From: henry@spsystems.net   
      
   In article <45022fc8.0401131734.96e35da@posting.google.com>,   
   Iain McClatchie  wrote:   
   >The mass of pressurized tanks holding gases scales with the moles and   
   >temperature of the gas held.  The same rule applies unchanged to both   
   >propellant and pressurant tanks.  So if the gas temperature isn't   
   >changed, and it doesn't undergo a chemical reaction changing the   
   >number of moles, then the pressurant tank has to weigh as much as the   
   >propellant tank...   
      
   This is mitigated somewhat by being able to use more aggressive tank   
   technologies (e.g. fiber-wound tanks) for pressurant without running into   
   minimum-gauge considerations.  But it's not an accident that big rockets   
   show a distinct trend toward using propellant-boiling pressurization   
   systems whenever possible, and even when they don't, they do tend to store   
   their pressurants cold and heat them before use.   
      
   >The book doesn't appear to discuss the mass of the pressurant tank.   
   >It does talk about storing liquid pressurant, but says this hasn't   
   >been done.   
      
   It's been done a little bit (notably by Goddard!).  Do bear in mind that   
   Huzel&Huang is very definitely Rocketdyne's take on engine design, *not*   
   an industry-wide survey.   
      
   For pressurization in particular, NASA SP-8112, "Pressurization systems   
   for liquid rockets", is worth reading.  It *is* more of a survey.  It's   
   available (as a scanned PDF) on the net.  (As are almost all of the   
   NASA 8000-series SPs, many of which are fascinating reading for rocket   
   designers.)   
      
   >It talks about heating the pressurant in the tank, but   
   >does not talk about keeping the pressurant tank cold before launch   
   >(except for the special case of liquid hydrogen pressurant).   
      
   That's actually an outright omission, perhaps the result of having the   
   book written by engine designers rather than vehicle designers.  Chilled   
   pressurant tanks are common, exploiting both the higher density of the gas   
   and the greater strength of tank materials at low temperatures.  The   
   Saturn V tended to locate pressurant tanks inside cryogenic propellant   
   tanks, for that reason, and if memory serves, Atlas's helium tanks had LN2   
   jackets.   
      
   >...The only alternatives are boiling your propellant,   
   >which is only interesting if your propellant is cyrogenic...   
      
   It can be done even for non-cryo propellants.  An extreme case is the   
   Titan II second-stage oxidizer tank, which has no in-flight pressurization   
   system!  It gets some helium (I think) from ground-support equipment, and   
   any pressure makeup that happens after that is from the oxidizer boiling   
   in the tank as pressure drops off.  Similarly, Centaur's tanks are mostly   
   self-pressurizing.   
      
   >or burning your propellant and reinjecting that.   
      
   Or variations thereon.  The original Arianes did some of their tank   
   pressurization using turbine exhaust gas, cooled with water injection.   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
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