THEORETICAL VARIATIONS IN SPT PERFORMANCES WITH THE USE OF KRYPTON AS PROPELLANT

摘要

The use of Hall Effect Thrusters in space operations is becoming more frequent as time passes, specially the Stationary Plasma Thruster class (SPT). The success of SMART-1 demonstrates their usefulness as primary propulsion system. Xenon is the propellant habitually used in SPT's because of its good cualities, such as low ionization potential and easiness of storage. However, it has some disadvantages: it is very expensive and the world annual production is only around 20 t. Therefore, it is interesting to search some alternative gas, able to provide similar performances, but cheaper and easier to obtain.rnIn the last years, experimental tests have been made with Krypton and Krypton/Xenon mixtures, in order to study their suitability as propellants for SPT's and the variation that generates in the thruster's performances. Kr is expected to provide higher specific impulse (Isp) and thrust (T) than Xe, due to its lower atomic weight; but it can also vary plasma parameters along the channel and some efficiencies, such as ionization or thrust efficiency. Some tests have shown that Isp and T do not increase as much as it was expected, and also there is a decrease in thruster efficiency.rnThis paper aims to apply a theoretical model in order to study the relations between the use of Kr as propellant and several thruster parameters (like those mentioned above: T, Isp), and also performance parameters like discharge voltage, discharge current and mass flow rate. A theoretical model will be employed to make a prediction of different parameters when using Kr, and comparison will be made between these predictions and experimental data. When both data disagree, different physical processes are taken into account in order to explain it (which can lead to a better comprehension of the phenomena involved). An estimation of the losses in the channel walls will be made, getting an idea of its relative importance in propellant utilization and propulsive efficiency. Aditional attention will be paid to the variation of ionization efficiency and propulsive efficiency when using Kr, and how they are related to losses in the global thrust efficiency.rnAt last, a look will be taken at variation on performance parameters needed to maintain good thrust efficiency, and the predictible consecuences it may have (erosion, thermal problems).