From: henry@spsystems.net   
      
   In article <792abaf9.0401220620.79f263c9@posting.google.com>,   
   Parallax  wrote:   
   >isp is given as the thrust produced/quantity of fuel/sec used.   
   >Pounds/(Pounds/sec) gives units of seconds for ISP.  Do the same in   
   >MKS (metric) units and you get:   
   >Newtons/(Kg/sec)=Kg*m/sec^2/(Kg/sec)=meter*seconds.   
   >Why do I never see isp expressed in MKS units?   
      
   If you read European papers, you will sometimes see it written that way.   
   (Computed that way, it is the effective exhaust velocity of the engine.)   
      
   However, there is some case for keeping Isp in seconds regardless, even   
   though that means introducing an arbitrary factor of g (9.81m/s^2) into   
   the way it's computed in metric units.  Engineering figures of merit are   
   often computed in fairly arbitrary ways, and consistency -- preserving the   
   ability to compare results -- is more important than strict ideological   
   purity or direct relation to physical properties.  (Consider EPA mileage   
   ratings, which use a test procedure that is now known to be overly   
   optimistic in predicting real mileage, but which continue to use that   
   procedure because it gives a consistent basis for comparison.)   
      
   Isp in seconds also has the incidental advantage that it is independent of   
   the choice of unit systems.   
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