11. Lesson: Paper Review of Designs
Specific design reviews which are based upon an analysis of drawings can inadvertently overlook important features such as operational incompatibilities.
Background:
During the early development phase of the Saturn S-1B stage, data became available showing that 7079-T-652 and 7178-T-6 aluminum alloys were susceptible to stress corrosion cracking. These materials were used extensively in the stage and an intensive effort was undertaken to eliminate the problem.
A drawing analysis was made to identify all components which were made with the suspect material and which were subjected to residual tensile strength. (Stress corrosion is, of course, a long-term material cracking which occurs when the aluminum is exposed to a corrosive environment while under tensile stress.) All parts which could be replaced without a large cost were made of a less susceptible material. Those items which were difficult to replace were replaced only if they could not be inspected on a periodic basis. The remaining items were required to be inspected on a regular schedule.
The Skylab launch vehicles were built in the 1966-68 time frame, and special reviews were undertaken to assure that all time-dependent problems were eliminated. One of the reviews covered the stress corrosion of the S01B. Another drawing review was held to assure that all susceptible parts were identified.
Skylab 4 completed the last propellant loading test prior to launch when a routine inspection uncovered a crack. Stress corrosion had caused a crack at a mounting bolt hole. All required factors were present: time, tensile stress, and a corrosive atmosphere. The unnerving factor was that the crack appeared from under the head of a bolt and the design inspection had not identified that this point of susceptibility was not capable of "routine inspection." Indeed, there was no way of inspecting for a failure after the flight load of fuel was put in the tanks. As a result, another design review was held. All of the mounting bolts were removed and special brackets were installed which carried the load over the suspect part.
It is quite possible that previous Saturn 1B vehicles were launched with cracked bolt holes.
These lessons learned are from SKYLAB LESSONS LEARNED AS APPLICABLE TO A LARGE SPACE STATION, A dissertation submitted to the faculty of The School of Engineering and Architecture Of the Catholic University of America For the Degree Doctor of Engineering by William C. Schneider, Washington, D.C., 1976.
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March 07, 2004
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Richard Katz
