Fuel Grains Research Articles

Superior Al90Mg10-Based Composite Fuel Grain for a Hybrid Rocket Engine

A paraffin-based fuel coupled with a nested helical matrix structure is a low-cost, high-performing alternative for hybrid rocket engine applications. The mechanical and combustion properties of the composite fuel grain can be enhanced by replacing the conventional polymer matrix with a metal skeleton. Three-dimensional printing was used to design an Al90Mg10 skeleton embedded with the paraffin-based fuel. The combustion characteristics of the composite fuel grain, including the ignition behavior, pressure oscillations, regression rate, and combustion efficiency, were comprehensively investigated. The properties of grains with and without a secondary perforated structure were compared. The flame structure and metal burning behavior of the hybrid rocket engine were monitored by endoscopic radioluminescence imaging, and the emission spectral characteristics of the plume were analyzed simultaneously. Good flammability makes the Al90Mg10 helical skeleton a promising candidate for enhancing the combustion performance of a paraffin-based fuel grain. The combustion process with rapid ignition was relatively stable, and no additional pressure oscillation frequency was observed. The metal-based composite fuel grains had a superior regression rate to that of a paraffin-based fuel grain (up to 100% higher using the perforated skeleton). Introducing a secondary structure into the fuel grain promoted the reaction and thereby enhanced the combustion performance.

Supersonic Combustion of Solid Fuels

Supersonic combustion of solid fuels is investigated experimentally using a rectangular-cross-section combustor in the context of understanding ignition characteristics, studying fuel regression, and determining operating conditions. Fuel grains consisted of 3D printed polymethyl-methacrylate of varying geometry. Specifically, the cavity flameholder dimensions are systematically studied to develop a flammability map. Initial examinations reveal that a longer and larger cross-sectional area of the flameholding zone in the fuel grain creates the most optimal conditions for ignition and sustained flameholding. The longer and larger flameholding zone also creates conditions for faster time-averaged regression rates. The ignition characteristics and fuel regression are also examined by varying inlet temperatures of the air, which showed that a higher inlet temperature is conducive to sustained ignition, as well as higher fuel regression rates. This work extends the available literature on supersonic combustion of solid fuels to lower temperatures than has previously been reported.

ENHANCING HYBRID MOTOR THRUST BY A HELICAL-PORT FUEL GRAIN

Hybrid rockets can be good propulsion alternatives to solid and liquid rockets due to their combined safety, controllability, and high energetic performance attributes. However, their characteristic low fuel regression rates, implying low thrust, may be a significant drawback in the application of hybrid rocket systems. In this research study, we experimentally investigated employing a fuel grain with a helical-shape port as a method to increase motor thrust. A series of static firing tests using gaseous oxygen as the oxidizer and polyester fuel grain with a helical port showed an increase up to 2.5-fold in the regression rate compared to a regular straight cylindrical port. Moreover, the addition of 5% expandable graphite (EG) to the fuel revealed a threefold increase in the regression rate. By relating the fuel consumption solely to the gain size (length), an even further increase in the apparent regression rate was observed-up to fourfold for plain polyester and sixfold for polyester with the 5% EG additive. The enhancement in the regression rate along with the longer internal flow path implies a substantially higher fuel mass flow rate from a given fuel grain, enabling a remarkable increase in the motor thrust or noticeable shortening of the fuel grain and motor, which results in a lighter and smaller rocket motor with improved performance.

