Speaker
Description
Mechanically connected fluid joints are virtually unavoidable in complex cryogenic system designs. In the case of spacecraft where repair or replacement may be difficult, dangerous, or impossible, the performance of these mechanical joints is critical to mission success. This is especially true for long-duration exploration class space missions where even very low leak rates can eventually lead to significant propellant losses or failure of vital, active cryogenic cooling systems. To aid in future designs, NASA has recently undertaken an effort to quantify the leak rate of Vacuum Coupling Radiation (VCR) fittings from the Swagelok company over the temperature range from ambient to 20 K, both before and after exposure to a launch vibration profile. VCR fittings were chosen based on a survey of NASA cryogenic users that showed these fittings to be routinely used when performing cryogenic thermal vacuum work. A test apparatus employing a cryocooler and a calibrated helium mass spectrometer was developed and validated and used to obtain quantifiable leak rates at a fitting test pressure of 31 bar (450 psig). Three different fitting sizes were tested, 25.4 mm (1/4 inch), 12.7 mm (1/2 inch), and 6.35 mm (1 inch) and two gasket materials, stainless steel, and silver-plated nickel. Each fitting configuration (size/seal material) was subjected to two consecutive cryogenic thermal cycles and measurement tests, followed by a launch vibration test profile at ambient temperature, and then two additional cryogenic thermal cycles and measurement tests. The extensive test program showed exceptional performance for the VCR fittings, with a maximum leak rate on the order of 10-7 sccs across all configurations and test conditions. The design and development of the test apparatus, and test data are presented and discussed in detail.
