Boron-based Fuel-rich Propellants Research Articles

Theoretical analysis on applying steam reforming to the primary combustion of the boron-based fuel-rich propellant

The primary combustion products of boron-based fuel-rich propellant contain a significant amount of condensed boron, which poses challenges for complete burnout of the primary combustion products during the secondary combustion process. To enhance the ignition and combustion characteristics of these primary combustion products, a pioneering theoretical analysis on applying steam reforming to the primary combustion of the boron-based fuel-rich propellant was conducted. The results demonstrate that steam reforming improves the ignition and combustion characteristics of the primary combustion products through three main aspects: optimizing the composition of combustion products, improving energy distribution between the primary combustion and secondary combustion process, and elevating combustion temperature, respectively. It theoretically confirms the feasibility of this innovative combustion organization method for solid rocket scramjet. Furthermore, it has been found that there is an optimal steam-fuel ratio (the value is 1.0 for B35), which facilitates the conversion of combustible components in the primary combustion products from a gas–solid state to a pure gaseous phase. Simultaneously, the temperature of combustion products after steam reforming reaches the highest. Therefore, theoretically speaking, the steam-fuel ratio of solid rocket scramjet with steam reforming should not exceed the optimal steam-fuel ratio to ensure better ignition and combustion characteristics of the combustion products after steam reforming. This analysis provides valuable theoretical support for the design of solid rocket scramjets with steam reforming.

Point-to-Point Efficient Grafting Improved Compatibility of Boron-Based Fuel-Rich Propellants.

Due to the high mass calorific value, boron powder is widely used in energetic material systems such as solid propellants and ignition powders. Especially, boron powder has very broad application prospects in the field of fuel-rich propellants. However, due to the cross-linking reaction between the boric acid contained on the surface of boron powder and an adhesive called hydroxylated polybutadiene (HTPB), the viscosity of the propellant mixture system increases sharply, which seriously affects the preparation of the propellant. In this paper, "click chemistry" is intended to be used to graft the functional groups on the surface of boron powder to reduce the viscosity of the boron powder and HTPB in the initial mixing stage. In addition, a rheometer was used to test the viscosity of the boron powder and the HTPB system. The test results showed that compared to the viscosity of the raw boron powder system at 24.1 Pa·s, the mixing termination viscosity of the grafted sample was 17.1 Pa·s, a decrease of 29.0%.

Effect of azide polyether pyrolysis property on combustion and heat release of boron-based fuel-rich propellant

Overcoming the severe particle agglomeration of unreacted fuel boron (B) in combustion process to enhance the heat release efficiency of B-based fuel-rich propellants is still a great challenge. Glycidyl azide polyether (GAP) was introduced to synthesize GAP-based energetic thermoplastic elastomer (GAP-ETPE) binder, which was used to fabricate the B/GAP-ETPE propellant. High GAP content (85 wt%) can be used to synthesize 85%GAP-ETPE as the binder. With the increase of GAP content, the decomposition conversion rate of GAP-ETPE was increased and the apparent activation energy (Ea) of B/GAP-ETPE propellant was decreased. The heat of explosion and combustion of B/85%GAP-ETPE were increased from 4004.1 kJ/kg to 4546.2 kJ/kg and 24,056.5 kJ/kg to 24,938.3 kJ/kg, respectively. The combustion results showed that the burning rate of B/85%GAP-ETPE (3.1 mm/s) was higher than that of B/TPE (1.8 mm/s) using a thermoplastic elastomer (TPE) without GAP, and the particle injection effect was significantly improved. The pressurization rate of B/85%GAP-ETPE (85 wt% is maximal GAP content in binder in current study) was 2.6 MPa/s during combustion in a closed bomb, which was higher than that of other prepared B/GAP-ETPE, showing the propellant using GAP-ETPE with high GAP content exhibited better gas formation and higher heat release, which provided good dispersion and less particle agglomeration of B particles when burning. This strategy enables B to be ejected more effectively in the combustion process to achieve a better combustion performance and heat release.

