Cryogen acquisition in orbit

Orbital cryogen acquisition is a definite requirement for engine restart and/or propellant transfer on all major future missions. The most promising methods, especially for large scale vehicles, are acquisition by linear acceleration and acquisition by capillary systems. Intermittent acceleration and propellant settling requires some state-of-the-art extension since it has not been attempted on a large scale cryogen vehicle. Capillary acquisition systems offer the advantages of reusability, passiveness, low weight, and mission flexibility. The major disadvantage is that of an undeveloped technology. The method does not lend itself to verification through ground testing.