Trafficability analysis of lunar mare terrain by means of the discrete element method for wheeled rover locomotion

An irregularly shaped particulate system for simulation of lunar regolith is developed using discrete element modeling based on the fractal characteristics, particle shape, and size distribution of returned Apollo-14 samples. The model parameters are determined by dimensional analysis and biaxial test simulation with an improved boundary condition. Under terrestrial conditions, the trafficability of lunar mare terrain is estimated in terms of wheel-terrain interaction by experiment and simulation in order to validate the applicability of the wheel-terrain model employed here. The results show that the discrete element method combined with the wheel-terrain model is sufficiently accurate for mare terrain trafficability analysis without consideration of lunar environmental effects. To predict the trafficability of in situ lunar mare terrain, the non-contact forces attributed to the lunar surface environment are discussed and the initial mechanical model of discrete elements is modified by introduction of lunar gravitational force as well as electrostatic force. In the modified model, wheel-terrain interaction is analyzed under the same travel conditions as that of the experiment. The result shows the trafficability of the in situ lunar mare terrain is worse than that obtained by experiment and simulation with the initial model according to the value of horizontal force at any slip ratio. However, the wheel requires less drive torque on the moon than that on the earth. An explanation for these phenomena may be that lunar subsurface regolith particles are arranged in a looser manner under local environmental effects that effectively decrease the bearing and shearing strength of regolith.