Drilling problem. Most state of the art rock cutting processes are accomplished by the shearing action or grinding motion of some cutting tool. With no exception, these cutting actions result in noisy work environment coupled with the undesirable excitation of vibrations that are transmitted to the drill unit home structure. A parameter of paramount importance in any drilling process is the “weight on bit” which is the axial force acting on the bit during the cutting process. Normally this force is relatively large and may be generated via proper anchoring of the drill machine to the drilled surface or as an alternative, weight on bit may be provided by the self-weight of the drill unit structure.
State of the art drilling methods involve the utilization of a drill-string that carries the cutting tool at its front end while being connected to a surface driving mechanism on its other end. Such a drilling method becomes unattractive for space missions due to weight restrictions on equipment and the high power driving machine needed to overcome, in addition to the drill process torque, also the unavoidable friction generated along the long drill string in a deep hole drilling operation.
Instead of plunging an entire shaft deep down into the ground, an alternative strategy may be considered using a detached, self-driven underground autonomous tethered drill system. In contrast to state of the art drilling methods, such a system may be quite lightweight that needs to use only enough power to accomplish the drilling task while propelling itself downward, trailing a thin cable for power and communication, An auxiliary thin wire rope connected to a surface winch may be linked to the system for lifting and clearing of scientific samples and the rest of the drill process cuttings. The elimination of the notorious drill-string from the drilling process dramatically reduces the weight of main system components, along with reduction of power consumption for drilling task. While drill-string systems are limited by the ultimate depth they may achieve, autonomous tethered system may reach almost any desirable destination.
The Low Reaction Force Drill (LRFD), shown in Figure 1, is a new low-energy, low mass, self-advancing drilling system concept for drilling on Mars or other planetary bodies (U. S. Patent # 5,641,027, 1997). Energy savings have been demonstrated in this study to be five times less than current systems competing for selection as the tool for drilling on Mars. Table 1 provides an energy consumption comparison between the LRFD and other state of the art drilling methods. The distinct advantages of the LRFD are its low energy drilling capability as a function of its unique rock cutting mechanism, its essentially unlimited depth capability due to its tethered downhole motor and bailing bucket configuration, its self-advancing capability by self-contained torque and weight on bit by counteracting multiple concentric drill bits and bracing against rock or regolith. Additional LRFD advantages may be found in its large non-thermally degraded intact sample production (> 1 cm3) with position known to within 15rnm, and finally, the large diameter hole it produces that allows for down hole instrumentation during and post drilling. The system has application for shallow drilling (1 to 200 meters) through kilometer class drilling in a broad range of materials.
The LRFD is a departure from conventional drilling technology in mode of excavation and thus in power consumption and advance technique. Prototype testing to date confirms system feasibility and demonstrates TRL 4 capability identifying the LRFD as a viable candidate for near term Rover and Lander missions. (Hill et al, October 2001; Hill et al, February 2001; Hill et al, 2000; Kishoni, 1998; Amini et al, 1998).
The LRFD can be adopted to meet a broad range of NASA mission requirements including: