Boron-based Propellants Research Articles

Experimental investigation of combustion mode transitions on solid rocket scramjet combustor

Based on the design configuration of a solid scramjet combustor, the direct-connect experimented investigation of combustion modes transition was carried out, simulating inflow conditions of Mach number 6.0 and flight altitude 25 km. It is the first experimental investigation using boron-based propellant as fuel to realize the transition between scram-mode and ram-mode, and to assess different operating modes of the combustor and their impacts on performance. In addition, the experimental pressure data are used to distinguish the combustion modes accurately. The experiment shows that, the structure of the shock train changes substantially during the mode transition. The combustion efficiency η, total pressure recovery coefficient δ and the specific impulse of bench thrust Isp increase greatly when the combustion mode transforms from scram-mode to ram-mode. In addition, for inflow condition of Mach number 6.0 and flight altitude 25 km, the performance of the solid rocket scramjet combustor operating in ram-mode is better, and the isobaric combustion also occurs in the combustor.

Experimental investigation on the ignition delay of fuel-rich mixture in solid rocket scramjet

In this paper, the effects of equivalence ratio and solid propellant composition on the ignition delay time of fuel-rich mixture in solid rocket scramjets were experimentally investigated for the first time. The experiments simulated a flight condition with Mach number 5.5 at 23 km. The equivalence ratio was studied in the range of 0.39–0.58, and three different solid fuel-rich propellants were used. The experimental results show that the ignition delay time of fuel-rich mixture decreases with increasing equivalence ratio under the studied conditions. This is because that the mass flow rate and momentum of the fuel-rich mixture are increasing with the equivalence ratio, and the primary combustion in the gas generator is more efficient with the higher value of equivalence ratio. Additionally, the solid propellant composition also plays an important role in influencing the ignition delay, which may be closely related to the temperature and composition of the fuel-rich mixture, with the temperature believed to be the more critical factor. Compared with hydrocarbon-based propellant, the ignition delay when using boron-based propellant is shorter. These conclusions will provide helpful guidance for shortening the ignition delay of fuel-rich mixture in solid rocket scramjets.

Optimizing combustion performance in a solid rocket scramjet engine

To enhance combustion efficiency of a boron-based propellant solid rocket scramjet (SRS) engine, this study attempted a new organization scheme combining cavity and pneumatic slope. To optimize the proposed system, a series of numerical simulations were performed, and through orthogonal design and single factor analysis, the key factors were determined. The developed two-phase flow simulation has been verified by the full-scale ground test, given an error within 10%. The results show that for both combustion efficiency and total pressure recovery, the influence of gas injection mode is much greater than the ratio of the length to depth of the concave cavity as well as the cavity depth. And the combustion performance has proved to be optimized via the enlarged low speed area near the cavity and the strengthened particles reflux.

Combustion characteristics of magnesium borides and their agglomerated particles

Boron is an attractive fuel for propellants because of its high energy density. However, its combustion is inhibited by the oxide layer that covers the particles. The use of magnesium borides instead of boron was shown to promote the combustion of boron-based propellant. The combustion behavior of magnesium borides and their agglomerated particles in CH4/O2/N2 premixed flames were studied using flat-flame burner, high-speed camera and spectrometer. The experimental results showed that the combustion of magnesium dominated the early stage of magnesium borides combustion process. As the combustion reaction proceeding, the combustion of boron gradually replaced magnesium to dominate the combustion of magnesium borides. When oxygen mole fraction was 0.13, MgB2 and MgB4 agglomerated particles would extinguish after ignition within the temperature range explored (1364K–1541 K). When oxygen mole fraction increased to 0.22 and 0.31, the burning time of MgB2 and MgB4 agglomerated particles were much shorter than that of amorphous boron agglomerated particles between 1400 K and 1600 K, which could improve the combustion performance of boron-based propellant. When temperature increased to 1600 K, the burning time of MgB4 agglomerated particles was close to that of amorphous boron agglomerated particles, while the burning time of MgB2 agglomerated particles was still shorter than amorphous boron agglomerated particles.

Effect of gas generator pressure on the physicochemical, oxidation and combustion characteristics of boron-based propellant primary combustion products

To clarify the effect of gas generator pressure on the physicochemical, oxidation and combustion characteristics of the condensed primary combustion products of boron-based propellants, the elemental, composition and morphology of the primary combustion products collected under chamber pressure of 0.2–8 MPa were investigated by energy-dispersive (EDS), X-ray photoelectron spectroscopy and scanning electron microscopy with energy-dispersive (SEM–EDS) individually. The oxidation, ignition and combustion behaviors of these products were further studied by laser ignition system and thermogravimetry–differential scanning calorimetry. We found out that high-pressure condition lowered the content of elementary boron and elementary carbon while raised the content of boron carbide. Numerous spherical carbon particles with a diameter around 100 nm were observed in the products. Boron lumps were partially or almost fully covered with carbon particles on the surface. The reaction mechanisms of thermal oxidation of primary combustion products were given. The onset temperature of boron in the products kept at 500 °C when pressure ranged from 3 to 8 MPa but increased to 583 °C at 0.2 MPa. As the pressure increased from 0.2 to 8 MPa, the emission spectrum intensity of both boron and carbon got enhanced by ~25%, and the ignition delay of boron was significantly shortened by 515 ms. In conclusion, high gas generator pressure is favorable to the secondary ignition and combustion of primary combustion products.

