XPost: sci.space.shuttle   
   From: dg411@freenet.carleton.ca   
      
   JazzMan (No_Spam@airmail.net) writes:   
   > khatcat@hotmail.com wrote:   
   >>   
   >> I think I understand why objects heat up when going through the   
   >> atmosphere and therefore why re-entry vehicles need heat shielding.  My   
   >> question is why we don't need heat shielding when launching.  Aren't we   
   >> going through the same atmosphere?   
   >>   
   >> BigKhat   
   >   
   > Most of the accelleration that occurs with the shuttle is after it's   
   > left the thickest part of the atmosphere,   
      
   Correct.   
      
   > they keep the throttle down until they've cleared that.   
      
   Wrong. At the moments of " Max Q ", which is maximum aerodynamic   
   pressure on the vehicle ( Due to thickness of air times velocity;   
   later on, the shuttle is flying 3-10 times faster, but the air is   
   10-50 times thinner ), the SSMEs throtle down to about 65% thrust,   
   but the SRBs keep burning flat out.   
      
   After well under a minute, the SSMEs throttle back up to full rated   
   power.   
      
   > Once above the atmosphere the shuttle   
   > goes mostly horizontal and accelerates to orbital velocity.   
      
   Not quite. The trajectory is more slanted than that, for most of that   
   time.   
      
   > When re-entering the shuttle has full speed as it hits atmosphere and   
   > it has to scrub off speed using the atmosphere. If you had unlimited   
   > fuel you could fire the engines and mostly stop the shuttle in orbit,   
   > then let it drop pretty much straight down with no atmospheric friction   
   > heating at all.   
      
   If you are in any physical object at 160 km up, and your burn has   
   eliminated your forward velocity, the acceleration DOWN will swiftly   
   give you much of that speed back, just in a different direction.   
      
   Not only will you need the tiles, you'll need double thickness, as   
   the shuttle's re-entry course is designed to shed a lot of speed   
   *before* you hit denser air. Free falling straight *down*, you   
   get no such benefit.   
      
   Andre   
      
   --   
   " I'm a man... But, I can change... If I have to... I guess. "   
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