★ Ultrasonic horns are optimised to maximise momentum transfer to free-masses on impact. ★ Large free-masses deliver more effective impulse for drilling. ★ The number of free-masses and the mass of the cutting bit is not highly significant.
Traditional rotary corers and sample retrieval mechanisms for planetary drilling suffer from a variety of technical difficulties. A heavy and rigid drillstring must be assembled on-site and deployed with considerable applied preload and torque, and these mechanical loadings are difficult to react in a low gravity environment. Furthermore the entire drillstring must often be removed to retrieve samples, unless an augering approach is taken, in which case stratigraphic sequencing is lost. Ultrasonic tools which operate by converting an ultrasonic frequency to a low impacting frequency at the tool end can resolve the mechanical problems because they require very low applied preload and no torque to operate. In developing such a tool, however, several fundamental design decisions must be taken regarding the architecture of the transducer, horn and stack. These include the choice of solid or hollow transducers and the employment of single or multiple free-masses at the ultrasonic to low frequency conversion location. This paper addresses the layout of such a system by contrasting the pros and cons of these architectural choices and concludes that a solid system with a single free-mass provides the best performance in the parameter range here discussed.