Cryogenic Propellant Thermal-Fluid Management for Chemical and Nuclear Spacecraft Propulsion: Importance, Physics, and Zero-Boil-Off Strategies

Cryogenic propellants such as liquid hydrogen (LH₂), liquid oxygen (LOX), and liquid methane (LCH₄) are critical to high-performance chemical and nuclear propulsion systems. However, their storage, transfer, and utilization in space environments are fundamentally constrained by heat ingress from external environment, phase-change boiloff loss, and complex two-phase flow dynamics under reduced gravity and microgravity. This paper outlines the importance of cryogenic thermal-fluid management (TFM), identifies the governing physical processes, and presents key system elements required for reliable long-duration propellant storage and transfer. Particular emphasis is placed on Zero-Boil-Off (ZBO) technologies, which are essential for enabling in-space refueling, propellant depots, and nuclear thermal propulsion missions.