Capacitance Mass Sensing of Boiling Propellants

No pressurizing gas is introduced into the propellant tanks of the Centaur vehicle during engine firing. The consequent pressure decay causes the propellants to boil, and the resulting gas bubbles change the effective density of the propellants. For liquid hydrogen in the Centaur, this change is 1.4% and would show up as an error in any level sensing propellant utilization system. A perforated capacitance probe shows a net 1/2% error because the bubble population is not the same inside and outside the probe. However, a manometer type capacitance probe senses propellant mass without a bubble induced error. This paper outlines the theoretical background for this effect, and presents the results from medium-scale tests in a general form applicable to vehicles other than Centaur.