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Ringworlds, also called Niven Rings, are an advanced type of megastructure built around stars to house a large population on a super-habitable surface.

Overview Edit

Ringworlds are enormous bands encircling stars (usually red, orange, or white dwarfs) that are spun to generate gravity. Depending on the type of star built around, the mass requirements (usually) range from 0.01 to 10 times the mass of the earth, but they provide between 120 and 1,500,000 times the living area of the earth. While at first glance the enormous forces needed to spin the ring to generate artificial gravity would rip the ring apart, active support can be used to fix this. A stationary ring can be placed outside the first ring to provide an inward force from the star's gravity, and the two rings can be suspended apart by neodymium magnets. However, any star G-type or larger will have very little gravity at the habitable zone, and thus is not a good candidate for ring world construction.

Ringworlds also have a maximum diameter for any given material since the material would eventually crush itself due to the nonzero wall-wall component of the stellar gravitational force at the edges of the ring. Walls are needed at the edges of the rings to keep the atmosphere on the inside from spilling over the edge (1,000 km is an ideal height for these walls.). "Shadow squares" are sometimes needed to orbit inside of the ring to periodically block the sun to simulate the day-night cycle.

Empires which use ringworlds can grow to be millions of times denser than empires who do not, especially since more ring worlds can be built at angles to catch more sunlight, usually up to a maximum of about 50 to 70.

Construction techniques Edit

The construction of a ringworld is no small matter, and many factors have to be taken into consideration. First, what are the mass and luminosity of the star in question? Second, what temperature do you want to have on the ringworld? Third, how much room do you have to expand otherwise? Any empire with FTL should not build ringworlds unless they have already colonized and terraformed every planet with the right gravity within their territory, and expanded their territory as much as is possible. Terraforming planets is between 10 and 200 times less expensive than building ringworlds, so ringworld construction is not viable when there are still new planets to colonize. With that in mind, only STL or extremely powerful and populated FTL empires are inclined to build ringworlds.

Mathematics Edit

To calculate how to construct a ringworld, some math is required. As much math as possible will be omitted, so feel free to check the calculations yourself if you are skeptical. Let us define the parameters of the ringworld that you would like to build as m, the mass of the star (or close binary) that you are building your ringworld around, L, the luminosity of the star (or close binary), g, the gravity preferred for the surface of the ringworld, S, the amount of stuff you want on the surface once finished, and R, the temperature of the ringworld in distance units (the radius of a ringworld around a star with a luminosity of 1 Lsol where the temperature would be just right), with the units being msol, Lsol, m/s^2, kg/m^2, and m. Three more constants are needed, the specific strength of the construction material (make sure to use a value quite a bit lower than the actual specific strength of the material to include a margin of error), C, in N*m/kg, the density, p, in kg/m^3, and the thickness, t, in m. Note: while planets have a quarter of their radiating area as absorbing light, and ringworlds seem to have half of their radiating area as absorbing light, they still only have a quarter since most ringworlds make use of sunshades to block the light half of the time.

The radius of the ringworld, R, can be calculated from Rt by multiplying Rt by the square root of L. The gravity of the star at the ringworld distance in m/s^2 can be calculated by G=530462345040000000000*m/R^2. If the ringworld is to be supported using active support, then a second, (usually) more massive stationary ring must be placed just outside of the first one, with neodymium magnets holding the two apart. The ratio of outer ring mass to inner ring mass is g/G. Another constant, effective breaking length, can be calculated as follows: E=(C*t*p/(t*p+S))/g. Maximum ringworld width is an important piece of information, so the linear algebra and calculus required to derive it will be spared. Maximum width, or w, can be calculated by w=2*R*((-E^4+12*R^2*G^2*E^2+16*R^3*G^3*E)^0.5)/((E^4 -8*R^2*G^2*E^2+16*R^4*G^4)^0.5). Finally, material needed is (t*p+S)*(1+g/G)*2*pi*R*w, and the amount of light absorbed and maximum number of ringworlds around a given star can also be calculated.

