XPost: sci.space.policy, sci.physics, sci.astro   
   From: fjmccall@gmail.com   
      
   Lofty Goat  wrote on Mon, 04 Jun 2018 22:38:58   
   -0500:   
      
   >On Sun, 03 Jun 2018 14:26:20 -0700, Fred J. McCall    
   >wrote:   
   >   
   >>Lofty Goat  wrote on Sun, 03 Jun 2018 11:05:53   
   >>-0500:   
   >>   
   >>>On Sun, 3 Jun 2018 03:58:11 -0400, "Robert Clark" wrote:   
   >>>   
   >>>> ... a functional aerospike engine ... vaporware....   
   >>>   
   >>>Perhaps someone can clear something up for me:   
   >>>   
   >>>In a conventional rocket engine with bell-type nozzles the pressure of   
   >>>expanding gas against the inside of the nozzle propels the rocket.   
   >>>   
   >>   
   >>Incorrect.   
   >>   
   >>>   
   >>>The greater the area of said nozzle, the more energy is recovered from   
   >>>the expanding gas which explodes within, up to a point.   
   >>>   
   >>   
   >>Incorrect.   
   >>   
   >>>   
   >>>In an aerospike rocket engine, what does the pressure of the expanding   
   >>>gas act against to propel the rocket?   
   >>>   
   >>   
   >>Newton's Law:  For every action there is an equal and opposite   
   >>reaction.  Throw gas out back, rocket goes forward.  No requirement to   
   >>'push against' anything.   
   >>   
   >>>   
   >>>BTW, I've read all sorts of very imaginative explanations of aerospikes,   
   >>>yet none has shown a believable diagram of the forces at work.   
   >>>   
   >>   
   >>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'd have snipped all of the above but I think ti should be preserved for   
   >posterity.   
   >   
   >How the do you think the mass of gas being pushed backwards applies   
   >force to the rocket which it is intended to push forward?  It's gas   
   >pressure against the nozzle inside of which it is exploding.   
   >   
      
   It's not 'exploding'.   
      
   >   
   >That's how the "throw gas out the back" part pushes the rocket towards   
   >the front.  That equal-and-opposite force gets applied to the rocket   
   >somehow.  With a bell nozzle, that's how it is applied.   
   >   
   >Now, try again.   
   >   
      
   Oh, you can think of it that way if you like, but it's really not a   
   useful way to do so.  It's more useful to just think of it as total   
   momentum of the system.   
      
   >   
   >Or better still, just post a link to a free body diagram of the gas and   
   >the nozzle, and a similar diagram of an aerospike, and I'll look at it   
   >myself.   
   >   
      
   Sure.  Just send me a check to cover my consulting fee, since you seem   
   to think I work for you.   
      
   >   
   >I'm not that choosy.  But the ones I've seen of aerospikes are Popular   
   >Mechanics-type diagrams, which are fanciful at best.  "The air forms one   
   >side of a virtual bell."   
   >   
   >Really?   
   >   
      
   Yeah, really.   
      
   >   
   >I don't doubt that they work, but every explanation I've seen of *how*   
   >they work reeks ever so faintly.   
   >   
      
   Perhaps that's because you're a bit of a thickie?   
      
   If it helps you, think of the flow field as 'attached' to the   
   vehicle...   
      
      
   --   
   "Some people get lost in thought because it's such unfamiliar   
    territory."   
                                         --G. Behn   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)