XPost: sci.space.shuttle   
   From: schillin@spock.usc.edu   
      
   kitplane01@gmail.com writes:   
      
   >I understand that the basic structure of the Shuttle is aluminum,   
   >rather like a Boeing 747.   A titanium structure would weigh less.   
      
   Not that much less.  Titanium alloys have only 30-40% better strength   
   to weight than aluminum alloys, and slightly *less* stiffness to weight.   
   Also higher density and more difficult fabrication, which complicate   
   structural design.   
      
      
   >Further a titanium structured craft could have a less massive thermal   
   >protection system, since things like the wings and tail could be allowed   
   >more heat.   
      
   The temperature behind the bow shock of a space shuttle on re-entry is   
   vastly higher than either aluminum *or* titanium can handle.  You're   
   still going to need the tiles, and you're not really going to be able   
   to shave that much weight off them.  It's like suggesting that you   
   could make it easier for people to walk through a blast furnace if you   
   were able to tweak the natural body temperature up to 180 F.   
      
   Aluminum also has an edge in this respect in that, A: it has a very   
   high heat conductivity, and B: it has a moderately high heat capacity.   
   So any local overheating will be diffused throughout the structure and   
   absorbed, and you mostly just have to worry about the average.  With   
   titanium, local overheating leads to local failure, and you have to get   
   your thermal protection system right *everywhere*.   
      
      
   >NASA did study this option.  Why did they pick aluminum?   
      
   Because it was almost as good, and a lot cheaper and easier to deal   
   with.   
      
      
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