Subject:      Re: beanstalk cable
From:         Jorj Strumolo <>
Date:         1996/12/31
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References:   <5a296e$> <> <>
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JS> The support lengths assume no tapering. He figures tapering would > be used, but a safety margin is also required, so they cancel out. > The necessary support length for a Terran beanstalk is about 4,940 km. (Beth and Richard Treitel) writes: RT> I have a real problem with the last paragraph. Probably more with my condensation than what Sheffield wrote. (The article appears in his 1979 _Vectors_ collection, BTW.) "The cable that we need must be able to withstand a tension at least equal to its own weight from a height of 35,000 kilometers down to the surface. In practice, it must be a good deal stronger than that. [...] The tension in the cable at a height of 35,770 kilometers, where upward and downward forces exactly balance, is less than the weight of a similar length of 35,770 kilometers of cable down here on Earth [since] the downward gravitational force decreases as the square of the distance from the center of the Earth, and the upward centrifugal force increases linearly with that distance. Both these effects tend to decrease the tension that the cable must support. A straightforward calculation shows that the maximum tension in a cable of constant cross-section will be equal to the weight of 4940 kilometers of such cable, here on Earth. This is in a sense a 'worst case' calculation, since we know that the cable will be designed to taper. However, the need for a safety factor means that we need to be conservative, and the figure of 4940 kilometers gives us a useful standard in terms of which we can calibrate the strength of available materials." Better? He then goes into a discussion of 'support length' or 'characteristic length' being a common term, not just a concept derived from beanstalk wondering, and reference works use it, etc. RT> The *only* way to build a beanstalk with > anything less than "fictionite" is to taper it As you'll see he said. My condensation simply cut too much out. RS> I have seen a suggestion that we could build a 1000-km tower > (resting on the ground: in compression, not tension) and > hang a beanstalk above it, which reduces the material requirements; > but I'm not engineer enough to say how well this might work. There are quite a few ways of doing it. The tower can be actively-supported, with metal slugs shot up its evacuated walls magnetically, with upper floors stealing momentum (I don't think that's really the term I want, but you can probably figure it out) from the slugs, which then fall back and get reaccelerated. There's the launch loop, basically a larger, sideways version of that. Spacewhales, nets venturing partway into the atmosphere to grab suborbitally lofted parcels. Pinwheels, or untethered beanstalks that "roll" around the planet, the end periodically dipping into the atmosphere. A few more. These don't need magic materials, tho the tradeoff is that more energy input is needed. You have to make up for the drag the pinwheel gets, and so on, whereas the ideal beanstalk is much more efficient, with loads moving down almost enough to make up for loads moving up.