ME 415 Failure Analysis and Prevention Failure of the Fortnight #4 H-1 Engine LOX Dome Failure. Evrim ERSU (ee51) Due Date: Thursday, March 29!!

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1 ME 415 Failure Analysis and Prevention Failure of the Fortnight #4 H-1 Engine LOX Dome Failure Evrim ERSU (ee51) Due Date: Thursday, March 29!!

2 Space travel and exploration has always been one of humanity's greatest aspirations. Every human being lived on earth looked up to the stars and thought what is out there?. Some named it as God, some said Allah, and some called it the Sky. As time went on, with increasing curiosity, people started building machines that would enable them to observe and see the particles scattered among the sky, particles being stars in our solar system. In 1609, Galileo Galilee, one of the greatest engineers that have ever put feet on earth, invented a magnifying glass called the Telescope. With the use of telescope, people started seeing the stars and observing their shapes. This ability was quite satisfactory at the time, but still was not enough. People had to fly up to space and see everything with their own eyes. On October 4, 1957, Soviets fulfilled this unreachable aspiration by sending Yuri Gagarin into space. This was the first mission to outer space and created a whole new category of engineering technologies to be explored. As people continued discovering more and more planets in space, another limiting variable came into the equation of space travel. This variable was called efficiency. To increase a spacecraft s travelling efficiency, different kinds of fuels were tested. At first, solid fuels were being used in the form of sand particles. Although this idea was used in many space missions, it was not efficient enough to travel long distances. Later on, the usage of liquid propellants in rocket engines was proposed, and proved to be a much more efficient fuel compared to solid fuels. These liquid propellant rocket engines have tankage and pipes to store and transfer propellant, an injector system, a combustion chamber, and multiple rocket nozzles. The liquid systems enabled higher impulse than solids and therefore, increased the thrust per fuel ratio of engines drastically. The mission Saturn S-I-7 was designed to use liquid oxygen (LOX) as a propellant for its rocket engines. A company called Rocketdyne in August 1962 originally built these engines. To test the engines reliability, Rocketdyne test fired the engine once in June 1963, and once in January Both of these firings lasted approximately seven minutes and were successful. In June 6, 1964, the day of the launch, the engine was test fired during pre-launch checks and failed because of a crack present in the dome of the rocket engine. The dome in this rocket engine was made out from one of the toughest aluminum alloys, Aluminum 7079-T6 die forging, and acted as a cap by closing the H-1 engine combustion chamber through which LOX entered the injector. LOX feed lines were connected to the dome inlet with a feed pressure of 2000 psi. The diameter of the dome and LOX inlet was 36 and 6.5 inches respectively. The average thickness of dome was 1.5 inches around the cracked section. The chemical composition of die forging Aluminum 7079-T6 in weight percent (wt %) generally consists of Zinc, Magnesium, Magnesium, Chromium, Manganese, max 0.3 Silicon, max 0.1 Titanium, and rest being as aluminum. The operation of artificial aging performed on the aluminum alloy speeded up the chemical composition change over time resulting in an increased quality and accuracy in close tolerance 2

3 specifications. Since the alloy was die forging, it had better fatigue resistance, improved microstructure, continuous grain flow, finer grain size, and most importantly, greater strength. To observe LOX s temperature effect on the aluminum alloy, the notched and un-notched tensile and yield strengths were calculated. At room temperature (around 70 ), the unnotched tensile and yield strength of the alloy were found to be and psi respectively. The notched tensile strength at room temperature was a lot higher than the unnotched strengths, with strength of psi. At cryogenic temperatures (around 320 ), the unnotched tensile and yield strengths increased to and psi respectively, but the notched tensile strength reduced to psi. In order to accurately determine the causes for the crack, the microstructure of the material and grain flow direction on the cracked surface was analyzed. Grain flow, in its core, is a directional orientation of metal grains and any inclusions that have been deformed by forging. When the crack propagation and grain flow direction are perfectly aligned, fatigue strength, impact strength, and ductility can be significantly increased. But, from Figure 5, the grain flow direction around the cracked section of the dome was seen to be parallel to the crack edge. The grain flow lines were not aligned with the principal stress applied before the crack and therefore, optimum alignment was not reached. By analyzing the microstructure of the fracture area from Figure 6, it could be concluded that the non-alignment of the grain flow directions were caused by the deformity and non-uniformity in the microstructure. When it comes to the causes for the H-1 engine LOX dome failure, stress corrosion cracking (SCC) seems to have the highest occurrence probability. The material used to produce the engine dome was 7079-T6 die forging aluminum, which is susceptible to SCC and is characteristically intergranular. The environment was ambient air mixed with liquid oxygen propellant, and had a tensile stress of 2000 psi applied by the propellant. Since 7079-T6 is susceptible for SCC under oxygen-filled environments, the occurrence of SCC is apparent. Since intergranular impurities are present in the fracture surface of the aluminum alloy and the dome is in an oxygenated environment, there is going to be electrochemical potential difference over the surface of the alloy. The generation of electrochemical potential is a crucial factor that increases the occurrence probability of SCC. Stress corrosion cracking (SCC) is generally defined as the cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses. It happens under the interaction of corrosion and mechanical stress to produce a failure by cracking. The occurrence of this phenomenon depends on the simultaneous achievement of a susceptible material, an environment that causes SCC for that material, and tensile stress sufficient enough to induce SCC. The causes for SCC, however, are much more complex. The first cause for SCC is called active path dissolution, which is the process of accelerated corrosion 3

