From: henry@spsystems.net   
      
   In article <11efmfvfckscee3@corp.supernews.com>,   
   Ron Webb  wrote:   
   >- While orbiting at 17000mph or so to dip down into the upper atmosphere -   
   >the very edge -then deploy a large parachute similar to the modern sport   
   >chutes that forms an airfoil.   
      
   It's been proposed, actually.   
      
   >These things generate lift, just like any other airfoil. It seems to me that   
   >an inclined plane should still generate lift, even when the medium is a very   
   >thin gas - or even independant molecules.   
      
   Correct.  The rules are somewhat different up in the region of molecular   
   flow -- where the molecules are indeed pretty much independent -- and at   
   hypersonic speeds, but lift is still available... at the usual price of   
   drag.  (See below.)   
      
   >Then just skip along in the upper atmosphere for a long time (maybe as much   
   >as a full "orbit" changing angle of attack - slowly slowing and dropping   
   >into thicker air enough to balance temperatures and lift.   
      
   Alas, here you propose a numerical impossibility.  *It can't be done.*   
      
   When you buy a certain amount of lift, you pay with a certain amount of   
   drag.  And the L/D ratio of reasonable shapes is not that good at   
   hypersonic speeds in molecular flow.  If you're getting enough lift to   
   hold you up, you are *not* decelerating very slowly and gradually.  Oh,   
   initially, yes, because at just below orbital speed you don't need much   
   lift... but the situation doesn't stay that good for very long.  At half   
   orbital speed, "centrifugal lift" is only 1/4 as strong, and aerodynamic   
   lift must do most of the work, and that means you're getting a *lot* of   
   drag and slowing down rapidly.   
      
   In fact, when you study the details, it turns out that the large surface   
   area of something like a parafoil doesn't really make any difference to   
   how *quick* reentry is.  That is determined almost solely by the L/D ratio   
   of the shape, and there are real limits to how good that can be.   
      
   A large surface area does buy you something:  you decelerate earlier, in   
   thinner air, and the heat is spread out over a larger area.  This lowers   
   temperatures and makes materials problems much easier.  But things still   
   happen just about as quickly.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)