# Effect of Structural Materials on Monopropellant Thruster Propulsion Performance in Micro Scale
## 1 Introduction  
One of the most innovative aspects of today's space technology has stemmed from micro-electro-mechanical system (MEMS) fabrication technology, which has contributed to the miniaturization of spacecraft, once considered very costly due to their weight and the expenses associated with launch vehicles. MEMS technology has expanded the horizon of miniature satellites in space missions, including contemporary deep space missions involving nanosatellites [1, 2, 3, 4, 5, 6], which weigh less than 10 kg. As satellites become smaller, propulsion subsystems should also be downsized accordingly, for attitude control, orbit maintenance, and orbit transfer of miniaturized satellites. In fact, a micro-propulsion system is a prerequisite for the successful operation of downsized satellite systems on various space missions, particularly in terms of affecting compatibility with multiple satellites on constellation operations, as well as in regard to improving revisit time (i.e., time taken between observations). There are various types of space propulsion, which are normally classified as either chemical or electric. In chemical propulsion, there are the following propellant types: monopropellant, solid propellant, hybrid propellant, and bipropellant. Electric propulsion types include electrothermal, electromagnetic, and electrostatic propulsion. While electric propulsion relies on an electric energy source to produce thrust by accelerating charged particles of ionized gas or conductive liquids using an electrostatic field, electromagneticfield, or thermal energy converted from the electric energy, most chemical propulsion utilizes chemical energy stored in a propellant as its main energy source, and only a limited amount of electric energy is required for its operation. Thus, for a miniature satellite without a sufficient electric energy source, chemical propulsion represents a good option in terms of securing attitude control and orbit maneuvering capabilities. In order to engender compatibility with nanosatellite systems such as CubeSat and PocketQube, the micro-propulsion system configuration is expected to be systematically simple, with appropriate functions for thrust generation, such as reignition and throating ability. Monopropellant-type thrusters require a propellant to be decomposed via a catalyst to enable the conversion of chemical energy, stored in a propellant, to thermal energy, and generate thrust by accelerating a high-temperature and -pressure decomposed gas product through a nozzle. Such thrusters have reignition and throttling capabilities, with a simple system configuration consuming only one propellant, and thus it is believed they are among the most optimal systems to downsize into the micro scale and make compatible with miniaturized satellites. Efforts have been made to miniaturize monopropellant thrusters in various manufacturing processes, particularly the MEMS process, which involves different material substrates for thruster fabrication. Those materials are required to endure high-temperature and -pressure environments in the thruster chamber, and they have mainly been silicon, glass, and ceramics, different from some materials compatible with other microfluidic devices such as micropumps [7,8] based on polymers. Numerous studies [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] have proved that several materials utilized in other fabrication processes present no issues in terms of meeting the need for chemical robustness without any potential adverse reactions with propellants. However, these materials inevitably possessed different performance characteristics in terms of thermal energy loss.