From: nuny@bid.nes   
      
   On Tuesday, October 4, 2016 at 7:25:08 AM UTC-7, Mikkel Haaheim wrote:   
      
   (snip)   
      
   > BTW: another option, which also decreases efficiency a little, is to curve   
   > the radiator concavely.   
      
     If by that you mean to reduce the emission angle by making the radiating   
   surface concave, that won't work for two reasons.   
      
     First, every point on the radiating surface is radiating in all directions.   
   Curving the radiator just effectively reduces the radiating surface area as   
   seen by distant, heat-absorbing space. It does not reduce total emission angle.   
      
     Second, you now have all points on the radiating surface "looking at" other   
   points on the radiating surface reducing their ability to lose heat. Heat   
   emitted by any point that impacts those other points is reabsorbed and has to   
   be re-emitted, raising    
   the temperature of the whole emitting surface. While that helps radiativity   
   because heat flows better from a hotter radiator to cold space, it becomes a   
   choke point for heat flow.   
      
     To see my point better, continue curving the radiator into a long, skinny   
   paraboloid. Its open end still emits heat into a half-sphere of space, but   
   from a distance it's become a small, hot circle.   
      
     (This also increases the load on the insulation for the cold side of the   
   radiator.)   
      
     Both of my objections become moot if you can come up with a material that   
   only emits heat perpendicularly (or at any controllable angle) to the emitting   
   surface with e. g. some metamaterials trick, but I'm not suspending my   
   disbelief from that without    
   at least one current technology "laboratory curiosity" example. With such   
   materials a radiator can be flat or any shape dictated by available places to   
   hang it on the spaceframe. Another limitation- such technologies are typically   
   limited to a narrow    
   band of wavelengths while heat is by definition wideband.   
      
     Speaking of metamaterials, I've read of some that can turn heat into light a   
   la the unphysical "cooling lasers" found in some SF, but they're still subject   
   to thermodynamics; they generate more waste heat than they emit.   
      
      
     Mark L. Fergerson   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)