From: dietz@dls.net   
      
   Jon Watts wrote:   
   >   However from these data the RS-68 achieves a   
   > (slightly) higher sea level Isp despite having less than half the   
   > chamber pressure and less than one third the expansion ratio.  How is   
   > this possible?   
      
   I believe the SSME is considerably over-expanded at sea level,   
   which reduces the Isp.  The RS-68 appears to be more optimized   
   for use at sea level.   
      
   	Paul   
      
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