Monday, November 18, 2013

Rotating Tube Reference Design

Without doing much new analysis, I want to add a post in which I formalize the parameters associated with the tubes that rotate for gravity in my vision.  For gravity balloons, you start out getting to a usable object (like an asteroid), sealing off the pressure boundary, and then adding air.  After that, some rotating living area will be needed.  Many layers of sheets surround this structure in my vision, which reduce the drag on the construction.  This makes it a practical proposal, and that will not be true without the friction buffers.

So, just for the sake of completeness, I will present a set of workable parameters here.  These are all highly tunable.  You could change anything, but that will affect others.  I hope the links of what affects what is somewhat obvious.  First, here are the images, and more detail is included in tables below.

If you will, take this 1 km cubed box and repeat it spatially in your head.  I wanted to give a sense of how close they would all be, so I'm trying to illustrate this with the image below.

To be perfectly clear about what can be specified, and what follows from other values, I'm presenting the "independent" variables first.

Specified ParameterValueDescription
Inner Radius500 metersThe radius of the rotating tube, where people would be standing. This is basically the same for the floor and the structural supports.
Length500 metersTaking inspiration from the Kalpana One, this is a conservative choice to eliminate possible rotational instabilities. Longer tubes can be problematic.
Friction Buffers20 sheetsThis was selected from a balance of the number of sheets and power dissipation, discussed in a previous post.
Buffer Width103 meterAlso from the prior post in the subject, it's mostly constrained by volume constraints and diminishing marginal value of wider regions.
Population21,000 peopleYou could set this to a range of values, but this demands a reasonable power consumption for rotation and affords enough floor space.
Ramp Slope45 degreesThis is the rise over run slope for the ramp to the ends, which leads to the zero gravity space.
Access Diameter20 metersDiameter of the area open to the zero-gravity atmosphere at the end. The edges would have a slow speed and acceleration for moving in and out.
Mass Density2 tons/m2Assumed mass per unit area. This includes all lifestyle-associate things.

Variables which can be calculated from the system specified so far:

Derived ParameterValueDescription
Power for Rotation5.2 MWThis is the mechanical power needed to keep the tube spinning. It could be more when corrected for motor efficiency.
Usable Area0.79 km2Area with 1 g of gravity, so this does not include the access ramp.
Total Area1.32 km2Internal area, including the ramps. Obviously a good deal of this would still be usable.
Displaced Volume0.26 km3Volume including the friction buffer space. This means that 74% of the total space would be unoccupied in the repeating lattice.
Lattice Shape1 km3 boxThe arbitrary bounding box I'm using so that a repeating pattern of these can be discussed, converting volume metrics to more tangible things.

To make this more personable, I'm also including some per-capita parameters.

Per PersonValueDescription
Rotation Power250 WattsThe power needed to keep the construction spinning for each person. This is similar to an appliance, so it wouldn’t be overly burdensome.
Area37 m2Livable area per person, which corresponds to a relatively high density city. However, roads and other things can be through the center.
Mass per Person75 tonsThis is the total mass in the area that corresponds to one person's living space. For reference, a house may weigh 60 tons, so this is still relatively low, but workable.

For further information, I have looked into the farmland necessary to sustain a human.  The National Space Society has made a variety of claims, which result in parameters you can use, but the range is large.  Anyway, it would be reasonable to assume about 50 to 200 square meters would be necessary to grow food for one person.  This is obviously a problem if the rotating tubes include farm production.

I would advocate a different method of growing crops.  For some algae (and others), it would make the most sense to just have the suspended in zero gravity.  For most crops, however, I think it would make sense to grow them in low gravity, such as 1/10th Earth gravity.  With such a selection of parameters, you could have farmland with no friction buffers at all, which would be much more economical.

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