From: eripe.dk@gmail.com   
      
   On Tuesday, September 17, 2019 at 10:43:47 AM UTC+7, Sjouke Burry wrote:   
   > On 17.09.19 2:03, eripe wrote:   
   > > On Tuesday, September 17, 2019 at 5:09:00 AM UTC+7, Bob Jenkins wrote:   
   > >> Space elevators won't work.  Well maybe they will but they're more   
   complicated than we've been thinking.   
   > >>   
   > >> A space elevator would be anchored at one point on earth, usually over   
   the ocean on the equator, then would extend straight up to about 1/5th the   
   distance to the moon.   
   > >>   
   > >> A minor overlooked matter is the moon.  It's 1/81th the mass of the   
   earth, and on average 10x further from the elevator than the earth, so the   
   elevator would be pulled up at 1/8000th its weight when the moon is overhead.    
   So the connection to earth    
   has to be strong enough to resist a force of 1/8000th the elevator's total   
   mass pulling up.  Like tides, it'll be 1/8000th heavier when the moon is   
   opposite the elevator, so it's more like 1/4000th fluctuation daily.   
   > >>   
   > >> But the major overlooked matter is that the earth's equator is not   
   aligned with the moon's orbit.  It's off by 18 to 29 degrees.  Two issues.   
   > >>   
   > >> 1. The elevator's orbit wants to be in the plane of the moon's orbit,   
   because the moon tugs it that way once a day.  On planets without a moon, the   
   sun will have a similar effect.  The plane of the earth's orbit around the sun   
   and the plane of the    
   moon's orbit around the earth are pretty close, off by about 5 degrees.  If   
   the elevator isn't in the moon's plane, it'll fall towards it, and keep   
   falling past it then fall back, oscillating back and forth.  "Precession."    
   From earth this looks like    
   tilting north, then south, then back to north, once a day.  If you start with   
   an elevator pointed up, after a few years it'll have fallen 2*29 degrees to   
   the side, forming a 32 degree angle with the ground, then it'll rise back to   
   straight up, back and    
   forth.  If you want to prevent it from precessing, you have to actively push   
   it sideways, which is a thrust of about 1/100000th the elevator's mass times g   
   all the time.  I can't see doing that.  So instead you'd build the elevator in   
   the moo   
   > n's orbital plane in the first place, and it would tip north and south daily   
   by 18 to 29 degrees.   
   > >>   
   > >> 2. The elevator no longer goes straight up from the nearest point on   
   earth: the connection oscillates north and south from the nearest point   
   between -18 to -29 and 18 to 29 degrees daily.  This implies a change of up to   
   800km of elevator length    
   twice daily.  Perhaps you could spool it up then reel it out again.  You could   
   drive a power plant off of reeling it out.  The elevator cable will be moving   
   pretty fast: 800km in 12 hours is an average of 66km/hour.  Alternatively, you   
   could keep the    
   elevator connection fixed, just tugging harder when the attachment point is   
   pulling in and tugging less when the attachment point is going out.   
   > >>   
   > >> Any pictures of a space elevator going straight up from earth are as   
   wrong as a stopped clock: they're only right for an instant twice daily.   
   > >   
   > > The force from the moon is tiny compared to the self correcting force to   
   the rotating plane. If the station is 1° off that plane it will be pulled   
   back by a force of sin(1)= 1,7% of its mass.   
   > > The moon can only make it wobble a widdle.   
   > >   
   > There is no cable able to carry its own weight , and a few percents will    
   > make   
   > no difference whatsoever.   
   > Any cable of any type of matter will break.   
      
      
   Lets see,    
   Force is L A g rho    
   Strength needed is A s   
   so s = L g rho   
      
   L = 40.000.000 m   
   rho = 2200 kg/m3 (graphene)   
   g = 4 m/s^2 (best guess)   
      
   = 352 GPa   
      
   Graphene has a strength of 130 GPa so yes, you would have to taper the   
   structure to make it work.   
      
   --- SoupGate-Win32 v1.05   
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