From: del@branta.demon.co.uk   
      
   On Sat, 22 Jan 2005, in sci.space.tech,   
   Perplexed in Peoria  said:   
      
   >"John Smith"  wrote   
   >> As a bit of whimsy - how about two long thin contra-rotating masses,   
   >> with an electric motor at the pivot point that makes them contra-rotate.   
   >> (Motor is powered by sun.)  Two chopsticks spinning against each other.   
   >   
   >More whimsy, but with a serious point:  Consider a propellantless   
   >launch system consisting of a huge roll of toilet paper.  Set the   
   >roll rotating.  Set it rolling with a large horizontal velocity.   
   [...]   
   >But here is the hidden point in this whimsy.  Since angular momentum   
   >is conserved as the roll unwinds, it must be the case that the   
   >ultimate escaping cardboard tube is rotating faster than the original   
   >roll consisting of tube and paper.  The moment of inertia has decreased,   
   >Right?  (It took me a long time to figure this one out.  Dooooh!)   
      
   I think the system you're describing is mathematically equivalent to a   
   whip.  As a wave travels down the length of the whip, a conserved amount   
   of momentum is stuffed into a smaller and smaller mass, until the end   
   breaks the sound barrier.   *Crack!*   
      
   But I think you could almost get the same benefit from a rocket being   
   followed by a slow "vacuum cleaner", provided the rocket's exhaust was   
   carefully tuned to leave the nozzle at zero speed in the solar system's   
   frame of reference.  It's all quite a bit of effort just to recover some   
   mass; I'd go and get more mass, since it's probably cheaper to do that.   
      
   --   
   Del Cotter   
   Thanks to the recent increase in UBE, I will soon be ignoring email   
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