Influence of the Multicavity Shape on the Solid Scramjet

In this paper, a modular solid scramjet combustor with multicavity was proposed. The influence of multicavity shape on the performance of solid scramjet was investigated by the direct-connected tests. The experiments simulated a flight Mach 5.5 at 25 km. The boron-based fuel-rich propellant was used. The microstructure of combustion products was analyzed by SEM. The experimental results show that the fuel-rich mixture produced by the gas generator would ignite rapidly in the solid scramjet combustor. The combustion process showed a typical characteristic of establishment-development-maintenance-attenuation. Compared to the flame-holding cavity, the other shapes of cavities, e.g., narrow and lobe, can improve mixing and combustion. In our experiment, the combustion efficiency increased from 0.41 to 0.48, and the total pressure recovery was 0.36. In summary, the proposed solid scramjet combustor can effectively solve the ignition delay problem of the fuel-rich mixture, and the narrow/lobe cavity shows the ability to improve the mixing and combustion of the fuel-rich mixture.

Experimental study on the influence of the injection structure on solid scramjet performance

Injection structures can affect the ignition and combustion of fuel and supersonic air in scramjets, strongly impacting their performance. The influence of three different injection structures on the performance of a solid scramjet was investigated using direct-connect tests. The experiment employed a boron-based fuel-rich propellant and simulated a flight Mach number 5.6 at 25 km. The results show that, compared to a circular nozzle (test no.1), the elliptical nozzle (test no.2) can significantly improve the ignition and combustion of the fuel-rich mixture produced in the gas generator. This improvement is even more significant when a two-stage elliptical nozzle is adopted (test no. 3). The combustion of the fuel-rich mixture presents the typical cycle of establishment, development, maintenance, and decay. The wall ablation area and particle deposition distribution, show that the gaseous components in the fuel-rich mixture burn rapidly, and that particles deposition increases when an elliptical nozzle is used. The combustion efficiency of test no.3 reached 0.60, which is larger than the 0.48 obtained during test no.1. The low particle combustion efficiency is a key factor limiting solid scramjets performance.

Composition of solid and gaseous primary combustion products of boron-based fuel-rich propellant

The investigation of the primary combustion products of boron-based fuel-rich propellants is conducive to understanding the mechanism of the two-stage combustion process and provides important information for the prediction and organization of secondary combustion processes. The present investigation methods include numerical simulation and experimental analysis. The former deviates considerably from the actual conditions. The latter is currently a reliable and practical method, although it is difficult to quantify when the components are complex. In this work, SEM, LPSA, XRD, XPS, IC, ICP–MS, AAS, EDS, and chemical analysis methods were employed to qualitatively and quantitatively analyze the 3 solid products (aggregations, condensed-phase products, and flocculent products) at 1, 5, and 10 bar. GC, GC–MS, and IC combined with chemical tests were adopted to investigate the gaseous products (including chlorine-containing gas) for the first time. The solid products primarily included B2O3, B(HO)3, unburned boron, KB5O8(H2O)4, KCl, C, NH4Cl, NH4Mg(H2O)6Cl3, B13C2, Fe3O4, Mo, MgO, Mg(OH)2, MoO3, C3N4, BN, MgCO3(H2O)3 (or MgCO3), MoO2, Fe4(Fe(CN)6)3, and FeB (or BFe4), and the first 7 were dominant. The reaction degree of boron was less than 50 wt% (increasing with increasing pressure), indicating the significance of the secondary combustion process. The reacted boron was likely to react with O and C but not with N. The generated B2O3 vapor easily combined with H2O to form B(HO)3 or reacted with KCl (from the thermal decomposition of KP) and H2O to produce KB5O8(H2O)4 after cooling. There was less B13C2 (its content increased with increasing pressure, resulting in increases in particle size), since it was easily oxidized to generate B2O3. NH4Cl was produced by the oxidation-reduction reaction between the NH3 and HClO4 released from large-sized AP, and some of it crystallized with MgCl2 and H2O to form NH4Mg(H2O)6Cl3. The gaseous products primarily included N2, H2, CO, CH4, CO2, C2H4, C2H2, HCl, C4H6, C3H6, Cl2, C6H6, and other hydrocarbons. N2 and H2 had the highest content. The N2 content increased with increasing pressure, while the content of the combustible gases decreased, indicating that high pressure promotes the propellant reaction. The results also show that high pressure was conducive to the low-temperature decomposition of AP (producing more HCl and Cl2) and the reaction of boron and hydrocarbons (generating more B13C2), while it had an adverse effect on the C reaction (producing less COx and mostly CO). Nevertheless, the increasing pressure had little effect on the combustion product components but did affect the content within the pressure range.

