural gegenschein is at Sun-Earth Lagrange 2, These are much closer to Earth and should show up while civilization is still "small".

    The largest disruptions will be in low Earth orbit. If there is no regulation to prevent it there will be advertisements flying overhead near sunrise and sunset. In Summertime a banner in LEO could shine most of the night. The adds might be focused at cities. North-South orbital ring systems will be very bright after sunset or before sunrise if you are looking away from the Sun. Space elevators will be in sunlight for most of the night. The station will be at a fixed location and should resemble a star. The cable would be a feint line for part of the night. The cable will seam to fade in the direction of the station.

  • Oct. 11, 2019, 7:29 p.m.

    I thought I remember a video where Isaac discusses concerns with mobility while in spin gravity. For example, there was an example about throwing a ball in the direction that the rotating platform is spinning that would cause the ball to "fall" up or down, but I cannot remember. I am not well studied in physics and would like a refresher on these forces and was hoping someone could point me to the video and/or other topics like this to help me learn more.

  • Oct. 19, 2019, 7:52 p.m.

    Reply:
    i'll look through the transcription in hopes of finding the video you had in mind and add it here.
    I don't know how much curious you are about the subject so I'll start by pointing you to video format themselves put in increasing order of comprehensiveness:
    Straight to the point, 5 min video for a very quick overview form Real Engineering www.youtube.com/watch?v=im-JM0f_J7s
    Most impressive summary of the subject, 30min long but you won't feel em go by. It goes over the problems of lack of gravity then proceeds to detail the possibilities to deal with it. It gradually explains 99.99% of everything you would want o know. From Cool Worlds www.youtube.com/watch?v=b3D7QlMVa5s
    If you've watched the previously mentioned video and are wondering about that weird flipping effect,
    i believe this video from Veritasium should help you out a bit: www.youtube.com/watch?v=1VPfZ_XzisU

    Finally, if you're still curious about the details it's worth noting that all of these videos and Wikipedia quick formulas are glossing over many aspects in order to simplify things a bit (a lot actually). For instance all of these are for the most part considering a rotating habitat far more massive than anything that needs to move inside it. Now no one has yet made a nice graphic video explaining the details so the most intuitive examples would be the slight slowing down/speeding up of the structure (and thus spin gravity) when something of non-negligible mass accelerate(tries to move) in the same/opposite "direction" * that the habitat is spinning. These can be compensated via carefully placed reaction wheels with relatively low response time. But this can potentially imply that some part of the structure will have to suffer from very high variations in tensile strength due to the fact that in the real engineering world things don't magically transfer forces across their volume and said forces are always looking to find a weak spot somewhere in the structure... This and other things could be added to the graph showing the relationship between size and rotation rate shown in the 30 min video, further decreasing our options considering only the technologies available to us in 2019.
    *i used the common folks definition of "direction" similar to the one used in the 30 min video, not a proper one.

  • Oct. 27, 2019, 3:12 a.m.

    @yvonm Thank you for the time you put in to this detailed response. These were indeed the videos I was remembering. I must have been on a science and engineering binge with the SFIA videos thrown into the mix. Now that you mention force-transfer I feel as if I've gone back to step 1 in understanding how all of this works. Thanks again! These are very good things for me to go look in to.

  • Dec. 24, 2019, 12:23 a.m.

    I am working on a story including O'Neill cylinders (OC) and I am tripping over the math for determining the load an OC can handle based on the construction material.

    I know the formulas for determining the maximum radius I can use for various materials but I don't see how to determine how much mass I can put inside the OC without it failing (or causing it to fail based on the story). How does the thickness of the tube affect the mass it can hold?

    On a related question, how much mass would be necessary for a typical suburban environment?

    Can anyone point me to a resource so I can study up?

    Thanks!

  • Dec. 30, 2019, 5:17 a.m.

    An O'Neill Cylinder is basically a thin walled tube with end caps. Bursting pressure for the cylinder portion is: yield strength of the material x material thickness divided by the radius of the tube. Spheres can take twice the pressure if everything else is the same, or use half the thickness at the same pressure.

