From: norman.yarvin@snet.net   
      
   In article ,   
   Wallace Berry   wrote:   
      
   >In article ,   
   > henry@spsystems.net (Henry Spencer) wrote:   
   >   
   >> In article ,   
   >> Wallace Berry   wrote:   
   >>   
   >> >What experiments have been done to examine incorporating some sort of   
   >> >fiber reinforcement into the grain to prevent cracking? How about the   
   >> >obvious: carbon fiber?   
   >>   
   >> Carbon fiber in particular is probably a bad choice, because it's quite a   
   >> good thermal conductor, and solid fuels want low thermal conductivity   
   >> (partly to insulate the outer case, partly to concentrate combustion heat   
   >> near the inner fuel surface to encourage the surface to boil off).   
   >>   
   >> I think fiber reinforcement of solid fuels has been done, but the mass   
   >> penalty is substantial if you want good reinforcement.   
   >   
   >Ah, I hadn't thought about the thermal conductivity of carbon fiber   
   >being an issue. I suggested carbon fiber mainly because I've been laying   
   >up some parts out of it lately and it was on my mind.   
   >   
   >It did belately occur to me that the mass of fibers required to achieve   
   >reinforcement might be significant. I wonder if fibers could be made out   
   >of something that would be a relatively efficient fuel as well as   
   >provide reinforcement? On second thought, that ain't gonna work as the   
   >fuel and oxidizer need to be mixed at a very fine scale.   
      
   The larger the motor, the less finely the ingredients have to be mixed.   
   But they still do have to be mixed, and I think this, rather than the   
   mass penalty, would be the main difficulty with fiber reinforcement.   
   Solid rocket mixes are usually upwards of 80 percent oxidizer, and mixing   
   80 percent of solids into 20% of rubber binder is a very difficult task.   
   If, for instance, the oxidizer were composed of perfect spheres all of   
   the same size, it would be impossible to make such a mix; a perfect   
   packing of spheres has more than 20% space between them.  Just for it to   
   be possible, you need oxidizer that has a mixture of different particle   
   sizes, so that the small particles can fill in the spaces around the big   
   ones.  And possible does not mean easy: even uncured, the rubber binder   
   is viscous stuff, and once you start mixing it there is a limited time   
   available before it starts to cure.  For good burning, the batch has to   
   be very well mixed, and with no bubbles.  All these difficulties have   
   been overcome; but that does not mean that they would still be   
   surmountable if you threw a load of Kevlar fibers into the mix; for   
   starters, the fibers would quickly clog the paddles of your mixer.  If   
   you kept the fibers really short, maybe you could get away with mixing   
   them; but then you'd have to worry about what their orientation turned   
   out to be, in the poured grain, and you might well have to include so   
   many of them that their mass would be a significant factor.  If you try   
   to use the fibers as fuel, then their mass reduces the amount of rubber   
   you can use, which makes the mixing even harder.   
      
      
   --   
   Norman Yarvin						norman.yarvin@snet.net   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)