From: henry@spsystems.net   
      
   In article <45022fc8.0410081608.561ccec8@posting.google.com>,   
   Iain McClatchie  wrote:   
   >Then I stopped to consider the Minuteman or MX.  Both of these   
   >missiles are three-stage solid rockets.  Does either need a   
   >precision thrust cutoff from the third stage (or other two)? ...   
   >I assume both have some sort of throttleable propulsion on the   
   >bus that delivers the MIRVs.  Is this enough to overcome   
   >uncertainties in the boost delta-V?   
      
   Yes, but the uncertainties are kept down by having thrust termination on   
   the final stage, so it can be cut off on command.  The final stage has   
   thrust-termination ports up near the top of the motor, which are blown   
   open to drop the chamber pressure sharply.  The outgassing from the fuel   
   surface continues at full speed momentarily, since it's driven by heat   
   already transferred into the fuel, but the combustion rate in the gas   
   drops with the pressure drop... and so if the pressure drops quickly   
   enough, the flame is blown away from the surface and goes out.  (Caution,   
   I oversimplify a bit.)  Done carefully, with attention to details, this   
   can give a relatively clean and predictable cutoff.  And then the little   
   fourth stage, aka warhead bus, cleans up any remaining error.   
      
   And no, alas, you can't apply this to the shuttle SRBs.  Solids were   
   chosen for the shuttle on the assumption that thrust termination was in   
   fact possible... but shutting off a solid this way is a fairly violent   
   process, and it turned out that the orbiter and ET are not strong enough   
   to survive it.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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