From: cphoenixNOSPAM@crnano.org   
      
   In SS1's Sep. 29 flight, a roll built up near the end.  It did not   
   appear to be atmospheric, and presumably RCS didn't get stuck, so the   
   only place it could have come from is the main motor.  Something must   
   have been making the hot gas swirl inside the motor (assuming there was   
   no unevenness in the nozzle bell).   
      
   In a normal solid fuel motor, all the gas is being produced by the   
   burning surface of the grain.  The gas motion near the surface must be   
   more or less perpendicular to the surface.  As the gas moves toward the   
   middle of the chamber it starts to move toward the nozzle.  But the   
   surface of the grain will be more or less insulated from the nozzle-ward   
   flow of the gas.   
      
   In a hybrid, the oxidizer must be carried from the gas to the surface of   
   the fuel.  (Perhaps not quite to the surface, if the surface of the fuel   
   boils or sublimates from radiant heat, but the fuel vapor would be   
   pretty cool and wouldn't expand much, so combustion would happen near   
   the surface in any case.)  This implies a lateral or turbulent-mixing   
   flow across the surface.  The surface is certainly not insulated from   
   the dynamics of the gas.  And the flow of gas, from the oxidizer   
   injector to the engine nozzle, is quite directional.   
      
   Imagine a vertical wall with gas flowing down it.  Put a small pit near   
   the top of the wall.  Gas will dip into the pit and slam against the   
   bottom edge of the pit, eroding it.  Now slide the wall sideways, as it   
   might be if the wall were a section of a large rotating cylinder.  The   
   flowing gas has momentum, so will tend to erode the lagging side of the   
   pit.  (If you're facing a wall moving left to right with the gas flowing   
   down the near side, then the gas that goes in the top center of the pit   
   will come out slightly to the left of center, and so on.  The pit will   
   elongate downwards _and toward the left_.)   
      
   Now the pit is turning into a slanted gouge.  And the direction of the   
   slant is such that the flowing gas will tend to push the wall in the   
   direction it is already moving.  The effect will build on itself.  The   
   gouge also acts on the gas, inducing a swirl, which will make any   
   downstream pits even more slanted.  And the swirl (and possibly   
   turbulent vortices) may even press the gas closer to the wall,   
   increasing erosion even further in the problem area.   
      
   Note that this effect depends on rotation of the engine, and therefore   
   of the craft.  It will be much weaker on a test stand--and even in   
   atmospheric flight--where the engine cannot rotate.   
      
   If I'm right, this implies several things:   
   1) A hybrid engine, due to strong gas flows across its surface, may   
   develop uneven spiraled surface erosion more easily than a solid engine.   
   2) This effect may only be seen in exo-atmospheric flight where the   
   whole engine (and craft) can rotate, not on a test stand or in atmosphere.   
   3) The effect may start more or less randomly, and may get worse rapidly   
   as the swirling gas and rotating engine combine to further damage the   
   grain surface.   
   4) The effect might be reversed by applying a counter-rotation to the   
   vehicle (using RCS) while the spiral erosion on the fuel grain is still   
   small.  Note that simply stopping the rotation will not fix the problem;   
   once a spiral erosion pattern develops, it will impart a spiral to the   
   gas, creating more spiral erosion downstream.  But slowing the rotation   
   will at least help.   
   5) I can't predict whether trying to "counter-steer" manually (apply a   
   counter-rotation to create counter-spirals) would lead to overshoot and   
   pilot-induced oscillation.  But in any case, reducing the speed of   
   rotation during the burn rather than waiting till the end to cancel it   
   would appear to be a very good strategy, as long as this procedure did   
   not interfere with calculating the must-abort point where you run out of   
   RCS fuel.   
   6) The effect might be countered in future engines by angled oxidizer   
   injectors that can be switched on and off as needed.   
      
   Chris   
      
   --   
   Chris Phoenix                                  cphoenix@CRNano.org   
   Director of Research   
   Center for Responsible Nanotechnology          http://CRNano.org   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)