Results Edit

In general, the more luminous the host star, the harder it is to build a ringworld per unit area, and the more massive the host star, the easier it is to build a ringworld per unit area. As luminosity increases faster than mass, M- and K- type stars are the best main-sequence candidates for ringworld construction, but white dwarfs and neutron stars are very good candidates too. Anything larger has too high of a luminosity and is unfeasible to use for ringworld construction.

In most cases, between 0.1 moon masses and 200 earth masses in materials can create an optimized Dyson swarm of several dozen to a hundred ringworlds absorbing more than half of the star's light (any more would yield diminishing returns at this point), creating between 16,000 and 60,000,000 times the earth's area, depending on the star.

For a more user-friendly description of the results of the mathematics, let us examine possible ringworlds around well-known stars. Several assumptions have to be made when doing this. Namely:

  1. The formulae stated in the previous section are correct.
  2. g = 8 m/s^2
  3. Rt = 125 Gm (This is slightly inside earth's orbit to ensure a tropical climate for maximum habitability.)
  4. The material used, after a 75% margin of error, has a specific strength of 1,681,714 N*m/kg. This can be achieved using several existing materials, such as poly(p-phenylene-2,6-benzobisoxazole). The material will also be assumed to have a density of 1.5 g/cm^3.
  5. One technician being paid the equivalent of $45.00/hour can control enough robots to complete a 1000 kg structure in 1 hour.
  6. For every increase in the mass of a structure by a factor of n, the cost and labor required increase by n^0.84, not n, due to economies of scale.
  7. The random material on the surface will have a mass of 175,075 kg/m^2.

We will cover several stars in these examples: Barnard's Star, Sirius B, 61 Cygni B, a hypothetical neutron star with a mass of 2.5 Msol and luminosity of 0.40 Lsol, and Sol itself. Note that these scenarios are NOT part of the fiction universe, they are examples to help the reader.

Barnard's Star Edit

  • Mass - 0.144 Msol
  • Luminosity - 0.0035 Lsol
  • Ringworld radius - 7,395 Mm
  • Stellar gravity at ringworld distance - 1.3968 m/s^2
  • Outer ring/Inner ring ratio - 5.7274
  • Maximum width - 150 Mm
  • Area (km^2) - 6.96841 trillion
  • Area (Earth areas) - 13,663
  • Mass (Earth masses) - 0.00373
  • Cost of Construction per hectare - $3,820.54
  • Man-years of labor required for construction - 1,972.36 Trillion
  • Maximum number of ringworlds - 68

Sirius B Edit

  • Mass - 1.018 Msol
  • Luminosity - 0.056 Lsol
  • Ringworld radius - 29,580 Mm
  • Stellar gravity at ringworld distance - 0.61716 m/s^2
  • Outer ring/Inner ring ratio - 12.963
  • Maximum width - 451 Mm
  • Area (km^2) - 83.8656 trillion
  • Area (Earth areas) - 164,442
  • Mass (Earth masses) - 0.0932
  • Cost of Construction per hectare - $4,738.41
  • Man-years of labor required for construction - 29,440.4 Trillion
  • Maximum number of ringworlds - 90

61 Cygni B Edit

  • Mass - 0.63 Msol
  • Luminosity - 0.085 Lsol
  • Ringworld radius - 36,443 Mm
  • Stellar gravity at ringworld distance - 0.25163 m/s^2
  • Outer ring/Inner ring ratio - 31.793
  • Maximum width - 784 Mm
  • Area (km^2) - 179.612 trillion
  • Area (Earth areas) - 352,181
  • Mass (Earth masses) - 0.469
  • Cost of Construction per hectare - $8,594.01
  • Man-years of labor required for construction - 114,356 Trillion
  • Maximum number of ringworlds - 64

Hypothetical neutron star Edit

  • Mass - 2.5 Msol
  • Luminosity - 0.4 Lsol
  • Ringworld radius - 79,057 Mm
  • Stellar gravity at ringworld distance - 0.21218 m/s^2
  • Outer ring/Inner ring ratio - 37.703
  • Maximum width - 1,258 Mm
  • Area (km^2) - 624.926 trillion
  • Area (Earth areas) - 1,225,345
  • Mass (Earth masses) - 1.92
  • Cost of Construction per hectare - $8,091.03
  • Man-years of labor required for construction - 374,593 Trillion
  • Maximum number of ringworlds - 87

