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The World of Metals Combustion Testing Is Heating Up
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 October 2005 Feature
Stephen D. Herald, Integrated Concepts and Research Corporation, is a member of ASTM Committee G04 and chairs the task group writing the heated promoted combustion test standard. As senior test engineer at NASA’s Materials Combustion Research Facility, he leads the development of new test systems and methods.
It Is Rocket Science

New heated promoted ignition/combustion tester will help NASA build better rocket engines and prevent fires on earth and in space.

Engineers at NASA’s Marshall Space Flight Center nicknamed their new tester “The Hulk” because it is the largest and most advanced heated promoted ignition-combustion system ever constructed. “The Hulk” is designed to replicate the extreme temperatures and pressures inside rocket engines.

“‘The Hulk’ provides an important and unique test capability within NASA,” says Ralph Carruth, manager of the MSFC Engineering Directorate Materials and Processes Laboratory, which operates the Materials Combustion Research Facility. “It is a national asset, and we’ve already had inquiries from industry and the Department of Defense regarding access to this test system.”

Materials used in high performance propulsion systems are exposed to high temperatures and pressures with many critical components also exposed to a pure oxygen environment — a potential formula for explosions and fires. To mitigate these risks, materials must be tested to ensure they can survive the extreme conditions inside propulsion systems.

“For the first time, ‘The Hulk’ makes it possible to test materials in conditions they will actually experience,” says Eddie Davis, the NASA engineer who manages Marshall’s Materials Combustion Research Facility where the new tester is located. “Before we built ‘The Hulk,’ the metals used in NASA engines were tested at room temperature. When these materials are used for rocket engines, they are heated to temperatures in the thousands of degrees. We can now simulate these conditions.”

“The Hulk” makes the Marshall Center the only place in the world that can heat materials up to 1093°C, while exposing them to gaseous oxygen atmospheres at high pressures up to 69.0 Mpa — more than 680 times the pressure at sea level on Earth.

“Before, the only way to test engine materials at these temperatures and pressures was with a costly full-scale engine firing or a subtest of engine components,” explains Davis. “Now we can test the materials before they are selected for use in a rocket engine. This lowers testing cost, gives designers better data for selecting engine materials, and improves the safety and success of component and full-scale engine tests.”

Researchers using this facility are continuing Marshall’s pioneering research on materials and oxygen systems that began with the development of rockets like the Saturn V, which first carried explorers to the moon more than 35 years ago. Now, MSFC engineers are testing the super alloys often used in modern engine design, including nickel-based super alloys such as INCONEL®Alloy718, HAYNES®Alloy214, and MONEL®Alloy400, as well as several different 300-series stainless steels. Until now, there was no place to test metals under the combination of elevated temperatures and pressures that can change the combustion characteristics of even the strongest, most stable metals.

“We already have surprising results,” says Davis. “Materials that didn’t burn in high pressure oxygen at room temperature are igniting at elevated temperatures and much lower pressures than we expected. These test results will not only benefit the aerospace industry but also medical and manufacturing industries that use oxygen tanks and equipment.”

As NASA strives to fulfill the nation’s vision for exploring the moon, Mars, and beyond, Marshall Space Flight Center will continue to push the limits of propulsion design, and the heated promoted ignition-combustion test system will help to define those limits.

The World of Metals Combustion Testing Is Heating Up

In an example of using a private-sector standards developing organization to enable technology transfer between a federal government agency and industry, a new test system created by NASA is being standardized for widespread use by ASTM Committee G04 on Compatibility and Sensitivity of Materials in Oxygen Enriched Atmospheres.

The ignition of metals in oxygen has been a topic of much discussion, research and testing for more than 40 years. While this interest has led to a decrease in the number of accidents in the aerospace and industrial gases industries, a new test system and ASTM standard are allowing researchers to “heat up” metals to never-before-explored regimes of extremely high temperatures and pressures.

