Under zero-gravity conditions, liquid propellants adhere to the inside of fuel tank walls due to surface tension and capillary effects. This unpredictable spatial distribution makes fuel levels hard to determine. Propellants are also free to slosh about, float and form bubbles – none of which happens on Earth.
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“The flexible and inexpensive design and fabrication of these sensors may allow several of them to be used in a spacecraft or satellite fuel tank working independently to create a composite image of the total fuel volume,”
