XPost: misc.misc   
   From: zenadsl6186@zen.co.uk   
      
   Harmon Everett wrote:   
   > Peter Fairbrother wrote:   
      
   >> To give some idea of the problems, first, the layup would be terribly   
   uneven,   
   >> no matter how skilled the operatives. That means there would be places where   
   >> the layers are seperated by thick layers of resin - this a) uses a lot of   
   >> heavy resin to no purpose, and b) makes the finished composite much weaker -   
   >> the resin cracks and layers, allowing the tension to be taken on only one   
   >> layer of reinforcement, which will fail if it's not strong enough. There are   
   >> other failure modes too.   
   >>   
   >> Slightly worse, there will also be gaps between layers which are filled with   
   >> air rather than resin; and worst of all places where the fibres are not   
   >> impregnated with resin.   
   >>   
   > Is it that much of a problem in this circumstance?  The force across the   
   > fiber/film layer - from inside of the sphere to the outside, is just the air   
   > pressure at that point isn't it? The reason we are using the fiber blanket is   
   > to take up the tension along the length of the fibers -warp and woof. That   
   > doesn't depend on the uniformity of the resin application.   
      
   Yes, it is a problem.   
      
   The composite structure helps to even out the loads on the reinforcement.   
   This requires even and thorough dispersal of the reinforcement in the   
   matrix. This evening-out prevents all the load being put on part of the   
   composite and breaking that part - then the load goes on a new part, and   
   breaks that, and so on until the whole fails.   
      
   It also requires that any layers be well bonded together and that they not   
   delaminate - for instance, if you had just two layers, one flat and one with   
   a wrinkle, any tensile forces will only be on the flat layer, the wrinkled   
   layer will not contribute. The overall strength will be halved.   
      
   The matrix also provides some extra tensile strength, esecially when the   
   bond thickness is near the optimum. This requires a ratio of resin to   
   reinforcement of usually around 30% for maximium strength _from a fixed   
   amount of reinforcement_. Adding more resin actually weakens the final   
   structure, even if the amount of reinforcement is constant.   
      
   > The time between different layers could be months.   
      
   Leaving the economics aside, that means that the layers will not bond   
   together easily. They could delaminate, which would be a big problem.   
      
      
      
   > Unfortunately, once you crack the UV protective barrier, the prepreg will   
   > harden, whether you get the blanket spread out in time or not. Other than a   
   > process which takes place automatically - such as the original inflation and   
   > hardening, which could be done automatically, there isn't any real way to   
   > guarantee that the workers won't be interrupted by something more important,   
   > like a puncture or something before they get the blanket spread out and   
   rolled   
   > into place.   
      
   The UV barrier goes on the outside of the sphere. It is only "cracked" when   
   you want to harden resin repreg which has already been spread out and   
   heated. It is in place all the rest of the time.   
      
      
      
      
   One more problem I should have mentioned, resins stink, and most are toxic   
   until set.   
      
      
      
   >> A large pressuriseable and _depressuriseable_ volume might be more useful   
   for   
   >> building work. You would need air pumps, air storage tanks, and a long   
   zipper   
   >> to get things in and out of it. No need for 100m sphere though, I'd guess,   
   >> something smaller would be enough.   
      
   > We've got a ISS, its not big enough to lease out volumes to independent   
   > organizations. A space station needs to be big enough to be able to bring in   
   > enough income to support itself.   
      
   I wasn't thinking of using the volume as a space station, but as a   
   construction and repair bay, just part of a station (or a garage/workshop   
   attached to a station).   
      
      
      
      
   > You mean the Shuttle External Tank?  It ends up in space with lots of   
   internal   
   > stuff that needs to get refitted, and cut out, and fuel that gets nasty if   
   you   
   > don't dispose of it well.   
      
   The tanks are 4.2 m dia x 10 m long foam insulated steel (yes, steel) LH2   
   tanks. Leftover LH2 is scavenged, the inside is cleaned by warming and   
   opening to vaccuum, and the slosh baffles can either be removed or left in   
   place as floor supports.   
      
      
   --   
   Peter Fairbrother   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)