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Message: <blockquote data-quote="Umbran" data-source="post: 6879745" data-attributes="member: 177">You're being hyperbolic, and that obscures the point.On Earth, when you want to build a structure, one of the first constraints is the size of the plot of land you have to work on.  If you want to maximize how much value and/or use you can get out of that plot of land, you need to build up or down, because your reach sideways is limited.  You are driven by constraints to build layers to get the most out of a particular footprint.In space, you are not limited in footprint.  No.  The ISS is the physical size it is because of limits of *mass* (specifically, the cost of lifting that mass), not limits of available space to put it in.    The ISS isn't the size of the moon because we can't lift that much stuff into orbit!But, consider - The ISS has 32,333 cubic feet of pressurized space.  If being compact were really the driving factor, then that could be fit in a cube about 32 feet on a side.  Or in a sphere of about 20 foot radius.  But, instead, it's in a string of modules over 160 feel long.  Compared to what it could have been, it is a sprawling structure.  Having the pressurized space be compacted into minimal external dimensions wasn't driving the design.  It was driven instead by weight considerations, and being able to build segments on the ground, and merely attach them to each other once in orbit.  For this huge station, we can't build functional segments on the ground at all, so that's not a concern.  Mass is the issue.On Earth, yes.  However, when considering these structures, there's reason to consider making two cylinders, or making one twice as long, rather than make one multi-level cylinder.  And it is related to what I mentioned about the forces involved - tension.We are going to make a spinning cylinder.  On Earth, where most structures of any size don't spin, a building has to support itself under it's own weight - the controlling engineering issue is whether the foundation will support the pressing weight of the building.  And, with our terrestrial building materials, we can build a foundation that will support two stories for just about the same cost as a foundation that will support one story.  We have the option of simply throwing more reinforced concrete at most building designs.With the station, as we spin this cylinder, it has to hold together not against forces that are going to crush it, but against forces that are trying to fling it apart.  The result is basically that, in terms of engineering, this cylinder is really like a suspension bridge - take a length of bridge, and pull the ends up until they meet - the cables become like spokes on a wheel.  Spin that wheel, and the cables hold the thing together.  Stack these wheels side by side, and put caps on the end, and you have a cylinder.The controlling issue is the strength of those cables.  How much tension can they support?  Since you *only* lift to space the amount of materials you need, with as little waste as possible, your cables are not terribly over-engineered - they are as light as you can get them.  They're only as strong as you need them to be.  So, you don't get a whole new layer for free - that layer must be supported by more cable.  So, if you need to effectively build a separate "foundation" of cables to support that second layer, you don't get much cost advantage to making the layered form.  If you want a low-G area, it is just about as cost effective as to build a separate cylinder of smaller size.Which is all to point out, in the markedly different environment, what counts as "efficient" may not be the same as it is on the ground.</blockquote>

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