Pyrolysis Behavior of Hybrid-Rocket Solid Fuels Under Rapid Heating Conditions

Abstract

An experimental investigation of the thermal pyrolysis behavior of several hybrid-rocket solid fuels under rapid heating conditions was conducted to determine pyrolysis laws and to identify and quantify the products of fuel pyrolysis. The study focused on four fuel formulations: pure hydroxyl-terminated polybutadienes (HTPB), 80% HTPB/20% Alex, 80% HTPB/20% Al, and the Joint Industrial Research and Development fuel formulation. A rapid conductive-heating technique was developed and employed to determine Arrhenius-type pyrolysis laws. All four fuels displayed two sets of Arrhenius parameters, depending on the range of surface temperature. For pure HTPB, E a = 4:91 kcal/mol and A = 11.04 mm/s above 722 K, while E a = 13:35 kcal/mol and A = 3965 mm/s below 722 K. These results agree well with those obtained previously using a lab-scale hybrid motor operating under realistic conditions. The gas chromatograph/mass spectrometer tests of the nonmetalized fuels, using a ash-heating oven, indicated that the relative concentrations of the pyrolyzed species depended strongly on temperature. For pure HTPB seven major products were identi ed, with 1,3-butadiene representing the dominant product at all temperatures tested, up to 1073 K. The measured mole fractions of the pyrolysis products and deduced pyrolysis laws of the fuels studied can be utilized in a comprehensive model simulation for combustion performance predictions.

Estimating the Enthalpy of Gasification of Acrylonitrile–Butadiene–Styrene Hybrid Rocket Fuels

“…When the system biases have been removed, these data closely match Chiaverini et al's [18] original HTPB pyrolysis data. The trendline plotted in Fig.…”

“…As mentioned previously, initially, the system apparatus was calibrated using HTPB test specimens, and these results were compared to Chiaverini et al's [18] results for HTPB to assess the overall fidelity and accuracy of this simple test apparatus. Following these initial tests, both FDM manufactured and extruded ABS specimens were tested.…”

“…Evaluating the HTPB samples allowed the system to be "calibrated" against well-accepted experimental pyrolysis data collected by Chiaverini et al [18] for HTPB. An accurate comparison would lend confidence in the applicability and fidelity of the alternative experimental apparatus and procedures used for this testing campaign.…”

“…Although the photogrammetric-based regression measurements of Chiaverini et al [18] exhibited good run-to-run consistency, the test apparatus was quite expensive. Developing such a one-time test apparatus was beyond the budget of this project, and Chiaverini et al's photogrammetric methods were discarded in favor of a lessexpensive piezoresistive test apparatus that allowed direct regression measurements of the pyrolyzed fuel specimen.…”

“…Following Chiaverini et al's approach [18], the regression rate data are interpreted using an Arrhenius-type process [23] model where the linear regression rate is analogous to the chemical rate of reaction, and the molar heat of gasification is analogous to the activation energy of the chemical reaction:…”