From: schillin@spock.usc.edu   
      
   kenw@kmsi.net writes:   
      
   >This is probably a dumb question, but...   
      
   >Would it be possible to use conventional jet engines to attain sufficient   
   >airspeed for scramjets to start to function?   
      
   Yes.  Almost trivially so if your scramjet doesn't *need* to operate in   
   supersonic-combustion mode, but it should be possible to go straight from   
   afterburning turbojet to supersonic-combustion ramjet with the crossover   
   at Mach 3 or so.   
      
      
   >SCRAMjets would seem to have limited value if they can only be launched   
   >via rocket.   
      
   Why?  If the only purpose of the rocket is to accelerate the scramjet   
   to scramjet operating speeds, well, rockets are the absolute best choice   
   there is for accelerating quickly to hypersonic speeds.  Turbojets can   
   do it, but A: barely and B: only by using heavy turbocompressor hardware   
   that you are then stuck hauling around while you do the scramjet thing.   
   Or, alternately, C: expensive turbocompressor hardware that you jettison   
   after using for one short burst of acceleration.   
      
   Conventional jet engines have only one single advantage over rockets,   
   that being their relative economy in propellant consumption.  But that   
   only wins if you are going to be using them for an extended period of   
   time.  If it's just a two-minute run up to Mach 3 and then turn off   
   the turbojet for the duration, it was a waste to bring the turbojet   
   in the first place.   
      
   Also, this being sci.space.tech, one has to think in terms of using the   
   scramjet as part of a space launch system.  In which case, you're going   
   to need the rocket *anyway*, as scramjets can't get you more than half   
   way to orbit.  If your scramjet space launch system absolutely has to   
   include a rocket good for boosting halfway to orbit, and it does, you   
   probably want to try real hard to use that same rocket to get up to   
   scramjet operating speed from the start, rather than adding a third   
   propulsion system to the mix.   
      
      
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