From: EG*nospam*Ruf@cox.net   
      
   On 29 Jan 2004 10:05:39 -0800, in sci.space.tech asherian@aol.com   
   (Asherian) wrote:   
      
   >Suppose there was a "hypervelocity squirt gun" that could inject fuel   
   >into a scramjet at the same speed that the air was rushing by. This   
   >would mean no supersonic combustion. Of course, the "squirt gun" would   
   >have to be able to shoot fuel at Mach 10+. If such a thing existed,   
   >would it make scramjets drastically easier to develop?   
      
   Several points. First,  the flow entering a combustor in a scramjet is   
   nowhere near Mach 10 for any flight Mach number. IN fact in the   
   "hypervelocity" region, say Mach 10+, the combustor entrance Mach number   
   tends to be relatively constant, say around Mach 5. At lower flight Mach   
   numbers, 4-7 or 8, the scramjet may operate in what's called dual mode,   
   where there is a thermal sonic throat in the combustor. Several factors   
   guide the design towards this. Rayleigh (heat addition)  losses increase   
   with increasing Mach number. There must also be sufficient diffusion   
   (compression) in the inlet process such that the static pressure and   
   temperature entering the combustor are high enough and the velocity is low   
   enough to allow time for the mixing and combustion process to take place.   
      
   If the fuel is to be mixed with the air, having it at the same velocity of   
   the air is not helpful as the velocity difference or shear is one mechanism   
   which may drive mixing. Mechanical means may also be employed, such as ramp   
   or strut fuel injectors which also may inject the fuel at supersonic   
   velocities. A portion of the scramjet's net thrust is the impulse of the   
   added fuel. How large a fraction this is compared to the overall net thrust   
   increases with increasing flight Mach number as the fraction of the energy   
   derived from combustion becomes a smaller percentage of the total kinetic   
   energy of the vehicle.   
      
   So yes, the limit of this is as you may suggest, minimize mixing and   
   combustion losses and just maximize fuel impulse after extracting heat from   
   the vehicle. The problem is how to do this in a real system, which must   
   operate differently at lower Mach, where good mixing and combustion are   
   desired.   
      
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