Untangling the\nEfficient Boron-Initialized Hydroxyl-Terminated\nPolybutadiene Combustion for High Energetic Solid Propulsion Systems

Highly energetic boron (B) particles embedded in hydroxyl-terminated\npolybutadiene (HTPB) thermosetting polymers represent stable solid-state\nfuel. Laser-heating of levitated B/HTPB and pure HTPB particles in\na controlled atmosphere revealed spontaneous ignition of B/HTPB in\nair, allowing for examination of the exclusive roles of boron. These\nignition events are probed <i>in situ</i> via simultaneous\nspectroscopic diagnostics: Raman and infrared spectroscopy, temporally\nresolved high-speed optical and infrared cameras, and ultraviolet–visible\n(UV–vis) spectroscopy. The emission spectra unravel two stages\nof the B/HTPB ignitionthe exoergic ignition of boron followed\nby HTPB combustion. It was found that HTPB readily absorbs the energy\nfrom the irradiating carbon dioxide (CO<sub>2</sub>) laser but efficiently\ntransfers that thermal energy to the densely arranged boron particles\ndue to the lower heat capacity of the latter. This transferred energy\ncauses a surge in temperature for the boron particles, leading to\nignition (in an oxygen environment) in B/HTPB, unlike the case with\nHTPB alone. The accumulated energy from the second stage of boron\nignition triggers the decomposition of HTPB in conjunction with hydrogen\nabstraction to produce radical precursors via boron oxides (BO and\nBO<sub>2</sub>)the key emitting intermediates detected. Along\nwith conventional combustion products such as carbon dioxide (CO<sub>2</sub>) and water (H<sub>2</sub>O), the formation of partially oxidized\ngaseous products such as methanol (CH<sub>3</sub>OH) and methyl vinyl\nether have also been detected as a tracer of diverse oxidation events,\nsuggesting a complex oxidation chemistry within HTPB and overall depict\ncrucial insights for its use as a solid rocket fuel.