Previously on rec.arts.sf.tv.babylon5.moderated, Chris Adams said:   
      
   > >> 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?   
      
   Well, there are the jets as used on Boeings and Airbuses which attain   
   speeds of ~500mph, then there are the jets as used on the Concord,   
   which attained speeds in excess of Mach 1, and then there are the jets   
   as used on the SR71 which attained speeds well over Mach 2.  All of   
   which are air-breathing jet engines used from take-off through cruising   
   altitude to landing.   
      
   Granted, none of these will attain orbital velocity, but that is not   
   the purpose of the take-off jets here.  These are to only be the first   
   stage propulsion.  Orbital velocity would be attained with second or   
   third stage propulsion.   
      
   > >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.   
      
   My point is that you can carry less of the heavy rocket oxidizer in the   
   first place if your actual launch engines breath atmosphere for the   
   oxidizer.   
      
   What I'm asking is if the weight trade-off works.  In my semi-informed   
   and fannish mind, if it results in a weight savings at launch, then the   
   trade-off works.   
      
   > 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.   
      
   Since we're only talking about a surface-to-orbit vehicle here, I do   
   not see that as a huge problem.   
      
   > >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.   
      
   I've never heard of that, I'll look it up.   
      
   --    
   Jeffrey Kaplan                                         www.gordol.org   
   Double ROT13 encoded for your protection   
      
   Peter's Top 100 Things I'd Do If I Ever Became An Evil Overlord, #177.   
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