Kepler and Bucky habitat clusters
Habitats that get their "gravity" from rotation are constrained in size, by material strength which limits their diameter, and by their cylindrical length. Cylinders spinning about their central axis are only passively stable if the axial length is not significantly greater than the cylinder's radius (technically, the moment of inertia about the central axis must be larger than the moment of inertia about a spin axis passing through the center perpendicular to the central axis). If this condition is not met, the cylinder will eventually end up tumbling end over end without constant active stabilization. This tends to favor passively stabilized spin habitats shaped like wheels while those shaped like cigars or soda cans use active stabilization.
Clusters of habitats arranged in polyhedral or geodesic arrays
More often than not this active stabilization is in the form of a counter-rotating mass the habitat is some how linked to. In the early "hatbox" design the counter-rotating mass was an external sleeve of radiation shielding. In the classic O'Neill colony the cylinders are twinned, laid side-by-side, and connected at their hubs through long compression/tension spars. A McKendree cylinder has a radius so large that most of its interior is in a vacuum so a second, smaller, cylindrical habitat can be counter-rotated within it. This kind of stablization allows cylinders to be up to five times as long as their diameter. In the extreme example of cylindrical length, a topopolis, the habitat becomes its own counter-rotating mass when it loops around a star to link its two ends to each other. A topopolis can even be looped around the star several times in a configuration known as a torus knot.
Cylinders connected into hoops are quite common; two well-known examples are Evermore and the Rungworld. These rely on dynamic rings to avoid collapse. But in other designs the cylinders themselves are used as structural members.
If you have three or more cylinders they can be linked hub to hub, through special jointed sections called knuckles, to form a small ring as a triangle, a square, etc. Knuckles can be built to hold more than two hubs and this leads to habitat clusters configured like polyhedral space frames, with the cylinders placed as the straight edges. For example, six cylinders form a tetrahedron, thirty can be used to build a dodecahedron and ninety can be formed into a buckyball-like megastructure. Linking cylinders in three dimensions like this has three advantages. For one, they can be built around a hotpoint with energy collecting panels placed as the faces of the polyhedrons. The second advantage is the cylinders can be even longer. Here the limit to length is not the moment of inertia but the strength of the structure needed to transmit the forces generated to the other cylinders. And third, the megastructures themselves can be made larger just by adding greater numbers of smaller components.
The classing of these habitat clusters, into Bucky or Kepler habs, is flexible but generally it is based on the information that the Old Earth German mathematician - Johannes Kepler formulated a hypothesis of planetary spheres in terms of the five platonic solids. Because of this, and the fame of the Kepleria dyson structure, any polyhedral configuration can be call a Kepler hab by association, even if it is not based on (nor even derived from) one of those five polyhedrons. Bucky habs on the other hand are more clearly defined. The knuckles and cylinders in these habitats either copy the arrangements of the atoms and bonds found in the molecules known as "fullerenes" (buckyballs and bucktubes) or they copy the arrangements of the connectors and spars found in the geodesic spheres popularized by the Old Earth thinker - R. Buckminster Fuller. As a result, any of these clusters that can't be classed as a Bucky hab is classed as a Kepler hab by default.
The largest Bucky habitat to date is Buckminster dyson sphere under construction at YTS-7742-6691-E45 in the MPA. It is a triangulated framework six AU across, reinforced by dynamic compression members.. Each strut of the frame is actually a 100,000 km long McKendree cylinder that rotates around the dynamic compression member at its axis. While the structure of this dyson is simple and repetitive it does provide real creativity in ecopoesis; when it is completed there will be nearly six hundred million of these self-contained, planetary scale habitats available for experiments in biospheric design.
Text by AI Vin
Initially published on 03 November 2010.