Composition and characteristics of primary combustion products of boron-based propellants

Primary combustion products of boron-based propellants are incomplete combustion products that are emitted from the gas generator of a solid ducted rocket. Studying the composition of primary combustion products provides valuable information about the primary combustion process and also helps to better understand the secondary combustion process. The particle size of the primary combustion products is analyzed by a laser particle size analyzer. The qualitative analysis of the sample composition is performed by using x-ray diffraction, x-ray photoelectron spectroscopy, and thermogravimetry–differential scanning calorimetry experiments. Based on these results, an integrated quantitative analysis of the sample composition is conducted. The quantitative analysis methods include tube furnace heating, ion chromatography, infrared spectroscopy, and inductively coupled plasma chromatography. In addition, scanning electron microscopy and energy dispersive spectrometry are also used to analyze the micro-morphology and distribution of different components in the sample. The primary combustion products mainly contain B, C, BmCn, H3BO3, B2O3, BN, Mg, MgCl2, and NH4Cl. BmCn (22–24%), H3BO3 (20%), and B (16.8%) are the three major components, while BmCn, B, and C (9.8–11.8%) are the three combustible components present in the highest amounts. The oxidant NH4ClO4 is completely consumed during the primary combustion, while the metal additive Mg does not show much reactivity. The micro-morphology and distribution of BmCn, H3BO3 (or B2O3), B, Mg, and C in the sample are investigated. Some components in the primary combustion products are found to be agglomerated, while some components are dispersed. Large particles in the sample mainly include B and Mg, while BmCn, H3BO3 (or B2O3), and C particles are small. In general, the combustion completeness of the primary combustion products is rather low. Therefore, better understanding and controlling of the secondary combustion process is very important to improve the performance of B-based propellants.

Improvement in energy release properties of boron-based propellant by oxidant coating

The energy release properties of a propellant can be improved by coating boron (B) particles with oxidants. In the study, B was coated with four different oxidants, namely, NH4ClO4, KNO3, LiClO4, and cyclotetramethylenetetranitramine (HMX), and the corresponding propellant samples were prepared. First, the structural and morphological analyses of the pretreated B were carried out. Then, the thermal analysis and laser ignition experiments of the propellant samples were carried out. Coating with NH4ClO4 showed a better performance than mechanical mixing with the same component. Coating with KNO3 efficiently improved the ignition characteristics of the samples. Coating with LiClO4 was the most beneficial in reducing the degree of difficulty of B oxidation. Coating with HMX was the most beneficial in the heat release of the samples. The KNO3-coated sample had a very high combustion intensity in the beginning, but then it rapidly became weak. Large amounts of sparks were ejected during the combustion of the LiClO4-coated sample. The HMX-coated sample had the longest self-sustaining combustion time (4332ms) and the highest average combustion temperature (1163.92°C).

Experimental Investigation and Numerical Simulation of Secondary Combustor Flow in Boron-Based Propellant Ducted Rocket

Experimental Investigation and Numerical Simulation of Secondary Combustor Flow in Boron-Based Propellant Ducted Rocket

Study of ignition process of boron particle with considering Stefan flow effects

A one-dimensional model about the ignition process of boron particle in boron-based propellant ducted rocket is systemically investigated. The gas flow around the boron particle, the heat transfer and the mass transfer between the boron particle and the surrounding are included in the model. And the effects of Stefan flow are also proposed. The changing regularities of important parameters in the two typical cases, viz., the successful ignition case and the degenerate ignition case are studied in detail. And their reasons are analyzed. The result shows that both the evaporation of the liquid boric oxide layer and the oxidation of the boron are remarkably accelerated as the result of the self-heating exothermic oxidation in the successful ignition case, and the mass fraction profiles of the oxygen gas and those of the B2O3 gas also dramatically change in that case. However, both the mass flux of the evaporation of the liquid boric oxide layer and that of the consumption of the oxygen gas are relatively small, and both of them tend to be nearly constant in the degenerate ignition case. The mass fraction profile of the oxygen gas and that of the B2O3 gas change little in the degenerate ignition case. In the two typical cases, Stefan flow on the boron particle surface undergoes the change of flow direction, viz., Stefan flow initially comes from the surrounding and then it flows from the particle surface to the surrounding.