Megastructures: Harnessing Nature's Design for Space Elevators
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The human desire to create ever bigger and more impressive structures is insatiable. The pyramids of Ancient Egypt, the Great Wall of China and the Burj Khalifa in Dubai are a consequence of pushing engineering to its limits. But huge buildings aren’t just monuments to human ambition: they might hold the key to humanity’s progress in the space-faring age. Proposals are now circulating for a free-standing tower or ‘space elevator’ that could reach up into the geosynchronous orbit around the Earth. Such a tower would be an alternative to rocket-based transport, and drastically reduce the amount of energy it takes to get into space.
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In recent years, engineers have managed to build on grander scales thanks to the strength and reliability of substances such as novel steel alloys. But as we enter the realm of megastructures – those of 1,000 km or more in dimension – maintaining structural integrity will become a fiendish challenge for structures made of any material presently available because the bigger something becomes, the more stress it experiences due to its own weight and shape. (‘Stress’ is a measure of mechanical tension, like when you pull something apart from either end, or squeeze it together. ‘Strength’ is the maximum tension a structure can withstand before it breaks.)
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It turns out that biological design, equipped with around 3.8 billion years of experience, might help solve this puzzle. Before the age of materials science, engineers had to find their solutions in nature – wood, leather, stone, weaker metals that could be cast through melting and iron that could only be beaten into shape. But then substances such as steel and concrete arrived, and became successively tougher and lighter, allowing designers to make structures that were much stronger than the maximum possible load they needed to bear. However, once structures turn into megastructures, this risk-averse approach places a cap on their size.
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However, neither the bones nor tendons in our bodies enjoy the luxury of being capable of carrying more than the maximum possible burden. In fact, they’re often compressed and stretched well beyond the point at which their underlying substances begin to break. That is why our bodies constantly repair and recycle their materials. For example, collagen fibres in tendons are replaced in such a way that, while some remain damaged, the overall tendon is safe. This constant self-repair is efficient and inexpensive and can change based on the load.
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Recently, this self-repair paradigm was applied to a possible model of reliable space elevators with available materials. One proposed design features a 91,000 km-long cable (called a tether), extending out from the Equator and balanced by a counterweight in space. The tether would consist of bundles of parallel fibres, similar to collagen fibres in tendons or osteons in bones, but made from Kevlar, a material used to manufacture bullet-proof vests. Using sensors and artificial intelligence, it would be possible to predict when, where and how the fibres would break. And when they do, speedy robotic climbers patrolling up and down the tether will replace them, adjusting the rate of maintenance and repair as needed. Despite operating at very high stress compared to what materials can sustain, we showed this structure would be reliable and would not demand exorbitant rates of replacement. Moreover, the maximum strength the material would need to possess to achieve a dependable structure was cut by 44%.
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