From: henry@spsystems.net   
      
   In article ,   
   Gordon D. Pusch  wrote:   
   >Note that, while the ultimate cause of the accident was the thermostat   
   >contacts welding themselves closed, it was the _TEFLON ELECTRICAL   
   >INSULATION_ around the wiring to the above devices that initially   
   >caught fire. (In a supercritical oxygen environment, even relatively   
   >"inert" substances such as teflon burn quite cheerily!)   
      
   Normal Teflon is not too bad even in such an environment... but the Teflon   
   in the Apollo 13 oxygen tank had been roasted severely when the tank   
   heaters were earlier used (improperly) to try to empty the tank on the   
   pad.  The result was a crumbly material that was very flammable in   
   supercritical oxygen.   
      
   >Unlike the Apollo Service Module, the ISS does not appear to use   
   >"slushed" supercritical oxygen cooled by the excess heat capacity   
   >of liquid hydrogen, so it presumably does not need internal heaters   
   >nor stirring fans...   
      
   The conclusion is generally correct -- bulk storage of LOX for propulsion   
   use would store it at or around its boiling point, and wouldn't need   
   anything fancy inside the tank -- but the first part is messed up.   
   Supercritical oxygen isn't slush, and the Apollo oxygen tank wasn't   
   cooled by the hydrogen tank.   
      
   "Supercritical" just means that storage pressure is high enough that the   
   distinction between gas and liquid has vanished.  This avoids the various   
   complications of managing a tank which contains both liquid and gas in   
   free fall, so supercritical storage is generally preferred when quantities   
   are small and easy trouble-free operation without gravity is wanted.  The   
   price is that the high-pressure tanks are relatively heavy, so it's not   
   practical for bulk storage.   
      
   The S-IVB's LOX tank was cooled by heat leakage from the LH2 tank, but the   
   Apollo SM oxygen tanks weren't.  They just relied on good insulation, plus   
   the fact that oxygen was constantly in use and hence a modest boiloff rate   
   was perfectly acceptable.   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
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