What size are these Imperial capitals?
I have previously described one of the SHQs as three kilometres in internal diameter and nine kilometres long, but I’m not sure that that cuts the mustard. I’ve calculated that such a structure (spinning to provide 1 gee and one bar of air on the deck at standard laboratory temperature and pressure, and with ten tonnes per square kilometre of landscaping, buildings, under “ground” facilities, and contents) could be built out of 28 billion tonnes of mild steel, 4.8 billion tonnes of high-tensile steel, 600 million tonnes of aerospace-grade aluminium alloy, or 110 million tonnes of carbon-fibre-reinforced epoxy.
The problem with that isn’t really with capacity. I estimated in Gross demographics of Flat Black that in 606 ADT a typical SHQ would hold a population of half a million. With 85 km² of floor space (not including the lower reaches of the end-caps) that’s a population density slightly under 6,000 head per square kilometre (15,000 per square mile). That’s a truly urban sort of population density, but much less than Manhattan, Paris, or Barcelona. No, the question is whether such a structure is going to make the awesome impression that the designers will have wanted. It ought to, on anyone who comprehends a billion-tonne structure. But these things are have to work at a sub-conscious level, and I’m not confident that 85 km² and sight-lines no longer than about 9.5 kilometres is going to have the right effect on a visitor who might be used to flying over a metropolis the size of Los Angeles or Sydney.
So let’s consider the hundred square miles of the Federal District in the USA or Australia. 256 square kilometres. An oneill 16 km long and 5.1 km in diameter, with Earth-like conditions on the deck could be built of 2.6 billion tonnes of aerospace alloy or 420 million tonnes of carbon-fibre-reinforced epoxy. The SHQs would be urban but not crowded at 2,000 head of population per square kilometre. The Old Capitol will not have been truly outgrown — 3.3 million people in 256 km² would be the density of Barcelona, only starting feel overcrowded. I have a feeling that this is about the size I ought to be looking at. Perhaps the effect of awe didn’t work as intended,
So how big should the New Capitol be? Ten times the area of the Old Capitol seems barely adequate, though the floor area would be about the size of Rhode Island or Dorset. Ten times its length and width?
An oberth cylinder 51 kilometres wide and 160 kilometres long seems feasible with unidirectional-carbon-fibre-reinforced epoxy, but the hull would be 13 metres thick and mass 590 billion tonnes. Using a material based on long carbon nanotubes, on the other hand, might allow a structural hull less than half a metre thick and 25 billion tonnes in mass. It would be larger than Maryland or Vermont, larger than Burundi. That might be going overboard.
20 kilometres in diameter and 60 kilometres long? That’s feasible with reinforced polymers and would give 4,000 square kilometres of floor area. 3.3 million people could live there at a population density of 834 people per square kilometre: 13% Copenhagen and 87% wilderness.