This study proposes a one-dimensional droplet model to predict the jet-tip height of a subcooling mixing jet issuing from the bottom of a cryogenic propellant storage tank. Cryogenic liquids, such as liquid hydrogen and liquid oxygen, are used as propellants and oxidants in spacecraft propulsion systems that require long-term storage in a closed tank. However, thermal stratification forms near the gas-liquid interface during long-term storage of cryogens due to heat flowing into the tank from the surrounding environment. In addition, boil-off gas (BOG) is generated from the interface, which causes increased pressure in the tank. To reduce the BOG, it is effective to destroy the thermal stratification by mixing in the cold jet issuing from the bottom of the tank. Ground experiments using FC-72 and water as test fluids are conducted to investigate the behavior of the jet using the proposed one-dimensional spherical droplet model as the tip of the jet. The jet behavior is visualized using the Shadowgraph system and the height of the jet-tip is investigated under various experimental conditions. The proposed model is also verified by comparison with experimental data available in the literature. The results show that the proposed model aligns well with the experimental data.