A New Standard for a New Test System

ASTM standard G 94, Guide for Evaluating Metal for Oxygen Service, is an excellent guide for the selection of metallic materials for use in oxygen systems, providing explanations of different test methods and presenting tables that rank various metals by each test method. The best known and most useful of these tests is the promoted ignition-combustion test, as described in ASTM G 124, Test Method for Determining the Combustion Behavior of Metallic Material in Oxygen-Enriched Atmospheres. This standard has served the oxygen compatibility community for many years, but the state of the technology has prevented addressing the critical parameter of material temperature. Elevated material temperature was simply not possible at pressures greater than 6.9 MPa until now.

In the last year, researchers have conducted the first tests with metals exposed to enriched oxygen atmospheres at extremely high pressures up to 69 MPa (10,000 psig) and temperatures up to 1093° C. Figure 1 shows the results of preliminary research with stainless steel. This research is only possible inside a new test system — the heated promoted ignition-combustion system — designed and built by engineers at the Materials Combustion Research Facility at the U.S. National Aeronautical and Space Administration’s George C. Marshall Space Flight Center in Huntsville, Ala. Marshall engineers are working within ASTM Committee G04 on Compatibility and Sensitivity of Materials in Oxygen Enriched Atmospheres in the process of writing a new standard, WK7946, Test Method for Determining the Combustion Behavior of Metallic Materials in High-Pressure Oxygen-Enriched Atmospheres at Elevated Temperatures. The standard and test system will allow researchers to explore metals combustion under conditions never before attainable.

Promoted Ignition-Combustion

What is promoted ignition–combustion, and why is it important to people who use and test oxygen systems? Promoted ignition-combustion simulates the kindling-chain effect, which is the most common ignition mechanism for metals. The kindling-chain effect is the process by which a material with lower oxygen compatibility ignites and promotes the combustion of a more combustion-resistant material. For example, to light a campfire, fine tinder is ignited, which then ignites small twigs, which in turn ignites larger sticks, and finally this kindling ignites large logs. This same concept is used to test metals.1 A kindling chain for metal combustion testing is created by using Pyrofuse (the trade designation for aluminum/palladium coaxial wire fuse) to ignite a small cylinder of aluminum or magnesium, which then ignites the test sample.

Many studies of the metal combustion phenomenon conducted in recent years focused primarily on steels, both carbon and stainless, and alloys in the nickel, cobalt, and copper families — all metals often used in oxygen-rich systems that operate at high temperatures and pressures.

Notable data, however, have been missing from these high-pressure oxygen tests. All of these tests were conducted at ambient (room) temperatures. While very little published data exist for promoted ignition-combustion testing performed at the combination of elevated temperatures and pressures, applications involving oxygen-enriched atmospheres at elevated temperatures have grown dramatically in recent years.

New propulsion systems being developed for the aerospace industry and new industrial processes have highlighted this shortage of combustion data for high-temperature and high-pressure oxygen applications. Yet, at the time of writing, only three test facilities in the United States are capable of performing the testing needed to ensure safe operations at extreme pressure and temperature regimes, and only one facility, Marshall’s Materials Combustion Research Facility, can test at pressures above 6.9 MPa.

Test Methods

At present, two methods are available for achieving the high temperatures required for the high-temperature, high-pressure promoted ignition-combustion test. The first method requires heating the entire test chamber, test atmosphere, and sample to the target temperature.2 The second method involves directly heating only the sample. Previous research performed by NASA and industry to develop a heated promoted ignition-combustion test system resulted in systems that were limited to less than 6.9 MPa maximum pressure.

The new test system at the Marshall Materials Combustion Research Facility overcomes many of the problems of previous designs. This tester uses a direct heating method — an induction coil (Figure 2) housed inside the promoted ignition-combustion chamber. The chamber is similar to that described in ASTM G 124.

The induction-heated high-pressure promoted ignition-combustion chamber can sustain pressures of up to 69.0 MPa in pure gaseous oxygen. The induction coil in the chamber is capable of preheating a metal specimen to near its melting temperature (possibly in excess of 1093° C), depending on the test pressure. This improved test system makes it possible to obtain important data at actual material use conditions and to observe material combustion behavior at these conditions. Older test equipment has not been able to produce this information.

Better Observations and Other Enhancements

The improved induction-coil design is not the only enhancement to the new standard for the high-temperature high-pressure promoted ignition-combustion test. Several other design improvements will aid researchers in high-temperature metals combustion testing.

