Metallurgical investigation of cracked stainless steel plumbing tubes used in engine gimbal control system of satellite launch vehicle

The attitude control of satellite launch vehicle employs various control modules. Gimbling the engine nozzle within a specified angle through actuator modules is commonly used to control the deviation in predetermined trajectory path of satellite launch vehicles. During one of the hot test of a liquid engine stage, a drop in pressure in actuator module was noticed due to cracking of plumbing line. This called for disabling the gimbal control during the test.Subsequently to the test, while opening the leaked joint, a through circumferential crack was seen at the neck of the flared tube. Detailed metallurgical investigation revealed that the failure was due to fatigue. This paper brings out the detail of investigation carried out and conclusion arrived on.

► Drop in pressure in actuator module was due to cracking of plumbing line.
► Cracking of plumbing line called for disabling the gimbal control during the test.
► A through circumferential crack was seen at the neck of the flared tube.
► Detailed metallurgical investigation revealed that the failure was due to fatigue.