Using physics-based simulation towards eliminating empiricism in extraterrestrial terramechanics applications

TL;DR

This paper addresses the fallacy of gravity-offset Earth testing for extraterrestrial terramechanics. It introduces a GPU-accelerated SPH based Continuous Representation Model (CRM) simulator implemented in Chrono and validated against NASA SLOPE lab data, with a scaling framework that links Earth and Moon conditions. Granular Scaling Laws show that nondimensional mobility metrics such as $P/(M g sqrt(L g))$ and $V/sqrt(L g)$ collapse across gravity, enabling Earth tests to predict Moon behavior when key dimensionless groups are matched. It demonstrates that single-wheel tests can predict full rover slope and slip behavior under Moon gravity and that the predicted results align with the scaling predictions, eliminating the need for gravitational offset. It argues for a paradigm shift toward physics-based terramechanics simulation for rover and lander design and mission planning, with open-source tools to support resource extraction tasks.

Abstract

Recently, there has been a surge of international interest in extraterrestrial exploration targeting the Moon, Mars, the moons of Mars, and various asteroids. This contribution discusses how current state-of-the-art Earth-based testing for designing rovers and landers for these missions currently leads to overly optimistic conclusions about the behavior of these devices upon deployment on the targeted celestial bodies. The key misconception is that gravitational offset is necessary during the \textit{terramechanics} testing of rover and lander prototypes on Earth. The body of evidence supporting our argument is tied to a small number of studies conducted during parabolic flights and insights derived from newly revised scaling laws. We argue that what has prevented the community from fully diagnosing the problem at hand is the absence of effective physics-based models capable of simulating terramechanics under low gravity conditions. We developed such a physics-based simulator and utilized it to gauge the mobility of early prototypes of the Volatiles Investigating Polar Exploration Rover (VIPER), which is slated to depart for the Moon in November 2024. This contribution discusses the results generated by this simulator, how they correlate with physical test results from the NASA-Glenn SLOPE lab, and the fallacy of the gravitational offset in rover and lander testing. The simulator developed is open sourced and made publicly available for unfettered use; it can support principled studies that extend beyond trafficability analysis to provide insights into in-situ resource utilization activities, e.g., digging, bulldozing, and berming in low gravity.

Figures (25)

• Figure 1: Schematic view of the workflow: experimental test results are used to validate the simulator, which is subsequently used to predict the VIPER rover's performance on the Moon. The experimental data was generated at NASA's SLOPE lab with tests performed under Earth gravity. The same tests were conducted in the physics-based Chrono simulator, to judge its predictive traits before using it to produce results under Moon gravity.
• Figure 2: The co-simulation framework employed: the rover was modeled as a multibody system whose dynamics was solved using a multicore CPU. The deformable terrain was modeled as a continuum whose dynamics was solved using GPU computing. The two modules communicated passing force and torque information from the terrain to the rover; and position, velocity, orientation, and angular velocity coming from each wheel and going to the terrain CRM solver.
• Figure 3: Schematic view of the single wheel test under velocity control mode (VV-mode). Both translational velocity and angular velocity of the wheel can be controlled using the test rig modeled in Chrono. Excess mass can be added on the wheel assembly to model wheel-soil interaction under various loads. The wheel used in the simulation shown here has the geometry used in NASA's SLOPE lab -- the radius is 0.25m, the width is 0.2m, and there are 24 grousers. The height of each grouser is 0.025m.
• Figure 4: Single wheel physical testing & simulation results on GRC-3 he2013geotechnical lunar soil simulant using the 17.5kg wheel. Tests were performed under VV-mode. In simulation, three different sets of GRC-3 material proprieties associated with the lunar soil simulant were chosen -- with bulk densities 1627, 1734, and 1839 $kg/m^3$, and internal friction angles $37.8^{\circ}$, $42.0^{\circ}$, and $47.8^{\circ}$, respectively. In this, and all subsequent images, curves listed with dotted lines correspond to experimental data. The experimental data is connected with a dotted line to emphasize the physical measurements, which can at times be hard to discern against the simulation results. Note that the experimental measurements might list multiple results for the same experimental setup, reflecting the uncertainty in physical measurements.
• Figure 5: Time history of DBP force measured on the wheel at each slip ratio. The simulations were performed on GRC-3 lunar soil simulant in VV-mode. Steady state can be observed in each of the experiments. The density and internal friction angle were 1734 $kg/m^3$ and $42.0^{\circ}$, respectively.