Burnback Simulation of Fuel Grains with Dispersed-Reinforced Propellant Composition

Burnback Simulation of Fuel Grains with Dispersed-Reinforced Propellant Composition

Extended Development of a Fission Gas Release Behavior Model Inside Spherical Fuel Grains for LWR Reactors

Fission gas plays a significant role in fuel rod performance following accidents. The amount of fission gas increases dramatically under accidental conditions. This leads to a subsequent rise in the fuel rod internal pressure and temperature due to aggravated gap conductance between the fuel pellet and cladding. As a result, fuel rod performance degrades. Therefore, studying fission gas behavior is crucial for accident assessment and evaluating fuel rod performance. Minimizing the Impact of fission gas on fuel rods is essential for maintaining their integrity and safety within nuclear reactors. One important aspect of ensuring safety is predicting fission gas release (FGR). In this study, we presented an extended model to be used in light water reactors (LWRs). The FGR can be modeled using COMSOL Multiphysics with the finite element method. This modeling approach considers both normal and abnormal conditions, with the latter categorized as Class-II type incidents. The model assumes that the gas diffusion inside a spherical grain varies over time. By examining perfect sinks with gas production, perfect sinks without gas production, and imperfect sinks under steady-state conditions, different initial and boundary conditions are set. To validate the accuracy and universality of expressions used in the model, input parameters from other models and experiments are utilized. By comparing the model’s results with these inputs, the accuracy and applicability of the expressions can be confirmed. This validation process ensures that the model provides reliable predictions for fission gas behavior in fuel rods under both normal and abnormal operating conditions. Based on our findings, it is evident that the FGR fraction displays an upward trend as diffusion coefficients and temperatures rise. Conversely, larger grain sizes and higher linear heat generation rates are associated with a reduction in the FGR fraction. Notably, enhanced resolution leads to a postponed onset of FGR. Furthermore, the influence of the diffusion coefficient on the FGR fraction primarily stems from the interconnected effects of temperature and linear heat generation rate.

Combustion Characteristics of a Swirl-Radial-Injection Composite Fuel Grain with Applications in Hybrid Rockets

The combustion characteristics of a swirl-radial-injection composite fuel grain were experimentally and numerically investigated. This composite grain permits swirl-radial oxidizer injection based on three hollow helical blades, each having a constant hollow space allowing uniform oxidizer injection into the main chamber along the axial direction. The oxidizer enters from channel inlets located along a hollow outer wall. This wall, together with the three blades, is fabricated as one piece from acrylonitrile-butadiene-styrene using three-dimensional printing. Paraffin-based fuel is embedded in the spaces between adjacent blades. Firing tests were conducted with gaseous oxygen as the oxidizer, using oxidizer mass flow rates ranging from 7.45 to 30.68 g/s. Paraffin-based fuel grains using conventional fore-end injection were used for comparison. Regression rate boundaries were determined taking into account the erosion of the oxidizer channels. The data show that the regression rate was significantly increased even at the lower limit. Images of the combustion chamber flame and of the exhaust plume were also acquired. The flame was found to be concentrated in the main chamber and a smoky plume was observed, consistent with the high regression rate. A three-dimensional simulation was employed. The present design was found to improve fuel/oxidizer mixing and combustion efficiency compared with a fuel grain using fore-end injection. Both the experimental results and numerical simulations confirmed the potential of this swirl-radial-injection fuel grain.

Acoustics in Flameholding Solid-Fuel-Ramjet Fuel Grains

Experiments are conducted to determine how acoustic perturbations affect the performance and flameholding of solid-fuel ramjets with nonstandard combustion chambers. The focus is on the effect of wall cavities carved in the fuel grain using additive manufacturing. An improved understanding of how the wall geometry contributes to the establishment of acoustic modes is sought. A novel combustion mechanism was developed using a counterflow burner to study the combustion and regression of solid model fuel polymethyl methacrylate. The diffusion flame between the fuel and oxidizer was studied numerically using a solid-fuel decomposition and melt layer model to simulate convection and pyrolysis of the material. This model was validated using new experimental data as well as previously published works. The foam layer parameters are critical to the success of the validation, showing that the increased residence time of the gas in the bubbles facilitates the fuel breakdown. Fourth-order computational simulations of ramjet combustion without regressing fuel walls using a novel discontinuous Galerkin approach are performed with a fully conjugate solution for the thermal wave in the solid. Turbulent transport strongly affects the heat feedback to the walls, and low-frequency vortical modes (e.g., with a vortical wavemaker) associated with a recirculation region at the injector upstream wall are linked to an increase in chamber pressure and fuel mass flux.

