Moreover, prototype rovers such as MAMMOTH and Sherpa both seek to innovate with their novel active suspensions, but conduct these studies using only one wheel type \cite{reid2016actively,cordes2014active}. Work has also been conducted to better understand grousered wheel interactions with regolith \cite{moreland2012soil}. Furthermore, studies into the excavation force requirements for space environments are conducted \cite{zeng2007calculation}. The large excavation forces required for excavation, coupled with the low weight and traction forces experienced on planetary surfaces imply that the excavation system should minimize the excavation force, and maximize the amount of traction provided by the mobility system in order to enable efficient excavation for ISRU \cite{wilkinson2007digging}. Nonetheless, the effect of wheel design and mobility system on excavation has not been studied.
Screwed-Propelled Vehicles (SPVs) use pontoons with helical protuberances that dig through granular media. This displaces the media and pushes the vehicle forward. They have traditionally been used in terrestrial applications in amphibious or otherwise challenging environments \cite{neumeyer1965marsh,evans2016history,ju2010experimental} where marshes, clay, or ice are present. These environments prevent vehicles using wheels or tracks from being reliably mobile. However, SPVs are well suited to those terrains because they are mechanically simple, provide a large surface area for traction, and work in both dry and wet conditions.
Screw generated forces in granular media have been analyzed both experimentally and computationally \cite{thoesen2019screw,thoesen2018screw}. Furthermore, screw-driven mobility platforms have been tested in a lunar regolith analogue and computationally tested using gravity variant coupled discrete element method and multi-body dynamics simulations \cite{thoesen2020revisiting,thoesen2020comparative,thoesen2019helically,thoesen2020granular}. Results indicate that screw propelled vehicles work well across a wide range of gravities. In addition, scaling laws have been developed that relate gravity and craft size to the craft's velocity and power draw \cite{thoesen2020revisiting,thoesen2020comparative,thoesen2019helically,thoesen2020granular}. Building upon this prior body of work, a screw-propelled excavation rover is developed and serves as the test subject for this analysis. This vehicle aims to meet the requirements of in-space excavation by using the screw propulsion system to provide high tractive force and reduce excavation force through churning the granular media and reducing its compaction.
The Counter-rotating Archimedes Screw-Propelled Excavation Rover (CASPER) is a novel platform that combines screw-propelled mobility with a discrete scooper excavation method. CASPER uses four screws to move itself forward while a centrally located ramp excavates material during rover movement. This combination of excavation technique augmented by a screw mobility system has not been investigated before, and will be compared to other techniques using the parameters described above. We will first discuss the design of the CASPER rover and the experimental procedures. Next we will present our experimental data on CASPER mobility and excavation performance. Finally, we will conclude with the major takeaways from this analysis and potential future directions.