XPost: misc.misc   
   From: del@branta.demon.co.uk   
      
   On Fri, 4 Feb 2005, in sci.space.tech,   
   "Cray74@gmail.com"  said:   
      
   >Kent Paul Dolan wrote:   
   >> An alternative that has lots of obvious arguments   
   >> going for it, like redundancy for safety reasons,   
   >> more reasonable single-launch lift requirements,   
   >> and a stageable startup, would be to replace the   
   >> "one big sphere" concept with a "bunch of grapes"   
   >> design.   
   >   
   >That's a reasonable point.   
   >   
   >By halving the diameter of the sphere to 50m, you   
   >quarter the mass of shell that needs to be launched   
   >at once.   
      
   Even more than that.  The larger the sphere, the stronger the skin has   
   to be to contain the same pressure.  In general, wall thickness goes up   
   linearly with diameter, if pressure is the overriding factor determining   
   wall thickness.  In other words, for a given pressure and material   
   strength, wall thickness/diameter is a design constant, and shell mass   
   goes up with the diameter cubed, not squared.   
      
   Halving the diameter probably wouldn't reduce the mass by a factor of   
   eight, because there are other drivers to the wall thickness than just   
   pressure containment, but if pressure containment is not irrelevant to   
   the design, then it will be a factor of more than four.   
      
   --   
   Del Cotter   
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