Experimental Firing Test Campaign and Nozzle Heat Transfer Reconstruction in a 200 N Hybrid Rocket Engine with Different Paraffin-Based Fuel Grain Lengths

Firing test campaigns were carried out on a 200 N thrust-class hybrid rocket engine, using gaseous oxygen as an oxidizer and a paraffin-wax-based fuel. Different fuel grain lengths were adopted to extend the fuel characterization under different operating conditions, and to evaluate rocket performances and internal ballistics in the different configurations. In addition to data collected under a 220 mm propellant grain length, two further test campaigns were carried out considering 130 mm and 70 mm grain lengths. Two different injector types were adopted in the 130 mm configuration; in particular, a showerhead injection system was used with the aim to contain high-amplitude pressure oscillations observed during some firing tests in this engine configuration. Parameters such as the chamber pressure and temperature inside the graphite nozzle, space-averaged fuel regression rate and nozzle throat diameter were measured. The results allowed for the investigation of different issues related to hybrid rockets (e.g., fuel regression rate, engine performance, nozzle ablation under different conditions). The focus was mainly directed to the nozzle heat transfer, through the reconstruction of the convective heat transfer coefficient for different tests in the 70 mm grain length engine configuration. The reconstruction took advantage of the experimental data provided by the nozzle embedded thermocouple. Then, the experimental convective heat transfer coefficient was used to validate the results from some empirical correlations. The results showed significant differences between the experimental convective heat transfer coefficients when considering tests with different oxidizer mass flow rates. Furthermore, the predictions from the empirical correlations proved to be more reliable only in cases characterized by oxidizer-rich conditions.

Experimental Research on Reconstruction Techniques for Instantaneous Regression Rate of Hybrid Rocket Motor with Single-Port Wagon Wheel Fuel Grain

This study investigated reconstruction techniques for building the instantaneous fuel regression rate of the hybrid rocket motor (HRM). Specifically, an experiment in a laboratory 500 N-class hybrid rocket motor with single-port wagon wheel fuel grain, operated with hydrogen peroxide (HP) and hydroxyl-terminated polybutadiene (HTPB) based fuel (including Al), was carried out. A piece of post-processing software was developed to reconstruct the instantaneous regression rate and other performance parameters of the HRM during the firing test. The results produced by the reconstruction techniques are in good agreement with experimental data obtained by traditional methods, with a maximum error of less than 5.75%. Moreover, compared with the traditional endpoint method, the reconstruction method had a significant advantage, which could ascertain the sensitivity of the regression rate to changes in the oxidizer mass flux and fit the formula of regression rate in a single firing test. Additionally, digital image processing techniques were employed to determine the axial distribution of the fuel regression rate after the test using computed tomography (CT) scanning. This served to verify the accuracy of the instantaneous reconstruction calculation. The error in the average regression rate between CT scanning and the reconstruction calculation was 1.91%, proving that the CT scanning and pixel statistic method of the calculating regression rate was practical for characterizing the axial distribution of the average regression rate during the firing test. In summary, the main objective of this study was to reconstruct the transient parameters of hybrid rocket motor with single-port wagon wheel fuel grain using reconstruction techniques, and to fit the formula of the regression rate through a single-firing test. Furthermore, this paper proposes a modified reconstruction method that is essential for investigating fuel regression rate during the firing test of HRMs.

Regression and Flame Structure in Cavity Flameholding Solid-Fuel Ramjet Fuel Grains

Introducing cavity flameholders into a solid-fuel ramjet fuel grain demonstrated increased fuel loading with sustained combustion in previously unfavorable geometries. Volumetric fuel loading improvements of up to 26% were demonstrated to sustain combustion. Regression patterns of cavity fuel grains are presented and show that the effect of implementing a cavity flameholder is to change the location of maximum regression and the reattachment point. The addition of a cavity flameholder does not appear to have a significant effect on combustion efficiency. However, it is noteworthy that longer cavities increased the chamber pressure above what was observed for a center-perforated fuel grains as a result of the increased mass addition and higher equivalence ratio associated with the higher regression rate. Large-eddy simulation computations were performed using a fourth-order discontinuous Galerkin finite element solver with a novel flamelet and progress variable formulation. The predictions agree well with the experiments and point to the increased heat transfer for longer cavities as the main flameholder mechanism. The larger heat feedback is supported by the formation of a stronger recirculation region, which leads to increased coherent fluctuations due to the transition between local and global instabilities.

