how critical is passive stability?

> Then the obvious solution is to ensure that the
> rotating body has length = to or < than the width.

It's actually a bit more complex than that. A cylindrical shell with no endcaps is stable if length <= 2R (where R is the radius), but one with even flat endcaps must be shorter, length <= 1.3R. Virtually any other sort of endcap would make it even worse (except for concave ones, perhaps).

But this is a severe design constraint, so before applying such a solution, we should make sure we really understand how much of a problem it is.

> The rotating cylinders can be stacked or strung along a
> common non-rotational axis, but unconnected from each
> other, allowing counter rotating cylinders of, say, 5
> km diameter X 5 km length and strung along a 500 meter
> diameter non-rotating tube.

How are they both strung along this tube, and also unconnected from each other? If they're all connected to the tube, then I'd say they're all connected. However, that's OK since in this case each individual habitat is stable (well, not if they're 5 km long, but perhaps if 2.5 km long for a 5 km radius).

I do think connecting multiple habitats together, like cities combining to form a state, is a good idea. I also think that by the time we're talking about habitats with linear dimensions in the kilometers, that the stability constraint isn't that big a deal -- living in a valley 2.5 km wide is no big hardship.

But I'm more concerned about early habitats, with a radius of 250 m (about the smallest you can make it for a comfortable 1G), and a length on the order of 250 or 300 m. Now you're living in a much narrower valley. If you launch Google Earth and turn on the scale bar, and then look at small towns, to see what you can fit into a 250 m strip, it's not very much.

In fact, there's perhaps an interesting challenge for the group: can you find any town or village of at least 5000 people, which is constrained by geography into a narrow strip on the order of 250 m wide?

Best,
- Joe

Joe Strout -- joe@...