Propellant mass gauging in microgravity has posed a challenge for decades. Various methods have been applied, including ultrasonic, capacitance probes, point level sensors, thermal detectors (thermistors, thermocouples, etc.), Michelson interferometry, and nuclear devices. All have problems in terms of how to provide accurate measurements irrespective of the fluid orientation in the tank.
A new method for determining propellant mass in tanks under microgravity conditions resolves the problems of high measurement uncertainty in mass due to the liquid position in the tank being unknown and not easily controlled. The method is relatively insensitive to propellant orientation and configuration. It is easily integrated with tanks having complex internal hardware, as well as tanks that have essentially no internal hardware. Analysis and preliminary test results with simulant liquids show that accuracy is of the order of a few percent, irrespective of propellant position or the tank fill fraction. The instrumentation required is essentially available as commercial off-the-shelf (COTS) hardware or relatively easily modified versions of such hardware that would be required for space applications.
In principle, the dielectric constant of a liquid is known for a given temperature and/or density for a given pressure. Normally, gases have dielectric constants that are very close to one at standard temperatures and pressures, but at cryogenic temperatures and high pressures, more accurate measurements of capacitance would take into account this slight increase in dielectric constant. The mass of liquid is directly related to the capacitance measurement throughout the entire tank, and this measurement is reasonably accurate no matter where or how the liquid is oriented within the tank. The tank simply requires a few wires, rods, screens, etc. — any of which can be configured so as to form a capacitor throughout the entire tank, with the tank walls grounded. There are no high voltages, no EMI, and measurements are easy to make with COTS hardware.
The running average of total tank capacitance improves measurement accuracy as the propellant location and configuration changes due to such disturbances as attitude control, station keeping, orbital changes, roll control, ullage gas venting, onboard disturbances due to operation of equipment, etc. For instances in which the propellant position is approximately known, such as with propellant “settled” at the bottom, the measurement accuracy is improved.
Certain internal hardware can be used as electrodes. For example, the channels of liquid acquisition devices or thermodynamic vent system tubes can form the non-grounded electrode simply by having this hardware electrically isolated by simple non-conductors, such as Teflon couplings.