scope:

Description of Driving Event:

When the shuttle orbiter main propulsion system oxygen manifold was pressurized to obtain gas samples, oxygen line to engine No. 3 also pressurized. The prevalves to all engines were closed and pressurization should not have occurred downstream of the prevalves. A small check valve is provided parallel to each prevalve and allows expanding fluid trapped in the manifold legs of each engine, between the prevalves and the engine valves, to relieve back into the main manifold. Upon removal of the No. 3 check valve, it was discovered that the spring holding it closed was broken and had been installed backwards. The error in installation contributed to additional stresses on the spring during earlier flow tests that lead to this failure. If the faulty installation of the spring had remained undetected and the spring had failed during cryogenic operations, uncontrolled flows into the engine leg could have caused launch delays. An X-Ray technique was developed to inspect existing valves for proper installation of the springs. A mandatory inspection point was added to the fabrication assembly procedures to preclude installation errors for this spring.