XPost: comp.theory.cell-automata   
   From: henry@spsystems.net   
      
   In article <375b1119.0407191221.49a88316@posting.google.com>,   
   Hephaestus  wrote:   
   >...Absent any heating, it would come to equilibrium   
   >with the sun's photosphere (approx 4500 K)...   
      
   Not *quite*.  At about six times the radius, it has about 36 times the   
   surface area, so it doesn't need to be as hot to radiate away that much   
   heat.  Since radiated heat scales with temperature to the fourth power,   
   that cuts the temperature by a factor of roughly the fourth root of 36,   
   about 2.5.  Still pretty warm, though. :-)   
      
   >...above melting point of tungsten (the highest melting element)...   
      
   Nitpick Dept:  the highest melting *metal*.  Graphite is still solid   
   after tungsten has melted.   
      
   >> (At least we're talking about compression mode forces for the most   
   >> part...,  the insane tensile strength needed for spinning stuctures has   
   >> always bothered me.)   
   >   
   >Forces should be very, very low.  It only has to support itself against 1g   
   >of gravity, after all.   
      
   Suppose you're standing on the outside of the sphere, and you jump up and   
   down.  What prevents the local surface from sagging under the load?  That   
   takes it closer to the Sun, which will encourage it to sag farther, until   
   the whole side of the sphere collapses...  This is how compression-mode   
   structures fail, in buckling, with the material squirming out from under   
   the load rather than actually breaking.  This is prevented only by the   
   stiffness of the shell material, and by the *very slight* arch of its   
   spherical shape.  (The arch means that sagging is resisted by compression   
   in the shell... but that won't be very effective with such gentle   
   curvature, especially with a relatively thin shell.)   
      
   Compression-mode forces are *bad*, not good.  It's very hard to prevent   
   buckling without using large masses of material to add stiffness.  Tensile   
   structures use materials far more effectively.  What we're talking about   
   here is, structurally, much like an infinite arch bridge.  But there's a   
   reason why very long bridges are not arches, but suspension bridges:   
   those make all major forces tensile except in the towers, which can easily   
   be made very stiff because they're quite compact.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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