The Relationship between Cone Penetration Resistance and Wheel-Soil Interactions in Lunar Gravity

Abstract

One of the major challenges faced by planetary exploration rovers today is the negotiation of difficult terrain, such as fine granular regolith commonly found on the Moon and Mars. Typical Earth-based testing methods cannot accurately predict rover mobility in reduced gravity environments (i.e. the Moon and Mars) as they fail to account for the effect of reduced gravity on the soil itself. Preliminary efforts have been made to account for effects of gravity on granular materials, at least indirectly, through simulant design. The soil simulant GRC-1 is designed to produce cone penetrometer readings comparable to those collected on the Moon (i.e. in lunar gravity) during Apollo. The assumption is that replicating the mechanical properties of lunar soil in terms of cone penetration resistance will also replicate the response to vehicle loading in terms of traction. The present research was designed to thoroughly characterize this assumption by exploring the relationship between rover mobility and cone penetration resistance experimentally in lunar gravity (1/6-g) and terrestrial gravity (1-g). The cone penetrometer response of GRC-1 was measured at relative densities (DR'S) of 46%,63%, and 69 % in both 1-g and 1/6-g aboard parabolic flights producing effective lunar gravitational accelerations. Cone index gradient (G) values measured in 1/6-g were 4 x lower on average than those measured in 1-g, indicating a significant decrease in shear strength in lunar-g. Wheel-soil interactions between a smooth rigid wheel and G R C-1 were also characterized at 69% DR in 1/6-g. The wheel experiments measured wheel-soil interaction data at controlled slip values of 20 % and 70 %. Wheel-soil interaction data collected include subsurface soil imaging, force/torque sensor data, wheel sinkage, and motor current, with drawbar pull (DP) and sinkage data reported here. Average DP/W values observed at 20% slip and 70% slip were 0.11 ± 0.02 and 0.32 ± 0.03, respectively, and maximum observed sinkage was 9.0 ± 1.9 mm and 16.2 ± 2.1 mm at 20% and 70 % slip, respectively. These results will be compared to 1-g experiments in a lower-density soil that produces an equivalent cone penetrometer response in order to test the hypothesis that equivalent wheel performance will be observed in soil with equal G values across differing gravity levels.