From: henry@spsystems.net   
      
   In article ,   
   Eric Fenby  wrote:   
   >Aluminium is a very cheap material but has to be inappropriate for building   
   >the Shuttle's wings etc.   
      
   Aerospace aluminum alloys are not all that cheap, actually...   
      
   >If the Shuttle had been constructed out of titanium, which has a 2000c   
   >degree melting point instead of the 400-C of aluminium, how much lower would   
   >it have been before breakup?   
      
   Possibly not any lower at all.   
      
   First, let's get the numbers right.  Aluminum melts at 659degC, and   
   titanium at 1670degC.  Moreover, those may not be the relevant numbers:   
   the highest *usable* temperature for a structural material is typically   
   rather lower, because most metals are quite weak by the time they're about   
   to melt.  Aluminum alloys are generally considered structurally useful up   
   to 250-300degC, titanium alloys up to 500-600degC.  The number we care   
   about is probably somewhere in between the usable temperature and the   
   melting point.   
      
   Now, as for a titanium Columbia...   
      
   For one thing, the outer thermal protection would have been different --   
   generally thinner -- since the interior could run hotter.  (The big   
   advantage cited for titanium structure, when NASA was considering what   
   material to use, was that the development of the tiles etc. would be   
   easier.)  So the hole might well have been bigger.  Also, the structure   
   inside would have been hotter to begin with, since the whole point of   
   using titanium would have been to permit that.   
      
   Then too, the titanium would have been considerably thinner, since it's a   
   stronger (and denser) material.   
      
   Also, the combination of thinner material and titanium's *much* lower   
   thermal conductivity makes a titanium structure much more vulnerable to   
   localized overheating, since it's not nearly as good at conducting heat   
   away from a hot spot.   
      
   Put all this together, and it's not clear that you get much advantage,   
   especially given the horrendous conditions involved.  This wasn't a case   
   of aluminum being almost good enough.   
      
   Finally, even holding together a little bit longer confers no real   
   advantage in such a situation.  There would still be no realistic chance   
   of the wing holding together all the way to the ground (which is what it   
   takes to do a successful bailout from an orbiter -- the last man out   
   leaves at quite low altitude).   
      
   >Perhaps a titanium wing structure with the interior box sections filled in   
   >with a lightweight refractory foam to exclude the superheated gasses?   
      
   If you want to spend considerable weight on improving tolerance to faults   
   in the thermal protection, probably much the most effective way would be   
   to forget screwing around with the structure, and put a layer of ablator   
   behind the leading edge.   
      
   Although I'm not up on everything that's done in advanced materials, I'm   
   not aware of any "lightweight refractory foams".  Certainly not ones that   
   were available in the early 1970s.   
      
   >Titanium's STW ratio is favourable enough for fighter aircraft...   
      
   Fighter aircraft are invariably mostly aluminum.  The only exceptions are   
   the MiG-25 and MiG-31, which are steel (heat-resistant but very heavy).   
   The only operational titanium aircraft have been a few specialized   
   high-speed types, notably the Blackbirds.   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
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