From: mikkelhaaheim@gmail.com   
      
   Le mercredi 21 septembre 2016 18:50:47 UTC+2, elie....@gmail.com a écrit :   
   > >So far, H2 is still superior... but at this point, heat capacity becomes a   
   disadvantage, as it takes 7x the amount of energy to heat gaseous H2 as it   
   does to heat water vapour.   
   >    
   > I am not sure how this is a disadvantage. Given that the ship has weeks,   
   months or even years of coast time, low acceleration is not a problem: small,   
   constant acceleration is about as good as high point acceleration, as seen   
   with probes using ion    
   drives.   
   > In specific short-duration missions, or on the outer reaches of the system   
   where solar-thermal simply don't have enough energy, higher thrust may be   
   needed for the entire course, though. Maybe Kuiper Belt models would use water   
   for that reason. They    
   might still keep liquid hydrogen to cool the hull down, though.   
      
   After reading through your earlier posts, I have one modification to make to   
   my response. You are correct that LH would be better suited exclusively for   
   the specific task of actively cooling the outer skin in order to prevent   
   blackbody emissions from    
   your low-albedo coating. However, this assumes that you are not using heat   
   pumps. The consequence of this assumption means that you will NOT be able to   
   maintain the shell temp below 14°K to 19°K (the LH will only absorb the heat   
   of the shell if the    
   shell is warmer than the temperature of the LH). IF you ARE using a heat pump,   
   then the temperature range of H2O will again render it better suited to the   
   task of absorbing the shell temp.   
      
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