Design Procedure of a Movable Pintle Injector for Liquid Rocket Engines

Abstract

Variable-area injectors are suitable choices for developing throttleable rocket engines because it is difficult to efficiently control thrust when fixed-area injectors are used. A pintle injector is a variable-area injector that can control the mass flow rate of propellants, thereby replacing a large injector plate having several injector elements with a single injector unit. To achieve proper performance under all throttling conditions, a design procedure was established for a pintle injector based on spray characteristics. The spray angle and Sauter mean diameter of the droplets were critical performance parameters. In spray experiments using water and air as simulants under atmospheric conditions, backlight imaging and laser-diffraction methods were used to measure the spray angle and Sauter mean diameter. The velocity ratio, nondimensionalized opening distance, and Weber number were considered as major nondimensional numbers. Empirical correlations were formulated between performance parameters and nondimensional numbers; these correlations were applied to the design procedure herein. Consequently, a 500 N combustor with a pintle injector was designed using this procedure. A control plan for the pintle-opening distance under lower throttling conditions was also obtained for designing a small combustor. Furthermore, the most sensitive parameters in the injector geometry were confirmed via sensitivity analysis.