Strength Analysis of a Micro-Rocket Combustion Chamber
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Strength Analysis of a Micro-Rocket Combustion Chamber Erin E. Noonan, Christopher S. Protz, Yoav P. Peles, S. Mark Spearing Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT This paper presents the fabrication, testing and analysis of the MIT micro-rocket combustion chamber structure. The structure is a rocket chamber with a closed throat and pressure feeds. It is fabricated with the same processes as the full micro-rocket builds. Mechanical test results are correlated with inspection of failed devices for flaws and with finite element modeling of the test condition. This analysis provides recommendations for modifications to improve the strength of the micro-rocket chamber. These recommendations are discussed in the context of the current status of the micro-rocket. INTRODUCTION The micro-rocket is one of several MEMS power generation devices currently under development at MIT [1, 2]. The performance of the micro-rocket is currently limited by structural failures of the combustion chamber at pressures below the design pressure of 125 atmospheres. The maximum pressure achieved in a cold test is 40 atmospheres. Hot tests have yielded a maximum pressure of 12.3 atmospheres [3]. The cold test failures suggest that even with improvements to the combustion chamber cooling, the device will continue to fail unless changes are made to the structure itself. The strength of silicon structures is critically dependent on the flaw population of the material. These strength-controlling flaws are most commonly introduced during processing and handling. To identify the most significant weakness in the structure, strength data must be collected from a statistically significant sampling of devices [4]. There are many possible factors contributing to the failure of the micro-rocket chamber. The strength of silicon structures is critically dependent on the flaw population of the material. This dependence has been investigated in many variations and is well documented [4, 5, 6]. In single crystal silicon such flaws are most commonly introduced during handling and processing. In micro-fabricated devices common structural issues include poor wafer bonds, bonded wafer misalignment, and various stress concentrations. Stress concentrations can develop from processed surface roughness, fillet radii, inadequate load distribution, or geometric features. The micro-rocket could be experiencing increased stress levels from any one or combination of these conditions. The failure occurs consistently about the circumference of the combustion chamber (Figure 1. (a)). This provides some indication of the most likely failure causes. In order to address the failure to achieve design performance, the contributing factors must be identified. STRUCTURE OF THE MICRO-ROCKET The MIT micro-rocket is fabricated using semi-conductor grade n-type silicon wafers. The device features are deep reactive ion etched (DRIE) using a Surface Technology B4.6.1
(a) (b) Figure 1. (a) A failed micro-rocket device
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