Yuichi Nakagawa, Pale Blue Inc.
Akira Iwakawa, Pale Blue Inc.
Kazuya Yaginuma, Pale Blue Inc.
Yuichi Nakagawa, Director, Pale Blue Inc.
Hall Effect thruster is one of the most popular electric propulsion systems, and it is commonly driven by xenon or other noble gases. It has more than 1000 s of specific impulse, and a relatively high thrust-to-power ratio, typically more than 50 mN/kW. Since these performances fit the actual application very well, it is widely used for orbit raising and station keeping of conventional satellites and expands its coverage to small satellites.
These high-pressure gas systems have disadvantages for applying them to small satellites given miniaturization and cost. Solid structures should be required to keep the high pressure, and it leads to a low mass-to-structure ratio when miniaturizing thruster systems. Also, it costs a lot to fill the propellant on the launch site. That’s why a lot of research on alternative propellants is being conducted. For example, using iodine as a propellant is one of the solutions, and some research was conducted. It can be operated and its performances, which indicate its thrust and specific impulse, were comparable to the xenon one, but have not been operated in orbit yet due to the problem of a lifetime.
Water is one of the promising candidates for alternative propellant because of its safety, affordability, and ease of handling. In general, the lower atom or molecule size of the propellant is a disadvantage when using it as a propellant for electric propulsion because its high sonic speed leads to faster escape of the propellant and lower mass utilization efficiency. In addition, water has a larger latent heat compared with other alternatives, and it is also a disadvantage as a propellant. However, since the satellite size got smaller and the importance of cost got relativity higher, the water as a propellant has been focused on corresponding with the small satellites arising.
Many types of thrusters were developed and tested, such as pulsed plasma thruster, resistojet thruster, and gridded ion thruster. Resistjet thrusters which have a simple principle for the operation compared with the other thruster systems, were already used practically, and some cases of the operation on orbit existed.
On the other hand, the oxidization and corrosion of the thruster when using water are problems to be solved for applying it. Most electric propulsion systems use plasma and its thermal, electrostatic, and electromagnetic force to generate the thrust, and electrodes are commonly used for generating plasma. However, when electrodes are used with molecules that include oxygen, the degradation of the electrode caused by oxygen can be a problem. Also, atomic oxygens included in the plasma attack the surface of the plasma source and affect its performance and lifetime. These are examples of the reasons why water is not commonly used as a propellant for electric propulsion, especially using plasma.
Pale Blue has developed a water-gridded ion thruster with a technique of electrodeless plasma source and successfully operated on the ground for more than 1000 hours. It has not been proven only the possibility of using water plasma practically, but also enough technique for controlling the electronics for operating the thruster to complete the thruster as a system. The thruster system has high specific impulse, such as 1700 s, and low power consumption, such as 60 W, as a gridded ion thruster. The system has already passed all the qualification tests and is ready for the launch.
Based on the technique and know-how, Pale Blue is also developing the 500 W-class water Hall Effect thruster. The whole system of the thruster was composed of a power processing unit, mass flow control unit, thruster unit, and system control unit. The power processing unit should be operated with oscillation of plasma resistance due to discharge oscillation. It is being developed based on the power source of a gridded ion thruster, which can deal with high-voltage sparks. The mass flow control unit feeds the water to the thruster with a certain constant mass flow, and it was combined with the high mass flow control system used in the resistojet thruster and the precise mass flow control system used in the gridded ion thruster. The thruster unit has been developed with several universities, and the performance measurement and its improvement are ongoing activities. Some ground tests on the thruster and the system were already conducted. In this presentation, we will present these results and the development status of the thruster.