SPACE-SHUTTLE SIMPLIFIED LO2 CHECK VALVE DEVELOPMENT TESTS

The coil spring in a Space Shuttle liquid oxygen check valve failed du e to cyclic fatigue in September, 1998. The dual-flapper, swing check valve is used to prevent reverse flow to the Space Shuttle Main Engine s. Upon inspection of the failed component, the spring tangs were foun d to be missing and heavy wear was observe on the i.d. of the spring c oils. The fracture surfaces revealed that the metal had been steadily worn away until a simple overload caused the final fracture. A series of flow tests using water and a water/gas mixture was conducted to ide ntify the flow phenomenon that produce the cyclic wear. A Plexiglas(R) outlet housing was utilized to view the flapper behavior under differ ent flow conditions and to aid in high-speed photography. The tests re vealed that flow instabilities induced two oscillatory flapper respons es: 1) a rocking mode and 2) a chattering mode. Initially, attempts we re made to reduce the spring-flapper oscillations. However, the final solution to the problem was a springless configuration that satisfied the valve's design requirements and eliminated the oscillations. The s pringless design relied on the inherent ability of the reverse flow mo mentum to close the flappers.