HOW DO you measure the amount of fuel a satellite or space probe has left?
Earthbound methods are non-starters in zero gravity. Now engineers at satellite
maker Lockheed Martin in California and at Purdue University in Indiana have
found a way. Knowing exactly what you鈥檝e got left will extend a craft鈥檚 useful
life, they say, saving satellite operators millions of dollars.
Their trick is to use a computer program that models the surface tension of
hydrazine fuel. This should give them a better idea of how hydrazine behaves
inside the satellite鈥檚 fuel tank. 鈥淚n automobiles, we have a float that鈥檚
attached to the fuel gauge. But floats don鈥檛 work up there,鈥� says Steven
Collicott, an aeronautical engineer at Purdue.
Engineers currently have two ways of figuring out how much fuel is left in a
satellite. Either estimate the amount by keeping track of how long the
satellite鈥檚 thrusters have fired, or else assume the fuel is an ideal gas in a
perfect sphere, and use the size of the fuel tank and its temperature to
calculate the amount used over time.
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The new surface-tension technique takes advantage of free software called
Surface Evolver, written by Ken Brakke, a mathematician at Susquehanna
University in Pennsylvania. Surface Evolver uses the exact shape of the fuel in
the satellite to make very accurate estimates of the amount of fuel left.
Satellite fuel tanks are very different from their earthbound cousins. They
contain vanes to channel the liquid fuel using surface tension, and absorbent
material to wick the fuel away. By feeding temperature readings from inside and
around the fuel tank into Surface Evolver, engineers can estimate what shape the
remaining fuel is adopting, and then calculate the amount left. Space scientists
at Lockheed Martin have already begun using the new method, says Collicott.