From: daellis94@gmail.com   
      
   Sadly, I don't have any kind of truly specific understanding of the mechanics,   
   but vague is better than nothing.     
      
   So, the question of what the bell does for the rocket exhaust:  Simply, it   
   causes the exhaust to leave with a greater velocity.  As far as I know, it is   
   related to the Principle of Continuity in fluid flow.  A mass, m, of a fluid   
   moving through a channel    
   with a given sectional area with a flow velocity of v will flow with a greater   
   velocity when that sectional area is decreased.  This is true, at least, with   
   incompressible fluids.  Gas isn't typically incompressible, so this is where   
   you'll find one of    
   those gaps in my understanding, but as far as I can tell, the comparison works   
   well enough.     
      
   So, as the high-temperature, high pressure exhaust moves through the rocket   
   engine toward the throat of the nozzle, the area of the channel decreases, and   
   reaches its minimum at the throat.  This is, if the rocket is designed   
   properly, the point at which    
   the flow velocity of the exhaust is equal to the speed of sound in the exhaust   
   gas.  That is, the flow velocity is sonic at the throat of the rocket.  After   
   that point, due to physics magic that, like I said, I don't really understand,   
   the mechanics of    
   supersonic fluid flow cause the gas to be FURTHER accelerated by a channel   
   that expands rather than contracts.  That is where the bell comes in.     
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)