From: schillin@spock.usc.edu   
      
   fairwater@gmail.com (Derek Lyons) writes:   
      
   >schillin@spock.usc.edu (John Schilling) wrote:   
      
   >>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.   
      
   >The real question is...  Why add a *second* propulsion system in the   
   >first place?  If you need a rocket to get from to the scramjet range,   
   >and then a rocket to get from scramjet range to orbital range...  What   
   >is the scramjet adding?   
      
   More a matter of what it is subtracting, which is a decent fraction of   
   the propellant you would have otherwise needed for the first half of   
   the trip to orbit.   
      
   Not that we care about the propellant in and of itself, most sorts of   
   rocket fuel being quite cheap.  But building a vehicle with a propellant   
   mass fraction of 0.9 or better, seems to be a rather hard problem.  A   
   vehicle with a 0.8 propellant mass fraction, is much easier and is more   
   than sufficient for SSTO performance if you've got a Mach-3 to Mach-15   
   scramjet in there.   
      
   Except that a lightweight Mach-3 to Mach-15 scramjet, *also* turns out   
   to be a rather hard problem.  So pick your favorite rather hard problem.   
      
   If you want to get to orbit, of course, you pick the problem that seems   
   the least hard.  If you want lots of research funding, you pick the   
   problem that seems the most hard.  From the observed behavior of people   
   on both sides of that one, it would seem that there's a pretty good   
   consensus that the scramjet is the hardest way to make it to orbit.   
   But it's not a sure thing, and at the conceptual level at least the   
   argument for the scramjet has merit.   
      
      
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