Comparison of propulsion methods for regolith thruster

In recent years, manned/unmanned missions are under the spotlight for the lunar science and exploration among the space agencies, the private companies and the academic institutes. In addition, a permanent base for the astronauts are planned to be built in lunar orbit as well as the surface of the Moon. As a result, there are several targets for the utilization of the lunar resources such as the soil-like material called the lunar regolith. Due to the irregular nature of the lunar gravity field, spacecraft navigating along the lunar orbit require a propulsion system to provide velocity increments to maintain the orbit altitude. Therefore, utilization of the lunar resources with electric propulsion technologies could significantly increase the capabilities of small satellites in lunar orbit. In this study, novel regolith thrusters are investigated as electric propulsion devices, which could use dust grains from the lunar surface. So far, three types of regolith thruster have been considered: "breathing-type," "storage-type," and "vaporization-type". In this presentation, the results of our comparative study of the three propulsion methods are discussed, especially in terms of the significant advantage of the vaporization type. Satellite velocity increment $\Delta$v, rather than thrust, is more significant with the thrusters that are used in the lunar orbit. $\Delta$v was assumed as 40 m/s, 80 m/s, 120 m/s, and a dry weight of 6U satellite as 12 kg. The breathing-type thruster accelerates the dust particles floating in the lunar orbit by passing them through a grid that is energized. The result of calculations indicates that the acceleration grid having an area of 1 m$ ^{2}$ is required to circle the lunar orbit at least 10$ ^{8}$ times for the breathing-type thruster to reach $\Delta$v of 40 m/s. Therefore, the grid is required to increase in size to collect a sufficient number of the floating particles from the lunar dust exosphere. In the storage-type thruster, the regolith is stored in the propulsion device, and the particle deposition layer is irradiated with an electron beam to charge particles. In addition, the electrostatically charged particles are accelerated and jetted by a grid. Ejection of the dust grains from the container is initialized by the secondary electron emission within the micro-cavities between the neighboring dust grains. On the other hand, vaporization-type thruster is under the study most recently. The thruster heats and vaporizes the regolith dust to obtain thrust by supersonic nozzle, and this type solves the problems that low exhaust velocity and grid potential as the dominant parameter of the breathing and storage types. The thermal conductivity of regolith is considerably low as 10$ ^{-2}$ W/mK. Whether a heat transfer wire is used, 76.4 W/cm$ ^{2}$ of power will be necessary under a vacuum condition of 10$ ^{-3}$ Pa. Storage-type requires an optimized specific impulse of 10$ ^{4}$$\sim$10$ ^{5}$ seconds, 10$ ^{-9}$$\sim$10$ ^{-10}$N thrust at assumed $\Delta$v of 40, 80, 120 m/s and a dry mass of 12 kg 6U satellite; however, the loaded dusts takes 1.07 times as much as the particle storage at $\Delta$v of 40m/s. This type can obtain minimum 10$ ^{-9}$ N. Breathing-type requires an oversized grid area in order to obtain a sufficient thrust. If the dust weight, which could be described as the propellant weight, is defined as total weight of regolith dust passing through grid at an altitude of 50km at a given time, the requirements can be listed as 10$ ^{9}$ m$ ^{2}$ grid area and an operation time of 10$ ^{6}$ s. For the required thrust, the grid area requirement is approximately 140 m$ ^{2}$. The vaporization-type can obtain the highest specific impulse of 10$ ^{5}$ s for a specific case of parameters while oxygen is emitted from the regolith particles during the heating. Therefore, the exhaust velocity is estimated as 10$ ^{2}$ km/s, and the thrust is 10$ ^{-7}$ N. In addition, the nozzle opening ratio is 4.23 whereas the gas store chamber pressure is 10$ ^{-5}$ Pa. As a result, the best specific impulse is determined as the vaporization-type thruster.