XPost: sci.physics, sci.space.policy, rec.arts.sf.science   
   From: fjmccall@gmail.com   
      
   "Robert Clark"  wrote:   
      
   >A problem with scramjets, which attempt to achieve combustion at hypersonic   
   >speeds, is that the high speed means the air is producing so much drag that   
   >it is difficult to achieve net thrust when combusting with just the oxygen   
   >in the air.   
   >   
   >So what if we also combusted with the nitrogen? Nitrogen makes up 80% of the   
   >air mass so perhaps this could provide sufficient thrust. The problem is   
   >nitrogen is commonly present in the atmosphere as N2, a molecule that is   
   >very stable, i.e., non-reactive.   
   >   
   >But at hypersonic speeds so much heat is produced that the N2 is "cracked",   
   >generating various nitrogen species, such as N, NO, NO2, etc.   
   >   
   >So how much energy could be produced by reacting hydrogen fuel with the   
   >various nitrogen species? How much for methane fuel?   
   >   
   >The energy density and thus Isp would be less than for reacting with oxygen   
   >but conceivable the thrust could be greater because of the greater mass of   
   >the nitrogen.   
   >   
      
   Uh, Bob?  You don't get to 'pick' what your fuel reacts with when your   
   oxidizer is ram air.  It all reacts.  Energy of formation for things   
   like NH3 is tiny when compared to H20, so that sort of reaction is   
   going to make your energy lower.  The nitrogen in the atmosphere   
   doesn't just go away.  All that mass is still in the exhaust whether   
   you 'combust' it or not.  If you spend a lot of fuel 'combusting' with   
   nitrogen you are burning more fuel to get less energy; you are burning   
   50% more fuel to 'combust' nitrogen to NH3 and only getting 12% or so   
   of the energy you get from combusting with oxygen.   
      
      
   --   
   "Some people get lost in thought because it's such unfamiliar   
    territory."   
                                         --G. Behn   
      
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