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Solid rockets are an important component of the defense capabilities of the United States, in particular, in the Air Force. One of the major components of solid rockets is the motor, which consumes solid rocket propellant as its fuel. The successful operation of the rocket depends on a predictable burning rate of the solid propellant. However, this is not always the case. As Dr. Wojnar explains: “A typical solid propellant has the consistency of a pencil eraser. However, as we observe with pencil erasers, over extended periods of time, the material becomes stiffer and more prone to cracking. Such cracks in propellants can drastically increase burn rates due to the increased surface area and thus a higher pressure build-up in the motor that can lead to an explosion.” This stiffening and cracking of the propellant over time is referred to as aging. Predicting aging of solid propellant in a rocket that sits in storage for decades is thus a crucial need when maintaining the Air Force’s missile stockpile.
Predicting the lifetime of solid rocket propellant thus requires extensive experimental testing; that is, propellant samples are artificially aged for different lengths of time and the change in stiffness is measured and the formation of cracks is observed. However, carrying out experiments using samples of propellant can be dangerous; the flammability and explosive nature of the material makes laboratory testing extremely difficult as numerous safety precautions must be followed and special testing facilities are needed. Very few laboratories exist around the country for carrying out experiments with solid propellant.
However, Dr. Wojnar and his coworkers have found a way to circumvent many of the difficulties arising when carrying out experiments with solid propellant, “The safety risk of handling propellant material is substantially reduced when the amount of material you are handling is small. If the amount of solid propellant is sufficiently small, you no longer need to find explosion-resistant testing facilities and equipment, which can be very costly and difficult to operate.” To experiment with smaller specimen sizes, Dr. Wojnar and his colleagues have developed an experimental method using dynamic mechanical analysis (DMA) to measure the change in stiffness of propellants over time, which appeared in the Journal of Propulsion and Power published by the American Institute of Aeronautics and Astronautics. Using the method developed by Dr. Wojnar and coworkers, scientists can now test significantly smaller specimens than used in previous standardized testing methods and it was shown that the measured specimen properties were more accurate. “We realize that current testing standards for solid propellants have been around for decades and have been accepted by the community. Therefore, in our work, we carefully investigated our new approach to ensure its accuracy and repeatability so that other scientists can have confidence in the new method.”
After being published this year in the Journal of Propulsion and Power, Dr. Wojnar’s new method was picked up by writers at Aerospace Defense & Technology magazine for its potential for great impact on the field. In particular, the magazine highlighted the research in their technical brief as a technique that requires less material and gives more accurate results. “We are happy to see potential for the technique to help scientists better understand solid propellant aging” Dr. Wojnar says.
Dr. Wojnar’s research interests include the development of experimental methods for characterizing the mechanical properties of materials. Using these methods, Dr. Wojnar also seeks to understand and predict the properties and behavior of materials by developing new and efficient mathematical models.
The paper, “Measuring Propellant Stress Relaxation Modulus Using a Dynamic Mechanical Analyzer” was published online on April 14, 2017. The co-authors with Dr. Wojnar were Dr. Timothy Miller, a materials research engineer in the propellants branch at the Air Force Research Lab at Edwards Air Force Base, California, and Jeremy Louke, an engineer with ERC incorporated, Huntsville, Alabama. The research was carried out by Dr. Wojnar at Edwards Air Force Base, which was supported by the AFOSR’s summer faculty fellowship program.
Dr. Wojnar’s research website: http://mae.mst.edu/~wojnarc
Appearance in magazine: http://viewer.zmags.com/publication/356b6a67#/356b6a67/44