Liquid Hydrogen Mass Gauging: Sensor Concepts for Future Hydrogen Aircraft Tanks

The safe and precise measurement of liquid hydrogen (LH2) tank levels is a key challenge for the use of hydrogen as a sustainable aviation fuel. The envisioned large cylindrical hydrogen tanks differ greatly from today´s integrated wing tanks. While traditional Fuel Quantity Indication Systems (FQIS) rely on tens of capacitive probes, temperature sensors, compensators and high/low-level sensors to achieve the required robustness, the necessary sensor systems for an LH2 FQIS that meet future requirements remain unknown. Although multiple LH2 level measurement principles have been applied in industry, automotive, and space applications, no complete system has yet been developed at the scale and performance required for future LH2 aircraft. Historically, there have been test flights powered by turbines or fuel cells using LH2, but for these prototypes no full FQIS was developed; instead, fuel duration was estimated, or a single probe was used. This paper focuses on identifying the requirements of today´s FQIS, addressing both robustness and precision. While regulations regarding LH2 fuel systems for civil aviation (ISO/AWI 19888, SAE AS6679) are still under development, existing standards for LH2 systems in the automotive sector can be combined with the requirements of modern FQIS and current LH2 aircraft design studies to provide insights into the operational requirements and challenges of LH2 FQIS systems. In addition, an overview of measurement concepts for LH2 fill-level sensors is presented, along with initial experimental results obtained from tests with capacitive probes in cryogenic nitrogen, non-invasive vibro-acoustic methods, and temperature-based fill-level sensors.