Orbital rings are a proposed megastructure intended to dramatically reduce the cost to enter or exit a planetary gravity well. The ring is a solid structure, belting around the planet like a hula hoop.

Because it is solid, any loss of gravitational potential energy (falling) on one side would be result in a potential energy gain on the opposite side. Instead of support beams holding the ring aloft in compression, support cables in tension keep any part of the ring from floating away. The weight of an orbital ring is carried by the structure of an orbital ring.

The math for an orbital ring is --

• d: Diameter of the Ring Cross Section
• t: Wall Thickness (hollow tube)
• A: Cross Sectional Area of the Ring $ \approx \pi d t $
• R: Radius of Planet
• L: Length (Circumference) or the Ring $= 2 \pi R $
• V: Volume of Ring Material $ = AL $
• $\rho$: Density of Ring Material
• m: Mass of Ring $ = \rho V$
• g: Acceleration Due to Gravity
• P: Load on Ring Cross Section $ = {{m g} \over {A}}$

For a typical ring construction (3 meter tube, 0.001 m thick walls, made of steel (5,000 kg/m^3) or aluminum (2,000 kg/m^3), going around Earth at ~6,500 km radius and 9.8 m/s/s, the Cross Sectional Load on a proposed orbital ring is in the order of 800 GigaPascals. The strongest materials known only come in at a few hundred megapascals... so the discussion is over at this point.

I was playing with this design last night, and decided to substitute aerogel. The stupendously lightweight material has a compression strength in hundred megapascal range, but weighs one-onethousandth as much (1.5 kg/m^3).

Using aerogel, the same orbital ring is only bearing 600 MPa (which is within the strength of some existing aerogel formulations)

The entire mass of the assembly is 577 tons, which would need to be supported by at least three tether cables or something similarly strong and lightweight (silk).

A silk cable 0.08 meters thick can bear the peak load (the entire unloaded ring). However, silk cables are no good. A single tether reaching 100 km up to the arbitrary edge of the atmosphere made of silk would weigh more than the entire orbital ring.

There's no question here. I'd like to start a discussion, however. It does seem like this once fanciful megastructure might actually be within our grasp.

For discussion starters, can anyone recommend a good tether?

Also, what services should an orbital ring provide? I imagine, at a minimum, elevators to lift payload to the ring platform.

posted about 2 years ago

CC BY-SA 4.0

2y ago

James McLellan‭

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