New spacecraft fuel gauges could hold satellites operating for longer

As a spacecraft flies, it uses about 75-90% of the propellant to reach orbit. The residual fraction decides how long it will last up there, so it is no easy feat in zero gravity to calculate the amount of fuel that remains in the tank. Research teams at the United States National Institute of Standards and Technology now have a solution based on a sensor suite that measures the liquid’s capacitance within the fuel tank of a spacecraft and uses this data to recreate a three-dimensional map of the remaining fuel. The prototype model could allow satellites to work for longer, according to the team, while also helping to deter harmful end-of-life collisions.

Liquid propellants bind to the interior of fuel tank walls under zero-gravity environments due to surface friction as well as capillary effects. This volatile spatial distribution makes it impossible to determine fuel levels. Propellants are also free, none of which exists on Earth, to slosh around, float, and form bubbles. 

Several methods for calculating onboard spacecraft propellants have been developed. One of the most popular, known as the methodology of bookkeeping, includes calculating how much is consumed with each thrust as well as subtracting this from the amount of fuel remaining in the tank. At the outset of a task, but although this methodology is extremely precise, each calculation error carries over to another. It accrues with each thrust, explains team member Nick Dagalakis, a mechanical engineer. “The calculations become more like approximate guesses by the period a tank is close to zero and can miss the point by as much as 10 percent,” he says.

Satellite operators are in a dilemma, Dagalakis adds, without accurate fuel calculations. It is a waste of resources to delete a satellite while it still has a lot of fuel remaining, but leaving the tank to run empty could leave the satellite stuck, with no remaining fuel to avoid other craft or travel to a secure orbit. NASA technology transition chief Manohar Deshpande has developed a new fuel gauge based on a 3D imaging technology known as electrical capacitance volume tomography (ECVT). In general, tomography is a means of analyzing an object’s internal structure without destroying it; familiar examples comprise positron emission tomography (PET), magnetic resonance imaging (MRI), and routine hospital-based X-ray tomography.

ECVT is a more modern version that uses a variety of electromagnetic wave-emitting sensors. The other sensors in the array will sense these waves, and how far they are distributed relies on the capacitance of whatever lies between them. If there’s nothing, the transmission is going to be big. However, if an object is available, the transmission will decrease as some electromagnetic waves will be absorbed by the object. Therefore by positioning these sensors around a container and measuring the signal at several places, a 3D image of the container’s objects can be generated.