The "Space Runway" is a counterintuitive idea. Many people struggle to grasp it properly, even those who know some physics. It's a tube in low orbit around earth, weighing between 1000 and 1,000,000 tons, that is used to move mass from down here to up there. (2/)
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This series of movie-frames shows what it does: it captures a slow spacecraft from behind and "drags" it up to orbital speed. (3/)pic.twitter.com/MwcjjEMGQL
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The coupling between the spacecraft and the runway is electrodynamic: the spacecraft is magnetic and the runway is a conductive tube. The magnetic field from the spacecraft induces eddy currents in the runway, which slow the spacecraft: https://www.youtube.com/watch?v=5BeFoz3Ypo4 … (4/)
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That is, in the frame of reference of the runway, the spacecraft is slowed. But the runway starts out in orbit, so in the frame of reference of the surface of earth the spacecraft is actually speeding up. The electrodynamic braking process requires no power input! (5/)
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It also doesn't require any wires or electronics in the runway itself. The runway is functionally just a conductive tube with no frills. Admittedly it masses 1000+ tons and is in orbit, but during the acceleration phase there is nothing going on apart from passive effects. (6/)
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The runway loses momentum in order to bring a spacecraft up to speed. Its orbital parameters change, and some energy is lost as heat. There are also various structural considerations, sound waves, etc. Many details need to be filled in here, but we think it's doable. (7/)
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To mitigate the various hardships that the capture causes to the runway, it will mass somewhere between 150 and 1000 times the mass of the ship it captures, with my guess being around the higher end of that range. For a 1000 ton BFR, this would be up to 1 megaton! (8/)
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The runway will regain its momentum gradually. That's the key. Over the course of 90 minutes or more, electric propulsion will reboost the runway's orbit. A crude drawing shows what the runway looks like: (9/)pic.twitter.com/qYhz3qavww
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The main component is a strong vertical tether with a dense counterweight at the lower end and solar panels at the top. The runway is shown as a tube inside a truss structure, supported by cables that attach to the upper and lower counterweights. (10/)
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The runway needs these counterweights to be passively stable in earth's gravity gradient. Without them, it would naturally orient itself vertically! (11/)pic.twitter.com/sbChYJWFaQ
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Accelerating satellites would also add a rotational velocity to the runway if it didn't go through the center of mass
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