Concept and performance study of turbocharged solid propellant ramjet

This study proposes a turbocharged solid propellant ramjet (TSPR) propulsion system that integrates a turbocharged system consisting of a solid propellant (SP) air turbo rocket (ATR) and the fuel-rich gas generator of a solid propellant ramjet (SPR). First, a suitable propellant scheme was determined for the TSPR. A solid hydrocarbon propellant is used to generate gas for driving the turbine, and a boron-based fuel-rich propellant is used to provide fuel-rich gas to the afterburner. An appropriate TSPR structure was also determined. The TSPR's thermodynamic cycle was analysed to prove its theoretical feasibility. The results showed that the TSPR's specific cycle power was larger than those of SP-ATR and SPR and thermal efficiency was slightly less than that of SP-ATR. Overall, TSPR showed optimal performance in a wide flight envelope. The specific impulses and specific thrusts of TSPR, SP-ATR, and SPR in the flight envelope were calculated and compared. TSPR's flight envelope roughly overlapped that of SP-ATR, its specific impulse was larger than that of SP-ATR, and its specific thrust was larger than those of SP-ATR and SPR. Attempts to improve the TSPR off-design performance prompted our proposal of a control plan for off-design codes in which both the turbocharger corrected speed and combustor excess gas coefficient are kept constant. An off-design performance model was established by analysing the TSPR working process. We concluded that TSPR with a constant corrected speed had wider flight envelope, higher thrust, and higher specific impulse than TSPR with a constant physical speed determined by calculating the performance of off-design TSPR codes under different control plans. The results of this study can provide a reference for further studies on TSPRs.

Ignition and Combustion Performance of the Primary Condensed-phase Combustion Products from Boron-based Fuel-rich Propellants

Ignition and Combustion Performance of the Primary Condensed-phase Combustion Products from Boron-based Fuel-rich Propellants

Rate constants determination for reactions of B2O3 with H2O and HCl at high temperature using ab initio calculations

Primary combustion of boron-based fuel-rich propellants produces large amounts of HCl(g) and H2O(g) which may react with B2O3(g) during the secondary combustion at high temperature. The reaction paths of the two reactions were expedited with Gaussian-09 program at CCSD(T)/aug-cc-pvtz//M062X/aug-cc-pvtz level, and the rate constants and Arrhenius parameters were estimated by using conventional transition state theory, variational transition state theory and variational transition state theory with interpolated single-point energies. The calculation results show that rate constant of the two reactions is in the range of 6.17×10−16 to 3.57×10−15cm3molec−1s−1 and 1.58×10−16 to 1.47×10−15cm3molec−1s−1 at temperature 2500–3500K with activation energy of 118.01–136.29kJ/mol and 155.12–168.28kJ/mol, respectively.

Attaining Hypersonic Flight with Aluminum-Based Fuel-Rich Propellant

In this paper, experimental results of an aluminized fuel-rich propellant having energetics similar to boron-based fuel-rich propellant and having no residue after combustion are discussed. Using this aluminum-based fuel-rich propellant, which has higher density than the current fuels used for hypersonic flight, this paper looks at enhancing the Mach number envelope of a ramjet. Nondimensional thrust and drag calculations were carried out to identify the air–fuel ratio required for a Mach 6 flight, with different intakes accounting for different pressure recovery. Calculations were carried out with three different intakes, and it was observed that, even with the lowest pressure recovery of 9% at Mach 6, a hypersonic mission was possible. Values for various efficiencies were chosen with great care, and a perturbation analysis was carried out to make the calculations more robust. The specific impulse obtained with the aluminum-based fuel-rich propellant in ramjet mode was 515 s at a 25 km altitude. Comparison of an aluminum-based fuel-rich propellant in ramjet mode was made with a kerosene and hydrogen fueled scramjet from the specific impulse data available in the literature. The specific impulse of the aluminum-based fuel-rich propellant was nearly an order of magnitude higher as compared to a hydrogen fueled scramjet when adjusted to the density of hydrogen. Upon considering a system size, it was also shown that the aluminum-based fuel-rich propellant could meet the burn rate requirement for a Mach 6 flight.