Flame Dynamics in the Combustion Chamber of Hybrid Rocket Using Multiangle Chemiluminescence

The flame dynamics in the combustion chamber of a hybrid rocket motor were visualized using novel chemiluminescence imaging. A multidirectional visualization system employing endoscopes generated images based on methylidyne chemiluminescence (CH*), with one endoscope in the precombustion chamber and two in the postcombustion chamber. Images were collected with a high-speed camera using a 1 ms exposure and a 1 kHz frame rate. Fuel grains having a helical or a conventional circular port structures were assessed, and combustion trials were conducted using a laboratory-scale hybrid rocket motor with oxygen as the oxidizer at mass flow rates from 10.43 to : equivalent to combustion chamber pressures ranging from 0.7 to 1.24 MPa. Flame structures were observed during the ignition, combustion, and shutdown stages; and the helical grain generated a larger, more intense flame zone. A proper orthogonal decomposition analysis showed that the helical grains also produced a greater degree of turbulence and stronger oscillations. These results confirm that a helical structure increases the flow turbulence and convective heat transfer in the combustion chamber. These effects lead to higher regression rates and better mixing efficiency that may, in turn, provide greater combustion efficiency at optimized oxidizer/fuel ratios.

Development of Hybrid-Rocket Solid Fuel Grain with a Star-Fractal Counter-Swirl Port

Development of Hybrid-Rocket Solid Fuel Grain with a Star-Fractal Counter-Swirl Port

Numerical Simulations of Fuel Shape Change and Swirling Flows in Paraffin/Oxygen Hybrid Rocket Engines

The objective of this work is to describe and validate a numerical axisymmetric approach for the simulation of hybrid rocket engines (HREs), based on Reynolds-averaged Navier–Stokes simulations, with sub-models for fluid–surface interaction, radiation, chemistry, and turbulence. Fuel grain consumption is considered on both radial and axial directions and both axial and swirl injection of the oxidizer are simulated. Firing tests of two different paraffin–oxygen hybrid rockets are considered. First, a numerical rebuilding of fuel grain profile, regression rate and pressure for axial-injected HREs is performed, yielding a reasonable agreement with the available experimental data. Then, the same numerical model is applied to swirl-injected HREs and employed to analyze both the flowfield and the regression rate variation with swirl intensity. A validation of the model through the rebuilding of small-scale firing tests is also performed.

Improving the Combustion Performance of a Hybrid Rocket Engine using a Novel Fuel Grain with a Nested Helical Structure.

A technique to improve the combustion performance of a hybrid rocket engine using a novel fuel grain structure is presented. This technique utilizes the different regression rates of acrylonitrile butadiene styrene and paraffin-based fuels, which increase the exchanges of both matter and energy by swirl flow and recirculation zones formed at the grooves between the adjacent vanes. The centrifugal casting technique is used to cast the paraffin-based fuel into an acrylonitrile butadiene styrene substrate made by three-dimensional printing. Using oxygen as the oxidizer, a series of tests were conducted to investigate the combustion performance of the novel fuel grain. In comparison to paraffin-based fuel grains, the fuel grain with a nested helical structure, which can be maintained throughout the combustion process, showed significant improvement in the regression rate and great potential in improvement of combustion efficiency.

