>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.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)