Boron-laden solid-fueled hybrid gas generator: Its feasibility for potential application in ducted rockets

This study presents a novel approach to enhance the performance of ducted rockets by using a hybrid gas generator (HGG) as an alternative to conventional solid propellant-based gas generators. The HGG (primary combustor) consists of a single perforated solid fuel through which the oxidizer is passed during combustion, akin to a hybrid rocket. However, by intentionally maintaining the fuel-rich combustion enables the byproducts/effluents of the gas generator to be combusted again in a secondary combustor (ram-combustor) along with the incoming ram air, thus improving the overall performance. The primary benefit of this type of gas generator is its ability to be actively throttled. This study aims to test the proof of concept and feasibility of the HGG for its potential application in ducted rockets (DRs). Hence, in this investigation, the emphasis is made on characterizing the gas generator without ram-combustor, i.e. hybrid rocket mode, with a comparative analysis between hydroxyl-terminated polybutadiene (HTPB) and 10% boron-laden HTPB (HB10) solid fuels with gaseous oxygen. The equivalence ratio is maintained above one during its operation for all the cases, aligning with ducted rocket requirements. The results indicate that HB10, operated with gaseous oxygen as the primary oxidizer, generally demonstrates slightly improved performance in regression rate (2–7%) and chamber pressure (3–8%). Also, successful ignition and combustion of boron within the HGG are observed. Additionally, the study evaluates static motor performance in hybrid rocket mode (HGG), revealing HB10’s marginal superiority over HTPB in terms of specific impulse (for the equivalence ratio greater than 1.2). The trend of characteristic velocity (c*) for the HB10 fuel increases with increasing equivalence ratio and outperforms HTPB after reaching 1.4 (approximately), whereas the c* efficiency shows the reverse trend. The HGG operating on HB10s also has relatively higher thrust (0.6–10%). Although these parameters do not directly determine ducted rocket performance, they offer insights into HGG feasibility and potentiality. These modest performance enhancements of the boron-laden solid-fueled gas generator hold significant promise for enhancing ducted rocket capabilities, particularly due to the possibility of two-stage combustion, a promising technique for enhancing boron combustion efficiency.