There are 38 primary reaction control system (RCS) thrusters on each space shuttle orbiter that are used to provide attitude (pitch, yaw, and roll) maneuvers, as well as translation maneuvers along the orbiter axis. Cracking was discovered in an RCS thruster in April 2004 after a chamber repair and rejuvenation was performed. Relief radius cracking had been observed in seven thrusters in the 1970s and 1980s prior to flight. Because of the concern that hot combustion gases could leak through the cracks and damage the space shuttle orbiter, RCS thruster cracking was considered a "critical 1/1 failure," meaning loss of vehicle with no redundancies. This concern was compounded by the fact that the extent of cracking damage in the thruster fleet was unknown, since no reliable nondestructive evaluation methods were available to detect cracks in thrusters installed on the vehicle.
RCS thruster being examined optically for cracking in the relief radius.
The NASA Engineering and Safety Center (NESC) requested that a materials consultation on the thruster cracking problem occur in parallel with the efforts of the Space Shuttle Orbiter Program Office’s Materials and Processing Division. The NESC team, which was led by the NASA Glenn Research Center, was composed of Glenn, NASA Langley Research Center, and NASA Marshall Space Flight Center scientists and engineers who conducted a detailed review of the relevant literature and of the documentation from the previous RCS thruster failure analyses from the 1970s and 1980s. The NESC team concluded from the review that the prior failure analyses lacked sufficient documentation to support the conclusions that stress corrosion cracking or hot-salt cracking was the root cause of the thruster cracking. Subsequently, the NESC team identified and conducted new materials characterization and mechanical tests focused on (1) substantiating the root cause of the cracking in the RCS thrusters and (2) determining if the cracks were likely to grow in service. Electron microprobe analyses and field emission scanning electron microscopy performed on thruster hardware (serial number 120) and on niobium test specimens demonstrated that the thruster material was not susceptible to the root cause proposed in the earlier failure analyses. These in-depth analyses coupled with mechanical test results supported the conclusion that hydrogen embrittlement was the likely cause of cracking.
Electron microprobe dot maps showing the crack tip of the relief radius crack. Levels of fluorine (F), carbon (C), oxygen (O), and nitrogen (N) are shown within the crack, with brighter areas indicating higher concentrations of each element. Qualitative levels (in weight percent) are shown in the keys.
In the second phase of the NESC consultation, an independent failure analysis of another thruster (serial number 132) was conducted. Examination of the thruster crack surfaces confirmed that the oxide features on the intergranular crack surfaces were similar from the crack mouth to the crack tip and that no changes in crack mode were observed across the fracture surfaces. This work contributed to the conclusions that the thruster cracking was produced during original manufacturing as a result of processing with fluoride-containing acids and that no appreciable crack growth was observed for the thrusters after manufacturing. This extensive examination enabled the most comprehensive data to be obtained on crack-depth profiles for any RCS thruster hardware to date and will aid in the development of both stress analyses and non-destructive evaluation methods for in situ crack examinations. More detailed information on the NESC materials consultation of the thruster cracking issue can be found in reference 1.
Left: Thruster fracture surface near the tip of the intergranular relief radius crack. The intergranular crack surface contains reaction products that were chemically analyzed. The lab overload region denotes the region of the fracture that was opened in the lab to more clearly observe key features on the crack surface. Right: Energy-dispersive spectrosocopy scan showing elevated levels of C, O, and F associated with the oxide nodule on the intergranular crack.
Last updated: October 10, 2006
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