    So, assuming standard earth sea level air pressure (14.7 PSI), and an equal amount of internal mass mounted on the cylinder, plus a little for uneven pressure and safety factors, the pressure on the shell would be about 40 PSI. Using lunar aluminum at 40,000 PSI yield strength, a 40000 in radius (slightly larger than 1000 m or 1RPM spin rate for 1 g) cylinder would need to have a skin of 40 inches. Note that the shell pressure includes the weight/unit area of the skin material.

    There is another limitation on the size of the cylinder though. The circumference of the cylinder cannot be longer than the breaking length of the material (en.wikipedia.org/wiki/Specific_strength). So the cylinder made out of this medium grade aluminum could be no more than ~10km in circumference (or 3100m in diameter). As an engineer, I'd want some margin on that, so I'd keep the 1000-1200 m radius as my limit - not a particularly large OC.

    Also, due to spin instability, single OC's can't be ridiculously long compared to their length. If you group them together rigidly (say with transport tubes), with half spinning in each direction, or if the outer radiation shell is spinning in the opposite direction from the inner shell, this is not a significant issue.

  • Jan. 9, 2020, 1:45 p.m.

    Following up on Spaceswimmer's wiki link, I was surprised that basalt fiber has about 8 times the breaking length of aluminum.

    Basalt is really common in raw Lunar regolith (cheap!)

  • Jan. 25, 2020, 8:15 p.m.

    Basalt?
    Since when could you construct a Tensile based structure out of Basalt? Basalt is a stone, like granite or shale.

    Are you sure that you did not confuse Compression strength for Tensile strength?

  • Jan. 27, 2020, 3:56 p.m.

    Mechanical behavior of basalt fibers in a basalt-UP composite core.ac.uk/download/pdf/82815068.pdf

    basfiber.com/ for a simpler overview

    Or maybe just google-ing something before questioning someone else's thinking : en.wikipedia.org/wiki/Basalt_fiber#Properties

  • Feb. 1, 2020, 11:03 a.m.

    The idea of having ad-hoc composable O'Neill cylinder colonies where individual cylinders can be added or removed doesn't actually work.

    The problem basically comes down to physics. The cylinders are huge - anywhere from the size of a large county to the size of the state of Vermont. Since they're rotating, that means huge inertial forces, and their mass is easily large enough to generate enough gravity to be problematic. The sheer forces involved mean that any collision between cylinders is guaranteed to be catastrophic, so any collection of them absolutely must ensure that no collisions occur.

    Additionally, in order to spin up the cylinders you need some sort of counterweight. The simplest solution is simply to wrap the cylinder with another, slightly larger cylinder. The outer cylinder can be used for extra radiation and meteorite shielding, and additionally serves to counteract the natural tendency of the cylinders to precessing due to the rotation. However, this also significantly increases the mass and inertia of the system even further.

    So for example, take a tetrahedron. A tetrahedron is the simplest 3D shape you can build, and uses only 6 cylinders.
    A Tetrahedron

    The only way to hold the cylinders in this shape is to apply tension at the vertices. Applying tension at the vertices pulls the cylinders together, causing them to inevitably collide. This is a guaranteed disaster.

    To solve this problem you need a configuration which holds the cylinders apart through a balance of tensile and compressive forces. In other words, a tensegrity structure. Here is a simple tensegrity structure using the same 6 cylinders:
    Tensegrity Icosahedron

    In this example the cylinders are arranged into 3 pairs of parallel units, which each pair orthogonal to the others. None of the cylinders are touching, and the cables which hold them together also keep them apart. I honestly doubt you could build a colony much larger than this, simply due to the stress and breaking forces that increase as the distance between the units grows. Any imbalances in the forces that occur tend to be amplified the less compact the structure is, so you really want to keep it as simple and compact as you can, not to mention that gravity rapidly becomes problematic the more massive the cluster becomes.

    Obviously in a tensegrity structure you can't just pack up one of the cylinders and run off with it. Nor can you just tack on a new cylinder without completely rewiring the cables that hold it together and changing the shape entirely. Even modifying the design into something that works would be non-trivial, and quite frankly something you would probably avoid at all costs with cylinders that are already spinning. The hazards involved just aren't worth it, and it's far more convenient to move individual people around than to pack up a state-sized colony and try to fidget it around.

