Space isn’t the place to run out of fuel

Keeping an eye on your fuel levels in space can be tricky – and that can be costly.

Let a satellite’s tank run dry and you leave it stranded in its original orbit with no fuel to avoid smashing into things. Hold back more than you need, and you risk wasting quite a bit of money by retiring a satellite with fuel still on board.

The problem, of course, is that microgravity inside a spacecraft allows liquid to freely slosh about.

A common way to keep track of it is to estimate how much fuel is being burned with each thrust and subtract that amount from the volume of fuel in the tank.

However, engineers at the National Institute of Standards and Technology (NIST) in the US say that while this is quite accurate when a tank is close to full, estimates become more like rough guesses and eventually can miss the mark by as much as 10%. The error of each estimate carries on to the next.

The answer could lie in their new idea for fuel gauge, described in a paper in the Journal of Spacecraft and Rockets, which can digitally recreate a fluid’s 3D shape based on its electrical properties.

The concept, originally devised by NASA’s Manohar Deshpande, makes use of a low-cost imaging technique known as electrical capacitance volume tomography (ECVT).

Like a CT scanner, ECVT can approximate an object’s shape by taking measurements at different angles. But instead of shooting X-rays, electrodes emit electric fields and measure the object’s ability to store electric charge, or capacitance.

Deshpande then worked with Nick Dagalakis and colleagues in the NanoFab clean room at NIST’s Centre for Nanoscale Science and Technology to develop a prototype.

They first produced sensor electrodes using a process called soft lithography, in which they printed patterns of ink over copper sheets with a flexible plastic backing. Then, a corrosive chemical carved out the exposed copper, leaving behind the desired strips of metal.

Next they lined the interior of an egg-shaped container, modelled after one of NASA’s fuel tanks, with the flexible sensors. Throughout the inside of the tank, electric fields emitted by each sensor could be received by the others, but how much of these fields ended up being transmitted depended on the capacitance of whatever material was inside the tank.

“If you have no fuel, you have the highest transmission, and if you have fuel, you’re going to have a lower reading, because the fuel absorbs the electromagnetic wave,” Dagalakis says.

“We measure the difference in transmission for every possible sensor pair, and by combining all these measurements, you can know where there is and isn’t fuel and create a 3D image.”

It’s still early, Dagalakis says, but it’s “a good starting point” – and the ECVT system could help overcome other challenges in space.

“The technology could be used to continuously monitor fluid flow in the many pipes aboard the International Space Station and to study how the small forces of sloshing fluids can alter the trajectory of spacecraft and satellites,” Deshpande adds.