On October 1, 2022, 07:26 a.m. local time, the TEXUS-57 sounding rocket – a cooperative project between Airbus, the European Space Agency (ESA) and the German Space Administration (DLR) –lifts off from the launch pad at the Esrange Space Center in Sweden with four experiment modules on board. A moment of relief for the entire science team. At least for a short while. Whether the experiment in space is a success will only be known later, after the data has been evaluated. Many things can go wrong. The experiment can be damaged during launch. The rocket can have a hard landing, or it can crash into a lake – in northern Sweden, twenty percent of the area is covered with water or swamps. These are the risks and hurdles of space exploration. But if all parts function – from the tiny experiment valves to the large rocket engines – a real data treasure from space can be obtained. This data treasure was now comprehensively utilized by the CDIC-4 science team (CDIC stands for Chemically-Driven Interfacial Convection). Scientists from Helmholtz-Zentrum Dresden-Rossendorf and TU Dresden, University of Szeged and Université libre de Bruxelles worked together with the microgravity data.

 Knowledge for planet Earth

New findings in this field could help improve environmentally relevant technologies, such as CO₂ storage or soil remediation. In soil remediation, active ingredient solutions are introduced into soils under pressure to bind pollutants or solidify the soil. In the case of CO₂ sequestration, a carbonate-rich solution is injected into alkaline soil, where it encounters a solution of alkaline ions that mineralize the CO₂.Central to both methods is the reaction of two liquids as one flows into the other – dynamic reaction fronts with complex flow patterns form in the process.

Calculating such reaction fronts in scientific models, including the complete flow dynamics, is currently still very complex and time-consuming. Therefore, the science team from Germany, Hungary and Belgium is investigating whether certain scenarios can be handled with simpler models. Excluding influencing factors can reduce the computational effort tremendously. One such factor is buoyancy in liquids since it has little influence on porous media, such as soil materials. Put simply, this is because the individual pores in the material are so small that wall friction and flow pressure dominate what happens.

 Turning off gravity

This is where gravity enters the picture, as it is the physical cause of buoyancy. The team in Brussels already developed respectively simplified theoretical models. They describe the injection flow, the chemical reaction of two liquids, and their diffusion, i.e., the random self-motion of the molecules. However, they do not take gravity into account. With these models, the reaction front can be calculated on a normal PC. With the effects of gravity taken into account, high-performance computers are needed to solve the complete flow equations in direct numerical simulations. But: Even the simpler models without buoyancy must first be validated. Experimentally, this is much easier to do in weightlessness than in porous media. Microgravity basically switches off all the buoyancy force, and effects can be observed that are otherwise overshadowed by the gravity on Earth.

 In the first step, this was achieved by a parabolic flight campaign. The state of weightlessness is reached several times, at least briefly, during the free fall of the aircraft: the research group had 22 seconds per parabola. Porous-media flows where replicated using a simplified setup, the so-called Hele-Shaw cells. Each of these cells consists of two planar acrylic glass plates stacked parallel to each other with a very narrow gap in between, into which two different liquids are injected one after the other. A ring-like flow pattern develops as a result.

 By comparing the experimental data obtained in weightlessness with the simplified models, the researchers were able to compare product formation trends with theoretical predictions. But this comparison was limited because the 22-second intervals of weightlessness were not long enough for the diffusion process to take place. Therefore, the parabolic flights were just a first step, and the rocket flight a huge opportunity. In 2016, the group presented the first technical design of an experimental module for the European sounding rocket program. Three years later, the flight ticket was granted.

 Six years of preparation for a six-minute experiment

The space mission placed new demands on the hardware set-up once again. During the rocket's launch and landing, enormous forces affect the experiment, and space is extremely limited. Moreover, once launched, experimenters can no longer intervene, so every centimeter and every second had to be meticulously planned. On board the rocket, three cameras were recording the reaction fronts live. Reassuringly, for the first time a "digital downlink" was provided for this mission. The team could have saved at least a few images if something had gone wrong during the landing.

 Since the beginning of 2020, the researchers and engineers met regularly with their development partner, Airbus, in Bremen for testing. Then, unexpected circumstances unfolded: the Corona pandemic, a major fire at the launch pad, and eventually, in February 2022, one day after the first test countdown, the launch was cancelled due to the war in Ukraine. Finally, in October 2022, the delayed launch took place. Six years of preparation for thirteen minutes of flight time and six minutes of weightlessness - it was worth the effort.

 Value of microgravity data

With the new data from the rocket experiment, the research group was able to compare the influence of diffusion on the reaction fronts with the predictions of the models. The joint evaluation showed that simple one-dimensional models can be used in very shallow reactors with a slow flow. For larger reactors or faster flow, however, two-dimensional models are necessary. Within these validity ranges, the corresponding correlation relationships can now be applied to predict product formation. This is used in the design of innovative reactors, the targeted synthesis of particles and fluid transport in geological layers, but also in the supply of space stations, where gravitational conditions differ from those on Earth.