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#101
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Towards routine, reusable space launch.
On Jun/22/2018 Ã* 5:45 PM, JF Mezei Wrote :
On 2018-06-22 07:18, Jeff Findley wrote: Because it's flexible, duh. Imagine that you took 10 meters of fishing line with a weight at the bottom then cut it in the middle. Would it stay straight as it fell? Try it! This does not consider/test the issue that the top of the cable is travelling at faster speed and as it is being pulled down, will accelerate further. If the top of the cable wants to travel faster than the bottom part of cable, a tension will exist which would not exist in your fishing line example, and that tension should keep the cable straight. That tension you talk about is caused by the centrifugal pseudo-force. In the fishing line example Jeff gave there is the gravitational force that should pull the line down. In both cases, when the fishing line or the elevator cable snaps the elasticity pulls back with more force than the gravity or centrifugal force. With the caveat that we don't know what material would be used for the hypothetical space elevator, so we don't know how elastic it would be. But I think it is a safe assumption that if such an elevator was to ever be built, the material would be sufficiently elastic for tens of thousands of kilo-meters of it to give a very strong pull back. Alain Fournier |
#102
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Towards routine, reusable space launch.
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#103
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Towards routine, reusable space launch.
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#104
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Towards routine, reusable space launch.
On Jun/23/2018 at 3:39 AM, JF Mezei wrote :
On 2018-06-22 19:58, Alain Fournier wrote: In both cases, when the fishing line or the elevator cable snaps the elasticity pulls back with more force than the gravity or centrifugal force. Centrifugal force is equal to gravity at genostationary orbit for this setup. below that, gravity is bigger so it pulls cable down. But what your argument does not consider is that the cable at geostationary is travelling at roughly 9370 km/h. But throughout the cable, all portions have the same radial speed (15° per hour, 360° per 24 hours). As the topmost portion of the cable is pulled down, its speed increases and it now has a radial speed greater than 15° per hour. The lower end will pull cable down (gravity) and resist beiong pulled forward (either because still anchored or being dragged on ground (resistance). The higher end will respond to being pulled down by increasing forward velocity, thus tugging on cable to move horizontally. Those two forces should keep cable fully extended and straight. It won't be snaking around. Any elasticity in the cable means that when the initial break at geostationaly happens, the elasticity will pull cable down more than just gravity. But that extra force will also result in the top most portion accelerating horizontally. So it isn't clear that as the tension is released, the cable would "snake". You are just missing the end of you last sentence. It isn't clear that as the tension is released the cable would snake until you do simulations to see what happens. Alain Fournier |
#105
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Towards routine, reusable space launch.
On Saturday, June 23, 2018 at 8:34:11 AM UTC-4, Jeff Findley wrote:
That's not what I meant. I meant that you can't know these things intuitively. The example was a huge bridge made of concrete and steel that was destroyed by wind. That's not intuitive! Sure it is. A high enough wind will destroy almost anything. |
#106
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Towards routine, reusable space launch.
On 6/23/2018 2:39 AM, JF Mezei wrote:
On 2018-06-22 19:58, Alain Fournier wrote: In both cases, when the fishing line or the elevator cable snaps the elasticity pulls back with more force than the gravity or centrifugal force. Centrifugal force is equal to gravity at genostationary orbit for this setup. below that, gravity is bigger so it pulls cable down. nope. The orbiting object just needs to orbit faster to stay at the same height above the earth, and it does not go down. But what your argument does not consider is that the cable at geostationary is travelling at roughly 9370 km/h. But throughout the cable, all portions have the same radial speed (15° per hour, 360° per 24 hours). the cable will bend under these conditions if very long. as the top part is going too fast to be geostationary, and the bottom is going too slow to be geostationary for the cable to remain normal to the Earths surface. Short cable is ok, but not a 10,000 km one. As the topmost portion of the cable is pulled down, its speed increases and it now has a radial speed greater than 15° per hour. The lower end will pull cable down (gravity) and resist beiong pulled forward (either because still anchored or being dragged on ground (resistance). The higher end will respond to being pulled down by increasing forward velocity, thus tugging on cable to move horizontally. Those two forces should keep cable fully extended and straight. It won't be snaking around. Any elasticity in the cable means that when the initial break at geostationaly happens, the elasticity will pull cable down more than just gravity. But that extra force will also result in the top most portion accelerating horizontally. So it isn't clear that as the tension is released, the cable would "snake". |
#107
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Towards routine, reusable space launch.
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#108
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Towards routine, reusable space launch.
"Scott M. Kozel" wrote on Sat, 23 Jun 2018
21:28:31 -0700 (PDT): On Saturday, June 23, 2018 at 8:34:11 AM UTC-4, Jeff Findley wrote: That's not what I meant. I meant that you can't know these things intuitively. The example was a huge bridge made of concrete and steel that was destroyed by wind. That's not intuitive! Sure it is. A high enough wind will destroy almost anything. Except that isn't what destroyed it. Winds when the Tacoma Narrows Bridge collapsed were 40 MPH and less. -- "Ignorance is preferable to error, and he is less remote from the truth who believes nothing than he who believes what is wrong." -- Thomas Jefferson |
#109
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Towards routine, reusable space launch.
On Sunday, June 24, 2018 at 4:19:49 PM UTC-4, Fred J. McCall wrote:
"Scott M. Kozel" wrote on Sat, 23 Jun 2018 21:28:31 -0700 (PDT): On Saturday, June 23, 2018 at 8:34:11 AM UTC-4, Jeff Findley wrote: That's not what I meant. I meant that you can't know these things intuitively. The example was a huge bridge made of concrete and steel that was destroyed by wind. That's not intuitive! Sure it is. A high enough wind will destroy almost anything. Except that isn't what destroyed it. Winds when the Tacoma Narrows Bridge collapsed were 40 MPH and less. "For over six decades, engineers have studied the collapse of the 1940 Tacoma Narrows Bridge. The experts disagree, at least on some aspects of the explanation. A definitive description that meets unanimous agreement has not been reached. The exact cause of the bridge's failure remains a mystery." https://www.wsdot.wa.gov/TNBhistory/...e/machine3.htm |
#110
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Towards routine, reusable space launch.
"Scott M. Kozel" wrote on Sun, 24 Jun 2018
21:31:15 -0700 (PDT): On Sunday, June 24, 2018 at 4:19:49 PM UTC-4, Fred J. McCall wrote: "Scott M. Kozel" wrote on Sat, 23 Jun 2018 21:28:31 -0700 (PDT): On Saturday, June 23, 2018 at 8:34:11 AM UTC-4, Jeff Findley wrote: That's not what I meant. I meant that you can't know these things intuitively. The example was a huge bridge made of concrete and steel that was destroyed by wind. That's not intuitive! Sure it is. A high enough wind will destroy almost anything. Except that isn't what destroyed it. Winds when the Tacoma Narrows Bridge collapsed were 40 MPH and less. "For over six decades, engineers have studied the collapse of the 1940 Tacoma Narrows Bridge. The experts disagree, at least on some aspects of the explanation. A definitive description that meets unanimous agreement has not been reached. The exact cause of the bridge's failure remains a mystery." https://www.wsdot.wa.gov/TNBhistory/.../machine3.htmr Disagree on SOME aspects. That doesn't change the fact that it was NOT high winds that brought it down. -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
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