XPost: misc.misc   
   From: zenadsl6186@zen.co.uk   
      
   Cray74@gmail.com wrote:   
   > Harmon Everett wrote:   
      
   >> The second layer would be thicker and more capable of   
   >> being pushed against with the squeegee, and so on.   
   >   
   > Speaking as a materials engineer, I *still* wouldn't trust a composite   
   > shell laid up by hand in that fashion even if you could press and roll   
   > it firmly.   
      
   Speaking as a person who does a lot of composite fabrication, nor would I.   
      
   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.   
      
   Hand layup under those conditions also means that the fibre/resin ratio is   
   far higher than optimum - optimum is about 30% resin or less, but it's hard   
   to get ratios of less than 70% or so with hand layup. The extra resin does   
   not contribute to the strength of the composite, and the larger gaps between   
   fibres actually weaken the composite.   
      
   Temperature control would be a bitch - mixed catalysed resins can only be   
   worked reliably in a very small temperature range.   
      
   Adhesion of extra layers would be a problem too, few resins like to stick to   
   hardened versions of themselves, and the best composites are made in one   
   piece, or if that's too difficult then a strict timing is used when applying   
   extra layers, so that the layers are only partly set when the next layer is   
   applied - usually less than 12 hours between layers, not really practical in   
   your scheme I suspect.   
      
      
      
      
   To get round some of the problems you could use a UV hardening prepreg.   
   That's fibre preimpregnated with resin in a semi-solid rubbery form, which   
   is formed into flexible sheets. These are melted together (either by heat,   
   solvents, or adding extra resin, though heat is best), and set when UV light   
   shines on them.   
      
   However, just using prepreg isn't enough, you would want to vaccuum-bag it.   
   On Earth that involves laying up the resin and fibre or prepreg, covering   
   with a mylar bag and sucking the air out, letting the air push the layers   
   together. Often you use a layer of mylar with tiny holes in it next to the   
   composite as well, with a layer of fabric (or of plastic with channels in   
   it) between the mylar layers, to ensure the air gets sucked out from the   
   middle of the piece.   
      
   Plenty of vaccuum available in space, the problem is the air. I use about   
   10psi for my best work - but perhaps 3-4 psi would be sufficient with a   
   suitably chosen prepreg.   
      
      
   You would have some UV opaque sheets to ward off the sunlight, and spread   
   the prepreg out where needed, let it get hot and melt together eg by shining   
   extra sunshine on it with mirrors, bag, then remove the UV opaque material   
   and let the sunshine set the resin.   
      
   You would then need to replace the UV opaque layer for long-term UV   
   protection, as most resins are badly affected by UV (there are exceptions,   
   such as some straight-chain polyurethanes, but these are expensive and hard   
   to use).   
      
   Still doesn't solve the multiple layer adhesion problem, but perhaps some   
   clever chemistry might help enough there. You are still talking about a long   
   and expensive development and testing program though.   
      
      
      
      
      
      
   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.   
      
      
      
   Might I recommend my scheme as an alternative source of living space? Every   
   flight puts a 10 meter long 4.2 meter diameter insulated tank into orbit.   
   That's about the size of a small house, fifteen times a day, 300 days a   
   year.   
      
   Or the same volume as a 100 m sphere every 14 months, fully pressurised and   
   divided into usefully-sized compartments. For free.   
      
      
   --   
   Peter Fairbrother   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)