1. INTRODUCTION

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1.1 Research motivation

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Various methods are employed to explore extraterrestrial bodies, including Mars, dwarf planets, and asteroids. Among these, the use of rovers is the most prevalent. For example, on Mars, where the surface is predominantly rocky, rovers are a practical and efficient exploration tool. In 2021, however, NASA introduced a new approach by deploying the drone Ingenuity to explore Mars. Drones offer the advantage of covering much larger areas than rovers and can access terrains that are inaccessible to rovers. Conventional drones, however, generate lift by pushing air downward using devices like propellers, which makes them impractical for exploring celestial bodies with little to no atmosphere.

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Interestingly, many dwarf planets, such as Pluto, and moons, such as Europa, have crusts composed of ice. This observation suggests that a drone, or "hopper," capable of using surface ice as propellant could significantly expand the exploration range beyond what rovers or stationary probes can achieve. Therefore, the goal of this study is to develop the Autonomous Propellant Harvesting Hopper (APHH), which utilizes an innovative propulsion system that processes surface ice as propellant.

 

1.2 Research summary

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In this study, the Autonomous Propellant Harvesting Hopper (APHH), which utilizes solid carbon dioxide (dry ice) as a propellant, was developed. Since the environmental conditions of dwarf planets and asteroids differ significantly from those on Earth, it is challenging to fully simulate these extraterrestrial environments on Earth. Consequently, the APHH was designed with Earth's physical conditions in mind.

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The APHH consists of two main systems: a harvesting system and a propulsion system. The harvesting system includes a grinding mechanism and a robotic arm for collecting and transferring ice. To minimize energy consumption, the grinder employs a claw-shaped shovel actuated by a single servo motor to grind the surface and collect fragments of ice. The collected fragments are then lifted 70 cm using a servo-actuated robotic arm and conveyed into a liquefaction tank.

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The propulsion system operates in three steps: (1) liquefaction of solid carbon dioxide, (2) heating the liquefied carbon dioxide, and (3) expelling the supercritical carbon dioxide through thrusters. Initially, the crushed dry ice enters the 4.5-liter liquefaction tank, constructed from lightweight aluminum 6061 alloy to withstand pressures up to 180 bar. Upon entering, the dry ice begins to vaporize as carbon dioxide cannot remain solid at room temperature. When the pressure exceeds 5 bar, the dry ice transitions to a liquid state. Once liquefied, the carbon dioxide is filtered through activated carbon pellets to remove impurities. The purified liquid carbon dioxide is then transferred to a heating tank, where it is heated to over 40°C, generating a pressure of 100 bar and converting it into a supercritical state. A pressure regulator reduces the pressure to 20 bar before the supercritical carbon dioxide is expelled through cold gas thrusters, each generating 50 N of thrust.

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Testing of the APHH was conducted on a surface densely covered with dry ice to simulate the conditions of an ice-covered planetary surface. The dry ice collection system successfully delivered a sufficient quantity of material to the liquefaction tank, which effectively liquefied the dry ice and filtered out impurities. However, due to cold weather conditions during testing, the generation of supercritical carbon dioxide was unsuccessful, and sufficient thrust for lifting the APHH was not achieved.

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Future research will focus on enhancing the APHH's performance. Flight duration will be tripled by reducing the weight of the hardware components. Additionally, four adjustable valves will be incorporated to independently control the thrust of each thruster, addressing safety limitations encountered in this study. The current APHH has a short flight duration due to being optimized for Earth's gravity. However, in environments with lower gravity, such as the polar regions of Mars, the APHH is projected to achieve a flight time of 50 seconds. On Europa, where gravity is even lower, the flight time could extend to approximately three minutes. These advancements could significantly expand the potential for extraterrestrial exploration.