On-orbit Propellant Transfer and Mass Gauging

Among the challenges posed by the proposed Lunar Gateway architecture is the storage and transfer of propellant and life support fluids between Gateway elements. Sustained human presence on the lunar surface will require on-orbit refueling of ascent/descent vehicles and the Orion Crew Capsule using technologies that do not yet exist at sufficient TRL. ECLSS systems require liquid volume knowledge and transfer rate measurements that also do not yet exist at sufficient TRL for Gateway. These vehicles and systems have unique gauging requirements driven by expectations of re-usability, long-term storage and transfer, and varying gravitational environments. As an example, Lunar Lander and Ascent Element vehicles will likely implement designs derivative of the Morpheus Lander prototype, which utilized four spherical propellant tanks in a horizontal cruciform configuration. Differential draining in the four tanks can lead to control instabilities that can only be mitigated by highly accurate propellant mass gauging.

The authors will review the state-of-the-art in low-gravity liquid mass gauging and describe the advances made in the Modal Propellant Gauging (MPG) project to provide on-orbit propellant mass gauging with 1-3% mass resolution during dynamic events such as engine burns and fuel slosh. Data from recent flight experiments with sub-scale tanks on both parabolic and suborbital vehicles illustrate the potential advantage of zero-g liquid mass gauging with sub-1% full-tank resolution in settled liquids and 1-3% gauging resolution during transfer events. Finally gauging data from recent tests on the Shuttle OMS tank during a fast drain simulation validate the authors’ computational approach to extracting contained mass from modal response measurements of the tank during drain operations. Proposed ConOps for liquid mass gauging on the Orion ESM will be discussed in the context of EM-3 and subsequent Orion missions.