Anton V. Kulchitsky

Intersecting dilated convex polyhedra method for modeling complex particles in discrete element method.

This paper describes a new method for representing concave polyhedral particles in a discrete element method as unions of convex dilated polyhedra. This method offers an efficient way to simulate systems with a large number of (generally concave) polyhedral particles. The method also allows spheres, capsules, and dilated triangles to be combined with polyhedra using the same approach. The computational efficiency of the method is tested in two different simulation setups using different efficiency metrics for seven particle types: spheres, clusters of three spheres, clusters of four spheres, tetrahedra, cubes, unions of two octahedra (concave), and a model of a computer tomography scan of a lunar simulant GRC-3 particle. It is shown that the computational efficiency of the simulations degrades much slower than the increase in complexity of the particles in the system. The efficiency of the method is based on the time coherence of the system, and an efficient and robust distance computation method between polyhedra as particles never intersect for dilated particles.

Geotechnical Properties of Asteroids Affecting Surface Operations, Mining, and In Situ Resource Utilization Activities

Abstract Geotechnical properties of a granular material affect all surface operations from mobility to landing and excavation. As such, significant efforts to study and model these properties are necessary before sending a spacecraft. Lack of knowledge of regolith material properties adversely affected Apollo, Lunokhod, and Mars Exploration Rover missions; hence additional measures need to be undertaken to prevent potential failures or delays of future missions, in particular missions to explore low-gravity asteroidal surfaces. Geotechnical properties of regolith include cohesion and friction angle, which affect material strength. Friction angle is gravity-dependent, whereas cohesion is not. It is therefore much easier to study and model surface regolith on planetary bodies with significant gravity such as the Moon or Mars. If gravity becomes extremely low, for example, on asteroids, cohesive forces start to dominate. This chapter addresses geotechnical properties of asteroid regolith and their implications for safe mission surface operations. The chapter starts with a high-level overview of soil mechanics followed by an overview of asteroids regolith from past and current missions. Models related to regolith are presented with specific emphasis on sources of cohesion. Several examples of surface operations are given (landing, boulder retrieval, excavation) to illustrate the effect of various properties on the hardware.

Thumper and shotgun: Low velocity kinetic penetrometers to estimate regolith and rock properties for NASA's Asteroid Redirect Mission (ARM)

NASAs Asteroid Redirect Mission (ARM) is a challenging mission to capture and bring back either an entire asteroid (Option A) or a large boulder from the surface of an asteroid (Option B) to a cislunar orbit. Options A and B have a range of risks; one of them relates to the unknown strength of the asteroid or boulder. This paper describes methods of estimating asteroid regolith strength and density, and the strength of boulders, using kinetic impactors: Thumpers and Shotgun. Thumpers are large, instrumented kinetic impactors specifically designed to measure regolith strength during impact deceleration. The Shotgun system, on the other hand, uses a large number of small projectiles (“balls”) fired at low velocity at the surface of the asteroid or at the boulder. If a ball impacts regolith, it will create a crater whose size is a function of regolith strength and density. If a ball impacts a coherent boulder, it will bounce back at a certain speed, whose value is proportional to rock strength. If the rebound speed cannot be measured, hollow balls packed with retroreflectors could be used instead (similar to paintballs). The shell of balls can be designed to crack open and release retroreflectors when impacting rock above the threshold strength required for successful boulder retrieval. This paper describes the concepts of Thumpers and Shotgun and demonstrates their feasibility through a series of experiments. These methods leverage many of the advantages of in-situ measurements of target properties- particularly the ability to accurately determine geotechnical measurements- at a considerably reduced cost and implementation effort, and will enable significant risk buy-down on the scope of the ARM mission.

Discrete Element Method Simulations of Mars Exploration Rover Wheel High-Slip Mobility Tests.

Mars Exploration Rovers (MERs) experienced mobility challenges when crossing wind-blown ripples and deformable sands while exploring Martian terrain. Analysis of MER wheel mobility using a 3D discrete element method (DEM) model indicate three stages of mobility: (1) low slip ( 60% slip) defined by residual soil strength and depth of wheel sinkage. MER wheel sinkage and drawbar pull increased then decreased during contact between soil and the wheel tie-down patch, compared to wheel cleats, in high slip conditions, but had little affect on overall wheel mobility. DEM simulations well describe intermediate and high slip average wheel sinkage and replicate the signature of drawbar pull and sinkage for the tie-down patch, but not its magnitude. Improved DEM simulation accuracy can be achieved by using smaller polyhedral particles to represent soil.