From: jeff.findley@ugs.nojunk.com   
      
   "Peter Card"  wrote in message   
   news:43D0CB72.1030408@jet.uk...   
   >   
   > The propellent fraction varies exponentially with the specific impulse   
   > for a given delta-vee, and the specific impulse for LH2/LOX is at least   
   > 25 percent better than any practical alternative. It's 16 percent better   
   > than the thourougly impractical liquid flourine / hydrazine combo   
   (shudder)   
   >   
   > See http://www.braeunig.us/space/propel.htm   
      
   What you say is true, but the above page also shows that the density of LH2   
   is absolutely appalling when compared to the alternatives.  0.071 g/ml for   
   LH2 compared to 0.749 g/ml for kerosene.  That's literally an order of   
   magnitude difference in density.  This difference makes comparing the   
   performance of a rocket powered stage using LOX/LH2 engines to one using   
   LOX/kerosene engines a lot more difficult than looking at ISP alone.   
      
   Jeff   
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