Once upon a time, Jeffrey Kaplan   said:   
   >Previously on rec.arts.sf.tv.babylon5.moderated, Chris Adams said:   
   >> Once upon a time, Jeffrey Kaplan   said:   
   >> >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   
   >   
   >Which jets are you referring to?   
      
   The jets you wrote about.  Are there some other jets I could have been   
   referring to?   
      
   >Overall, is it more dead weight that having to lift the oxidizer for   
   >your rockets for the atmospheric flight section?  Someone stated that   
   >it's the oxidizer that takes up so much of the tonnage of the fuel.   
      
   Oxidizer isn't dead weight; it is (mostly) gone by the time you get to   
   orbit.  The highest G-forces experienced by the crew are just before   
   main engine shutdown, when the fuel weight is almost gone.  That weight   
   loss during ascent is required; when the solid rocket boosters detach,   
   the shuttle is actually not generating enough force to overcome gravity.   
   If it were sitting on the ground, it wouldn't move (it is generating   
   around 1.2 million pounds of thrust but still weighs more than 1.2   
   million pounds).  It is only as the fuel burns off that the thrust   
   surpasses the weight.   
      
   Trying to put multi-stage engines on one vehicle means you have to carry   
   the initial weight of engines, fuel tanks, pumps, support structure,   
   etc. throughout the flight.   
      
   >An additional potential benefit of having a jet powered ascent stage is   
   >that if you can get a slow enough re-entry, you can then have a powered   
   >landing approach.   
      
   The Russians tried this with Buran (jet engines for landing); it didn't   
   work out very well.   
      
   Look, a very high-performance jet engine might generate 30,000 pounds of   
   thrust.  The Shuttle main engines each generate up to 400,000 pounds of   
   thrust, and the solid rocket boosters each generate up to 3,100,000   
   pounds of thrust.  At take-off, the Shuttle system has about 6,800,000   
   pounds of thrust, over 200 times what a jet engine could produce.  Even   
   if half the vehicle weight disappeared, jet engines still wouldn't get   
   it off the ground.   
      
   I'm not going to reply to any more of this; you don't have a grasp of   
   the basic physics involved.   
   --    
   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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