水下固体火箭发动机的负推力现象研究

Study on the negative thrust of the underwater solid rocket engines

针对水下固体火箭发动机工作环境压强高的特点,结合固体推进剂的燃烧特性,采用UDF方法定义喷管入口边界条件,建立了固体推进剂燃气质量生成与水下超音速气体射流的耦合计算模型。将该模型的计算结果与水下固体火箭发动机的实验测量结果进行对比,验证了该模型的合理性。研究发现,水下固体火箭发动机在点火初期会出现负推力现象,负推力产生的原因是发动机点火初期,喷管内被过度压缩的燃气冲出喷管后,在喷管尾部形成一个超音速燃气泡,超音速流动使泡内压强降低;同时受到流动惯性作用的影响,气泡持续膨胀使泡内压强进一步大幅降低,发动机前后端面上的压差最终导致负推力现象产生。

To simulate the working process of the underwater solid rocket engines,a coupling model between the process of gas generation and the process of gaseous jets injected into water was proposed.The burning rate of the solid propellant was considered in this coupling model and the User Defined Functions（UDF） were used to define the boundary condition of the nozzle inlet.The numerical method is validated by the experimental data.The results show that the negative thrust appears when the engines work in the water.The reason is as follows： at the initial stage of the propulsion process,the high compressed gas jets into water to form a supersonic ＂gas bag＂ behind the nozzle exit,causing the pressure in this area to drop suddenly.Furthermore,the inertia effect of the water causes the pressure in the ＂gas bag＂ to keep dropping,creating the negative thrust due to the pressure difference between the rear and front parts of the engine.