Hypergolic combustion of boron based propellants

Abstract

We investigate the potential of sodium borohydride (NaBH4) as reactive additive for ignition and combustion of boron and boron-paraffin wax solid fuels using hydrogen peroxide (H2O2) as liquid oxidizer. The new hybrid propellant system (solid fuel and liquid oxidizer) is investigated for a gas generator in ducted ramjets in which, the fuel-rich hot gases would be used for ramjet. Measurements of ignition delays along with visualizations of the ignition and combustion events were conducted by high-speed imaging on three propellant configurations; oxidizer drop on boron powder, oxidizer drop on boron-paraffin solid fuel slab, and oxidizer spray on a hollow cylindrical boron-paraffin fuel grain. These investigations revealed that the hypergolicity of boron with H2O2 is controlled by catalytically driven exothermic decomposition of H2O2 into superheated steam and oxygen in the presence of magnesium (∼5%) such that ignition and combustion of boron is delayed inordinately (7 s at 1.0 MPa). Boron hypergolicity improved dramatically by adding only 1% NaBH4 due to higher exothermicity of NaBH4H2O2 reaction. This reaction driven hypergolicity is very fast (∼1 ms at 1.0 MPa), rapidly gasifies boron oxide layer, and enables sustained combustion of boron. Ignition of boron-paraffin wax solid fuel slab with a single drop of oxidizer was achieved with mean delay ∼5.4 ms at 1.0 MPa. The optimum NaBH4 concentration (5.7 wt%) for achieving ignition and spontaneous burning of boron was evaluated as a baseline formulation. The spray-on-hollow cylindrical fuel grain achieved combustion of boron during ignition and reignition tests with an ignition delay of 12–15 ms under normal ambient conditions. Our investigations prove the feasibility of utilizing liquid H2O2 for the hypergolic combustion of boron based solid fuels for a green and energy efficient propulsion system.