Screw-Propelled Vehicles (SPV’s) have been historically used for terrestrial applications such as transportation over mud, snow, and amphibious environments. One early example for SPV’s was the Marsh Screw Amphibian, seen below1. This craft propelled itself through water and then quickly transitioned to a solid, muddy environment. Another vehicle is the Amphirol. This vehicle is capable of navigating through sticky wet clays, a nearly impossible task for other forms of transportation.
These types of vehicles were considered during the design of the first lunar rover, given their success in aqueous and arctic media, but ultimately passed over for wheeled vehicles. However, with space exploration becoming a larger field of science there is a need for transportation solutions in reduced gravity granular medias. Many small bodies such as asteroids are covered with a loose granular media called regolith. These environments include Mars, the Moon, and asteroids. Some other targets include Mars’ moons such as Phobos. This regolith is hard to characterize from a distance but most studies estimate small grain sizes of no larger than a few millimeters.2 Lunar regolith is categorized as even finer, going down to micron ranges. Navigating such a material is a challenge and of great interest to the space community. While tracked vehicles have ultimately provided a solution on Earth, they are dependent on gravity to maintain adequate friction forces.
Previous studies have looked at the mobility of SPV’s on the surface of granular media but there are not many modern computational and experimental studies on the propulsive forces of buried screws or SPV’s. Analyzing the performance of such a craft in a reduced gravity environment further complicates matters. J.Y. Wong, a pioneer in the field of terramechanics, addressed concerns about reduced gravity granular experiments in a 2012 paper3. He found significant differences between the performance of wheels in parabolic flight experiments and in equivalent experiments with simple gravity-equated weight reduction. The use of simulation techniques such as the Discrete Element Method enables us to simulate granular material behaviors and provides key insights that Earth experiments are not able to. With gravity adjustment, mass flow rate measuring, geometry force measurement, and other data we are better able to examine what would make a good low gravity vehicle.
The focus of my research is to do just that. By using EDEM as a DEM tool to simulate the granular media and Multibody Dynamics (MBD) software to drive craft dynamics, we can examine the physics of how the craft will move and look at non-trivial drag effects. However, it is essential to evaluate any differences between software and experimental results specifically with regards to screw propulsion in granular media. First, we must evaluate performance of the propelling screws in a well characterized media. For this experiment we’ve chosen Earth gravity and spherical glass beads of relative uniform size to minimize additional variables. In doing so we can begin to build a framework for more extensive design evaluation of screw propelled vehicles in more exotic materials and different gravities. Understanding the role of screw design and its angular velocity on thrust force is also key to the advancement and control of SPV’s.
Our study presents experimental and computational results of a submerged, double-helix Archimedes screw generating propulsive force against a bed of soda-lime glass beads. Thus, this research forms the basis for design of a future miniaturized exploration vehicle for space applications. In our study, we used three different screw designs (5 cm radius, 10 cm length, and 4 cm, 6 cm, and 8 cm pitches) submerged in 2mm glass beads (90\% roundness with 0.1mm std). These screws are subjected to three modes of analysis: experimental, DEM, and analytical modeling. In our experimental results, similar trends are observed between rotational speed and thrust force in the 45-120 RPM regime for all three screws. Using EDEM, we could directly evaluate the generated forces. EDEM’s results show trends which match to the experimental results.
Verifying that EDEM simulations behave in the expected manner for screws buried in characterized granular material, we continue with a two-fold path. The first is to characterize granular media which more closely resembles conditions on the moon or asteroids. Thanks to developments in material characterization for EDEM, we’re anticipating successful first time simulation of these materials. At the same time we perform this testing, we will evaluate the dynamic performance of a screw-powered craft in the glass bead media. Eventually we will combine the two to evaluate vehicle performance on the moon and asteroids. In this way, we could inform the development of mobile craft and other equipment in numerous extraterrestrial environments.
1 M. J. Neumeyer, and B. D. Jones. “The marsh screw amphibian.” Journal of Terramechanics 2, no. 4 (1965): 83-88.
2 Gundlach, Bastian, and Jürgen Blum. “A new method to determine the grain size of planetary regolith.” Icarus 223.1 (2013): 479-492.
3 Wong, J. Y. “Predicting the performances of rigid rover wheels on extraterrestrial surfaces based on test results obtained on earth.” Journal of Terramechanics 49, no. 1 (2012): 49-61.