IMPROVED PRESSURE-VOLUME-TEMPERATURE METHOD FOR ESTIMATION OF CRYOGENIC LIQUID VOLUME

Abstract

One of the most important issues in a liquid propellant rocket is to measure the amount of remaining liquid propellant for reignition under low gravity environment during space mission. This paper presents the results of experiment and analysis of pressure-volume-temperature (PVT) method which is simple, reliable and useful for gauging liquid quantity with uneven liquid-vapor interface. The experiment is conducted using 7.4 liter tank for liquid nitrogen with various liquid-fill level. To maximize the accuracy of PVT method with minimum hardware, the technique of helium injection with low mass flow rate is applied to maintain stable temperature profile in the ullage volume. The PVT analysis considering both pressurant and cryogen as a binary mixture is suggested. At high liquid-fill level of 72%~80%, the accuracy from the conventional PVT analysis is within 5%. At low fill level of 27%~30%, the gauging error is within 3.5% using mixture analysis of PVT with specific low mass flow rate of helium injection. It is concluded that the proper mass flow rate of helium injection and PVT analyses are crucial to enhance the accuracy of the PVT gauging with regard to various liquid-fill level conditions.