Comparison of USDC With Conventional Drilling Techniques

 

The USDC ability to drill using low axial load creates new options for the exploration of low gravity bodies (e.g., comets and asteroids).  A comparison between the USDC and conventional drills is given in Table 1.     Unlike conventional drilling rigs, the USDC can drill or core rocks from a very light and flexible platforms (e.g., Sojourner or Marie Curie rovers, robotic arms, etc.).  Since the USDC does not have electrical motors, it can be duty cycled without significant loss of efficiency. This facilitates operations under very low average power (operation at 2 watts average power has been demonstrated).  Unlike conventional drills, the drive mechanism of the USDC has only three moving parts, which are not physically connected and do not require lubricants.  This design eliminates common mechanical failure modes and makes it easy to constrain during launch.  The use of a piezoelectric stack as the USDC actuator permits the device to operate over a very wide temperature range.  The drilling/coring bit does not rotate and does not require sharpening.  The USDC can core arbitrary cross-sections (square, round, hexagons) and can accommodate drilling of vertical and overhanging rock faces. 

 

 

Text Box: TABLE 1: Comparison between the USDC and conventional Drills
	Conventional Drills	USDC
Axial preload	>100N (typically 150N) 	<10N
Drill walk at core initiation	>30N·m induced torques and >100N tangential forces	<1N
Average power to create 10-mm core	>20-30 W.  Can be reduced but the drilling efficiency goes down.	Can be as low as 2-3W (lower power requires longer drilling)
Duty cycling	Involve staggering loss of efficiency	Very little efficiency loss (2W average at 25W peak was demonstrated)
 Current Overshoot	3-4 times larger startup electrical currents than those during continuous operations	<20% even with duty cycling.
Drill chatter	Induces low frequency (2-10Hz) and high force perturbations on the drilling platform	Minimal
Support system	Requires stable and massive platforms with solid anchoring	Minimal
Drilling/Coring soft rock	Shearing and spalling	Compression failure
Drilling/Coring hard rocks	Grinding with corresponding 300% increase in energy consumed per unit volume of removed rock. Require frequent sharpening or replacement. Otherwise, 10 fold increase in heat generation and similar drop in efficiency.	·	Spalling·	No need for drill bit sharpening
The simplicity of the USDC based sampler reduces the number of possible contamination sources (e.g., lubricants and metallic filings from wear on the gears). The acoustic vibration of the bit provides for transport of the powdered cuttings away from the bit/sample interface.  This design minimizes cutting edge wear and prevents particle entrapment unlike conventional composite drilling bits (e.g., diamond cutters on silicon carbide substrate with metal holders). The USDC can also use bits manufactured from a single element (e.g. Tungsten) and could thereby minimize the sources of trace metallic and abrasive elements.

Non-traditional (or “modern” in the oil industry terminology) drilling technologies (laser, electron beam, microwave, jet, etc.) usually are competitive only in applications that are time limited (time is money in oil industry) and not power/mass limited as is typical for space science applications.  Typically, the down-the-well energy that is required to remove a unit volume of rock for “modern” technologies is the same as grinding and melting.  The required energy is 3 and 5 times higher, correspondingly, than that for shear drilling.  Generally, the ratio of power delivered down-the-well vs. input power generation for these non-traditional approaches is below several percent vs. 10%-30% for conventional drills (comparable to USDC).  Consequently, many space missions do not have enough power to employ these  “modern” drilling technologies.