09/11/2017

Second chance for PVT-GAMERS experiment


The improved Pressure-Volume-Temperature Gaugin Method for Electric pRopulsion System (PVT-GAMERS) experiment has been short listed for its inclusion in the 2018 Fly Your Thesis! Campaign. This is the second attempt to get a post in the European Space Agency’s parabolic flights programme, after having been discarded in the final selection last year. On this occasion, the maturity acquired by the experiment poses a new hope of being among those eventually chosen to be carried out in the ongoing edition of the programme.

Sketch of the PVT-GAMERS experiment proposed for
Fly Your Thesis programme.
Credit: Atmospheric Science Group.

PVT-GAMERS is an experiment related to the Improved Xenon Gauging Method for spacecraft, a project that is being carried out by Atmospheric Science Group to optimize the propellant payload in modern spacecraft equipped with Electric Propulsion (EP) thrusters. The xenon payload in spacecrafts equipped with Electric Propulsion (EP) thrusters for their attitude control and orbital transfer manoeuvres has been raising over the years, from the initial 200-350 kg to the 800-1500 kg modern spacecrafts carry. This increase has overwhelmed the capacity of the existing gauging methods for measuring the propellant content in the tanks in terms of mass and density with the appropriate accuracy.
Currently there are three main methods used for the calculation of the propellant content in a spacecraft tank. These methods are the Bookkepping method, the Pressure-Volume-Temperature (PVT) method and the Thermal Gaugin method, and all of them based on the ideal gas equation, which can poorly describe the fluid properties at supercritical state of a gas, so neither the mass nor the density can be correctly retrieved. A combination of them is applied throughout the spacecrafts operational life to assess the propellant content in different moments, but not of them alone is suitable for the whole mission and, in any case, they entail the accumulation of errors in the measurements, so the assessment of the amount of xenon is more and more inaccurate as the mission progresses. The problem becomes critical at the end of the operational lifetime of the spacecraft, making very difficult to know accurately what is the remaining amount of propellant in the tanks at this stage, and hence to determine reliably how much time of operation is left.
As can be guessed, this constraint means that the commercial or scientific exploitation period of spacecrafts is underrated nowadays and, therefore, there is a pressing need to count on a new method that makes it possible to know in every moment the remaining amount of propellant in the tanks, especially at the end of the spacecrafts’ scheduled missions, when the xenon is scant and the accurate calculation of the remnant can lead to the extension of the operative period and hence to the increase of the profitability or the scientific output of the mission. PVT-GAMERS is meant to provide a new method to estimate accurately the propellant content in the tanks in any given moment throughout a satellite lifespan.
The new concept, which is a hybrid approach combining the PVT and the Thermal Gaugin methods, is based on the Redlich-Kwong equation of state, an empirical, algebraic equation that relates temperature, pressure and volume of gases. From it, the team has obtained an extra equation that is non-sensitive to pressure and temperature systematic errors, providing a reliable description of the gas fluid behaviour in many situations.

Álvaro Soria Salinas during the presentation of PVT-GAMERS method
at IAC in Australia.

The method has raised the interest of the space community, as it appears from the fact that a lecture describing it was invited to be presented at the 68th International Astronautical Congress organized by the International Astronautical Federation, which was held at Adelaide, Australia. Álvaro Soria Salinas was on charge of the presentation as co-author of the work together with prof. Javier Martín Torres and prof. María Paz Zorzano.
With regard to PVT-GAMERS experiment, it has been conceived to validate the operational limits of the method in different gravitational scenarios which are representative of the on-orbit conditions. Fly Your Thesis! Programme offers an ideal opportunity to validate the method in an environment which is representative of that in which it will be eventually applied, namely, the microgravity situation in which a satellite operates. Furthermore, apart from the microgravity the parabolic flights reproduce, there are periods under hypergravity conditions, which can be assimilate to those caused by the attitude control and orbital transfer manoeuvres, completing this way the different factors that should be taking into account to improve the measurement of the propellant content.
As previously mentioned, this is the second chance to participate in the parabolic flight campaign, so the team has had the opportunity to fix the problems which marked its exclusion last year. The were related to some security concerns with regard to the heating of the gas in the pressurised containers of the experiment during the flight. This time this process has been eliminated avoiding any derived risk, so a different outcome of the final selection is expected.
The defence of the experiment will take place in November 2nd-3rd at ESA’s European Space Research and Technology Centre (ESTEC), and the 4 teams selected will be announced the 10th.