From: schillin@spock.usc.edu   
      
   jgreason@xcor.com (Jeff Greason) writes:   
      
   >schillin@spock.usc.edu (John Schilling) wrote in message news:<   
   d9kj6$t4u$1@spock.usc.edu>...   
   >> thomsona@flash.net (Allen Thomson) writes:   
      
   >>> If you're willing to take a ~20% performance hit, Nitrous Oxide and   
   >> Ethane a la the XCOR teacart engine looks pretty good across the board.   
   >> Plus, in the right temperature range it's self-pressurizing.  But, there   
   >> is that performance hit, and a consequence of self-pressurization is that   
   >> your tanks may be heavier than you wanted for a low-performance system.   
      
   >Actually, the survival of the nitrogen tetroxide/hydrazine derivative   
   >combination has at least as much to do with its extreme maturity than   
   >with its inherent superiority, especially for deep space missions.   
      
   Yes, but deep space missions represent such a small fraction of the   
   market for spacecraft propulsion that they pretty much have to stick   
   with the technology base developed for Earth orbiting spacecraft unless   
   there is a *very* compelling reason to do otherwise.   
      
      
   >To pick the first example to come to mind, LOX and methane are   
   >readily space storable, *especially* as far out as Saturn.  And they   
   >have noticably superior performance to tetroxide/hydrazine.  But   
   >it's got no flight history, you need igniters for the thrusters, you   
   >would have to refrigerate on the ground during the wait for a launch   
   >opportunity, etc.  Yes, it would make your spacecraft lighter -- but   
   >who cares -- if it fails, you lost your one chance in 20 years to get   
   >the mission, and everybody knows how to make the toxic hypergols work.   
      
   Precisely.  And the reason N2O4/N2H4 has that technical maturity is   
   that, while LOX/methane is nicely storable out as far as Saturn,   
   tetroxide/hydrazine is the one that's storable in Earth orbit.   
   Oh, you can keep LOX around for a few days or weeks, but months   
   or years that close to the big hot ball of fire and the small warm   
   ball of rock requires either active cooling or really heroic levels   
   of passive thermal control.  Neither of which scale down well enough   
   for the market.   
      
   The one compelling advantage N2O4/N2H4 has, is its excellent storability   
   over the temperature range of interest for most missions.  So that's   
   where most of the technology development goes, and unless you have a   
   really compelling reason to use something else that's what you are   
   stuck with.   
      
      
   Historically, some of the HNO3/hydrocarbon combinations were contenders,   
   and for the future there is some good work being done on the so-called   
   "green" storables.  And earlier this year I sat in on some meetings   
   where someone tried to resurrect ClF5 yet again.  But not yet anything   
   with a compelling advantage over tetroxide/hydrazine for most of the   
   space-storable, small thruster market.   
      
      
   --   
   *John Schilling                    * "Anything worth doing,         *   
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