XPost: sci.space.policy, sci.physics, sci.astro   
   From: jfindley@cinci.nospam.rr.com   
      
   In article <6fr9hdlraoer9ga9snksve8f9b49lv2gnf@4ax.com>,   
   fjmccall@gmail.com says...   
   >   
   > 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'.   
      
   Fred is spot on here.  Again, the idea is to make sure that as many of   
   the particles in the exhaust go in the direction you want.  Without some   
   sort of nozzle (bell or aerospike), all that high pressure exhaust   
   exiting the throat of a rocket engine is going to end up in a wide cone   
   shape, greatly reducing efficiency.  You can show that with high school   
   physics and geometry if you examine an individual particle that's   
   shooting off at a 45 degree angle instead of straight "down".   
      
   Note that with ion engines, you don't typically see a large bell nozzle   
   on the thing.  That's because an ion engine already shoots the ions out   
   in pretty much a single direction.   
      
   You could also make an engine out of a linear accelerator.  Shoot mass   
   out the back and the spaceship goes forward.  But to make this   
   efficient, you'd need a super high exit velocity and a super light   
   accelerator.  Those two things are not very compatible with each other,   
   so the whole thing isn't very practical, but it's a good thought   
   experiment.   
      
   Jeff   
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