In missile-loading and certain other transfer systems, large quantities of cryogenic liquids must be transferred in a relatively short time. Such systems must be cooled to the point at which transfer is accomplished as a liquid rather than as a gas in a fraction of a minute, or, at most, in a few minutes. Four significant factors influence the cool-down time of a transfer system: (1) transfer pressure; (2) heat leak to the fluid from the environment; (3) line flow impedance; and (4) quantity of system mass that requires cooling. These factors have been studied experimentally by Burke et al., who have developed an equation for predicting cool-down times of horizontal lines.