From: johnhare@tampabay.rr.com   
      
   "Len"  wrote in message   
   news:36dabe8a.0401241004.70700ff6@posting.google.com...   
   > dbohara@mindspring.com (Parallax) wrote in message   
   news:<792abaf9.0401220620.79f263c9@posting.google.com>...   
   > > I have a question that may have an obvious answer but I have lost too   
   > > many brain cells for it to be obvious to me.   
   > >   
   > >   
   > > 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?   
   >   
   > A pet peeve.  I always use m/s for specific impulse:   
   > specific impulse (Ns/kg = kg m/s^2 x s / kg = m/s).   
   >   
   > The unit of impulse is force x time;   
   > the unit of specific impulse is force x time per unit mass.   
   >   
   > In English units the correct units are ft/sec, or   
   > lbf x sec / lbm =   
   > (slugs x (ft/sec^2)x sec/lbm) x 32.174 lbm /slug   
   > = ft/sec   
   >   
   > Thus 300 lbf/lbm/sec   
   > = 9652 ft/sec   
   > or 2942 m/s   
   > = 300 kgf/kgm/s  (kgf is a bastard term, as Henry   
   > points out.   
   >   
   > I guess people have found "sec" convenient, since it appears   
   > to be the same number in both English and metric units.   
   > However, this usage is wrong and has caused serious engineering   
   > mistakes.   
   >   
   This is one of those things that has been brought up and argued   
   hundreds of times since I have been reading these groups. You   
   and most of the people I respect here seem to be in agreement   
   on this matter. Due to a self inflicted education in this subject,   
   I tend to think in seconds for specific impulse.   
      
   Could you explain again, in small words and short sentences,   
   what problems are caused by using seconds for specific impulse.   
   The way I think, it is difficult for me to want to change units   
   unless I have a clear understanding of why I should want to do so.   
   In this business, as in my day job, consistently using an amatuerish   
   definition brands one as ignorant to the pros. I wouldn't mind being   
   able to hide it a bit better.   
      
   > Best regards,   
   > Len (Cormier)   
   > PanAero, Inc.   
   > x@tour2space.com  (substitute len for x)   
   > (  http://www.tour2space.com  )   
   >   
      
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