The improved Pressure-Volume-Temperature Gauging Method for Electric Propulsion Systems (PVT-GAMERS) has been one of the two selected projects to be included in the ESA’s Fly Your Thesis! 2018 campaign, which will take place in Autumn 2018. The PVT-GAMERS is a technological demonstrator proposal, aimed at validating a new methodology by operating it in a representative environment, in this case a zero-g and a modified-g scenario. In the future, this method may be used in propellant tanks of any electric propulsion spacecraft to determine, in the absence of gravity, the amount of remaining propellant during the full duration of its mission.
In April 2017, ESA Education released a call to the student community announcing the opportunity to compete for a chance to fly on Novespace’s Airbus A-310 Zero-G which provides a total of 30 minutes of micro-gravity in bouts of 20 seconds. In September, a shortlist of the top 6 teams was made by ESA Education based on several criteria and finally the PVT-GAMERS team, from LTU-Atmospheric Science Group, was selected together with the G-STANDING proposal, from the Erasmus Medical Centre in Rotterdam, the Netherlands.
PVT-GAMERS has been developed within the Atmospheric Science Group as a new method to achieve more accurate assessments of the remaining amount of propellant in the tanks of spacecrafts equipped with electric propulsion thrusters for their attitude control and orbital transfer manoeuvres. It stems from a research line of the group and has been adapted to the requirements of this ESA Educational program by a team of LTU students together with their endorsing professor, María-Paz Zorzano. The Fly Your Thesis! (FYT!) programme gives master and PhD candidates, the opportunity to fly their scientific experiment or technological research in microgravity conditions. The programme launches a call for proposals once a year and proposals for experiments are tailored for the parabolic flight configuration and requirements .Now, after acceptance into the program, the LTU team shall prepare an adapted experiment that will be tested on ground and finally it will be operated during the zero-G flight.
In the current Electric Propulsion era, one of the most relevant propellants is xenon, which is generally stored in supercritical stage. Because of the increase in time of spacecraft lifetime, the amount of propellant stored on-board has been quadrupled in the recent years, and the need of more accurate gauging methods for measuring propellant usage along the missions has become more critical too. Thermal gradients affect the densities distribution of the stored propellants and this turns out to be critical in orbit because of the absence of convection in low-gravity environments.
Recently we have proposed a new gauging method (A Xenon Mass Gauging through Heat Transfer Modeling for Electric Propulsion Thrusters) that relies on the analysis of measurements from existing and operating technology (in TRL 9), i.e., this method does not imply the development of any new technology. This new method, the improved PVT method, improves by a factor 8 the accuracy of the standard PVT retrievals. A laboratory experimental validation has shown that, for CO2 at a pressure of about 70 bar, just below the critical pressure, the error of the mass retrieval using this new gauging method is only 0.1% of the initial mass at launch. However, for its complete validation, a microgravity study should be performed in order to quantify the effect of thermal gradients under the absence of convection in a low-g environment.
The accuracy of the classical methods used for the calculation of the propellant content in spacecraft tanks could be improved by describing the fluid properties at supercritical state of a gas, so that the mass and the density can be better retrieved. This is especially important at the end of the operational lifetime of the spacecraft, when it is very difficult to know accurately what is the remaining amount of propellant in the tanks, and hence to determine reliably how much time of operation is left.
This constraint means that the commercial or scientific exploitation period of spacecrafts is underrated nowadays and, therefore, it would be highly suitable to count on a new method that makes it possible to know in every moment what is the remaining amount of propellant in the tanks, especially at the end of the spacecrafts’ scheduled missions. Then, the accurate calculation of the remnant can lead to the extension of the operative period, and hence to the increase of the profitability from longer commercial operations, or the scientific output of the missions.
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, 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 has been derived, making it possible to know precisely the amount of propellant in every moment along the whole life of a mission.
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.
PVT-GAMERS has already 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, tests in microgravity and representative on-orbit conditions to refine the model were missing, and FYT parabolic flight will provide the chance to do it like any other platform could do, allowing the advance in the validation of the method by raising its Technology Readiness Level (TLR). In this sense, it is noteworthy that, besides the microgravity phases, the parabolic flight offers hypergravity intervals which mimic the accelerations suffered by the tanks during the spacecraft manoeuvres, enabling a complete depiction of the behaviour of the propellant within the tanks also in this common scenario.
This year, the call for experiments in the next campaign was more restrictive, with only two posts available in the scheduled flight, which were awarded among six pre-selected and short-listed proposals after a final selection held in November 2nd-3rd at ESA’s European Space Research and Technology Centre (ESTEC). Álvaro Soria Salinas, (from the Atmospheric Science Group), Riccardo Lucchese, and Erik Nyberg, all of them doctorate students at Luleå University of Technology (LTU), were on charge of the defence of the experiment as representatives of the team. The event consisted of a presentation of the experiment before representatives of ESA, of the European Low Gravity Research Association (ELGRA), and of the flight provider (Novespace company), followed by the correspondent question time.
After selection, the team starts now, together with the proposal endorsing professor, a detailed project management plan to make sure that the initial prototyping and testing activities are launched on time and that all the project goals are met within less than one year.
The first milestone of the progress towards the final performance of the flight, which goes on schedule until now, will be the ESA Gravity-related Training Week, to be held at the ESA Redu Station, Belgium, next January, during which the participants will get more specifications on the schedule of the general FYT! 2018 program and the requirements of the flight, documents, reviews and tests.