From: jfindley@cinci.nospam.rr.com   
      
   In article , sylvia@email.invalid   
   says...   
   >   
   > On 04-Sep-20 3:12 pm, Sylvia Else wrote:   
   > > How does this actually help?   
   > >   
   > > The aerodynamic forces on the craft are a function of its speed and air   
   > > density. There's not much one can do about the speed, given that the   
   > > craft is accelerating.   
   > >   
   > > If the craft accelerates more slowly overall, for each speed it will be   
   > > traversing less dense air than it would do otherwise. But in that case,   
   > > is there a benefit from high initial acceleration and then reducing   
   > > acceleration on the approach to max-q?   
   > >   
   > > Is it really about reducing vibration loads during max-q?   
   > >   
   > > Sylvia.   
   >   
   > The thing is, it's not as if the rocket thrust and aerodynamic forces   
   > are adding together to squeeze the vehicle from both ends.   
      
   Actually it is.  If I clamp a board in a vice, at the center of the wood   
   the forces from each side of the vice do cancel out to precisely zero   
   (i.e. it's not accelerating due to the forces).  But, the wood is under   
   stress due to the compression caused by the vice.  Tighten the vice   
   enough and the wood will be crushed (permanently deformed).   
      
   That's exactly what's happening during max-Q.  Thrust and drag are   
   acting opposite each other and are placing the vehicle under more stress   
   than it would be if it were operating under the same thrust in vacuum.   
      
   > I'm not suggesting that the throttle down is not required, but the   
   > rationale for it doesn't appear as straight forward as it might seem at   
   > first sight.   
      
   This bit is true.  Throttling down early decreases acceleration so that   
   max-Q happens at a higher altitude, where density is lower and therefore   
   aerodynamic forces are lower.   
      
   As was posted earlier in this thread, Apollo/Saturn V experienced max-Q   
   at a much higher altitude than the space shuttle due to its slower   
   acceleration.   
      
   Jeff   
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