From: ansible@typhoon.xnet.com   
      
   In article ,   
     wrote:   
   >henry@spsystems.net (Henry Spencer) wrote:   
   >>...   
   >>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.   
      
   Maybe this is drifting into sci.space.tech.psychology, but why does this   
   keep coming up?  Is it because we are used to living at the bottom of   
   our ocean of air, and we think of air as a fairly dense and useful   
   substance?   
      
   Where as, at high altitude, there isn't enough air to be useful, it only   
   slows you down.   
      
   Is it a lingering "jet fighter" mentality in the design groups?  Is it   
   that they all want to design highly-optimized, high-performance systems?   
   As opposed to systems which may not be optimal in some senses, but are   
   more practical and cost-effective.   
      
   >Which begs the question, why don't we use them for aircraft?  I'm not   
   >trying to argue that we should, it's just that every time I see this kind   
   >of discussion, that comparison is left hanging.   
   >   
   >What is it about space launches that makes rockets better, and what is it   
   >that is different about atmospheric flight that makes jets better?  Is it   
   >reusability?  Fuel economy (and why doesn't that matter for space   
   >launches)?  The altitude/pressure thing (surely that wouldn't be such an   
   >issue for first stages)?  Weight (why isn't that a problem for aircraft)?   
   >Raw peak thrust?  Maybe it's the fact that space launches are so brief we   
   >need less fuel?   
      
   At the speeds and altitudes that aircraft run, the air is useful.  It is   
   dense enough to use as lift for wings.  It has enough oxygen to use as   
   an oxidizer.  It has enough density to use as reaction mass.   
      
   The thing to keep in mind is that rockets have to go really fast to have   
   their payloads reach orbit.  You want to get _out_ of the atmosphere as   
   quickly as possible, so that it doesn't slow you down.   
      
   James Graves   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)