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.   
      
   Actually, thinking it through some more, neither H2 nor H2O will be suitable   
   if you want to try to suppress shell temp to 3°K, as both will be solid at   
   this temperature. You will probably need Helium, which (according to my   
   sources) does not have a    
   solid phase.   
      
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