Nitrous oxide is in a gas-liquid equilibrium state at room temperature, and its vapor pressure is very useful concerning devices such as a pressure-fed hybrid rocket engine systems. However, it is not easy to estimate the flow history of the equilibrium flow. The vapor pressure and the latent heat of the vaporization of nitrous oxide strongly depend on its temperature. Though many models have been proposed, we used a simple equilibrium model and examined important parameters and negligible parameters considering experimental results. Experimental results displayed a substantial difference between the top and bottom tank temperatures, and that the tank pressure greatly matched the saturated vapor pressure at the liquid-phase temperature. A simple analysis showed that the determination of initial temperature was the most important factor and the heat transfer from the tank wall to the nitrous oxide was negligible.

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Nitrous oxide (N2O) is a suitable propellant for small rocket engines (mostly kN class), and has been widely used in various countries given its high saturated vapor pressure (i.e., 6 MPa at 300 K), which enables self-pressurization. Because nitrous oxide exists with gas-liquid equilibrium in tanks, cavitation occurs when the pressure in the tanks and feed lines drops slightly, which easily forms a gas-liquid two-phase flow. Since accurately estimating the performance of rocket engines requires ascertaining the characteristics of their propellant flows, flow tests with self-pressurization using nitrous oxide were conducted, as were firing tests of hybrid rocket engines using nitrous oxide as a liquid oxidizer and acrylic as a solid fuel. This paper presets the results of those tests, along with findings obtained regarding the characteristics of the gas-liquid two-phase injector flow with self-pressurization. In the N2O flow test, the injector upstream pressure was approximately 1.5 MPa, while the injector downstream pressure was approximately 0.1 MPa. At 0.070, the ratio of upstream to downstream pressure of the injector was thus extremely large, which suggested that the gas-liquid two-phase flow was choked with the injector. By contrast, in the firing test with a chamber pressure of approximately 1.0 MPa and a thrust of approximately 650 N, the ratio of the injector downstream pressure (i.e., chamber pressure) and its upstream pressure was approximately 0.56. Although that ratio was relatively large, because the injector upstream pressure is relatively low (i.e., approximately 1.8 MPa) and the backpressure fluctuated due to combustion, it remains unclear whether the gas-liquid two-phase flow was choked.

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