Combustion Characteristics of Boron-HTPB-Based Solid Fuels for Hybrid Gas Generator in Ducted Rocket Applications

Metal/metalloid particles as additives in polymeric fuels have been investigated since several decades, among which boron has been remaining the most attractive one due to its high energy density. In the present study, a low-cost screening instrument called opposed flow burner (OFB) is used to investigate the salient features of various boron-HTPB-based solid fuels to understand the major performance parameters such as burning efficiency and regression rate. Pure-HTPB (baseline solid fuel) and HTPB loaded with boron nanoparticles (nB) at various concentrations (5-40% by wt.) have been tested. In the OFB system, single gaseous oxygen (GOX) jet has been applied on the surface of the solid fuel pallet at oxygen mass flux (G(ox)) range of 20-57 kg/m(2)-s in order to compare the performance of various solid fuels. High-speed videography and UV-VIS spectroscopy have been employed to realize the burning process of fuel samples. Emission spectra obtained from the experiments clearly identify the gas-phase intermediate species (BO and BO2) of boron ignition/combustion. Several material characterization techniques such as field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), bomb calorimetry, and thermogravimetric analysis (TGA) have also been applied on pre- and post-burn samples to identify the physiochemical changes. The residual active boron content is found out to be almost negligible in fine-condensed combustion products whereas significant amount of un-burnt boron is present in the coarse ejected agglomerates.