From: spam@merlyn.demon.co.uk   
      
   JRS:  In article <1111360637.632172.62070@o13g2000cwo.googlegroups.com>,   
   dated Sun, 20 Mar 2005 15:17:17, seen in news:sci.space.tech,   
   Cray74@gmail.com  posted :   
      
   >Dr. John Stockton wrote:   
   >   
   >> We want to add a sunshade, well clear of   
   >> the tank, which will sit at about 250 K   
   >> and prevent the tank seeing the Sun   
   >> directly.  Then, in between, we want   
   >> a sunshadeshade, which cannot see the   
   >> Sun itself and prevents the tank seeing   
   >> the sunshade ... and can have a big black   
   >> fin to help it radiate into the 3 K   
   >> Universe.  Don't overdo it, or the H will   
   >> slowly freeze.   
   >   
   >For a single mass of liquid hydrogen totalling 1000 to 10000 tons (a   
   >sphere or squat cylinder), with current technology, could the   
   >sunshields and radiator (excluding tank pressure vessel mass) be 10% or   
   >less of the hydrogen mass?   
      
   Neglecting factors of the order of unity, one ton of water is a cubic   
   metre, so 10000 tons of hydrogen is 100 m^3, cross-section of the order   
   of 25 m^2.   
      
   A layer of kitchen foil (Aluminium) is more than adequate as a shade; I   
   don't know its areal density, but it cannot be wildly greater than good   
   paper at 100 gsm.  So each layer of shield would be of the order of 2.5   
   kg.  In cruise, support structure mass could be less than that.  Of   
   course, there's deployment mass to consider too, but inflatable struts   
   could do (remember to inflate them with H, or to keep them out of the   
   shade).   
      
   Practicalities may increase that by a couple of orders of magnitude, say   
   to the region of a ton; still only a thousandth of your 10%.   
      
   As regards the fin, I don't know; the radiating area could be increased   
   to a multiple of the tank area without adding a multiple of the mass,   
   but one would need to work out heat flow and thermal gradient.   
      
      
   It could be of interest to know the approximate solar system ambient   
   temperature of a location carefully shielded from the Sun and unable to   
   see a visible disc of any other body.   
      
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