The first improvement makes it easier to observe the sample, allowing more accurate real-time measurement of the propagation rate of a burning sample. In the past, data recording has been limited by the number of ports through which the combustion event can be observed. The standard test system, described in G 124 has just one site port located at the bottom of the chamber. This one port requires the researcher to determine the propagation rate of the burning samples by the rate at which the molten metal droplets fall to the bottom of the chamber. Some researchers have used ultrasonic transducers to overcome this design limitation, but this solution is expensive and not always as reliable as direct observation. To overcome this constraint, six site ports were placed in the new chamber, arranged in a helical pattern (Figure 3) so that researchers can record images of the combustion event for the entire length of the sample — up to 305 mm long. Since extremely high pressures are used for tests in this chamber, Marshall engineers constructed the site ports of sapphire with a 33-mm thickness.

The second improvement to the new test system allows researchers to measure the temperature of the sample as it burns. Obtaining this measurement presented a special problem because thermocouple data are affected by the induction field causing data inconsistencies. The solution was to use an infrared temperature transducer to measure the sample temperature remotely through one of the six site ports. The transducer is calibrated to “see” through the sapphire site port, and a special sample preparation procedure eliminates the issue of varying emissivity, thus ensuring accurate temperature measurements at the target location. (Figure 4 shows test data from the infrared transducer.) The infrared temperature sensor, data acquisition hardware and system control hardware are routed through a robust data acquisition and hardware control program with several redundant fail-safes. These enhancements combine to make this promoted combustion-ignition test system one of the safest and most advanced in the world.

Impact on ASTM Committee G04

ASTM Committee G04 was formed in 1975 to deal with issues such as the combustion of metals in high-temperature and high-pressure oxygen environments. The committee, which currently has more than 90 members, has jurisdiction over more than 20 standards. These standards are preeminent in the selection of materials, configurations and applications intended for use in systems involving enriched-oxygen atmospheres.

Committee G04 members are committed to contributing new ideas to the existing body of knowledge on risk management concepts, practices and procedures used by individuals and organizations involved in the design, use, and maintenance of oxygen systems. In support of this commitment, G04 members are constantly striving for advancements in materials testing for oxygen compatibility. This new test system and the new ASTM standard demonstrate Committee G04’s commitment to expanding the boundaries of knowledge necessary for the safe operation of oxygen systems.

Conclusion

Each time researchers test in unexplored regions of temperature and pressure, they make discoveries that advance the understanding of complex metals combustion. High-temperature, high-pressure metals combustion data — especially for test pressures in excess of 6.9 MPa — have only recently become available. Committee G04 members consistently introduce new test methods and exciting new advances in understanding material behavior in enriched-oxygen environments. This is demonstrated by the maturity of the new technology for promoted heated-ignition combustion and the evolution from standard G 124 to the draft standard WK7946. The data from this standard and future related standards will give designers and users of high-pressure oxygen systems the data necessary to improve the safety of the systems that they design, build and operate.

Acknowledgments

The author thanks Eddie Davis, NASA engineer at the Materials Combustion Research Facility at NASA’s Marshall Space Flight Center; Dr. Carl Engel, Qualis Corporation; and Dean Byess, Qualis Corporation. Mr. Davis’s input to this article and his constant encouragement are greatly appreciated. Dr. Engel and the author have collaborated on exciting research with the new test system, and I look forward to our future discoveries. Mr. Byess has been the driving force behind the design and development of this new test system. His dedication to this project allowed us to overcome challenges that at times seemed insurmountable, and his continued dedication keeps the system running. //

References

1 NASA-STD-6001, Flammability, Odor, Offgassing, and Compatibility Requirements and Test Procedures for Materials in Environments that Support Combustion, Test 17, Upward Flammability of Materials in GOX, National Aeronautics and Space Administration, February 1998.
2 Zawierucha, R. K., Million, J. F., “Promoted Ignition-Combustion Behavior of Engineering Alloys at Elevated Temperatures and Pressures in Oxygen Gas Mixtures,” Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: Ninth Volume, STP 1395, ASTM International, West Conshohocken, PA, 1991.

 
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