From: spam@merlyn.demon.co.uk   
      
   JRS:  In article <1111161989.710878.41200@z14g2000cwz.googlegroups.com>,   
   dated Fri, 18 Mar 2005 08:06:29, seen in news:sci.space.tech,   
   Cray74@gmail.com  posted :   
   >For long range space travel with hydrogen reaction mass...   
   >   
   >Roughly how much power is required to keep hydrogen liquified per watt   
   >of heat leakage into the tanks?   
   >   
   >I've found one value on the 'net suggesting 50 watts of refrigeration   
   >system power per watt of cooling. Is that correct?   
      
      
   No as an answer to you, since the question you give is unanswerable for   
   insufficient data.   
      
   To keep the hydrogen at temperature T0 by pumping the heat up to a much   
   greater temperature T1 at which it can be let loose requires about T1/T0   
   watts per watt moved for basic thermodynamics IIRC, which needs to be   
   multiplied up to allow for electromechanical losses in the contraption.   
      
   So the figure may be reasonable for hydrogen being kept below its 1 atm   
   boiling point at about 20 K with heat being rejected into worst   
   reasonable terrestrial ambient of about 300 K.   
      
      
      
   But for long range space travel, for most of the journey there's over 12   
   steradians of ambient at about 3 K, and not much else apart from maybe a   
   Sun at 6000 K occupying under 0.0001 sr.   
      
   Now, allowing for T^4, that Sun is adequate hereabouts to maintain a   
   general body at over ten times the BP of H -- so, therefore, we don't   
   want just a general body, a sort of generalised oil-drum.  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; ISTM that it should be at around 40 K or better.   
      
   Then we make the sunshadeshadeward side of the tank nice and shiny;   
   whereas the antisunshadeshadeward side is nice and black, 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.   
      
   On the sunny side, multi-layer insulation might be more convenient if   
   less effective, and anyway would be wanted during manoeuvring.   
      
      
   Given that the tank has to be launched, possibly full, ISTM that it   
   should be possible to allow the pressure to exceed 1 atm in cruise,   
   which would also help.   
      
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