Research Paper
Investigation of effect of stones on lunar regolith drilling using DEM

https://doi.org/10.1016/j.compgeo.2020.103958Get rights and content

Abstract

In lunar soil drilling, stones have significant influences on sampling. Herein, stones are classified into greater stones and supramaximal stones based on their sizes. Five simulation conditions with different sizes and positions of the stone were designed. The stone trajectories, sampling efficiencies, velocity field, stress field and load curves were compared and analyzed. The results indicated that the resultant axial force and torque were mainly decided by the load of the bit. The stone affected the stress of the particles around it obviously, but had little effect on the velocity field. A greater stone, having diameter smaller than that of the virtual cutting circle of the drill bit, has little influence on the lunar soil drilling process. However, a supramaximal stone, with diameter larger than that of the virtual cutting circle, significantly affects the lunar soil drilling process. If supramaximal stones are near the edge of the virtual cutting circle, they are easily spun out and can cause obvious discrete pulse peaks of axial force. If the supramaximal stones are screwed inward, they can easily cause a jam in front of the drill bit, making the soil injection difficult. An experiment result verified the effect of the supramaximal stone.

Introduction

Lunar exploration has attracted the attention of numerous scholars in several countries. Studies have been performed on the moon environment (Konopliv et al., 2001, Chin et al., 2007, Colaprete et al., 2015) and landing and roaming tasks (Sun et al., 2005, Ip et al., 2014, Zheng et al., 2008). Lunar soil drilling sampling—one of the most important areas of lunar exploration—has only been achieved by the United States and the former Soviet Union, and will be performed by China’s Chang’E probes (Zhang et al., 2016).

The analysis results for soil samples obtained by Luna 24 indicated that no samples were obtained 60 mm below the lunar surface. For Apollo 17, no samples were obtained at depths of 38–40 cm and 71–80 cm. For Apollo 16, the samples obtained at depths of 72–120 cm were small and discontinuous. At present, no analysis of the sampling situation with intermittent, insufficient, or no samples has been reported in the literature. However, according to experiment and simulation experience, the presence of stones in lunar soil may be one of the reasons for the foregoing phenomenon. The presence of stones in the soil has always been one of the most important conditions in lunar soil drilling and sampling, because stones can affect the sampling characteristics and load characteristics in the drilling process. However, it is difficult to identify the stones in the lunar soil and analyze their effects.

Scholars often analyze the lunar soil drilling process via experiments on the ground using lunar soil simulants. They mainly focus on the load law in the common lunar soil case (Quan et al., 2018, Chen et al., 2019); the coring characteristics (Zhao et al., 2019); the soil removal characteristics (Tang et al., 2018); the drilling power consumption (Tian et al., 2017), or other special cases (Roush et al., 2018). However, experimental methods have the disadvantage that the dynamic changing process of the interior soil cannot be observed. Thus, the discrete element method (DEM) is widely used. In the field of lunar soil sampling, few scholars have investigated the drilling (Cui et al., 2018, Li et al., 2017) and excavation (Liu et al., 2014b, Liu et al., 2014a) processes, owing to computational limitations. Additionally, the effects of stones have not been investigated.

In this study, drilling simulations are carried out based on the previously established DEM model by the authors. The stones are classified into greater stones and supramaximal stones according to whether the size of the stone exceeds the diameter of the virtual cutting circle of the drill bit. The effects of stones with different sizes and positions on the drilling process were analyzed by comparing the stone trajectories, sampling efficiencies, and load curves.

Section snippets

DEM model of lunar soil drilling

Lunar soil exhibits a larger internal friction angle and less cohesion than Earth soil, and it is difficult to simulate these characteristics using the DEM. Scholars have used various methods to improve the accuracy of the lunar soil DEM model and have obtained significant results; however, some problems remain to be solved (Du et al., 2017, Nakashima et al., 2011, Nakashima and Kobayashi, 2014, Gao et al., 2017, Jiang et al., 2013, Modenese et al., 2012, Li et al., 2018, Li et al., 2018). The

Simulation design

The cutting range formed by the cutting teeth is defined as the “virtual cutting circle” and is shown in Fig. 8. For stones in surface lunar soil that are significantly larger than ordinary lunar soil particles, if the stones are smaller than the virtual cutting circle, they are defined as “greater stones,” and if the stones are larger than the virtual cutting circle, they are defined as “supramaximal stones.” The following analyses focus on the effects of these two types of stones on lunar

Effect of greater stones

  • (1)

    5-mm greater stone in center

In this condition, horigin = 31.1 mm, hfinal = 21.9 mm, so the sampling ratio is δd = 0.704. ms = 2.37 g, including the greater stone. The whole lunar soil sample is cylindrical and plump, and the top of the sample is arched. The greater stone is ultimately located at the top of the lunar soil sample, as shown in Fig. 16.

  • (2)

    5-mm greater stone in offset position

In this condition, horigin = 31.5 mm, hfinal = 22.1 mm, so the sampling ratio is δd = 0.702. ms = 2.01 g,

Conclusions

The effects of greater and supramaximal stones at different positions on the lunar soil drilling process was comprehensively analyzed. The results indicated that the resultant axial force and torque are mainly decided by the load of the bit. The stone affected the stress field of the several particles around it obviously, but had little effect on the velocity field. A greater stone whose diameter is smaller than that of the virtual cutting circle has little influence on the lunar soil drilling

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (grant numbers 51605114 and U1637208), the China Postdoctoral Science Foundation (grant number 2017M621286), and the Natural Science Foundation of Heilongjiang Province, China (grant number E2018039). This work was also financially supported by the China Scholarship Council (grant number 201806125048).

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