I posted this a couple of weeks ago, but it seems to have gotten lost.  I   
   think it's worth repeating, because I think it finally answers the other   
   side of the question.   
      
   =========================================================================   
      
   I was a bit confused by Henry's posting a while back in this thread, where   
   he said:   
      
   >>This would mean you do not have to lift the oxidizer for the 1st stage...   
   >   
   >Why is that an advantage?  Liquid oxygen is compact, relatively easy to   
   >store and handle, and so inexpensive that it's nearly free.  It *is*   
   >heavy... but with rockets, extra thrust is cheap.   
   >   
   >Design group after design group has come up with an elegant jet/rocket   
   >design, and as an afterthought compared it to an all-rocket approach...   
   >and been startled to discover that the all-rocket system looked to be   
   >simpler, more capable, and cheaper both to develop and to operate.   
      
   Whereupon I felt compelled to ask, if oxidizer is so cheap and easy to   
   carry, why don't we dispense with jets altogether for in-air flight, and   
   just use rockets? After all, I'd seen several postings here saying that   
   rockets are actually quite efficient.   
      
   After being tweaked to try it, I was surprised how easy the answer was to   
   find with Google: jets are far more fuel-efficient that rockets.  At least   
   that's what they call it; I'm not sure the word "efficiency" is appropriate   
   when it can't be expressed as a fraction or percentage.   
      
   I also came across some claims that rockets are considerably more energy-   
   efficient than jets, but couldn't find any good references.   
      
   So, unless I'm mistaken, that answers the question.  Jets have far higher   
   fuel efficiency (Isp) than rockets, so make more sense for commercial   
   atmospheric flight.  The reason rockets are better than jets for space   
   launches has already been covered here very well, by people who actually   
   know what they are talking about; I won't attempt it.   
      
   And no, this doesn't _directly_ help much for space launches.  But I do   
   think it helps to conceptually clarify the relative benefits and   
   limitations of the two types of engines.  And that is useful.   
      
   /kenw   
   Ken Wallewein   
   K&M Systems Integration   
   Phone (403)274-7848   
   Fax   (403)275-4535   
   kenw@kmsi.net   
   www.kmsi.net   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)