Centrifugal Casting of Paraffin and Beeswax for Hybrid Rockets

Abstract

© 2020, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. A high-level research framework is presented which seeks to navigate the barriers associated with reusing wax phase change material onboard a satellite as a hybrid rocket fuel for de-orbit or other in-space propulsion needs, while also conducting fundamental studies of the fluid mechanics and heat transfer phenomena which drive the cooling and solidification of wax within a horizontal rotating cylinder in various gravitational and thermal environments. A detailed review of past work in the area of beeswax fuel for hybrid chemical propulsion is reported and served to motivate consideration of this fuel for centrifugal casting efforts, due to previously reported values of regression rate comparable to that of paraffin wax. The production process of beeswax fuel from beekeeping detritus was perfected and documented. Analysis of the shrinkage of beeswax and the neat Candlewic FR5560 paraffin wax used herein determined a volume shrinkage percentage during liquid to solid phase transition of 18.7 ± 0.62 and 13.3 ± 0.22%, respectively. An image analysis routine was developed in order to automate the process of determining the instantaneous solidification rate for each one-second timestep through the centrifugal casting process of paraffin and beeswax fuel grain sizes common for small-scale hybrid rockets. Beeswax completed solidification in 22% less time than paraffin under identical conditions but exhibited more coning of resulting solid wax. Calculated time-and space-averaged solidification rates for paraffin and beeswax were 0.017 and 0.028 mm/s, respectively, within a 50.8 mm inner diameter, 57.15 mm outer diameter, and 254 mm length polycarbonate tube. Careful analysis, however, shows that instantaneous solidification rate increases very slightly but steadily over time for both paraffin and beeswax, though the rate increase is greater for beeswax. The image analysis routine was most effective when applied to the beeswax solidification process as compared to that of paraffin, as the solid/liquid interface is considerably more salient in beeswax due to a distinct color change upon solidification. Dye will be used with paraffin casting in the future with the goal of improving solid/liquid phase contrast.