Debris clouds behind plates impacted by hypervelocity pellets

Abstract

The effectiveness of two-plate meteoroid shields is determined by the character of the debris cloud ejected behind the front plate after a hypervelocity impact. The material velocities, distributions, and momentum intensities of these clouds have been determined experimentally for 6061-T6 aluminum, OFHC copper, and cadmium pellets impacting optimal thickness bumpers of the same materials at 7.0 km/sec. Debris dissection techniques, free suspended flyer plates, flash radiography, and high speed photography were used to make these measurements. The results were compared with numerical computations from a computer code that describes thin plate impacts as two-dimensional flow processes. Agreement was reasonably good with respect to debris trajectory, origin, and velocity comparisons (except for the Cd pellets), but predicted momentum profiles failed to agree with experiments near the cloud axes, and predicted cloud thicknesses were too great in all cases. On the whole, the discrepancies were considered to be sufficient to call into question any shield design based solely on the code predictions. Background T WO primary approaches are currently available for predicting the responses of spaced-plate shields to meteoroid impacts: 1) use of numerical analyses (computater codes that treat hypervelocity impacts as two-dimension al material-flow processes) and 2) extrapolation of the results of experimental impact data generated at lower velocities. Because the latter extrapolations require questionable assumptions regarding similarity of impact effects at different velocity regimes, and whereas the computer codes deal directly with the velocities of interest and are based on relatively plausible assumptions concerning materials response, the codes have been used widely in the design of meteoroid protection systems. In the present study, a typical code§ was used to predict general dynamic characteristics of clouds generated behind thin plates (bumpers) that were impacted by pellets traveling at velocities achievable with laboratory facilities.2 Particular impact situations were chosen to simulate the phenomena that are thought to be most important during actual encounters between meteoroids and spaced shields. These impact situations were then produced experimentally, the dynamic parameters were measured, and the results were compared with the code predictions. Results from this study are useful both for establishing the reliability of this code and other similar codes, and for guiding the development of more advanced versions with greater reliability.