Study on effects of chemical reaction rates on ram accelerator thrust performance

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Numerical analyzes and ram accelerator operation experiments were conducted with the aim of clarifying relationships between its thrust performance and chemical reaction rates. There exist two propulsive modes in which subsonic combustion occurs ; subdetonative and transdetonative modes. In this study, numerical analyzes were based on quasi-one-dimensional steady flow with finite-rate chemistry taken into account. The formulation yields ordinal differential equations solution of which can be obtained through a well-developed implicit method. Instability in the calculation due to sonic singularity was avoided by a method which was developed uniquely in this study. The analyzes shows a propulsive mode is determined by the relationship between a characteristic length of finite-rate chemical reactions and a projectile length ; in a large apparatus the transdetonative propulsive mode tends to appear at relatively low Mach numbers. This result suggests that a small apparatus has an advantage to obtain stable subdetonative mode. For a large apparatus, a similar operation is possible by adjusting the fill pressure of the mixture. The experiments were conducted using a world-smallest working ram accelerator with a 25-mm-bore. As was predicted by the analyzes, ram acceleration was realized with a relatively energetic mixture. This study points out the importance of a chemical reaction induction length in determining the thrust performance. This conclusion should lead to further development of quantitative clarification of the scaling laws on ram accelerators.