From: com.ntlworld@feditech.dante.retro.com   
      
   "Jonathan A. Goff"  wrote...   
   > What are the current options for cryogenic propellant transfer on orbit   
   (assume   
   > LOX temperatures, not LH2 temps)?  Has anyone actually succesfully   
   demonstrated   
   > this?  Are there any options that look reasonably from a   
   cost/mass/reliability   
   > point of view?   
      
   Depends on how you do it. Answers below for fuel are just as applicable for   
   oxident.   
      
   Tank. If you transfer the entire tank then it's easy. Think of it as a big   
   LNG canister.   
      
   Valve. If you connect the two tanks together then after a while you'll have   
   half the fuel in either tank by simple expansion.   
      
   Heat. If you heat the full tank and cool the empty tank then vapour will   
   evaporate and then condense in the cool one. This would probably get the   
   vast majority of the fuel out. But Apollo 13 demonstrates the dangers of   
   heating cryogenic substances.   
      
   Pump. If you place both tanks under acceleration (via spin or continual low   
   thrust) then all the fuel will settle to the bottom and you'll be able to   
   pump it. This is evffectively what they had to do with the RCS everytime   
   they fired the engine on the Apollo service module whenever they wanted to   
   'pump' fuel into the engine.   
      
   Pour. If you place both tanks under acceleration (again via spin or   
   continual low thrust) and one tank is 'above' the other, then fuel will   
   naturally flow 'downwards' into the empty tank.   
      
   Concievably the last two could even take place on a planetary surface. ^.^   
      
   > I know for room temperature propellants that propellant transfer has been   
   > demoed before, and that just using an expulsion bladder can often do the   
   > trick, but AIUI, it's hard to get anything elastomeric at LOX   
   temperatures.   
      
   Aside from 'heat' the answer is pretty much any tempreature/pressure that   
   can be supported by the tank and transfer equipment. The heat solution   
   naturally takes place at a variety of tempreatures. ;) To the best of my   
   knowledge, no-one has ever demonstrated this exact technique, but the ASM   
   had to pump fuel into it's engine, so that might count, depending on your   
   point of view.   
      
   John   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)