Improving the Combustion Performance of a Hybrid Rocket Engine using a Novel Fuel Grain with a Nested Helical Structure

Improving the Combustion Performance of a Hybrid Rocket Engine using a Novel Fuel Grain with a Nested Helical Structure

Evaluation of Hybrid Rocket Fuel Grain with a Star Fractal Port Using Combustion Experiments

Evaluation of Hybrid Rocket Fuel Grain with a Star Fractal Port Using Combustion Experiments

Hybrid rockets with mixed [formula omitted] oxidizers for Mars Ascent Vehicles

The objective of this research is to demonstrate the feasibility of sustained combustion in hybrid rocket motor configuration with oxidizers containing high concentrations of CO2, an in situ propellant suitable for Mars applications. Motor tests are performed by using N2O/CO2 mixtures as the oxidizer and paraffin fuel loaded with high concentrations metal powders. Fuel grains with aluminum and magnesium powders have been cast at 40% by mass in the paraffin wax binder. CO2 is mixed with liquid N2O up to 50% in weight percentage as a saturated liquid mixture. Experiments are performed in the blowdown mode using the self-pressurizing capability of the mixed oxidizers. A lab-scale hybrid motor with 70 mm grain length and 31 mm outer grain diameter is used to measure the key performance parameters such as regression rate and combustion efficiency. Successful ignition and sustained/stable combustion with the mixed oxidizer are achieved at CO2 concentrations up to 43% by mass in the liquid mixture. The primary conclusion from this study is that the operation of hybrid rockets with substantial levels of in situ propellant CO2 is feasible.

Mechanism of Stabilization of the Combustion in the Duct of the Solid Fuel Grain of a Solid Fuel Ramjet

The mechanism of stabilization of the combustion in the duct of the solid fuel grain of a solid fuel ramjet with a flame holder was analyzed for the first time. The flame holder can be either a shoulder in the inlet part of the duct, or an autonomous gas generator running on solid fuel capable of self-sustained combustion. A mathematical model for evaluating the stability of the combustion in the solid fuel ramjet grain duct was developed. It was shown that there are several parameters characterizing the solid fuel grain and the solid fuel ramjet as a whole that affect the stability of the operation of the solid fuel ramjet. The main control parameters are the temperature of the combustion products in the flame holder and the flow rate of the combustion products from the flame holder (which can be controlled by varying the sizes and shape of the flame holder), and also the degree of turbulence of the air flow in the solid fuel grain duct. The stability of the combustion in the duct of the solid fuel grain of the solid fuel ramjet was studied, and the possible modes of the operation of the solid fuel ramjet while varying the control parameters were determined.

Averaged Regression Rate Evaluation of Hybrid Rocket Fuel Grain with a Star Fractal Port

Averaged Regression Rate Evaluation of Hybrid Rocket Fuel Grain with a Star Fractal Port

Small-Scale Static Fire Tests of 3D Printing Hybrid Rocket Fuel Grains Produced from Different Materials

The last decade has seen an almost exponential increase in the number of rocket launches for sounding missions or for delivering payloads into low Earth orbits. The emergence of new technologies like rapid prototyping, including 3D printing, is changing the approach to rocket motor design. This project conducted a series of small-scale static fire tests of fused deposition manufacturing hybrid rocket motors that were designed to explore the performance of a variety of commonly available fused deposition manufacturing materials. These materials included acrylonitrile butadiene styrene, acrylonitrile styrene acrylate, polylactic acid (PLA), polypropylene, polyethylene terephthalate glycol, Nylon, and AL (PLA with aluminum particles). To test the performance of small-scale fuel grains, a modular apparatus with a range of sensors fitted to it was designed and manufactured. The small-scale testing performed static burns on two fuel grains of each material with initial dimensions of 100 mm long and 20 mm in diameter with a 6 mm straight circular combustion port. The focus of this study was mainly on the regression rates of each material of fuel grains. Acrylonitrile styrene acrylate and Nylon showed the highest regression rates, while the polyethylene terephthalate glycol regression rates were relatively poor. Also, the acrylonitrile butadiene styrene and acrylonitrile styrene acrylate demonstrating relatively high regression rates when compared to existing hybrid fuels like hydroxyl-terminated polybutadiene.