    An advantage of this configuration is that you can run trains along each of the cables to carry goods and people back and forth between the cylinders several times a day. In the basic configuration each cylinder is connected to 4 out of 5 of the others in the cluster, but you could easily add extra cables for full connectivity. Since the whole structure is balanced by tensile and compressive forces there's no need to expend reaction mass to get from place to place and any minor force imbalances between the mass of one train and another are absorbed by the structure itself. Unlike a more naive configuration, the movement of the trains does not drag the cylinders together or push them apart over time, and the trains can stay attached to the cables the whole time so there's no risk of missing the target at the far end like you might have if the trains were merely shot out by a mass driver and then "caught" at the far end.

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    The O'Neill cylinder Loading - How to do the Math thread has been merged into this thread.

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  • Moderator
    April 20, 2020, 7:17 a.m.

    @MultiTool
    This is the best I can do at the moment, since I do not have permissions to create new categories for threads.

  • April 25, 2020, 9:56 p.m.

    Thanks, that's really awesome.

    Why did you merge all megastructures in too?

  • Moderator
    May 1, 2020, 7:26 a.m.

    Oh, just the one. It seemed appropriate since the topic included things like O'Neill cylinders, albeit not them exclusively :).

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  • May 11, 2020, 7:52 a.m.

    Thank you Yvonm for the Youtube links you posted 7 months ago.

    It seems to be the assumption that an O’Neill cylinder will have 1g gravity once spun up. This is absurd. I am 75 and feel 1g, and even when I was 18 and did much more walking, it was tiresome to climb hills. Even young people can hurt themselves if they fall over.

    I suggest 0.3g. I know that astronauts in the ISS need to exercise a lot each day to avoid bone and muscle loss, before they return to Earth. I remember whenever a Russian capsule returned from the ISS, the astronauts were guided out of the capsule and onto armchairs set out for them.

    At 0.3g in an O’Neil cylinder, people can cycle long distances easily. It would be absurd to live under 1g, and move about the cylinder by battery golf carts. Little or no rain, light breezes at the most and good weather. Bicycles work well, and the ease of cycling under 0.3g means people would exercise a lot without thinking much about it.
    Ceilings could be another metre higher, allowing more storage higher up. Jump off the floor, rise to the ceiling, grab a grab handle and remove something from an upper cupboard, then let go and float down to the floor. A 75kg person weighs only 25 kg under 0.3g. There is little or no rain in an O’Neill cylinder, so expect flat roofs used as lounges. You may climb stairs to the roof, giving exercise, but you might come down by fireman’s pole. You would prefer not to use a fireman’s pole under 1g.

    Wikipedia tells me that O’Neill’s proposed cylinder would be 8 km in diameter, or about 25 km in circumference. With 0.3g, the centrifugal force (artificial gravity) will be one third that at 1g, so the diameter could be larger. Not 3 times larger, because centrifugal force depends on angular velocity squared. Coriolis forces, such as they are, will be less.
    Air pressure need not be sea level Earth pressure, which would exert a force of 14psi on the outer shell of the cylinder. Wikipedia tells me that air pressure on Earth halves with every 5500 metres of altitude, or every 18,000 feet. Air pressure on Earth is 90% of sea level at 1000 metres altitude, 80% at 2000m, and 70% at 3000m. Some people on Earth live above 3000m, or 10,000 feet, and have to move about under 1g. La Paz, Bolivia, Lhasa, Tibet, and Cuzco, Peru are all above 3000m. I suggest air pressure in an O’Neill cylinder be 70% sea level Earth pressure. People will adapt.

    At 0.3g, structures will experience less force and can be lighter. I suggest aerated concrete as a building material. Not much compressive strength, but at 0.3g, no problem.
    I live in Australia, and our early settlers tried to reproduce English buildings in a different climate. For one thing, the sun in Australia comes from the north, not from the south. We have, of course, adjusted, and I expect a similar adjustment on O’Neill cylinders.

  • May 12, 2020, 4:45 p.m.

    In the near term (next 10 years) moon industry, this is a very interesting concept. Water mining: youtu.be/t3ZnIQOmdG4
    I'd be interested in thoughts from anyone in this forum!