Suppression of Superfluid Film Flow in the XRS Helium Dewar

Abstract

Superfluid helium dewars for space missions use a porous plug phase separator to retain the bulk liquid while releasing boiloff vapor. Although the thermomechanical force is quite effective in holding the liquid against the internal vapor pressure, van der Waals forces will lead to a leakage of helium through the porous plug to feed the flow of a superfluid film in the vent line. Since the latent heat is generally not extracted from the remaining liquid, the film poses a serious problem for space missions such as the X-Ray Spectrometer (XRS) which have extremely low heat load budgets. A design for the XRS porous plug and vent system is presented which keeps film losses below 2 µg/s through a combination of a heat exchanger, to evaporate the film, and a knife-edge device. Tests of the vent have been conducted in a minus 1g configuration. At the expected on-orbit tank temperature of 1.22 K, film losses are negligible, but rise to 4 µg/s at the maximum tank temperature of 1.30 K. Design parameters and performance tests are discussed.