From: schillin@spock.usc.edu   
      
   alexterrell@yahoo.com writes:   
      
      
   >Henry Spencer wrote:   
   >> In article <41c459bd$0$9342$ed2619ec@ptn-nntp-reader02.plus.net>,   
   >> Ian Stirling   wrote:   
   >> >Argon is almost as cheap as nitrogen, at least in the quantities   
   >> >needed.   
   >>   
   >> Agreed.  There's no point in going to anything lighter, *especially*   
   >> diatomic gases like nitrogen, which can mess things up badly by   
   >> splitting into loose atoms.   
      
   >How much of a problem is this. I was thinking long term it would be   
   >nice to use a propellant readily available in space, and the most   
   >readily available one is oxygen.   
      
   Two problems.  Well, three - for electrostatic thrusters, such as ion   
   drives and Hall effect plasma thrusters, you want high molecular weight   
   propellants, and even diatomic oxygen at 32 isn't terribly impressive.   
   Going from xenon to oxygen is going to cost you ~25% in efficiency right   
   there.   
      
   The molecular breakdown effect, only costs you ~5% in efficiency for   
   straight diatomic molecules.  OTOH, if you used something like C60 and   
   saw 50% fragmentation during ionization, that would be a ~50% efficiency   
   hit.  Complex molecules are right out, but diatomic could work.   
      
   Third problem, and potentially crippling for oxygen, is that when an   
   oxygen molecule does fragment, you get oxygen atoms - and atomic oxygen   
   is one of the most corrosive substances imagineable.  This is particularly   
   problematic for ion thrusters, which have relatively fragile grids right   
   in the propellant flow.  Hall thrusters would be less susceptible to such   
   damage, but it would still be a major engineering issue.   
      
      
   >Should we looking at Vasimr for this?   
      
   Ion and Hall thrusters, for all their engineering issues, have the almost   
   overwhelming advantage of actually existing.   
      
   In the event that someone actually managed to make a real Vasimr, it also   
   would have major engineering issues, but we don't presently know what they   
   are.  On the propellant front, Vasimr is a thermal rocket, and so wants a   
   very low molecular weight propellant.  Hydrogen, helium, lithium, all of   
   which have their own storage and handling problems, *maybe* something like   
   ammonia at an unknown cost in performance.   
      
      
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