XPost: sci.space.policy, sci.physics, sci.astro   
   From: fjmccall@gmail.com   
      
   JF Mezei  wrote on Sun, 3 Jun 2018   
   22:17:47 -0400:   
      
   >On 2018-06-03 17:26, Fred J. McCall wrote:   
   >>   
   >> The same as any other rocket.  Mass thrown after equals motion   
   >> forward.  All the bell does is try to make more of the gas go directly   
   >> 'aft' as it exits.   
   >>   
   >   
   >I get the "acceletate 1gr of gas backwards and you get pushged forwards   
   >by same amount force.   
   >   
      
   That's good, but why am I left feeling there is a big unspoken 'BUT'   
   there?   
      
   >   
   >If this were just it, why need an engine bell since once the gase has   
   >been accelerated as it leaves the combustion chamber it has done all its   
   >pushing, right?   
   >   
      
   Look at it as a momentum problem, which is the whole 'equal but   
   opposite reaction' thing is about.  Without something like an engine   
   bell, gas coming out the back is headed in all sorts of directions,   
   with only some small portion of the momentum aimed 'aft', so you get   
   much less reaction 'forward'.   
      
   >   
   >Sinxe, as the highly compressed gas leaves the combustion chamber and   
   >wants to expand, doesn't the engine bell capture some of this energy as   
   >expanding gas hits the engine bell and is diverted towards the back and   
   >tush bell pushes rocket forward?   
   >   
      
   Not the way to think of it.  Again, think of it as a momentum problem.   
   You want the gas molecules headed 'aft' at essentially ambient   
   external pressure.  This is why 'sea level' engines have relatively   
   small bells while vacuum engines have much larger bells (because the   
   lower pressure allows the exhaust to 'spread').  The bell doesn't   
   'push' anything.  It's still about the momentum.  This is why   
   aerospikes work, by the way.  At low altitudes the air flow that makes   
   up the 'outside' of the virtual bell is at relatively high pressure,   
   which forces the exhaust to be 'directly aft' in a relatively short   
   distance.  As you gain altitude, the lower pressure equates to a   
   'larger' bell where the gas has a longer path before it is headed   
   'aft'.   
      
      
   --   
   "The reasonable man adapts himself to the world; the unreasonable   
    man persists in trying to adapt the world to himself. Therefore,   
    all progress depends on the unreasonable man."   
                                         --George Bernard Shaw   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)