Ultrasonically assisted penetration through granular materials for planetary exploration

[摘要] Space exploration missions often use drills or penetrators to access the subsurface of planetary bodies. Protected by the conditions experienced at the surface, these regions have potentially been untouched for millennia. As such, the subsurface is a very attractive option for scientific goals, be it the search for extra-terrestrial life, to examine the history of the planet, or to utilise underground resources. However, many issues arise in such a task. Every other rocky body in our solar system possesses a surface gravity lower than our own, resulting in a lower available weight for a spacecraft to ‘push’ on a penetrating device. Add to this the low power availability and complications regarding remote operation, and this becomes a very difficult process to achieve. Mole devices which burrow through the ground whilst tethered to a surface-station to provide power and data have shown great promise in this regard. Using an internal mass to ‘hammer’ themselves into the ground, special care is required to ensure internal components are not damaged, and that they can arrive at their target depth in a reasonable period of time. There is continuous development in these types of drilling and penetrating technologies and anything that can penetrate with a lower weight-on-bit (WOB), and consume less power, could potentially be extremely useful for these situations.High powered ultrasonic vibrations have been shown to reduce operational space and forces required in cutting bones for surgery. Additionally, they have been successful in reducing WOB requirements for drilling devices through rocky substrates. To maximise penetration depth, it is often favourable to progress though granular material rather than solid rock, however this also provides its own set of problems. This work looks at applying ultrasonic vibration to penetrating probes for use in granular material, with the aim of utilising it in low gravity or low mass scenarios.Before this can be done however, the regolith used for testing must be fully characterised and consistent preparation methods established, ensuring that all other effects are accounted for. An ultrasonically tuned penetrator was designed and manufactured, and the effects it had on the surface of sand were investigated using a high-speed camera and optical microscope. It was found that regions of sand immediately surrounding the penetrator were highly fluidised, localising any deformations to a small radial distance. Penetration tests were then conducted that showed ultrasonic vibration significantly reduces the penetration forces and therefore the overhead weight required, in some cases by over an order of magnitude. A similar effect was seen in power consumption, with some instances displaying a lowered total power draw of the whole system.Experiments were then conducted in a large centrifuge to examine the trends with respect to gravity. Gravitational levels up to 10 g were tested, and the general trend showed that ultrasonic penetration efficiency indeed increased at lower gravities, suggesting that the force reduction properties would be enhanced at lower levels of g. Finally, the first steps to applying this technique as a fully-fledged penetration device were conducted. These tests oversaw combining ultrasonic vibration with the established hammering mechanism used by mole devices. Comparing this against a purely hammering penetration, it was found that the addition of ultrasonic improved performance significantly, greatly reducing the number of strikes required to reach the same penetration depth.To conclude, the work presented in this thesis shows the potential that ultrasonic vibration can have with advancing low gravity/low mass penetrating devices. Reducing both the weight and power requirements can be a huge boon to small spacecraft, and the potential use as subsurface access or anchoring devices makes it an attractive avenue for future research and development.

[发布机构] University:University of Glasgow;Department:School of Engineering

[关键词] Ultrasonic vibration, granular materials, penetration, subsurface exploration, force reduction, power optimisation, centrifuge testing, hammer-action penetration, moles.