Shock Tube and Laser Measurements of HTPB/air Combustion at Solid Ramjet Conditions for Hypersonics

The Navy is currently developing hydroxyl-terminated polybutadiene (HTPB)/air system as a solid-fueled ramjet propellant for hypersonic weapons. In order to understand and control the combustion behavior of HTPB, it is necessary to have a good chemical kinetic model that describes the various pathways and reactions involved in burning. It is important to know the initial fuel breakdown steps and what products are formed from the propellant at the time scales of burning. Unfortunately, the fidelity of current HTPB combustion kinetic models has not been assessed at the conditions of interest to solid ramjet operation - mainly due to a lack of reliable and relevant data. HTPB combustion is limited by the rate at which solid HTPB pyrolysis forms gaseous hydrocarbon species. Several works in literature have studied HTPB pyrolysis using slow heating methods and without the micro to millisecond time-resolution needed to understand the propellant burning. Therefore, we have conducted novel experiments to study HTPB pyrolysis and oxidation covering a range of temperatures up to 3000 K, including UCF’s high purity shock tube. Hydrocarbon species data obtained using non-intrusive, time-resolved laser absorption spectroscopy at MHz rates from these experiments are utilized to create a gas-phase representative surrogate for HTPB. Further experiments were conducted in UCF’s shock tube with this surrogate mixture to obtain critical combustion properties like species time history measurements. The data obtained is used to generate an improved kinetic model for HTPB combustion in air and will directly feed into computational fluid dynamic (CFD) codes used by researchers from NRL.