Evaluation of Epoxy Terminated Polybutadiene in Ammonium Perchlorate Coated Boron Based Fuel Rich Propellant Formulations

Evaluation of Epoxy Terminated Polybutadiene in Ammonium Perchlorate Coated Boron Based Fuel Rich Propellant Formulations

Factors Affecting the Primary Combustion Products of Boron-Based Fuel-Rich Propellants

Boron-based fuel-rich propellants with different formulation were prepared, and the condensed-phase products under different chamber pressures were collected in this study. The composition of boride (elemental boron, boron oxide, boron carbide, and boron nitride) in the condensed-phase products was measured by chemical analysis and the particle size distribution was determined using a laser particle size analyzer. Moreover, the burning rate and combustion temperature of the propellants were measured using a strand burner. The experimental results show that both the propellant burning rate and combustion temperature played an important role in the production of boride. There might be large amounts of boron carbide in the condensed-phase products which appear to have a negative effect on the combustion efficiency of the propellants studied in this work. The production of boron carbide could be reduced and combustion efficiency was improved by lowering the combustion temperature while increasing the burning rate. This study also shows that reducing the chamber pressure also causes a decrease in production of boron carbide.

Effect of solid oxidizers on the thermal oxidation and combustion performance of amorphous boron

Amorphous boron is usually employed as the most important fuel of boron-based fuel-rich propellants, and NH4ClO4 (AP), cyclotetramethylenetetranitramine (HMX), KClO4 and KNO3 are the solid oxidizers of most used in solid propellants. The mixtures of boron and different solid oxidizers with mass ratio 1:1 were prepared in this paper, and the effect of these oxidizers on the thermal oxidation and combustion performance of amorphous boron was studied by simultaneous thermogravimetry–differential scanning calorimeter–Fourier transform infrared spectroscopy and CO2 laser ignition experiments. The experimental results show that the main reactions during the heating process of B/AP and B/HMX samples are the decomposition of oxidizers, and the decomposition process of oxidizers rather than the decomposition temperature is affected by amorphous boron; boron could react with KClO4 and KNO3 violently with the release of large amounts of heat, and then both of the oxidizers, especially KClO4, have positive effect on the oxidation and combustion performance of amorphous boron.

Ignition and combustion characteristics of compound of magnesium and boron

Compound of magnesium and boron (MB) is promising to be the ideal substitute of amorphous boron which is usually used as the raw material of boron-based fuel-rich propellants. In this study, the physical characteristics of MB and amorphous boron were studied by the scanning electron microscope, X-ray diffraction and X-ray photoelectron spectroscopy. The thermal reaction characteristics and the ignition and combustion characteristics were investigated through TG/DSC experiments and laser ignition experiments. The experimental results show that the MB particle is much more regular than amorphous boron, which favors for the preparation of boron-based fuel-rich propellants. Magnesium exists in the form of elementary substance, and boron oxide is produced during the preparation of MB which results in the longer ignition delay time of MB. The content of magnesium and the pressure have effects on the MB combustion performance. Although the combustion of magnesium can provide much heat for the combustion of boron, MB with moderate content of magnesium shows the best combustion performance. On the contrast, the ignition delay time of MB is independent on the content of magnesium and the pressure.

Energy release properties of amorphous boron and boron-based propellant primary combustion products

The microstructure of amorphous boron and the primary combustion products of boron-based fuel-rich propellant (hereafter referred to as primary combustion products) was analyzed by scanning electron microscope. Composition analysis of the primary combustion products was carried out by X-ray diffraction and X-ray photoelectron spectroscopy. The energy release properties of amorphous boron and the primary combustion products were comparatively studied by laser ignition experimental system and thermogravimetry–differential scanning calorimetry. The primary combustion products contain B, C, Mg, Al, B4C, B13C2, BN, B2O3, NH4Cl, H2O, and so on. The energy release properties of primary combustion products are different from amorphous boron, significantly. The full-time spectral intensity of primary combustion products at a wavelength of 580nm is ~2% lower than that of amorphous boron. The maximum spectral intensity of the former at full wave is ~5% higher than that of the latter. The ignition delay time of primary combustion products is ~150ms shorter than that of amorphous boron, and the self-sustaining combustion time of the former is ~200ms longer than that of the latter. The thermal oxidation process of amorphous boron involves water evaporation (weight loss) and boron oxidation (weight gain). The thermal oxidation process of primary combustion products involves two additional steps: NH4Cl decomposition (weight loss) and carbon oxidation (weight loss). CL-20 shows better combustion-supporting effect than KClO4 in both the laser ignition experiments and the thermal oxidation experiments.

