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A numerical model that predicts the helium mass required for propellant tank pressurization during propellant outflow was developed. The model has the feature of including the effects of the internal hardware of the propellant tank such as baffles, pressurant storage tanks, and other instrumentation. It used a finite volume method that divides the ullage and tank wall one-dimensionally along the propellant tank axis. A series of cryogenic propellant drainage tests were carried out to verify the developed numerical model. The required amount of helium mass predicted by the model showed very good agreement with test data within an accuracy of 2:27% under the operating conditions. The developed model was applied to the pressurization system of Korea Sounding Rocket-III, and the results were compared with the flight-test data. The comparison results showed that the developed model was satisfactory for the prediction of the required helium mass during flight. Additionally, a parametric study was performed to test the sensitivity of the developed model, and the results showed that the heat transfer coefficient between the ullage and the tank wall was the key factor in the accuracy of the model.

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Second throat supersonic exhaust diffusers (STEDs) were designed to simulate high-altitude conditions according to the normal-shock model. Experimental studies were performed on the STEDs to investigate how performance characteristics varied with the length and diameter of the STED using high-pressure nitrogen gas. The variation in performance due to length indicated that the performance of the STED could be very slightly improved by adjusting the diffuser inlet length ( ), and it could be significantly improved by optimizing the second throat length ratio (   ) and the divergence length ( ). The starting and vacuum chamber pressures exhibited the highest level of performance near     of the design point.