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.   
      
   I find that I DO have a correction to make in my calculations:   
   I neglected to take into account the effect of the diverse BP temperatures on   
   the molar mass densities. At their respective BPs, H2 actually has slightly   
   better than 2x the density of H2O. This would actually yield a thrust   
   advantage of about 38x in    
   favour of H2. Even with the energy disadvantage, the overall advantage up to   
   this point would still be about 6.5x in favour of H2.   
   OTOH, as a function of thrust efficiency per volume, H2O still yields a   
   slightly better than 2x advantage.   
      
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