Mobility performance of a rigid wheel in low gravity environments

To investigate influences of gravity on mobility of wheeled rovers for future lunar/planetary exploration missions, model experiments of a soil-wheel system were performed on an aircraft during variable gravity maneuvers. The experimental set-up consists of a single rigid wheel and a soil bed with two kinds of dry sands: lunar soil simulant and Toyoura sand. The experimental results revealed that a lower gravity environment yields higher wheel slippage in variable gravity conditions. In addition to the partial gravity experiments, the same experiments with variable wheel load levels were also performed on ground (1 g conditions). The on-ground experiments produced opposite results to those obtained in the partial gravity experiments, where a lower wheel load yields lower slippage in a constant gravity environment. In low gravity environments, fluidity (flowability) of soil increases due to the confining stress reduction in the soil, while the effect of the wheel load on sinkage decreases. As a result, both of these effects are canceled out, and gravity seemingly has no effect on the wheel sinkage. In the meantime, in addition to the effect of wheel load reduction, the increase of the soil flowability lessens the shear resistance to the wheel rotation, as a result of which the wheel is unable to hold sufficient traction in low gravity environments. This suggests that the mobility of the wheel is governed concurrently by two mechanisms: the bearing characteristics to the wheel load, and the shearing characteristics to the wheel rotation. It appears that, in low gravity, the wheel mobility deteriorates due to the relative decrease in the driving force while the wheel sinkage remains constant. Thus, it can be concluded that the lunar and/or Mars’ gravity environments will be unfavorable in terms of the mobility performance of wheels as compared to the earth’s gravity condition.