Liquid Oxygen Test Results for an Optical Mass Gauge Sensor

Abstract

Measuring the mass of propellants for long duration missions is desirable in order to avoid carrying excess propellant mass, and its associated tankage, to compensate for uncertainties in the propellant mass and propellant boil off calculations. Many of the stateof-the-art propellant gauging methods would not be appropriate for use with cryogenic propellants or would complicate the mission timeline with additional procedures including settling burns. This paper describes the design, construction, and testing of a propellant mass gauge suitable for low-g operation for various tank pressures, fill levels, and tank geometries, that does not interfere with vehicle operations. The OMGS has been designed with the intention of testing it in zero-gravity environment. The optical gauging technique relies on the optical absorption of the liquid oxygen. With no moving parts or valves required by the OMGS, reliability will be substantially increased relative to other mass gauging techniques. Bench testing previously indicated that accuracies better than 1% are achievable, independent of propellant thermodynamics and liquid-vapor interface configuration and position. Further, the propellant mass can be and has been measured on a continuous basis with better accuracy. Thus, health monitoring would be improved, mission support simplified, and propellant mass plus tank size/weight could be reduced with the improved accuracy. OMGS accuracy verification testing was performed in a government test facility utilizing liquid oxygen as the analyte. OMGS predictions were compared a load cell system supporting the weight of the half scale liquid oxygen tank. Liquid oxygen mass determinations were performed by the OMGS at a range of fill levels from zero to 1800kgs and at pressures as high as 265psia. The OMGS accuracy goal of +/3% of full scale was not met for all fill levels. One of the OMGS configurations yielded mass predictions within +/3% of the load cell measurements except for fill levels between 80 and 90%. Several OMGS configurations were utilized during accuracy verification testing. A discussion of test results and recommendations for accuracy improvements is contained within this paper.