We are facing global environmental problems such as greenhouse warning, desertization, appearance of the ozone hole, and so on. Furthermore, it is a proven fact that the environmental problems caused by growing activities of human beings, exist not only in the earth but also in the space around the earth. Since the first artificial satellite, Sputnik 1 was launched in 1957, the number of artificial satellites has grown up to about 5000. As the space development got into full scale, earth-orbiting man-made objects, that were out of control, sharply increased. These objects are called space debris, and consist of used satellites, rockets and their components, as well as fragments created by collisions between those components. Therefore, all earth-orbiting spacecrafts are susceptible to hypervelocity impacts (HVIs), which can in turn lead to catastrophic failure of the spacecrafts.
Since the space debris bigger than 10cm is trackable by radar observation, spacecraft structures can avoid the impact of these space debris by changing their orbit. However, there is a numerous number of space debris smaller than 10cm, and it is hard to detect such small space debris. On this account, it is impossible to keep all spacecraft structures out of the risk of space debris impact. Therefore, spacecraft structures are equipped with debris shield that is placed at a small distance away from the main inner wall, i.e. pressure wall of module.
The main objective of this dissertation is to aim to develop the protection system of spacecraft structures against space debris. To achieve the goal, the study of the numerical simulation of hypervelocity impact phenomena in which a solid projectile strikes to a solid target, is carried out. Moreover, based on the numerical simulation results, two evaluation methods for the protection capability of debris shield, and for the perforation resistance of pressure walls, are proposed respectively.
To simulate the HVIs phenomena, a two-dimensional axisymmetric smoothed particle hydrodynamics (SPH) method is applied in this study.
First, a technique for improving computational efficiency of the two-dimensional axisymmetric SPH method is shown. To improve computational efficiency, simple functions of only even-order terms are adopted as the kernel function. The comparison of the numerical results by using new kernel function and Gaussian kernel function, and the experimental result is shown. It is shown that the SPH analysis in two-dimensional axisymmetric coordinates can be conducted efficiently by using new kernel functions.
Secondly, the numerical simulations of HVIs of an aluminum projectile on aluminum plate targets and on laminated composite plate targets are conducted. Here, the treatment of the anisotropy of the composite plate in the axisymmetric coordinate system is shown. To simulate the delamination of the laminated composite plate due to the HVIs, an interface modeling technique for mixed-mode delamination is adopted in the present SPH method. The numerical simulation results are shown with the experimental results which support the validity of the numerical simulation results. In conclusion, using this technique, the delamination occurring due to HVIs can be simulated numerically, and the delamination area can be predicted.
Thirdly, the numerical simulations of HVIs of a projectile on a pressure wall protected by a debris shield are conducted. Through the numerical simulations, an energy-based parameter, which is the maximum value of the total kinetic energy of debris cloud per unit area at the position of the upper surface of a pressure wall, is calculated. Consequently, the evaluation methods for the protection capability of debris shields and the perforation resistance of pressure walls based on this parameter are proposed based on the energy-based parameter. Furthermore, using these evaluation methods, the protection capabilities of hybrid composite debris shields, multiple bumper debris shields, and perforation resistance of hybrid composite pressure walls, are shown. It is concluded that the composite plate is inferior to the aluminum plate as a debris shield and a pressure wall.
Finally, a simple method for designing debris shields is shown. Here, an empirical formula to predict the energy-based parameter without conducting complex numerical simulations, is proposed. It is shown that the protection capability of a debris shield can be obtained easily by using the formula.
edited by Ryo ITO