From: willner@cfa.harvard.edu   
      
   In article ,   
    henry@spsystems.net (Henry Spencer) writes:   
   > [Spitzer] solves it only in a loose sense.  With elaborate insulation wrapped   
   > around a modest-sized tank, it gets boiloff rate down to the point where   
   > it won't boil dry for at least a couple of years.   
      
   The passive boiloff rate for Spitzer is equivalent to 1 mW of heat   
   input to a 360 liter tank.   
      
   > Liquid helium is harder than liquid hydrogen, yes.  But on the other hand,   
   > liquid-hydrogen tanks for propulsion will be far bigger -- they will   
   > dominate the vehicle in a way that Spitzer's tank doesn't -- and they   
   > can't tolerate a lot of boiloff, and mass issues limit the insulation.   
      
   On the other hand, not only is the boiling point higher for hydrogen   
   than for helium, the boiloff gas has a lot more enthalpy.  This   
   enthalpy can counteract heat conduction through structural   
   components.   
      
   Many years ago, I posted the calculation of the equilibrium   
   temperature of a body at 1 AU.  Now I can no longer find it, but as I   
   recall the answer is about 5 K.  Of course this assumes complete   
   shielding from the Sun and no nearby warm objects; the heating comes   
   from light from zodiacal dust particles.  Because of the T^4   
   dependence of radiation power, the equilibrium temperature is   
   negligible compared to the 20 K liquid hydrogen temperature.  The   
   problem is that radiation at 20 K is awfully inefficient, but the   
   "reservoir" temperature is plenty low enough.   
      
   Quite a lot has been learned about space cryogenics in the past   
   several years.  I don't think long-term hydrogen storage in deep   
   space can _by itself_ be considered a difficult problem any more.   
   However, storage near a planet is much more challenging, as Henry   
   mentioned.  Also, for a real mission, other requirements may conflict   
   with the insulation and shielding requirements.   
      
   --   
   Steve Willner            Phone 617-495-7123     swillner@cfa.harvard.edu   
   Cambridge, MA 02138 USA   
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