Members of the Atmospheric Science Group have developed a new method to gauge the propellant content in the tanks of modern spacecrafts equipped with electric propulsion (EP) thrusters. It is called Improved Pressure-Volume-Temperature (PVT) method, and it allows for retrieving the amount of propellant throughout the whole operational lifetime of the spacecrafts with an accuracy 8 times higher than with the usually applied methods, which show increasing inaccuracy towards the end of the nominal missions, when the amount of propellant in the tank is scarce. The work, supported by experimental confirmation in laboratory, has been published in the International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering (A Xenon Mass Gauging through Heat Transfer Modeling for Electric Propulsion Thrusters).
The method is based on universal Physics and Thermodynamics principles according to the Redlich-Kwong equation of state, an empirical, algebraic equation that relates temperature, pressure and volume of gases. From this expression, the authors have developed an analytical formula which permits to obtain a reliable description of the state of the propellant and its density at each pressure within the tank, that is to say, in every moment along the whole life of the mission. The only inputs it requires are temperature and pressure measurements taken by means of sensors attached externally to the tank and, unlike the commonly used methods, Improved PVT is quite less sensitive to pressure measurements errors caused by sensors drift, which become more and more critical as the tanks empty. The accuracy with which it is possible to determine the remaining amount of propellant in every moment is a determinant factor when establishing the operative period of a given spacecraft, and hence, the Improve PVT method offers the possibility to extend it by allowing to confidently retrieve the propellant mass left in the tank even when it is scant.
Furthermore, this method provides a reliable depiction of the propellant density flow in any situation, which means that it can be used to improve the efficiency of the propulsion and the management of the required specific impulses in attitude control and station keeping manoeuvres.
The method has been validated in laboratory by using a specifically built chamber filled with CO2, but it is applicable to Xenon (which is the most used propellant for the EP thrusters) and to different other gases as well used as propellants, such as krypton (Kr), argon (Ar), helium (He) and nitrogen (N2), and it is scalable, so that it can be easily adapted to any tank size. Nevertheless, further tests in microgravity conditions and under accelerations mimicking the spacecraft movements should be performed to refine the model, which undoubtedly will mean a valuable tool for the development of the EP thrust.