Introduction
In situ space excavation is a highly researched and sought after branch of technology as it will help achieve numerous scientific goals and enable a sustained human presence via in-situ resource utilization (ISRU) \cite{Benaroya_2002,Labeaga_Mart_nez_2017,Cesaretti_2014,saxena2019sun,li2018direct,zuppero1997nuclear,blair2002space,jones2019cost,crawford2015lunar,lavoie2016purpose,sanders2011comparison,sanders2012progress,spudis2011using,mueller2008trade}. In order to achieve the goals of ISRU, robotic vehicles capable of traversing planetary surfaces and excavating material must be developed. According to the literature \cite{just2020,skonieczny2011advantageous}, excavation systems are either discrete, where one cutting surface makes repeated contact with the surface, or continuous, where multiple cutting surfaces successively make contact with the surface to excavate material. Just et al. \cite{just2020} also indicate that the key parameters that should be used to compare excavation performance are the excavation rate, vehicle traverse rate, total power consumption, and the geotechnical properties of the regolith analogue used. These parameters are indeed critical to characterizing the performance of ISRU excavation systems, however, this paper will examine two additional parameters that provide further insight into system performance: the cost of transport (COT), which is a measure of mobility efficiency and the excavation transport rate (ETR), a new parameter introduced in this work that seeks to combine excavation and mobility performance. In particular, ETR is the rate at which the excavation system collects material, normalized by the craft mass and cost of transport.
The majority of research on ISRU excavation systems has focused primarily on the development of wheel and track based excavation systems. Scarab \cite{wettergreen2009design,wettergreen2010design} and RASSOR \cite{mueller2013regolith} are two examples of wheeled excavation platforms, which both employ continuous excavation systems, a drill and bucket wheel, respectively. Conversely, Cratos \cite{greer2013cratos,caruso2013cratos} utilizes a discrete scraper excavation system. It employs a track based mobility system due to the large tractive forces required by the scraper. Furthermore, studies into the excavation force requirements for space environments have been conducted \cite{Zeng_2007}. 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{Wilkinson_2007}. Continuous systems have typically been found to be more advantageous for low-gravity planetary environments because they require less tractive force than discrete systems \cite{skonieczny2016advantages}.
One key mobility architecture that has been neglected until now is a counter-rotating Archimedes screw propelled mobility system, which uses rotating pontoons with helical protuberances to react against granular media and propel the vehicle forward. Screwed-Propelled Vehicles (SPVs) have been shown to provide large tractive forces even in reduced gravity environments \cite{thoesen2019screw,thoesen2018screw}. Additionally, a screw mobility system can be used to augment an excavation system by churning up material prior to being collected, thereby reducing the required force to excavate material. The high tractive force and potential for reducing the force required to excavate material make screw-propelled vehicles highly interesting as a candidate for an ISRU excavation system. On Earth, SPVs have traditionally been used in terrestrial applications in amphibious or otherwise challenging environments \cite{Neumeyer_1965,Evans_2016,Liang_Ju_2010} where sand, 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 and provide a large surface area for traction. 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{Thoesen_2020,Thoesen_2020a,Thoesen_2020b,thoesen2019helically}. Results indicate that screw propelled vehicles provide high tractive forces and are capable of mobility 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{Thoesen_2020,Thoesen_2020a,Thoesen_2019a,Thoesen_2020b}. 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. This initial iteration of CASPER is a prototype which is meant to serve as a proof of concept for a helical mobility system augmenting a discrete scooper excavation system, and is not intended to be used on planetary surfaces for ISRU excavation. Future iterations of the craft would require improved materials for the body, excavation, and mobility systems, in addition to a robust suspension system to be capable of traversing and excavating material from planetary surfaces. The vehicle developed and the analysis methods presented in this paper are new to the field of planetary excavation in the following ways: The CASPER prototype is the first proposed vehicle to use a counter-rotating Archimedes screw mobility system to augment an excavation system of any type. Particularly, the CASPER excavation system is the first discrete scooper to be studied and compared to continuous excavation systems \cite{skonieczny2016advantages}. In addition, the excavation transport rate is a novel parameter that can be used as a method to compare the combined mobility and excavation performance of ISRU excavators. 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.