From: vincent@triumfunspam.ca.retro.com   
      
   In sci.space.tech, on Fri, 26 Nov 2004 14:56:30 GMT, David Given   
    sez:   
   ` Kent Paul Dolan wrote:   
   ` [...]   
   ` > It's worth noting that with _sufficient_ strength to   
   ` > weight, balloons (with ribs) could be filled with   
   ` > cheap vacuum instead of expensive helium or dangerous   
   ` > hydrogen, and probably provide greater net lift.   
      
   ` You could, but it wouldn't gain you much.   
      
   ` Air has an average molecular weight of about 30. Hydrogen has a molecular   
   ` weight of 2. Lifting capability is (mass of displaced air) minus (mass of   
   ` gas replacing air); so hydrogen would give a lifting capability of 28. All   
   ` vacuum could do would be to raise the lifting capability to the theoretical   
   ` maximum of 30. a 5% increase.   
      
   ` That increase would almost certainly be soaked up by the added mass of your   
   ` envelope, which now has to be rigid to withstand a considerable inward   
   ` force. Hydrogen balloons aren't subject to this, because the gas on the   
   ` inside is at the same pressure as the gas on the outside, so there's a net   
   ` zero force.   
      
   ` Not only that, but using vacuum would introduce some rather unpleasant   
   ` failure modes (such as your vehicle imploding violently). You're probably   
   ` better off sticking with hydrogen. Helium if you're feeling rich.   
   ` (Molecular weight of 4, so a lifting capability of 26 --- not much worse   
   ` than hydrogen.)   
      
   Passive solar heating of lift gas, raising its temperature significantly   
   above the ambient, could get you a bit more lift, getting the internal   
   density a bit lower for little or no weight penalty - say a transparent   
   upper half and black bottom half to the balloon. Perhaps taking a   
   small weight hit to improve the surface insulation might be a win   
   above some volume point, as well. But then, all this requires the   
   gas, and optionally the skin, to expand in size, so you have to either   
   have more initial slack surface material (which will start out as   
   dead weight) or else use a such a quantity of lift gas that some   
   must be jettisoned at altitude.   
      
   --   
   ==========================================================================   
       vincent@triumf[munge].ca                            Pete Vincent   
           Disclaimer: all I know I learned from reading Usenet.   
      
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