SAWAI SHUJIRO

In this chapter, the development procedure and the operation of Hayabusa2 bipropellant chemical propulsion system are described in detail. Lessons learned from the original Hayabusa (sample return mission from the asteroid Itokawa) and Akatsuki (Venus climate orbiter mission) are described, and the countermeasures incorporated into the design of Hayabusa2 chemical propulsion system are detailed. On-orbit data is used to assess the impulse provided by each thruster. In particular, the cause of thrust variability under pulsed operation is investigated, and approaches to ensure stable and reliable thruster operation are proposed. Furthermore, the relation between thrust duration and generated impulse is modeled to enable reliable reproduction of the required ΔV.

In this paper, we introduce the 500N class bipropellant ceramic thruster for SLIM (Smart Lander for Investigating Moon). It has three main features. The first is a silicon nitride ceramic chamber. It is the only one used by MHI. Second, it has a wide operating range and provides stable performance. It can perform blowdown operation without requiring a high-pressure gas tank. The third activity is Pulse firing which is considered difficult in 500N class thrusters, is possible.

SLIM (Smart Lander for Investigating Moon) is JAXA’s new lunar lander mission under development. The system firing test was conducted to acquire experimental data of transient behavior of propellant pressure in order to verify the requirement for the propulsion system design and the simulation model, using real propellants and the flight like model of SLIM propulsion system. We observed three remarkable phenomena, which are (1) wide frequency distribution in pressure fluctuation, (2) water hammer when the main engine’s valves are closed, and (3) cross talking. From the evaluation of test results and the combination with simulation, we understood the physical phenomena with real propellants and the flight like model. As a result, we concluded that the current design of SLIM propulsion system is compatible to all the requirements and the expected operational mode of SLIM.

This study experimentally investigated the cause and the scattering mechanism of the phenomenon in which a celestial surface object scattered by a thruster injection has a spacecraft direction component. The phenomenon was first observed by Hayabusa2 touchdown. One of the mechanisms by which objects on the surface of the asteroid are scattered toward the spacecraft is the making crater by the thruster injection. We investigated the trends in the direction and amount of scattering of surface objects by injecting thrusters into a sandbox created in a vacuum chamber.

During the touchdowns of the "Hayabusa2" spacecraft on the asteroid Ryugu, some of the ejected surface materials were scattered toward the spacecraft. Apart from impact sampling by firing a metallic projectile, another mechanism of this phenomenon is surface material ejecta from formation of a crater by RCS thrusts of the spacecraft during its ascent from the asteroid surface after the touchdown. We conducted ground experiments of gas injection into simulated soil under vacuum in order to investigate the interaction between thruster plume and asteroidal regolith.