Once upon a time, Jeffrey Kaplan   said:   
   >I'm not an engineer, I never claimed to be.  I can't build the thing.   
      
   You said "any of us here".   
      
   >That's why it would have multiple engine types.   
      
   Multiple engine types are a dead-end.  You'd be lugging a lot of dead   
   weight (low-altitude engines, fuel tanks, etc.) all the way to orbit and   
   back.  There's no way to break even that way.   
      
   >> The amount of energy required to get to low Earth orbit is massive when   
   >> compared with air flight.  The top speed of a 747 is 567 mph; Shuttle   
   >> orbital velocity is about 17,500 mph.   
   >   
   >Use jet engines for horizontal lift-off and flight up to the upper   
   >atmosphere, and then light off the rockets for the push into orbit.   
      
   Did you read what I wrote?  The altitude isn't the problem; it is the   
   velocity.  The jets can get you up to maybe 3% of the necessary   
   velocity.  You haven't gained much, but now you are lugging a bunch of   
   dead weight in engines and fuel tanks.   
      
   >Or, if you are really strapped for weight, then make it a dual-stage   
   >craft, which I believe is the direction Virgin Galactic is going: a jet   
   >powered atmospheric flight first stage with the orbiter second stage   
   >piggy-backing.  First stage returns under power to the launch site and   
   >the orbiter continues up under rocket power, and later returns on a   
   >glide path.   
      
   Welcome to 1970; that's the original Shuttle design, but it didn't work   
   out from a performance standpoint (and that was a rocket booster, not a   
   jet).   
      
   Anyway, Virgin isn't going to orbit, so that's not really a useful   
   comparison.   
      
   >Because it is, or can be, bigger.  Economics of scale, etc.  Sure, a   
   >747 costs lots more than a Piper Cub, but the 747 can carry   
   >significantly more.  Assuming a full load, this makes the per-pound   
   >lift more economical and thus more viable.   
      
   Economies of scale don't override the laws of physics.   
      
   I suggest you talk to an actual rocket scientist.  There are lots of   
   people that have looked at lots of possibilities, including things that   
   are even slightly (but probably not really) theoretically possible.   
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   Chris Adams    
   Systems and Network Administrator - HiWAAY Internet Services   
   I don't speak for anybody but myself - that's enough trouble.   
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