Efficiency of aft diffusers in vacuum systems of test benches for high-altitude firing tests of low-thrust liquid rocket engines

The article presents the results of calculations and experimental studies of a vacuum system with an aft diffuser for high-altitude firing tests of low-thrust liquid rocket engines. The aim of the study was to determine the geometric dimensions and setting parameters of aft diffusers. The diffusers ensure operation of the vacuum system that implements the regime of unseparated flow of the combustion products in the nozzle of the engine being tested. We carried out the experiments using 50 N liquid rocket engines with the geometric area ratio equal to 50. We investigated the impact of the setting parameters of the aft diffuser relative to the engine’s nozzle exit: the position of the inlet section of the diffuser relative to the nozzle exit varied in the range from -2 to +10 mm in the axial direction. We analyzed the influence of geometrical parameters of aft diffusers that differed in the inner diameter of the channel. The relative channel length was equal to 10. We determined the structural variables of an aft diffuser and its position relative to the engine’s nozzle exit and the pressure ranges after the exit section of the diffuser that ensure the mode of  unseparated flow of the combustion products in the engine nozzle. The efficiency of the vacuum system of the test bench increases as the internal diameter of the aft diffuser approaches the diameter of the nozzle. Other things being equal, the vacuum system provides longer periods of engine tests maintaining the design mode of nozzle operation.