Another approach is to tie together several cylinders together to create a larger space station. Like many of these deep-future scenarios I write about on this blog, massive structures would be much better off without needing manufactured steel beams with high compression strength. At a glance, working around the constraints to achieve this ideal might sound impractical, but it's actually entirely reasonable. I will address both scenarios of 1.) the superstructure inside of large gravity balloons and 2.) a generic case for orbital habitats in typical vacuum conditions. These are quite different, but some of the underlying principles are the same.
Purposes of TethersLet's get the first things straight - what specifically the engineering role for a superstructure is.
- Rotation - spinning up and stopping the rotation of the tubes that creates gravity
- Contingencies - if a tube breaks, there is a space traffic accident, or a host of other scenarios, the tethers must be in place to arrest the motion and avoid a local conflagration from destroying the entire space city in a domino effect
- Position - must be maintained against tube-to-tube self-gravitation, external gravitational fields, momentum exchanges from transport, and so on
Scale-DependenceFor a small gravity balloon, it might be completely reasonable to pull against a tether anchored into asteroid rock to maintain or start up rotation. On any larger scales, however, you would just use force-exchange with the neighbor cylinders for this task.
Also, let me modify and expand on a statement I previously made regarding wall-connected tethers:
These must be anchored deep into the asteroid rockLarger gravity balloons will see different physics in this respect. The tethers can be (and likely would be) held in place by the air pressure itself. This will demand some curvature of the wall, as I illustrate in the sketch below.
Going further, there are many many more interesting things that need to be taken into account on larger scales. As scale increases, the self-gravitation effect becomes larger and larger and for something like Virga, you might very need to imagine something like carbon nanotubes in order to keep all the habitats from collapsing in on the center.
Tensioners and Enabling TechnologyFor the superstructure concept to work, we need some form of control input. I have made several references to the generic type of technology whereby a tether can have slack pulled in (thus making it tighter) on command. This is the control machinery side of the equation - we also need good control signals. Simply put, on a large enough scale, the system must have more give than what simple material properties would allow you.
As a counterpoint alternative, think about what the superstructure would be like if it were made of springs. This could accomplish most of what a control system could otherwise do.
Momentum will be fully looked at as a commodity in this system. This implies it can be stored and transferred from one place to another as needed. There are large-scale balance equations that will eventually be satisfied, and a gravity balloon would presumably have an agency that followed these numbers (like the various energy agencies are to us on Earth today).
A Reference CaseIllustrating what I have in mind - tethers extend from the pressure liner into the habitat. These fork into different directions to connect with the different connection points (and have a curve so they can reach closer to the wall) and also with the rotating cylinders themselves. All throughout the middle space you must imagine a vast number of tubes. Each one of these tubes are connected into the tether network - and the tether network will do the work to hold them safely in place, depending on what the particular needs at the time and location are.
I have drawn an empty space between the tethers and the liner. This isn't strictly necessary because you could just have more tethers connected to the liner. I did not go this route because I assume that additional connections would have an additional cost, but this is not strictly true. So you are free to imagine an innumerable number of tethers extending out from the wall and no "dead space" around the edges. Also, I am burdened by the need to illustrate something.
You must also use your imagination to picture a network of ever-thinner tethers that reaches out to each individual habitat in the entire volume, and connects (eventually) to the superstructure.
Comparison to Vacuum Orbital SuperstructuresIn the cases I've described, air pressure is the "ultimate" tension force. That allows you to pull something away from the center. In an arbitrary orbital location, tidal forces might be one of the easily available options. Even a space elevator could be an extreme version of this.
Going in another direction, an orbital ring can be used to pull in another direction. Combining tidal and the orbital ring tug, you could arrange habitats in a 2D plane which is essentially a disk around a planet. This structure could hold all the constituent colonies in place. But let me stress that in the radial directions, "anchors" are needed in order to get a good tug on the tether, while operating on habitats that have relatively small micro-g accelerations acting on them.
These could also be very useful for other mega-structure projects, like solar power arrays, or electromagnetic launchers and catchers. These projects might need precise alignment, and you need something with which to act on in order for them to be viable. The disc-type megastructure might be the ideal option for that.
Another option also presents itself (which I have sometimes briefly hit upon), you could have a large bag of low-pressure air that envelops a large space of orbital habitats just for the purpose of holding the tethers in place and keeping tension on them. This would enable massive 3D habitats, but would not allow free-air access to the different habitats. Since the latter (a full gravity balloon) requires access to a large mass source (a large asteroid), many locations would not have that option and may possibly find a space city of this type the best available option.