The design and development of systems for collecting and delivering liquid cryogens to a supply tank drain inlet presents unique problems in hydrodynamic and thermodynamic control. Solution of these problems requires perceptive interpretation of available technology and realistic appraisal of any need for new supporting technology. In general, the state of the art is sufficiently advanced at this time to warrant proceeding with designs to control liquid cryogens in low-g environments. There must be assurance that there are no failure modes that cannot be accommodated by design schemes. Analyses are necessary to determine the draining efficiency of these devices because low residuals in the supply tank are essential. Three types of fluid control have been demonstrated successfully in flight.