Dynamic and Contact Analysis of a Special Percussive Drill for Planetary Subsurface Exploration

Planetary drilling is a vital task in the challenge for space exploration. Drilling and planetary soil/rock sample acquisition provides information about history of past events, minerals and chemical composition of the soil/rock of the planetary body, available resources for future manned missions and the mechanical behavior of the planetary soil/rock. Several types of drilling devices have been proposed for lunar, Mars, and planetary subsurface exploration. However, these devices have limitations (e.g. heavy equipment and need for large axial force) that need to be addressed in order to be feasible for extraterrestrial bodies’ exploration. A special type of ultrasonic percussive drill have been proposed by Honeybee Spacecraft Mechanisms Corporation and NASA Jet Propulsion Laboratory (JPL) to address the limitations of current drilling devices. In this study, the mechanical system of the ultrasonic percussive drill and its interaction with the supporting medium is studied. The percussive mechanism consists of an ultrasonic horn, a free mass, and the drill rod. Special attention is given to the impact between the free mass and the drill rod, including the effects of structural damping, the supporting medium of the rod, plastic deformation in the contact area, and repeated impacts of the free mass on the drill rod. A general methodology to analyze the impact of the free mass on the drill rod, analogous to the longitudinal impact of a mass on a rod, is developed. The methodology involves uncoupling the nonlinear problem by determining the response of each body independently under the contact force to find the local indentation, then using Hertz force-indentation relation to find the contact force. This method is applied using mode superposition method and finite element technique for various support conditions of the rod (e.g. rigid, elastic, free, viscoelastic). Additionally, a model to account for drill rod penetration into the supporting medium due to impact of the free mass is also presented. It was found that for an undamped rod, the support condition of the rod does not affect the contact force if the contact ends before the arrival of the reflected wave to the point of contact. It was also observed that the contact force due to impact, for a given support condition, increases with increasing damping of the rod. Moreover, a modified Hertz equation is introduced to include plastic deformation on the rod due to impact of the free mass. The study was performed for identical repetitive impacts and it was found that the largest plastic deformation occurs in the first impact, with additional plastic deformation decreasing with the increasing number of impacts. The dynamic response of the overall percussive drill was investigated with a finite element model, including the interaction of the free mass with the ultrasonic horn and the drill rod. A numerical example indicated that the dynamic response of the ultrasonic drill is directly affected by the supporting medium. It was found that system with fixed support experienced a higher frequency of oscillation of the free mass and higher impact force compared to the system with the elastic support and the model accounting for penetration into the supporting medium