Pressure drop in spacecraft internal cavities at their arrival on orbit is not as quick as it could be inferred from ambient pressure drop. This induces risks of corona discharges or (re)condensation, in particular when turning on or heating up sub-systems. At the origin of this phenomenon, the outgassing of internal cavities submitted to vacuum, as well as the subsequent drop in pressure, were simulated with a mockup in a vacuum chamber at CNES.

An organic sample was outgassing in the upper cavity of the mockup. The pressure changes due to variable temperature profiles of the sample and of the various cavities were studied (in particular with occasional wall reheating). The physics involved was discussed and modeled.

In addition to the outgassing of the sample, adsorption and desorption on the walls of the cavities proved to be key phenomena to explain the pressure rises during their occasional reheating. Classical simple models resulting from macroscopic measurements such as evaporation (of water in particular), or single-energy adsorption models, failed to reproduce the data. Reproducing the order of magnitude of the measured pressure bursts required using more advanced models taking into account the specificity of the first adsorbed layers, typically with a whole distribution of adsorption/desorption energies, far from the binding energy of water molecules in the “bulk” (as derived from the evaporation of macroscopic condensates).