Sol Edit

  • Mass - 1 Msol
  • Luminosity - 1 Lsol
  • Ringworld radius - 125,000 Mm
  • Stellar gravity at ringworld distance - 0.033950 m/s^2
  • Outer ring/Inner ring ratio - 235.64
  • Maximum width - 3,950 Mm
  • Area (km^2) - 3,106.38 trillion
  • Area (Earth areas) - 6,090,933
  • Mass (Earth masses) - 58.5
  • Cost of Construction per hectare - $28,649.05
  • Man-years of labor required for construction - 6,593,130 Trillion
  • Maximum number of ringworlds - 43

Disclaimer Edit

There is an exception to the "only M, K, and degenerate stars" rule, though. When the star is being constructed too, it is best to choose a star size where the total mass of all of the ringworlds relative to the star is approximately the same as the relative density of the building material in the interstellar void near the proposed construction site (so usually G-type). However, this is only for VERY advanced and populated races, as the labor requirements are huge. An empire will usually only build their own stars if all other options have been exhausted to the point where they have Dyson swarms of ringworlds around every star for dozens of light-years, and switched almost entirely on almost all of them to hydroponics for farming, still without FTL travel. This requires a population in the septillions, and it is not economically viable otherwise.

Uses Edit

A ringworld is a huge structure, and can have many different uses. First, be sure to build low and wide, as too much of a mass disturbance (for example a large, high building) may cause problems in the support structure. Second, do not do ANYTHING that has the slightest possibility of nuclear explosion (E.g. use beamed-in solar energy, not fission or fusion. While these are awesome on planets, they are not the best choice for ringworlds.), as that would create a huge hole in the ringworld, and either of these have a small chance of destroying it. Third, do whatever you can to defend it. Defense rings and a moderately-sized defensive fleet would be best, as an attack an the ringworld itself would have disastrous consequences.

To minimize damage if the ringworld does break, place walls similar to the rim wall along the ringworld to divide it into at least several dozen air-tight sections. These sections can have bulkheads between them for transport, but the bulkheads must be airtight. This means that even if the ringworld breaks, only a small section of it will be drained of atmosphere.

As for how to best use the area, there are a few options in that regard that are shown in the table below:

Land usage Cost per hectare Population/square kilometer Food production/consumption
Automated farming $75-$200 ~0.01 500.0-2000.0
Conventional farming $500-$1,500 ~1 50.0-200.0
Land-minimization farming $6,000-$10,000 ~100 3.0-6.0
Greenhouse farming $50,000-$80,000 ~1500 2.5-4.0
Light-customized

hydroponic farming

$1,000,000-

$4,000,000

~7500 1.5-3.0

In this table, only land use for farming is shown, but the food production/consumption column helps to determine the area of ringworld that should be used for farming and other things, in each case.

It should also be noted that as ringworlds are constructed objects, there are no mining opportunities on them. Therefore, most of the infrastructure and industry on a ringworld will have more to do with the processing of raw materials into complex structures than the acquisition of raw materials in bulk. Warehouses are not an ideal use for ringworlds either, as one could instead create an unlit building spanning an entire planet, with thousands of kilometers of floors filled with endless warehouses, with the upper layers supported by orbital rings.

So what are ringworlds good for? Producing food, manufacturing, and population capacity. Food and other life support items can easily be removed from a ringworld with a vibrant ecosystem as that ecosystem will use sunlight to convert carbon dioxide and various excrement into near-potable water and food. Ringworlds with both a stable ecosystem and farming are best at this, as the ecosystem can be tailored by adjusting the proportions of different crops in the biospherecircle.

Maintenance Edit

Another consideration that should be taken into account when building a ringworld is that stuff does wear out and will eventually break. As the ringworld structural components are (ideally) bulk material of some kind, melting and re-solidifying ringworld pieces that are in need of repair is a good tactic. To do this, simply tape off the area of the ringworld that is in need of repair, pull out this area (do not fix too big of an area at once or the ringworld might break from the strain imposed by the un-counterbalanced weight of the active support section that keeps the ringworld from flying apart. Once the area in question has been removed, add a new piece from your stockpile of ringworld components. This technique is most effective when (a) your ringworld is built out of "tiles" designed to slide out and in, and (b) you have a stockpile of random extra "tiles" in your system. Broken tiles removed from the ringworld can be rebuilt in space stations near the ringworlds.

