From: henry@spsystems.net   
      
   In article <45022fc8.0401151248.7298891b@posting.google.com>,   
   Iain McClatchie  wrote:   
   >BTW, has anyone ever done a liquid neon system, or (more likely) liquid   
   >neon refrigerated helium?   
      
   Not that I've heard of.   
      
   >Also, do LN2-jacketed helium tanks dump the LN2 just before launch, or is   
   >the valve to do that heavier than the LN2 dumped?   
      
   On Atlas, I *think* the LN2 is dumped before launch, with a modest amount   
   of insulation around the tanks preventing too much warming up.   
      
   Incidentally, one reason why the classic Atlas doesn't put the pressurant   
   in the LOX tank is that the entire pressurization system is jettisoned at   
   staging -- it's mounted on the booster-engine ring.  Atlas is blowdown   
   during the sustainer-only burn, with the hydrostatic head from increasing   
   acceleration compensating somewhat for gas expansion in the tank.   
      
   >HS> Or variations thereon.  The original Arianes did some of their tank   
   >HS> pressurization using turbine exhaust gas, cooled with water injection.   
   >   
   >Isn't there a major problem with water in the pressurant condensing to   
   >ice...   
      
   Not on Ariane, which uses storable propellants.  With cryo fluids, yes,   
   that could get a bit problematic.   
      
   >...I read that NASA has to be very careful about particulates in   
   >the LOX in the shuttle, but also that they pressurize with turbine exhaust,   
   >which has H2O.  I'm astonished.   
      
   As well you should be.  SSME turbine exhaust goes into the combustion   
   chamber; it's not used for pressurization.  Shuttle tank pressurization   
   is by propellant boiling.   
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