Effect of sloshing on the mechanics of dewar systems in low-gravity Original Research Article

The mechanics of a spacecraft or space module, that contains a dewar system, is affected by sloshing of the cryogen. When a spacecraft is in orbit the acceleration level is very low and sloshing differs in several ways from that on Earth. Surface tension replaces gravity as the restraining force. The liquid–gas interface is curved and the depth is variable. Cryogens have low surface tension and this causes the resonant frequencies of sloshing to be very low. Programs were developed to calculate sloshing in low-g in dewars with annular and circular cross sections and with segmented and ring baffles. The programs correct for surface curvature and variable depth. The driving force for sloshing is the motion of the wall of the dewar. Dewar motion is an unknown because the sloshing force contributes to the motion of the support structure. Programs were also written for the dynamics of the system treating the motion of the dewar as an unknown. The combined program for sloshing and dynamics is called LOWGSLOSH. The calculations are carried out in the frequency domain so that both coherent and random driving forces can be used as input. The output is the motion of the spacecraft if the dewar is firmly attached to the vehicle. For modules suspended by a vibration-isolation interface, the output is the acceleration spectrum of the module. The calculations show the conditions when sloshing has an appreciable effect on the spacecraft or module. The timeline of spacecraft maneuvers is very important in determining the sloshing amplitude. In many cases, it is possible to reduce sloshing drastically by timing a maneuver properly. The results are illustrated using calculations on two NASA projects: the Space Infrared Telescope Facility spacecraft (SIRTF) and the Low Temperature Microgravity Physics Facility (LTMPF), a module for the International Space Station.