The biospherecircle and terrain will need maintenance too. As ringworlds have no plate tectonics, the dirt on the surface will mix with lakes and oceans and will eventually turn the entire ring into mud. On smaller ringworlds around late M-type stars, there is a simple system that works well. If the ringworld is less than about 10,000 km wide, funnels can be placed in the middle which lead to pipes that go through the structural material underneath to pump the slush onto "spill mountains" on the sides, which slowly slide back towards the middle. These spill mountains can be 20 km high in some cases, but making the active support layer thicker on the edges of the ring and thinner in the middle will solve the problems caused by an uneven mass distribution. Another option is to use a less even distribution of pipes and place the pipes a lot more randomly. If done right, this can allow for constantly maintained gradual changes in terrain while always staying within 20 meters of sea level, minimizing the extra construction costs that the addition of extra material for spill mountains incurs.

Another consideration is whether or not the ring will be partitioned. A ringworld can support huge walls of aerogel several hundred kilometers high if they are running lengthwise and the active support structure is thicker in these places. These walls can be used to section off the atmospheres of different sections of the ring, slightly decreasing vulnerability to a puncture and letting you put different amounts of greenhouse gases on different sides to simulate different climates, useful when housing multiple species.

It is important to remember that ringworlds are inherently unstable and will fall into the star if they get off-balance. This can be avoided, however, by placing a cloud of sensors outside the ringworld's orbit, and incorporating radio transmitters into the ringworld. Then, mirrors can be placed at regular intervals along the ringworld, and a low-radius Dyson swarm of expandable satellites can be placed near the star. This smaller swarm will use pushing lasers and mirrors to push the ringworld back into place if any discrepancy between where the ringworld should be and where the sensors say it is is detected. While pushing lasers are expensive in terms of energy, satellites and include mirrors that reflect the laser back and forth thousands of times. As the satellites that you are pushing with are on the same side of the star as the part of the ringworld that is moving toward the star, it is possible to take advantage of relativistic effects to blueshift the laser light every time that it bounces between the satellite and ringworld, effectively absorbing the kinetic energy into the laser beam. Ideally, these lasers would be constantly in operation at extremely low intensities, allowing the ringworld to stay in the correct place, possibly with sub-kilometer accuracy.

Design Edit

Aesthetics are always secondary to function, which is why this section was placed last. After all of the above considerations have been taken into account, the terrain of the ringworld must be decided on. As (most) people prefer beachfront property over non-beachfront property, coastal area should be maximized while still leaving some land to prevent overly high tsunamis. As such, landmasses should be as small as possible, and everything should have barrier islands to triple the coastal area, not to mention a lot of gulfs, bays, and peninsulas. To maintain a fairly ordinary climate, the landmasses should be in the range of a million square kilometers, and there should be as much land as possible without interrupting a continuous ocean. This ocean is important as shipping is less expensive than most other forms of surface-based transit and will be an important part of intra-ring commerce. One way of designing this is using a hexagonal pattern for land, with side lengths of 400 to 1000 km and waterways in between 100 to 300 km wide. In the middle (right between the two edges), one or two strands of hexagons can be removed to allow for increased shipping capacity.

Another consideration is climate. Ringworlds have no seasons or latitudinal variations, so no strong trade winds would exist on the surface. While weak trade winds and ocean currents would arise due to the orientation of the land in the ocean, strong trade winds and ocean currents arise due to temperature differentials caused by latitudinal variations. With property value in mind, always design the climate to suit the needs of the inhabitants. As the ringworld is built, not found, the builder will have absolute control over the atmospheric composition, average temperature, and average humidity. As such, ringworlds tend to be more habitable than almost all planets, unless they were designed by a different species or not designed to be habitable to begin with (although that would kind of be a waste of money, so it is very unlikely).

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