Modern space missions face the problem of storing fuel onboard the spacecraft. The use of cryogenic fuel and the constant thermal effect of the Sun on the tanks endanger successful long-distance space flights. There is currently no infrastructure for refueling spacecraft. Considering that the fuel must be stored in a cryogenic state and that the tanks in which it is stored are exposed to the heat of solar radiation, it is not possible to collect enough fuel for a flight to Mars. NASA has been working on this problem and recently published a detailed review of some of its research.

Most missions use cryogenic hydrogen and oxygen as fuel. However, the tanks in which it is stored warm up due to constant exposure to solar radiation. Due to the lack of atmosphere, it is not possible to dissipate this heat, which can cause the fuel to boil and some of the fuel to be lost. When heating occurs, the fuel begins to boil, and mission engineers opt to eject gaseous fuel into space rather than leaving it as a potential environment for the rest of the fuel to heat more quickly.

The sublimated fuel could reach half the fuel needed for a mission to Mars in just one year. Storage of fuel in space due to the use of current technologies makes flights to Mars impossible. However, alternative solutions such as zero boil-off (ZBO) or reduced boil-off (RBO) systems are also available. These systems use “active” processes to maintain pressure in the tanks and prevent large fuel losses. An “active” process needs to be actively controlled and usually requires a certain amount of energy. ZBO systems are based on two technological ideas in particular: jet fuel mixing and droplet injection technology.

In jet mixing, some of the fuel is mixed back into the vapor space to control phase changes. The droplet injection system uses a special type of atomizer to inject fuel droplets into the vapor zone and reduce the pressure. However, these processes are difficult to perform in microgravity conditions because droplet formation and mixing of liquids in space do not occur just like on Earth. That’s why NASA is conducting experiments on the ISS to better understand these processes.

In 2017, the ZBOT-1 experiment, which evaluated the behavior of jets in microgravity conditions, was launched on the ISS. But even after 30 tests, there is still no full understanding of how these systems work in microgravity. Although their behavior is different from that on Earth, they still obey the laws of physics. Additional experiments will help learn how to better use ZBO systems.

Two other experiments aim to gain a deeper understanding of this question. One of these, the ZBOT-NC Experiment, is planned to be launched on the ISS in 2025. The effects of microgravity on “non-condensable” gases, which can be used to control the pressure inside the fuel tank, will be studied. Observational data can also be used in computer models used for fluid and gas motion (CFD) simulation and analysis. This will allow scientists to better understand how the model differs from reality in microgravity conditions.

Therefore, NASA is actively exploring the possibilities of developing fuel storage systems in space that would allow flights to Mars. Experiments to be carried out on the ISS will help develop technologies used to maintain pressure and prevent fuel loss during long space flights.

The last test in the series will be devoted to the phase change of the droplets. The ZBOT-DP test, the most ambitious of the three, was designed to test a technology that has not been used before in microgravity. Main attention will be paid to studying the interaction of droplets with the environment, including superheated tank walls in microgravity conditions. This could lead to the development of a fully functional drip system and an active control system that would prevent fuel from overflowing.

Dates for ZBOT-DP testing have not been scheduled yet. Given the importance of this technology in missions such as manned Artemis planned in the coming years, the successful completion of these experiments and the development and testing of a fuel tank equipped with the ZBO system should be high priorities for NASA.