[PCG24-P07] Investigation of requirements for regolith thrusters
Keywords:Moon, Regolith, Lunar exploration, Electric propulsion
A problem in lunar-orbit missions is the short orbital life of low-orbit spacecraft due to the moon's irregular gravity field. To sustain the orbit, much propellant is required to be loaded, whereby the mission would be costly. In recent years, electric propulsion devises have been applied to small satellites, which are growing in demand because of their short time required for development and the low risk of mission failure. We here suggest the "Regolith thruster", a new electric propulsion system which collects propellant material during the lunar-orbit flight. The Moon surface is covered with clastic materials derived from the weathered surface rocks called regolith, and its dust particles are of micro-meters in diameter. The dust particles are suspended above the moon surface, as observed by American lunar explorers.
Two types of regolith thruster have been considered. The "breathing type" captures lofted dust particles in orbit and accelerates them directly with a voltage applied grid. Our theoretical studies suggest that a grid of several-tens-of-square-meters class will be necessary to obtain sufficient thrust with this type. The “storage type” thruster uses electron source to generate secondary electrons to release the dust particles by micro-cavity charging. Dust particles are released by electrification in micro cavities (microcavities) formed by each other. This is due to the very small size of the microcavities on the lunar surface, so that the patched surface charge causes a strong repulsion against adjacent dust particles, which moves to the grid and is accelerated. When using a regolith simulant with a particle radius of 45 μm or less, only a small thrust of about 10-9 N can be obtained with the storage type. The volume required for the “particle storage space” of a storage-type regolith propulsion system, assuming that particles of each size fill the storage space, exponentially increases as the particle radius increases. If the particle radius is 5μm or less, each side can be made to fit into a cube of 10×10×10 cm (= 1U size small satellite).
Assuming that the regolith thruster obtains the velocity increment ΔV used in the previous studies, the required propulsion and satellite specifications can be investigated: the weight ratio of propellant and regolith thruster increases as the particle radius increases. This tendency is significant with a large value of ΔV. The propulsion device gains thrust because it shoots mass from the fuselage and propels by reaction. Once the weight ratio required to obtain the ΔV of the launched object (and propellant) and the spacecraft is obtained, the weight ratio will increase as the particle radius increases. The particles are desired to be of 1.0μm or less in radius so that the weight ratio can be easily achieved for a small satellite.
The minimum required number of orbits per month to obtain a specific ΔV is 108, and it has been found that filling the particle storage in orbit takes a considerable amount of time. If a method other than loading regolith dust directly on lunar surface is wanted, it is necessary to consider collecting particles using tethers, etc. While regolith dust is being used in the lunar orbiter, it continues to fill the storage units with new regolith dust.
Two types of regolith thruster have been considered. The "breathing type" captures lofted dust particles in orbit and accelerates them directly with a voltage applied grid. Our theoretical studies suggest that a grid of several-tens-of-square-meters class will be necessary to obtain sufficient thrust with this type. The “storage type” thruster uses electron source to generate secondary electrons to release the dust particles by micro-cavity charging. Dust particles are released by electrification in micro cavities (microcavities) formed by each other. This is due to the very small size of the microcavities on the lunar surface, so that the patched surface charge causes a strong repulsion against adjacent dust particles, which moves to the grid and is accelerated. When using a regolith simulant with a particle radius of 45 μm or less, only a small thrust of about 10-9 N can be obtained with the storage type. The volume required for the “particle storage space” of a storage-type regolith propulsion system, assuming that particles of each size fill the storage space, exponentially increases as the particle radius increases. If the particle radius is 5μm or less, each side can be made to fit into a cube of 10×10×10 cm (= 1U size small satellite).
Assuming that the regolith thruster obtains the velocity increment ΔV used in the previous studies, the required propulsion and satellite specifications can be investigated: the weight ratio of propellant and regolith thruster increases as the particle radius increases. This tendency is significant with a large value of ΔV. The propulsion device gains thrust because it shoots mass from the fuselage and propels by reaction. Once the weight ratio required to obtain the ΔV of the launched object (and propellant) and the spacecraft is obtained, the weight ratio will increase as the particle radius increases. The particles are desired to be of 1.0μm or less in radius so that the weight ratio can be easily achieved for a small satellite.
The minimum required number of orbits per month to obtain a specific ΔV is 108, and it has been found that filling the particle storage in orbit takes a considerable amount of time. If a method other than loading regolith dust directly on lunar surface is wanted, it is necessary to consider collecting particles using tethers, etc. While regolith dust is being used in the lunar orbiter, it continues to fill the storage units with new regolith dust.