The long-term storage and in-orbit transfer of cryogenic propellants will be key technologies for enabling a European in-space transportation ecosystem. An in-space logistics system consisting of a range of depots and refuellable orbital vehicles will enable sustainable, long term manned missions to Mars and the moon. One of the main challenges to achieve this vision is the low saturation temperatures of cryogenic propellants which introduce additional challenges in their management. At present, neither long-term storage nor refuelling with cryogenic propellants has been demonstrated in-orbit, and the technologies and processes necessary lack maturity in Europe.

CRYSALIS (CRYogenic Storage And refueLling In-Space) is a collaborative EU funded project (101135431) between Absolut System, The Exploration Company, the Universitat Politècnica de Catalunya - BarcelonaTech, and the Université de Liège, to mature the technologies necessary for the storage and transfer of cryogenic propellants, in particular liquid oxygen and liquid methane. The project aims to perform a small-scale in-obit demonstration with liquid nitrogen on-board the Nyx Earth capsule in 2028, to permit the maturation of technologies that cannot be matured on earth and to improve the knowledge of cryogenic fluid behaviour in a microgravity environment.

The UPC-BarcelonaTech will develop three techniques based on acoustics in the framework of the CRYSALIS Project. These techniques aim at the control of boil-off in the tank [1], the measurement of propellant mass [2], and the management of bubble dynamics.

A preliminary feasibility study and design for the demonstration will be presented, including the main objectives, with a focus on the technologies to be matured by UPC-BarcelonaTech.

The commercial reusable sub-orbital rocket industry in the US provides approximately three minutes of weightlessness at 1-millig or better acceleration. Three automated and one human-tended sub-orbital two-phase fluids experiments, flying on two vehicles illustrate types of research feasible in these vehicles.

“Cryo-Gauging” is a room-temperature payload (Fig.1) to model positioning of a cryogen in the RFMG launched by others to ISS. The interior of the RFMG tank[1] lacked axisymmetry and thus, non-symmetric liquid positioning needed investigation. Cryo-Gauging flew a representation of the RFMG tank on Blue Origin’s New Shepard rocket. It demonstrated critical-like wetting in between a circular post and a flat, variable-position panel, Fig.2. Post-flight analysis shows agreement with Surface Evolver modelling.

“2D Slosh” flew on New Shepard in December, 2023 to investigate contact line motion in response to periodic excitations, Fig.3. Fig.4 is one example of zero-g liquid positioning (near-planar interface was the goal). Static contact angles are substantial, between 40 and 90 degrees, and the test vessels are designed to produce largely two-dimensional flow. Two liquids and two different solids produced four combinations.

“Rotational Slosh” is nearing completion as an automated payload for Virgin Galactic sub-orbital missions. It examines the damping time of liquid propellant excited rotational maneuvers, such as for docking. Tank geometry is from the Cassini spacecraft[2], Fig.5. Fig.6 shows cradles that each hold one tank to be rotated about tank center or end. Video records liquid response. Eight tanks in the experiment, four with a good wetting liquid and four with a poor wetting liquid. Each set of four has 20%, 40%, 60%, and 80% fill fractions. This arrangement provides more data than could be acquired by pumping liquid out of or into tanks.

In December 2021, NASA Flight-Opportunities Program selected the author’s proposal to fly a human-tended experiment on Virgin Galactic. All parties are working through steps preceding flight scheduling, there is not yet a flight date. The experiment examines non-linear contact line advance for three purposes: acquire high-quality zero-g data to advance CFD capabilities, learn to perhaps automate future experiments, and demonstrate a 1-g screening test to assess which combinations of liquid, solid, solid surface structure, etc., are likely to be dependable or undependable wetting in weightlessness.

Commercial re-usable sub-orbital spaceflight is a useful laboratory for at least fluids. Access is simpler and total cost is much lower than to orbit. Boost vibration is weak and thus experiment construction is simpler.

The spacecraft tanks may have to be refilled in space to carry out long-term space exploration missions in the future. The refueling can be done with the help of propellant depots that help to store and transfer propellants to spacecraft tanks. A complete understanding of the physics that control fluid flow in reduced gravity is necessary to successfully refuel tanks in space. The stronger body forces are responsible for a flat interface in normal gravity. On the other hand, a change in the shape and position of the interface is caused by stronger capillary forces in reduced gravity. During liquid refueling, the gas present inside the tank may or may not be vented. The determination of the stability limits of an interface during the filling of liquid into a tank will help to regulate the self-pressurization of the tank and ensure a liquid-free venting.

This work discusses the vented and no-vent filling of a storable liquid into an experiment tank. The vented fill tests, which are performed under isothermal conditions in microgravity with a multi-species system are flow rate-driven. The no-vent fill tests, which are performed under non-isothermal conditions in normal gravity with a single-species system are driven by the pressure difference between two containers. The temperatures and pressures are measured in the experiment setup with the help of the sensors and the flow patterns inside the experiment tank are recorded using high-speed cameras.

