From: jeff.findley@ugs.nojunk.com   
      
    wrote in message   
   news:1133213475.873821.181700@o13g2000cwo.googlegroups.com...   
   > I note several grammar errors in my Nov 23 post and I think they make   
   > it difficult to understand.  I've corrected the errors (that I see) in   
   > the text below.  Sorry!   
   >   
   > Dave W wrote:   
   > > I'm intrigued as to why there are no references to the above in any of   
   the   
   > > 'space' newsgroups.   
   >   
   > um, I think you may be under the false impression that doing this would   
   > somehow magically save fuel or something.  If I understand you   
   > correctly, you seem to think that if I move a large body (such as an   
   > asteroid), and if that large body's gravity pulls a smaller body (a   
   > towed asteroid), then I am pulling the smaller body for free (without   
   > expending additional fuel).  Sorry, but it don't work that way.   
   >   
   > The smaller, "towed" body pulls back on the larger body, and you expend   
   > exactly the same amount of fuel as you would expend if you connected   
   > then up via a cable and pulled them both.   
      
   Actually, using gravity to tow the body is likely less efficient, because   
   you have to angle your engine's exhaust off the centerline which goes from   
   the engine through the center of mass of the body you're trying to tow.  To   
   balance this out, you use engines in pairs.  This extra angle will result in   
   some loss of efficiency when compared to landing on the body and orienting   
   your engine(s) such that the exhaust is expelled along the centerline which   
   goes from the engine through the center of mass of the body.   
      
   The advantage of towing the body with gravity is that you don't have to   
   worry about other issues, like designing your spacecraft to not only land   
   on, but affix itself to a body which has potentially unknown surface   
   features and material properties.   
      
   Jeff   
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