Development and Characterization of Fast Burning Solid Fuels/Propellants for Hybrid Rocket Motors with High Volumetric Efficiency

The goal of this project is to formulate and characterize high-density, fast-burning, paraffin-based solid fuels for future applications in hybrid rocket propulsion to improve space launch efficiency, safety, and cost. The 4th quarter activities included modification of the casting chamber to run a vacuum pump for RDX casting, updating the RDX casting procedures and hazard analyses, casting of paraffin/RDX solid-fuel samples, testing of solid-fuel grains manufactured by the Aerospace Corporation, analyzing burn rates and SEM images from paraffin/MWNT fuel grains, writing a conference paper and presentation for the 9th International Symposium on Special Topics in Chemical Propulsion (9-ISICP), and completion of the master’s thesis and all degree requirements. The environmental chamber used for casting was modified to include a vacuum pump so that the dimethylformamide (DMF) solvent could be boiled out of the paraffin/RDX. In addition to verifying the operability of the vacuum pump with the environmental chamber, a cryogenic trap was constructed. This cryotrap was placed as an in-line section upstream of the vacuum pump. A reservoir was submerged in an ethanol/dry ice bath so that any solvent vapors would be condensed and collected in the trap. A total of four casts were made with the paraffin/RDX/DMF. Three of the casts (shown below in Figure 1) were generated, with typical RDX percentages between 2% and 4% of the total batch. Fuel samples containing MWNTs were imaged using scanning electron microscopy (SEM) at UT-Austin’s Center for Nano and Molecular Science and Technology. The general comments about the fuel samples were that the MWNTs were well distributed in the paraffin, but typically, the MWNTs were clumped together. The MWNTs may not have been getting fully debulked during the sonication in the toluene. This is possible because the toluene cannot be sonicated for long periods of time due to observed breakdown of the toluene itself. Another explanation for the clumping of the MWNTs was that they were being tangled during the extensive mixing process. Solid-fuel grains were tested in the 3rd quarter, and analyzed results of regression rates are shown in Figure 3. The fuel grains containing LiAlH4 particles are from an existing collaboration with the Aerospace Corporation. After several iterations, the paraffin/LiAlH4 fuel grains could be characterized and compared to baseline paraffin. Many more attempts with the RDX and MWNTs are necessary to achieve similar, favorable results. It should be noted that the MWNTs were run at O/F ratios between 1.65 and 1.75, while most other grains on the plot in Figure 3 were run at O/F values between 2 and 2.4. The MWNTs did have reduced regression which may have been from lower O/F values. It is more likely that the slight decrease in regression rate was due to the carbon content of the MWNTs replacing the paraffin (C32H66) fuel. Although there was slightly reduced regression rate, this may not completely disadvantageous, as the MWNTs could be used to replace carbon black and/or chemical dyes, while still improving the mechanical properties of the fuel. The results of the this year-end summary report were presented in detail at the 9th International Symposium on Special Topics in Chemical Propulsion (9-ISICP) in Québec City, Canada on July 10, 2012. The presented paper will be reviewed and considered for publication in the International Journal of Energetic Materials and Chemical Propulsion.