Paraffin & Beeswax Spaceflight Experiments for Improved Understanding of Centrifugal Casting

The Space Enabled Research Group at MIT is conducting a multiyear research effort to better understand the technical and logistical challenges posed by the implementation of a wax-based hybrid chemical in-space propulsion system. Paraffin and beeswax are being considered as candidate fuels. The overarching effort includes imagery analysis conducted on paraffin and beeswax centrifugal casting tests conducted onboard progressively higher-fidelity experimental platforms within transparent hardware which aids in optical investigations. Such platforms include a laboratory optical table and vacuum chamber, a parabolic trajectory microgravity aircraft (three flights to date), the Blue Origin New Shepard suborbital launch vehicle (three experiments onboard two flights scheduled for 2022), and the Destiny laboratory module of the International Space Station (ISS; launch scheduled for mid-2022). Each of these platforms allows for testing in a new environment or increasingly longer-duration microgravity. The parabolic aircraft flights allow 20 parabolas of 20 seconds, the New Shepard flight 3 minutes, and the ISS flight 30–45 days of continuous microgravity time for testing. Atmospheric vs. vacuum experiments allow for isolation of convective and radiative effects on cooling and solidification of the wax, while 1g vs. microgravity experiments allow for evaluation of the role of buoyancy in the convective cooling process. In order to determine the response of the liquid wax within the centrifugal casting chamber, an image analysis script was made to track the leading edge of solidification for both beeswax and paraffin wax. This script is able to track the solidification process in any environment, both in the lab and in microgravity, as long as there is video available that displays the solidification over time. Determination of expected solidification time is especially important in situations with tighter temporal constraints, such as choosing the optimal material to use for casting on a microgravity flight with less than 20–180 seconds of continuous reduced gravity. The imagery analysis of the experiments aids in understanding the solidification rate dependence upon rotation rate as well as environmental factors. Solidification rate may impact material properties or mission timing. In addition to experimental work related to casting, a chemical equilibrium solver was used to compare predicted performance of paraffin, beeswax, and hydroxyl-terminated polybutadiene (HTPB) hybrid rocket fuels under identical conditions which warrants continued study of beeswax as a candidate green hybrid rocket fuel. These results indicate that beeswax, paraffin, and HTPB exhibit very similar performance which corroborates that the reusability and cost advantages of beeswax warrant its further study as a high-performing hybrid rocket fuel.