Chemical analysis of primary combustion products of boron-based fuel-rich propellants

The content of solid primary products of boron-based fuel-rich propellants could be determined quantitatively through the analysis procedure.

Effect of Oxidizer on the Combustion Performance of Boron-Based Fuel-Rich Propellant

Different boron-based fuel-rich propellants were prepared by replacing part of ammonium perchlorate with different content of cyclotrimethylenetrinitramine, cyclotetramethylenetetranitramine, potassium nitrate, and potassium perchlorate. The burning rate of the propellants under different pressures was measured in the strand burner, the combustion temperature of the propellants under 1 MPa was measured by microthermocouple, and differential scanning calorimetry/thermogravimetric analysis experiments of the propellants in argon atmosphere were carried out. The experimental results show that the effect of each oxidizer on the burning rate of the propellant is different; cyclotetramethylenetetranitramine and cyclotrimethylenetrinitramine could increase the combustion temperature and the gaseous products, whereas potassium nitrate, and potassium perchlorate have the opposite effect. The composition of the combustion products under different pressures was calculated based on the minimization of Gibbs free-energy principle, and the results indicate that, although cyclotetramethylenetetranitramine and cyclotrimethylenetrinitramine could decrease the content of boron carbide in the combustion products, cyclotetramethylenetetranitramine and cyclotrimethylenetrinitramine have a lower effect on the heat release of boron because more boron nitride is produced during the combustion process; potassium perchlorate could decrease the content of boron nitride in the combustion products, obviously.

Calculation for Primary Combustion Characteristics of Boron-Based Fuel-Rich Propellant Based on BP Neural Network

A practical scheme for selecting characterization parameters of boron-based fuel-rich propellant formulation was put forward; a calculation model for primary combustion characteristics of boron-based fuel-rich propellant based on backpropagation neural network was established, validated, and then was used to predict primary combustion characteristics of boron-based fuel-rich propellant. The results show that the calculation error of burning rate is less than %; in the formulation range (hydroxyl-terminated polybutadiene 28%–32%, ammonium perchlorate 30%–35%, magnalium alloy 4%–8%, catocene 0%–5%, and boron 30%), the variation of the calculation data is consistent with the experimental results.

Boron and its Compounds in New Energy and Materials Fields

Boron and its compounds play an important role in modern energy and material sciences for the bight application prospect. The applications of boron and its compounds in material fields especially advanced materials including permanent magnetic, superconducting, boride cermets, boron-based fuel-rich propellants and borate whisker materials were summarized in this paper, and the new trend for the exploitation and application was also pointed out.

Effect of fluorinated graphite on combustion of boron and boron-based fuel-rich propellants

The effects of fluorinated graphite (CF^) with different fluorine contents (x = 0.08, 0.25, and 1.17) on the combustion of boron powder (B) and boron-based fuel-rich propellants were experimentally investigated. The propellants contain ammonium perchlorate (AP) (28%), magnalium alloy (8%), hydroxy-terminated polybutadiene (HTPB) (27%), and boron agglomerates (37%, in 300-, 600-, 850-, and 1180-/urn sizes), with immediate contact between CFA and amorphous boron (1:14 and 1:7 in mass ratio) by means of a small amount of polymethyl methacrylate binder. The reactions between boron and CF,,. were investigated by adiabatic calorimetry under oxygen, and thermogravimetry under argon, using different E/C¥X mixtures. The baselines of boron and different CFX were studied separately. Because of liberation of highly reactive products such as unsaturated fluorocarbon and fluorine from CFV decomposition, more heat is evolved due to boron reaction compared to the boron baseline without CFV. Possible exothermic reactions involved in the system containing C, B, O, and F quadruple elements were proposed. Propellants containing CF^ were found to increase the burning rate and/or pressure exponent compared to baseline propellants without CFX. An interpretation based on the intensive reactions among the decomposition products from CF,, AP, HTPB, etc., was proposed. Boron agglomerate size can affect the burning rate because of the mixed effects of boron as a heat sink, condensed phase reactions, and flame structures at different pressures.