This SBIR Phase I proposal describes a fiber optic mass flow gauge that will aid in managing liquid hydrogen and oxygen fuel storage and transport. The increasing use of hydrogen as a fuel places the safety of facilities and personnel at the forefront of concern, making an explosion proof fiber optic system a desirable option. Further, the actual available fuel can depart significantly from the expected quantity due to variable boil-off and leak rates and inaccurate measurements. Measurement accuracy can be complicated by low gravity, stratification of the liquid density and temperature, the presence of a gas phase, the wetting of tank surfaces by the liquid and the lack of suitable sensors and instruments. The Phase II objective is to deliver a prototype that will withstand the high pressures and flow rates at liquid hydrogen and oxygen temperatures demanded by rocket propulsion systems.