XPost: sci.physics, sci.space.tech   
   From: mooregr@greenms.com   
      
   "John Schoenfeld"  wrote in message   
   news:a98beaaa.0307110848.38e0a240@posting.google.com...   
   > Timo Nieminen  wrote in message   
   news:...   
   > > On Thu, 10 Jul 2003, George Kinley wrote:   
   > >   
   > > > if there is no Atmosphere, where do rockets that go in Space get   
   thrust from   
   > >   
   > > Stand on a skateboard on a smooth flat surface, with a bag of rocks.   
   Throw   
   > > the rocks in one direction, and you'll go in the other direction (except   
   > > for the inconvenient effects of friction - use bigger rocks and you   
   should   
   > > see an effect). The rocket works exactly the same way. Throw mass away   
   in   
   > > one direction, and you go in the other direction. "Conservation of   
   > > momentum" is what it's all about.   
   >   
   > The direction of the rockets acceleration is irrespective of the   
   > direction of mass explusion.   
      
   Umm.  No.  It's directly related to the direction of the mass expulsion.   
      
   > I could build a rocket that expels its   
   > mass upwards and it would still accelerate upwards.   
   Good luck.   
      
   > It is the   
   > explosion in the combustion chamber that pushes out in ALL directions.   
   > The upwards push makes contact with the top of the combustion chamber   
   > transferring a net upwards momentum to the rocket. However, the   
   > downward push from the explosion does not make contact with any   
   > structure from the rocket and escapes - so the net momentum is   
   > upwards. It should be noted that the actual upwards acceleration has   
   > nothing to do with the output direction of the expelled mass (I could   
   > very well route the mass from the explosion pushing downards via the   
   > top or side, although this is both extremely difficult and very   
   > inefficient).   
      
   Nope, because as soon as you did that, your piping would have be acting just   
   like the top of the combustion chamber.   
      
   >   
   > Also note that you don't even need mass to escape from a rocket.   
   > Inertial propulsion is not prohibited in physics if you think about   
   > it. All that needs to be done is to make the upwards push a greater   
   > impulse than the downwards push - the rocket would essential jerk its   
   > way upwards - after all there is no "conservation of displacement"   
   > with such an inertial system.   
      
   Yes, you do need mass to escape.  You can't design a rocket that pushes UP   
   more than down unless it can eject mass out the back.   
      
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