THE threat to spacecraft from fast-moving space junk could be lessened thanks to a new way to measure how much fuel satellites have left in their tanks.
Satellite operators need to know when a craft is running low on fuel so they can use the last of its propellant to thrust it into a high-altitude orbit out of the way of other spacecraft. Accuracy is everything though: ditch a satellite too soon and a multimillion-dollar resource is wasted early; leave it too late and you create hazardous space junk.
At present, the fuel level is calculated either by estimating how much is used on each thruster burn, or using a sensor to measure gas pressure in the tank. Neither method is accurate, particularly when fuel runs low.
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Now aerospace engineers Amit Lal and B. N. Raghunandan of the Indian Institute of Science in Bangalore have solved the problem by designing a fuel tank with a titanium alloy cone inside it (see Diagram).
In microgravity, the greatest forces come from surface tension, and the fuel tends to coalesce into a large mass, says Lal. As the fuel runs low “it eventually migrates towards the converging end of the cone to minimise its surface area”, he says. Meanwhile, fuel outside the cone moves towards the base of the tank for the same reason.
The researchers found the fuel inside and outside the cone eventually reached equilibrium, with the meniscus heights of both remaining constant relative to each other. So by bouncing an ultrasound beam off the fuel in the cone and measuring its return time, Lal believes they will be able to accurately calculate the fuel left in the tank (Journal of Spacecraft and Rockets, vol 44, p 143).
Todd Barber of NASA’s Jet Propulsion Laboratory in Pasadena, California, says the idea could also help planetary probes. “For a planetary mission such as Cassini, measuring the remaining propellant more accurately would enable better decisions to be made about potentially extending the mission.”