4 along a path of higher than normal corrosion susceptibility, with the dominant material being passive. Generally, the active path is the grain boundary, and the active path corrosion is limited by the rate of corrosion of the aluminum alloy at the crack tip. The second cause is called hydrogen embrittlement, which involves the dissolving of hydrogen in between the metal atoms. The dissolved hydrogen around high tensile stress regions assists in the fracture of the metal by making cleavage easier and assists in the development of intense local plastic deformation. The last cause for SCC is associated with the coating on the material. If a normally ductile material is coated with a brittle film, a crack initiated in the coating can propagate into the ductile material for a small distance before being arrested by ductile blunting. If the brittle part has been formed by a corrosion process, it can be reform again on the blunted crack tip. The common SCC systems for aluminum and titanium alloys are low temperature chloride environments with low concentration for aluminum and high concentration for titanium alloys. The other cause that influences the occurrence probability of SCC is referred to as the electrochemical potential of the alloy. The case of high strength aluminum alloys -specifically 7079-T6 Aluminum- also depends on hydrogen embrittlement, but the dominant effect of electrochemical potential comes from the protective aluminum oxide on the aluminum alloy. Since aluminum is a reactive metal, the contact with water immediately triggers the production of hydrogen in the material. Additionally, under situations where electrochemical is not being directly controlled, the potential difference is determined by the composition of the environment. The two test firings performed on June 1963 and January 1964 could have had a crucial role in the failure of the engine dome. The exposure of liquid oxygen to the aluminum surface after each firing could have initiated stress corrosion cracking on the surface. Consequently, intergranular corrosion over 7079-T6 aluminum could have occurred in between the test firings. The intergranular corrosion over the surface of the material could have led to the failure on the launch at Kennedy Space Center on June 6, Although there seems to be a variety of ways that one can prevent cracks in the dome, low temperature of the LOX inside fuel chamber brings many limitations to the functioning of the engine. One of the most important limitations of LOX temperature is called the Low temperature embrittlement. This is a condition that occurs when a material is subjected to temperatures that make it less elastic, resilient, and therefore brittle. Cold embrittlement, while being reversible, can be destructive and costly in the case of brittle failures. In the case of H-1 engine liquid oxygen (LOX) dome failure, the temperature of the liquid oxygen propeller used was 183 ( 297 ). Around temperatures this low, all of the 7000 series aluminum suffer a degree of embrittlement, primarily because of the segregation of impurities in its high alloying elements (Zinc (Zn), Magnesium (Mg), and Copper (Cu)). The alloying elements of 7079-T6 aluminum make the notched to un-notched tensile ratio less and less as temperature decreases, causing embrittlement. 4

5 To prevent cracks in the short term, a nonreactive corrosion inhibitor could be applied on the dome s surface that would prevent the aluminum from reacting with oxygen. Furthermore, in the short term, the system could be kept under a closed environment with increased cleaning and maintenance operations. This would keep the dome functional before testing and launch. Long-term solutions, on the other hand, come with permanent changes on the dome s structure. The easiest solution to prevent cracking in the long term could be changing the aluminum alloy with a material that is not susceptible to SCC. Another solution could be changing the liquid propellant with one that has a slightly higher temperature and one that do not react with 7079-T6 aluminum. If the cracking could have been prevented with another aluminum alloy instead of the current 7079-T6 die forging aluminum, any one of the alloys 2024-T6, 7039-T6, 2014-T6, and 5456-H343 could be used. Looking from a different perspective, the production of the engine dome with titanium alloys could result in the prevention of potential cracks. Although titanium alloys are a ten times more expensive and equally difficult to forge than aluminum alloys, it s one of the best materials that can be used in cryogenic engines. Titanium alloys have low densities but still have high melting points and high strength to density ratios, which in turn increases its structural efficiency. Moreover, titanium alloys have low modulus of elasticity s but at the same time high fatigue strength and fracture toughness in air and chloride environments. The most important properties of these alloys, however, are the excellent corrosion resistance, thermal properties, cryogenic properties, and low thermal expansion coefficients. Potential titanium alloys that can be substituted with 7079-T6 aluminum are heat-treated 6Al-4V-Ti, heat-treated 8Al-2Cb-1Ta-Ti, and annealed 6Al-4V-Ti ELI. These alloys are not only less susceptible to SCC, but can also sustain their mechanical properties under temperatures that are even lower than liquid oxygen s operation temperature. The roots of engineering come from the ability of maximizing the output, optimizing costs, and preventing further failures. In the case where there are 24 similar engines in the storage, with each one costing 3.6 million dollars, the most efficient bundle would be reached by at first salvaging or selling 4 of those engines. Then, with the money earned from selling 4 engines, the domes of the other 20 engines would be replaced with domes made out of titanium alloys. Although the usage of titanium alloy domes in the engines required the firm to sell 4 of the 24 engines, it would reduce the probability of cracks to nearly zero, and since titanium alloys don t react with oxygen, it would ensure profits for the firm in the future. 5