Advanced fluid management technologies are required to support upcoming space missions.
Front image: Artist illustration of the Lunar Flashlight’s lasers scanning a shaded lunar crater for the presence of ice. (Credit: NASA/JPL-Caltech)
Low-Gravity Fluid Mechanics in the Artemis Era
By: Álvaro Romero-Calvo
The space community is undergoing an accelerated transformation boosted by the commercialization of the sub-orbital environment and the ambition to make humans a multi-planetary species. In the public eye, this adopts the form of an astronaut landing on the Moon, a tourist reaching the edge of space, or a massive launch vehicle taking off from the ground. The critical role played by fluid mechanics in these and other activities is, however, not so well known. Fluid management is key for life support, thermal control, propulsion, sample analysis, and other space applications. Upcoming space missions are pushing the limits of space fluidics to the point where the suitability of traditional solutions is no longer clear. In this context, the Artemis Era brings an opportunity to deepen our understanding of low-gravity fluid systems.
Addressing the dynamics of liquids in microgravity and partial gravity has traditionally been considered almost the same as studying capillary phenomena, with one important distinction: the interest in the former arises from its applicability to space systems rather than the fundamental understanding of surface-tension-driven flows. From a historical perspective, most of our knowledge in the field comes from the parallel and almost independent* efforts conducted in the United States and the Soviet Union during the Space Race. Initial studies focused on the equilibrium, stability, and modal response of liquid interfaces subject to capillary and inertial forces and, therefore, modern fluid management devices are mostly based on these fundamental interactions. Is that all? Well… not really. As early as 1963 the USAF was already considering electric, magnetic, and acoustic forces to control liquid expulsion, and ferrofluids were invented at this time to position rocket propellants. These approaches are intended to overcome the limitations imposed by more traditional methods. For instance, wettability-based propellant management devices are unable to remove small gas bubbles nucleated over hot tank surfaces, a problem that has become particularly critical for recent cold-gas SmallSat missions and that can potentially be solved by inducing mid-range electromagnetic forces on the liquid. Why, then, are these alternative approaches barely known?
Caption: Astronaut Kjell Lindgren brews coffee in space using the Space Cup. Capillary forces hold the liquid but are unable to remove gas bubbles. The same applies to traditional propellant management devices in spacecraft propellant tanks.
Three reasons may explain the limited scope of traditional low-gravity fluid mechanics: (1) the early stage of development of related technologies at the time when these ideas were first formulated, (2) the lack of dedicated study plans at the undergraduate (and even graduate) levels, and (3) a simple matter of opportunity. The latter is, probably, of major interest to us. Upcoming missions planning on refueling massive amounts of cryogenics in orbit (SpaceX Starship), operating lunar vehicles, orbiting miniaturized laboratories, or sending humans in a Mars-transit are pushing the state of the art in this field. Do we really understand low-gravity cryogenic sloshing? Do we have methods to compensate for limited mass transfer in partial-gravity boiling? Can we guarantee the reliability of the life support system of a Mars transit vehicle? Or easily automate biological payloads requiring liquid sample management? The answer is, unfortunately, no. Some of my colleagues at NASA, industry, and academia are doing an amazing job exploiting capillary and inertial interactions to attempt to solve these challenges. Others, among whom I am included, have noticed that non-traditional approaches based on electromagnetic and hydroacoustic force fields may offer an alternative path. Turns out we can easily control bubble trajectories with small neodymium magnets, enhance boiling through dielectrophoresis and conduction pumping, drive bubble behavior with acoustic fields, or gauge propellant residuals using acoustic actuators. We do know the basic physics governing these mechanisms, but we still need to characterize their impact on liquids in microgravity and partial gravity. Most importantly, the industrial infusion of these new technologies must be supported to ensure the successful development of a new generation of fluid management devices.
Magnetic separation of gas bubbles in microgravity (L: non-magnetic, C: magnet at the right, R: magnet at the left). This mechanism could be used in a future space station to debubble liquid samples or automate complex biological experiments.
“…the entire economic proposition of sustained human presence in space depends critically on low-gravity fluid systems.”
Low-gravity fluid mechanics should not be just the technical equivalent of capillary phenomena. We must adopt a more general perspective on this field and start considering additional interactions that, for different reasons, haven’t gathered the attention they deserve. We must update our study plans to provide the next generation of space engineers with a solid background on the topic. We must make sure that the space industry participates in the development of these promising tools. And we must do it because the entire economic proposition of sustained human presence in space depends critically on low-gravity fluid systems. After all, if we are to play chess with Nature, why would we renounce half our pieces?
* Myshkis and coworkers bitterly lament in their reference textbook, Low-Gravity Fluid Mechanics (1987), that “unfortunately, many American works on this subject […] were not accessible to the authors”. What these and other pioneers could have achieved in a more favorable political context is only limited by our imagination. ‘
Further Reading:
Romero-Calvo, Á., Akay, Ö., Schaub, H. et al. Magnetic phase separation in microgravity. npj Microgravity 8, 32 (2022). https://doi.org/10.1038/s41526-022-00212-9.
Dr. Álvaro Romero-Calvo is an assistant professor at the Daniel Guggenheim School of Aerospace Engineering at Georgia Tech. A board member of ASGSR (American Society for Gravitational and Space Research) he is the vice-chair of the COSPAR Commission G (Material and Fluid Sciences in Space Conditions) and a member of the AIAA Microgravity and Space Processes Technical Committee.