From: alcore@uurth.com   
      
   On 27 Jun 2005 iain-3@truecircuits.com wrote:   
      
   >Roland> Why is Xenon the propellant of choice for an ion thruster?   
   >   
   >Henry> As others have noted, you want a monoatomic material --   
   >something   
   >Henry> that will produce ions of only one mass, rather than a molecule   
   >Henry> that might break up and might not -- and you want it heavy.   
   >   
   >I don't think I fully understand either of these points.  Care to   
   >check?   
   >   
   >1. The problem with a molecule that might break up is... that a   
   >population with two very different charge/mass ratios will deliver worse   
   >overall Isp and thrust to power than a population with the same average   
   >charge mass but a single ion species?   
      
   No.  It's because energy is needed to separate the ions.  This energy is   
   not thrust-producing.   
      
   Imagine it like this:   
      
   Pick up (ionize) and throw a baseball (our ion).   
      
   Now imagine that the baseball is glued to another baseball.  So it   
   becomes:   
      
   Pick up (ionize), separate (break the bond), and throw a baseball (our   
   ion).   
      
   The energy to break the bond is wasted as part of the process.  You   
   *could* try and throw the whole ion...  But as you pump energy into it to   
   accelerate it, at some point you will put in enough to break the bond and   
   that energy gets wasted anyway.   
      
   >2. Large ion mass.  ?  I assume any electric field you set up is   
   >   going to deliver a fixed number of electron-volts to each ion on   
   >   the way out.  Heavy ions would appear to need more eV to reach   
   >   the same exhaust velocity as lighter ions.  Why is this good?   
      
   You must store your propellant in a storage tank, yes?   
      
   A mole of Xenon is not going to take up significantly more space than a   
   mole of a lighter gas.  (Perhaps even less... but I don't know; I have a   
   vague memory that the noble gasses have more compact electron shells   
   because they are fully filled/balanced.)   
      
   Since the mole of Xenon stores more mass in the same space, it is a more   
   space efficient reaction mass.  You can get more thrust from it per unit   
   of fuel VOLUME.  While mass is a traditional limiting factor, increasing   
   the cubic volume per unit mass of a reaction mass *is* a mass issue since   
   you must increase the size of the storage tanks...  and thereby increase   
   mass in the form of extra tankage.   
      
   Use one mole of Xenon or several multiples of a mole of something   
   lighter.    In terms of storage efficiency the Xenon wins hands down.   
      
   Gene P.   
   Slidell LA   
      
   --   
   Alcore Nilth - The Mad Alchemist of Gevbeck   
   alcore@uurth.com   
      
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