  The interlock drive system generates traction by inserting narrow articulated
spikes into the ground and by leveraging the soil's strength to resist
horizontal draft forces. The system promises high tractive performance in low
gravity environments where tires have little traction for lack of weight. At
Earth and Space 2021 we reported the performance of such spikes on a silty clay
loam, a cohesive soil. We found that in such soil, traction below a critical
depth is provided by a zone of lateral soil failure. We also found that the
articulation translates a horizontal draft force into a vertical penetration
force strong enough to penetrate a narrow spike to a depth where the soil can
sustain the draft force, in a self-regulating way. It is conceivable that a
granular material like regolith or sand with little to no cohesive strength
provides less vertical penetration resistance and less resistance to a
horizontal draft force than a cohesive soil, which leads to the question of
whether and how much tractive force an interlocking spike can generate on a
granular material. Here we report on field trials that study different spike
designs in dry and unsaturated moist sand. The results demonstrate that a loose
granular material requires larger spikes than a cohesive soil, that these
larger spikes penetrate dry and moist sand reliably, and that they promise good
tractive efficiency. The trials indicate that on sand, a larger spike diameter
can improve the pull/weight ratio without a loss of tractive performance.
