Experimental and Theoretical Study of Hypergolic Solid Fuel Ignition and Combustion with Hydrogen Peroxide

Abstract

Hypergolic ignition provides significant advantages for hybrid rockets by eliminating the need for conventional ignition devices, enabling rapid and reliable startup. This study investigates the ignition and combustion behaviors of hypergolic solid fuels (HSFs) using rocket-grade hydrogen peroxide (RGHP) as the oxidizer. The HSFs used in this study were composed of low-density polyethylene (LDPE) as the fuel binder, along with two types of hypergolic additives: sodium borohydride (NaBH4) as the reactive additive and manganese acetate tetrahydrate (Mn acetate) as the catalytic additive. A combination of experimental investigations and theoretical studies were conducted to investigate the heterogeneous ignition and combustion processes, as well as the key factors influencing HSF performance. Droplet tests were performed to examine the ignition behavior and performance of pressed and baked HSF pellets with RGHP droplets. The results showed that ignition delay times (IDTs) decreased as the NaBH4 concentration increased; however, pressed HSF pellets composed of fine particles exhibited weaker reactions, leading to slightly longer IDTs. HSF pellets with sanded and cut surfaces demonstrated significantly shorter IDTs, indicating that the surface reaction was kinetically controlled. The initial fuel surface temperature also played a critical role, with higher temperatures significantly reducing IDTs. To further examine the factors influencing the IDTs of HSF pellets, a hypergolic ignition model was developed, incorporating transient heat conduction with nonlinear surface heat generation. The model accounted for the effects of additive concentration, particle size, and initial temperature, integrating a particle packing model to evaluate surface area, thermal mass, and surface layer properties. The estimated IDTs from the model showed strong agreement with droplet test data, demonstrating its predictive capability across varying fuel compositions and conditions. A modified RGHP counterflow spray experiment was introduced to investigate the ignition and combustion behaviors under realistic oxidizer flow conditions. The results demonstrated that while NaBH4 facilitated surface reactions, the reactions also caused structural damage in pressed HSF pellets. Flammability limits were identified, with at least 20wt% NaBH4 required for ignition in pressed HSF pellets and 40wt% for baked HSF pellets. Mn acetate was found to moderate explosive surface reaction and prevent pellet destruction; however, it also lowered the flammability limits due to its competitive RGHP consumption with NaBH4. Reignition tests indicated that char layer formation after the first ignition inhibited the reaction between additives and RGHP, leading to longer IDTs and combustion delay times (CDTs). Due to explosive surface reactions, pressed HSF pellets experienced structural damage during ignition and combustion, whereas baked HSFs maintained their integrity and exhibited superior reignition performance. Regression rate measurements in the RGHP counterflow spray tests revealed significantly higher values compared to gaseous oxygen (GOX) counterflow burner tests, with regression rates increasing with oxidizer mass flow rate (MFR) and additive concentration. The combustion process exhibited distinct surface features, including char spots and bulges. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses confirmed that the char layer primarily consisted of NaBO2 and its hydrates. The limited chemical reactivity between NaBO2 and RGHP explains the reduced surface reaction during reignition. A hypergolic motor test was conducted to investigate the ignition behavior and motor performance. Two types of tests were performed: atmospheric combustion tests (ACTs) and hot fire tests (HFTs). The ACTs served as preliminary evaluations of ignition and combustion characteristics prior to HFTs. During the ACTs, unstable combustion behavior was observed due to the oxidizer flooding and the accumulation of concentrated hydrogen peroxide (H2O2) vapor on the fuel surface. This issue was mitigated by incorporating Mn acetate into the fuel. Successful ignition was achieved by cutting the fuel grain surface when the additive concentration was greater than or equal to 30wt%. IDTs and fuel regression rates were measured and analyzed. IDTs ranged from approximately 40 to 210 ms for both ACTs and HFTs. However, no clear correlation was found between IDT and either RGHP mass flow rate or fuel composition. The fuel regression rate increased with both additive concentrations and RGHP mass flow rates, aligning with the trends observed in the RGHP counterflow spray tests. At higher mass flow rates, the motor consistently achieved a C* efficiency of 88%, demonstrating the capability of HSF to provide reliable ignition and favorable combustion performance. This study provides a comprehensive investigation of hypergolic ignition and combustion behaviors in hybrid rockets, combining experimental and theoretical approaches to enhance the understanding of hypergolic ignition process, HSF combustion behavior, HSF fuel regression rates, delay times, reignition behavior, and char layer composition. The novel RGHP counterflow spray experiment offers a valuable platform for future research, supporting the development of high-performance HSFs.