While the vented fill tests investigate the stability of the free surface under microgravity conditions, the goal of the no-vent fill tests is to test various scenarios related to handling a single-species system and to understand how thermodynamics and fluid mechanics influence each other. Furthermore, the vented fill tests are modeled in CFD and compared with the drop tower experiments. All of the information gathered is in favor of designing an international space station experiment that will demonstrate the liquid removal, transfer, and filling processes in microgravity conditions.

Advances in modern space propulsion systems heavily rely on understanding and managing the complex thermodynamic phenomena within their cryogenic propellant tanks. These fluids demand chilling storage temperatures, making the tanks susceptible to heat ingress and propellant boil-off. The challenges in thermal control are aggravated by sloshing, which disrupts the dynamic stability of the spacecraft and produces violent pressure fluctuations, potentially hindering the smooth fuel supply to the engine. Consequently, controlling the tank's operating conditions requires advanced control strategies tailored to each stage of the spacecraft's mission profile. However, this requires robust modeling of the underlying heat and mass transfer to accurately predict the system's thermodynamic evolution.
As a part of a broader research effort characterizing sloshing for space propulsion, this work focuses on the propellant's thermo-hydraulics during the ballistic flight phase. In this context, the presented work combines experimental methods and data-driven modeling techniques to characterize the tank's thermodynamic evolution in microgravity conditions. The experiments were conducted in the 83rd ESA parabolic flight campaign within the NT-SPARGE (Non-isoThermal Sloshing PARabolic FliGht Experiment) project. The setup consists of four major components: a 'donor' tank carrying superheated HFE-7000 vapor, a 'receiver' tank partially filled with liquid HFE-7000, a pressurizing line connecting these two, and an acquisition system collecting all sensor information. The setup is instrumented with pressure transducers and racks of thermocouples measuring the spatial distribution of temperature inside and outside both tanks and along the pressure line. The experiment comprises two stages: (1) pressurization and thermal stratification and (2) disruption of the 'receiver' through sloshing.
The system is modeled via a 0D thermodynamic model derived from conservation balances applied to the gas, liquid, and solid regions. This implementation expresses heat and mass exchanges between each control volume through flux terms, depending on some heat transfer coefficients. We write these terms through parametric functions depending on the model's thermodynamic state and a set of (unknown) weights. We employ the acquired data to derive and update these functions using an adjoint-based non-linear optimization that identifies the weights by minimizing the discrepancies between model prediction and system evolution. Enriched by the data-driven thermodynamic closure, the identified model tracks the thermodynamic history of the 'receiver' tank during microgravity sloshing. Given the model's negligible computational cost, it can be integrated into broader system models, enabling real-time predictions and opening the path toward model-predictive control and anomaly detection in cryogenic thermal management systems.

Introduction

Cryogenic fluid management presents formidable challenges, especially within the context of orbital launcher missions, where different dynamical behaviours of the cryogenic propellants can be identified as a function of the flight stage (Dreyer, 2009). In the propelled phase, the liquid settles at the bottom of the tank, and the launcher's lateral motion governs the excitation of sloshing. Conversely, in the ballistic phase, when the upper stage performs manoeuvres to release the payload into orbit, capillary forces dominate. Consequently, any slight disturbance triggers substantial liquid motion (Werner et al. 2019). This fluid motion in the launcher's tank is responsible for mixing the superheated gas and subcooled liquid, causing significant pressure fluctuations (Marques et al., 2023).

In the current NT-SPARGE project, sloshing-induced thermal mixing in cryogenic propellant tanks is characterised for different launcher phases: normal gravity and microgravity conditions. The on-ground investigation was performed near the lowest eigenfrequency, usually found in space launcher tanks during the ascent phase (Arndt, 2012), while the microgravity characterisation was conducted at the 83rd ESA parabolic flight.

Experiment

Figure 1 displays the experimental setup, which consists of an active-pressurisation system. It operates a pressurant reservoir containing superheated vapour to pressurise a small-scale tank. The tank is partially filled with liquid, and different liquid-gas interface dynamics arise depending on the acceleration conditions. These dynamics disrupt the cell's thermodynamic equilibrium by destroying the thermal stratification created during the pressurisation.

For the experimental campaign, HFE-7000 was used as a cryogenic propellant substitute and a single-species environment guaranteed by pre-conditioning the system. The measurements were carried out in a quartz sloshing cell instrumented with a pressure transducer and three racks of thermocouples to measure the temperature distribution.

Results/Conclusion

Figures 2 and 3 highlight the preliminary results for the non-dimensional pressure evolution. On-ground tests were conducted at the von Karman Institute's SHAKESPEARE shaking table. The sloshing experiments proved that the free surface motion is much stronger in microgravity, where the inertial wave at aircraft injection wets the ullage walls, triggering evaporation followed by an abrupt pressure drop as the subcooled liquid encapsulates the gas. On the other hand, only a characteristic pressure drop arises during on-ground tests since the liquid settles at the bottom of the cell.

In the extended version of this work, we present a detailed investigation of the free-surface dynamics and position both the microgravity and on-ground sloshing experiment within a non-dimensional map of sloshing regimes.