Materials outgassing rate decay in vacuum at isothermal conditions
As a laboratory for scientific research, the International Space Station has been in Low Earth Orbit for over 17 years and is planned to be on-orbit for another 10 years. The ISS has been maintaining a relatively pristine contamination environment for science payloads. Materials outgassing induced contamination is currently the dominant source for sensitive surfaces on ISS and modelling the outgassing rate decay over a 20 to 30 year period is challenging. Using ASTM E 1559 rate data, materials outgassing is described herein as a diffusion-reaction process with the interface playing a key role. The observation of -1/2 (diffusion) or non-integers (reaction limited) as rate decay exponents for common ISS materials indicate classical reaction kinetics is unsatisfactory in modelling materials outgassing. Nonrandomness of reactant concentrations at the interface is the source of this deviation from classical reaction kinetics. A t-1/2 decay is adopted as the result of the correlation of the contaminant layer thicknesses and composition on returned ISS hardware, the existence of high outgassing silicone exhibiting near diffusion limited decay, the confirmation of nondepleted material after ten years in Low Earth Orbit, and a potential slowdown of long term materials outgassing kinetics due to silicone contaminants at the interface.
N-Nitrosodimethylamine Release from Fuel Oxidizer Reaction Product Contamined Extravehicular Activity Suits
Before extravehicular activity (EVA) on the Russian segment (RS) of the International Space Station (ISS), the docking compartment (DC1) must be depressurized, because it is used as an airlock. It is preferred to use the U.S. control moment gyros (CMGs) instead of attitude-control thruster firings to compensate for disturbances and to maintain the ISS vehicle attitude. However, when the DC1 is depressurized, the CMGs’ margin of momentum is insufficient to compensate for the disturbance and the service module (SM) attitude-control thrusters’ need to fire to desaturate the CMGs. The SM roll-control thruster firings induce fuel‐oxidizer reaction products (FORP) contamination on the adjacent SM surfaces around the thrusters. One of the components present in FORP is the potent carcinogen N-nitrosodimethylamine (NDMA). Because the EVA crewmembers often enter the area surrounding the thrusters for tasks on the aft end of the SM and when translating to other areas of the RS, the presence of FORP contamination is a concern. FORP contamination of the SM surfaces is discussed, along with the potential release of NDMA in a humid environment from crew EVA suits, whether they happen to be contaminated with FORP, the toxicological risk associated with the NDMA release, and the implementation of flight rules to mitigate the hazard.
Analysis of a coaxial-to-waveguide adaptor including a discended probe and a tuning post
A field matching analysis of a coaxial-to-waveguide adaptor incorporating a disc-ended probe and a tuning post is presented. In this analysis, the tuning post is considered as a short-circuited second probe. A computer algorithm for calculating the input impedance of this structure is developed. This algorithm is used to investigate and improve the operation of a commercially available coaxial-to-waveguide adaptor. >