Liquids aren't as well behaved in space as they are on Earth. Inside a spacecraft, microgravity allows liquids to freely slosh and float about. This behavior has made fuel quantity in satellites difficult to pin down but a new prototype fuel gauge could offer a solution. The gauge can digitally recreate a fluid's 3D shape based on its electrical properties. The design could potentially provide satellite operators with reliable measurements that would help prevent satellites from colliding and keep them operational for longer.
Letting a satellite's tank run dry could leave it stranded in its original orbit with no fuel to avoid smashing into other satellites and producing dangerous debris clouds. To reduce the probability of collision, operators save the last few drops of fuel to eject satellites into a graveyard orbit hundreds of kilometers away from functioning spacecraft. They may be wasting fuel in the process, however.
For decades, gauging fuel in space has not been an exact science. One of the most frequently relied upon methods entails estimating how much fuel is being burned with each thrust and subtracting that amount from the volume of fuel in the tank. This method is quite accurate at the start when a tank is close to full but the error of each estimate carries on to the next, compounding with every thrust. By the time a tank is low, the estimates become more like rough guesses and can miss the mark by as much as 10%. Without reliable measurements, operators may be sending satellites with fuel still in the tank into an early retirement, potentially leaving a considerable amount of money on the table.
The concept of the new gauge makes use of a low-cost 3D 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.
Researchers 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. The team lined the interior of an egg-shaped container modeled after one of NASA's fuel tanks with the flexible sensors. Throughout the inside of the tank, electric fields emitted by each sensor can be received by the others. But how much of these fields end up being transmitted depends on the capacitance of whatever material is inside the tank.
To test out what the new system's fuel gauging capabilities might look like in space, the researchers suspended a fluid-filled balloon in the tank, mimicking a liquid blob in microgravity.
Many liquids commonly used to propel satellites and spacecraft, such as liquid hydrogen and hydrazine, are highly flammable in Earth's oxygen-rich atmosphere, so the researchers opted to test something more stable. They filled the balloons with a heat transfer fluid — normally used for storing or dissipating thermal energy in industrial processes — because it closely mimicked the electrical properties of space fuel.
The researchers activated the system and fed the capacitance data to a computer that produced a series of 2D images mapping the location of fluid throughout the length of the tank. When compiled, the images gave rise to a 3D rendition of the balloon with a diameter that was less than 6% different than the actual balloon's diameter.