Abstract
Paraffin wax has been identified as a hybrid rocket motor fuel, which offers enhanced regression rates and improved combustion performance. While various investigations into the performance of this class of fuels are being conducted around the world, the consideration of its structural performance is often overlooked. The research presented here establishes a simplified, yet accurate method of defining the structural performance of a paraffin wax hybrid fuel grain to be introduced early in the design phase of a motor. The use of the Johnson–Cook (J–C) material model has been verified to work within the “low speed” ignition range experienced in paraffin wax/N2O hybrid motors, and therefore is used to predict failure in a variety of motors. The resultant stress profiles within the grains indicate that the grain outer to inner diameter (OD/ID) ratio, as well as the outer diameter (OD) itself, play an important role in the grain ability to withstand the loading conditions applied. Additionally, the grain structural properties, and the stiffness of the combustion chamber affect the severity of the internal stresses in the grain. The feasibility of large-scale pure paraffin wax grains without structural enhancement additives is thus found to be poor. Fuel additives should be considered for structural enhancement.
Highlights
The structural performance of a solid fuel grain is a necessary factor to consider for the propulsion of any solid rocket motor
The results presented indicate the plausibility of using the J–C model within the regime presented in the tensile tests, and may be applied for the purpose of estimating the structural integrity of a paraffin wax fuel grain as the point of interest is the failure
While it is difficult to ascertain a direct correlation between the results obtained and the geometry of a motor grain given the current data set, these results do lead to the conclusion that that the J–C material and failure model and modelling method are suitable in the grain structural integrity validation during the initial design phases
Summary
The structural performance of a solid fuel grain is a necessary factor to consider for the propulsion of any solid rocket motor. The shapes of the stressstrain curved between the experimental and simulated results differ, with a discrepancy being evident near the yield point, while the elastic modulus, plastic region, as well as the failure point, correlate closely The reason behind this discrepancy is the development of the J–C model based on materials with more clearly defined transitions between the elastic and plastic regions, while the transition in paraffin wax is comparatively extended over a strain range. When there is a large material stiffness difference between the grain and combustion chamber, it is estimated, based on multi-layered thick-walled pressure vessels design calculations, that the stress distribution will look more similar to that presented, where both the tangential and radial stresses are compressive, while still exhibiting a similar trend This means that a smaller OD/ID ratio may result in primarily compressive loads within the motor, reducing the likelihood of failure. It should be noted that based on the results discussed below, the maximum strain-rates for all the scenarios simulated fall within the range of the test data, meaning that the “slow” rate of applied load in hybrid ignition makes the use of the J–C model suitable
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