(A) study on pyroshock reduction by optimizing low-shock pyrotechnic separation device and using elastic wave refraction

Abstract

Various of pyrotechnic devices and systems use explosive materials and compositions for the generation of gas, flame, heat, shock, smoke, light, or sound. Since the 1960s, pyrotechnics separation devices have been widely used in various engineering areas where reliable or quick separation is required

especially, the aerospace industry has been the forerunner in the pyrotechnology. They have many advantages such as lightweight, low input energy, high reliability, and fast operation, however, they also generate intensive shock, called pyroshock, which can be fatal to electronic devices on the system. As an effort to reduce the pyroshock when the separation event occurs, the pressure cartridge types low-shock separation devices have been developed and widely used in rocket and space launch vehicle systems because they have many advantages of high fastening strength, environmental resistance, and high separation reliability without generating a high pyroshock or fragments compared to high-explosive type separation devices. There are various examples of the application using this type of separation device. In some of the pressure cartridge type low-shock separation devices, the combustion of the charge, the expansion of the chamber, and the complex separation mechanism are coupled

therefore, separation characteristics are also influenced by the amount of propellant, the initial volume, volume increase rate, and mass of components. These complex phenomena, including the chemical and mechanical separating processes, are completed in a very short time, less than a few milliseconds. Because of these characteristics, it is very difficult to experimentally observe the behaviors of each component inside the separation devices. Therefore, analysis and prediction of the separation behaviors and performance using a mathematical model would be very useful. For the common case of payload on launch vehicles, electronic devices mounted in payload are exposed to pyroshocks generated from various sources such as stage separation, firing separation, and payload separation, etc. There have been various studies to reduce or attenuate pyroshock at a source or on a propagation path. In common with the refraction phenomenon of light and ocean waves, elastic waves in solids are also refracted depending on the velocity of the waves. There have been some studies on the refraction of elastic waves on a thin plate which is a widely used component in various spacecraft structures. In this dissertation, the mathematical model for separation mechanisms of the split-type separation bolt, which is a kind of pressure cartridge type low-shock separation devices, and reduction of pyroshock on a thin plate are studied. The mathematical model consists of simultaneous differential equations including a combustion model, a buckling force model, ring split behavior model related to static and dynamic friction, O-ring friction model, bolt deformation model, surface contact force model, and slip angle model. In order to consider amplitude of pyroshock due to the mechanical collisions inside the separation bolt, a numerical analysis was conducted on the major mechanical collisions during the separation process. Using the developed mathematical model, sensitivity analysis for major design parameters and optimization considering shock reduction were conducted. In the sensitivity analysis, the seven design parameters were considered, and for the optimization, most sensitive three parameters were selected and controlled within defined bound constraints. Finally, to reduce the pyroshock propagated through the thin plate generated after the operation of the pyrotechnic separation device, a novel pyroshock reduction method of refraction using shock wave in a thin plate was proposed and investigated. The presented method uses elastic patches which can be easily applied to the actual structure to refract bending waves for shock reduction. The performance of the shock reduction for various shaped (triangle, circle, and double-convex lens) patches were numerically simulated and also experimentally investigated for an aluminum plate and patches. The analytical and experimental results confirmed the effectiveness of the proposed methods in terms of the shock reduction performance and the shock reduction area of the various shape of the elastic patches.