Centrifugal Casting of Paraffin and Beeswax for Hybrid Rockets

Abstract

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.

Review of Alternative Sustainable Fuels for Hybrid Rocket Propulsion

“…Another very interesting natural fuel is beeswax [164][165][166][167][168][169][170]. Beeswax (cera alba) is a natural wax produced by honeybees of the genus Apis.…”

“…Its main constituents are palmitate, palmitoleate, and oleate esters of long-chain (30-32 carbons) aliphatic alcohols. Beeswax has a relatively low melting point range of 62 to 64 • C. The flash point of beeswax is 204.4 • C. The specific gravity of beeswax at 15 • C is from 0.958 to 0.975 [164][165][166][167][168][169][170].…”

“…have been tested in order to improve impulse density and/or mechanical properties. Centrifugal casting of beeswax grain, again in analogy with paraffin wax, has also been studied [170].…”

Optical and Thermochemical Analysis for Paraffin and Beeswax Centrifugal Casting

“…The first two flights were flown previously using water and oil as the working fluid, respectively, while a third flight was conducted in May 2021 and comprised four side-by-side centrifuges containing water, oil, paraffin, and beeswax. The experimental setup for the first two flights is seen in Figure 3 and a description of the results were previously presented by Stober et al [1][2][3][4] A solid model of the experimental setup for the most recent flight is seen in Figure 4.…”

“…Prior work by the authors introduced the concept of reusing phase change material (PCM) from satellite thermal insulation as fuel for deorbit, reviewed current and prior work in the area of beeswax as a hybrid rocket fuel, detailed the steps of the image processing routine for solidification rate characterization, and presented results from two microgravity aircraft flights alongside tabletop testing in the laboratory. [1][2][3][4] The overarching effort includes imagery analysis conducted on paraffin and beeswax centrifugal casting tests on 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; most recent May 2021), the Blue Origin New Shepard suborbital launch vehicle (three flights scheduled for 2021/2022), and the Destiny laboratory module of the International Space Station (ISS; launch scheduled for 2021/2022).…”