From: david@davsoft.com.au   
      
   After seeing the Armadillo crash, I thought to myself why didn't they just   
   have a fuel guage and monitor it? Then I thought some more and realised   
   that it wouldn't be trivial to monitor fuel levels in tanks, especially in   
   zero g, or with cyro fuel.   
      
   What are the techniques used to monitor fuel levels? I can think of several   
   possibilities:   
      
   - Float gauge(i.e. car gas tank method): Not going to work in zero g at all,   
   various g effects causing bad readings   
      
   - Mass sampling via load cells: Possible, but rather hard do to tanks   
   usually being structural in most rockets   
      
   - Ultrasonic methods: maybe   
      
   - Pressure: Knowing the size of the tank, how much stuff is in there, what   
   the pressurant is, the temperatures and bunch of other variables, estimate   
   the propellant quantity   
      
   - Counting: Knowing how much fuel you put in, and how much may have boiled   
   off, count how much fuel leaves the tank, and subtract to find the quantity   
   remaining. Maybe problems with this due to inaccuracy in flow gauges??????   
      
   - Mass of vehicle: Monitoring mass of vehicle by processing responses to   
   control input to find out the inertia???? maybe   
      
   How far off am I? What methods are really used? I guess you'd probably use   
   several different methods together to come to a reasonably accurate value.   
   Thanks,   
      
   David   
      
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