From: henry@spsystems.net   
      
   In article <40E5973B.F308B656@NOSPAMhobonet.com>,   
   From henry@spsystems.net  Sun Jul 04 12:30:32 2004   
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   Newsgroups: sci.space.tech   
   Path: henry   
   From: henry@spsystems.net (Henry Spencer)   
   Subject: Re: What does it make sense to bring back?   
   Organization: SP Systems, Toronto, Canada   
   Message-ID:    
   References: <375b1119.0406291928.74a2c325@posting.google.com>   
   Date: Sun, 4 Jul 2004 18:47:33 GMT   
      
   In article <375b1119.0406291928.74a2c325@posting.google.com>,   
   Hephaestus  wrote:   
   >Basically, once you decide not to go 100% reusable, you have the ability to   
   >decide certain components (tanks, structure, etc) just aren't cost effective   
   >to reuse.  Right?   
      
   Correct.  There have been a number of proposals for semi-reusable   
   launchers that would return engines and electronics but sacrifice the   
   tanks, either leaving them in orbit or expending them in some other way.   
      
   It's not a ridiculous idea, but it's also not one that gives you as much   
   gain as you might think.  The subsystem mass for reentry and landing --   
   heatshield etc. -- scales very strongly with the mass of the object and   
   only rather weakly with its size, and tanks just don't weigh very much.   
   An engine pod is much smaller than the whole vehicle, but it's also much   
   denser and makes a much harsher reentry, so the heatshield mass isn't   
   reduced much.   
      
   A lot depends on details, but it's not obvious that you really benefit   
   greatly from bringing "just the essentials" down.  There are advantages,   
   like greater configuration flexibility.  There are also disadvantages,   
   such as potentially lower reliability due to being unable to flight-test   
   the expendable components.  (This is probably a more important issue than   
   the actual *cost* of the expendable bits.)   
      
   Whether it's an attractive approach will depend on what your priorities   
   are and how you prefer to tackle certain issues.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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