Abstract

It remains a significant challenge to accurately predict the performance of wheels driven on loose granular terrains in low gravity. Planetary rover testing predominantly consists of reduced-weight testing, the use of specially designed soil simulants, or a combination of both; however, these methods overlook how gravity affects the soil during the wheel-soil interaction.

An emerging prediction method is Granular Scaling Laws (GSL), allowing the prediction of wheel mobility metrics (traction, sinkage, and power consumption) across two different gravity levels with sets of nondimensional parameters. However, a recently proposed cohesion term would constrain the wheel radius ratio to the inverse ratio of the gravity levels, indicating that a scaled test wheel on Earth in 1-g should have a radius 1/6 that of a full-scale wheel on the Moon in 1/6-g since Lunar regolith has non-zero cohesion. Furthermore, it is possible for cohesion to become a major component of the soil strength in low gravity. To investigate the cohesion term, this research conducted wheel slip experiments in high fidelity cohesive LMS-1 Lunar regolith simulant during a Lunar gravity parabolic flight campaign in 2023. Comparing the results of this campaign with those from the research group’s earlier 2020 campaign that used noncohesive soil, there is compelling evidence that the cohesion constraint can be ignored for soils with cohesion and bulk density values comparable to real regolith on the Lunar surface. Additionally, refined frictional corrections show that GSL overestimates the traction force, contrary to the result from the previous campaign.

These findings are fundamental to future rover testing and mission planning. Impractical 1/6-scale test units are likely not required for Lunar rovers, and all mobility metrics using GSL should be carefully interpreted despite it being the best physical prediction method currently available.