Special Issue "Advances in Hybrid Rocket Technology and Related Analysis Methodologies"

A special issue of Aerospace (ISSN 2226-4310).

Deadline for manuscript submissions: closed (31 May 2019).

Special Issue Editor

Dr. Carmine Carmicino
Website
Guest Editor
Università di Napoli "Federico II", Dipartimento Ingn Ind, I-80125 Naples, Italy
Interests: chemical propulsion, fluid dynamics
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Special Issue Information

Dear Colleagues,

Once perceived as a niche technology, for about a decade, hybrid rockets have enjoyed renewed interest from both the propulsion technical community and industry. Hybrid motors can be used in practically all applications where a rocket is employed, but there are certain cases where they present a superior fit, such as sounding rockets, tactical missile systems, launch boosters and the emerging field of commercial space transportation. The novel space tourism business, indeed, will benefit from their safety and lower recurrent development costs. The number of researchers dealing with this subject has increased more and more all over the globe along with the launch of student sounding rockets.

The key research areas include systems to improve the slow fuel regression rate, such as the selection of paraffin-wax-based fuel casting, the enhancement of wall heat transfer with nonstandard oxidizer injection methods and/or fuel grain configurations, the effects of the addition of energetic ingredients into the fuel, the suppression of combustion instability, and the optimization of engine components.

In this scenario, a broad spectrum of internal ballistics reconstruction techniques, CFD and numerical simulation strategies, as well as experimental methods have been developed to predict or assess motor performance with success. However, the real challenge facing researchers is probably inseminating the hybrid culture to enable the widespread adoption of this technology, which is still hindered, not for technical reasons, but due to societal factors like the stereotype represented by the mature solid and liquid propellant rockets.

This Special Issue addresses a broad area of topics, welcoming papers that will make a substantial contribution to the state of the art on: (i) motor performance and related issues, (ii) internal ballistics modeling, (iii) applied computational fluid dynamics, (iv) combustion stability, (v) analytical and computational acoustics, (vi) the design of novel hybrid rocket motor concepts, and (vii) experimental methods.

Dr. Carmine Carmicino
Guest Editor

Manuscript Submission Information

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Keywords

  • Fuel regression rate enhancement methods
  • Combustion instability
  • Motor internal ballistics
  • Numerical fluid-dynamic simulations
  • Oxidizer injection techniques
  • Nonstandard solid fuel grain configurations

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Editorial

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Open AccessEditorial
Special Issue “Advances in Hybrid Rocket Technology and Related Analysis Methodologies”
Aerospace 2019, 6(12), 128; https://doi.org/10.3390/aerospace6120128 - 26 Nov 2019
Abstract
Hybrid rockets are chemical propulsion systems that, in the most common configuration, employ a liquid oxidizer (or gaseous in much rarer cases) and a solid fuel; the oxidizer, stored in tanks, is properly injected in the combustion chamber where the solid fuel grain [...] Read more.
Hybrid rockets are chemical propulsion systems that, in the most common configuration, employ a liquid oxidizer (or gaseous in much rarer cases) and a solid fuel; the oxidizer, stored in tanks, is properly injected in the combustion chamber where the solid fuel grain is bonded [...] Full article

Research

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Open AccessArticle
Design and Test of a Student Hybrid Rocket Engine with an External Carbon Fiber Composite Structure
Aerospace 2020, 7(5), 57; https://doi.org/10.3390/aerospace7050057 - 13 May 2020
Abstract
The development of hybrid rockets offers excellent opportunities for the practical education of students at universities due to the high safety and relatively low complexity of the rocket propulsion system. During the German educational program Studentische Experimental-Raketen (STERN), students of the Technische Universität [...] Read more.
The development of hybrid rockets offers excellent opportunities for the practical education of students at universities due to the high safety and relatively low complexity of the rocket propulsion system. During the German educational program Studentische Experimental-Raketen (STERN), students of the Technische Universität Braunschweig obtain the possibility to design and launch a sounding rocket with a hybrid engine. The design of the engine HYDRA 4X (HYbridDemonstrations-RaketenAntrieb) is presented, and the results of the first engine tests are discussed. The results for measured regression rates are compared to the results from the literature. Furthermore, the impact of the lightweight casing material carbon fiber-reinforced plastic (CFRP) on the hybrid engine mass and flight apogee altitude is examined for rockets with different total impulse classes (10 to 50 kNs). It is shown that the benefit of a lightweight casing material on engine mass decreases with an increasing total impulse. However, a higher gain on apogee altitude, especially for bigger rockets with a comparable high total impulse, is shown. Full article
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Open AccessArticle
Nytrox as “Drop-in” Replacement for Gaseous Oxygen in SmallSat Hybrid Propulsion Systems
Aerospace 2020, 7(4), 43; https://doi.org/10.3390/aerospace7040043 - 12 Apr 2020
Abstract
A medical grade nitrous oxide (N2O) and gaseous oxygen (GOX) “Nytrox” blend is investigated as a volumetrically-efficient replacement for GOX in SmallSat-scale hybrid propulsion systems. Combined with 3-D printed acrylonitrile butadiene styrene (ABS), the propellants represent a significantly safer, but superior [...] Read more.
A medical grade nitrous oxide (N2O) and gaseous oxygen (GOX) “Nytrox” blend is investigated as a volumetrically-efficient replacement for GOX in SmallSat-scale hybrid propulsion systems. Combined with 3-D printed acrylonitrile butadiene styrene (ABS), the propellants represent a significantly safer, but superior performing, alternative to environmentally-unsustainable spacecraft propellants like hydrazine. In a manner analogous to the creation of soda-water using dissolved carbon dioxide, Nytrox is created by bubbling GOX under pressure into N2O until the solution reaches saturation. Oxygen in the ullage dilutes N2O vapor and increases the required decomposition energy barrier by several orders of magnitude. Thus, risks associated with inadvertent thermal or catalytic N2O decomposition are virtually eliminated. Preliminary results of a test-and-evaluation campaign are reported. A small spacecraft thruster is first tested using gaseous oxygen and 3-D printed ABS as the baseline propellants. Tests were then repeated using Nytrox as a “drop-in” replacement for GOX. Parameters compared include ignition reliability, latency, initiation energy, thrust coefficient, characteristic velocity, specific impulse, combustion efficiency, and fuel regression rate. Tests demonstrate Nytrox as an effective replacement for GOX, exhibiting a slightly reduced specific impulse, but with significantly higher volumetric efficiency. Vacuum specific impulse exceeding 300 s is reported. Future research topics are recommended. Full article
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Open AccessArticle
Nano-Sized and Mechanically Activated Composites: Perspectives for Enhanced Mass Burning Rate in Aluminized Solid Fuels for Hybrid Rocket Propulsion
Aerospace 2019, 6(12), 127; https://doi.org/10.3390/aerospace6120127 - 25 Nov 2019
Cited by 1
Abstract
This work provides a lab-scale investigation of the ballistics of solid fuel formulations based on hydroxyl-terminated polybutadiene and loaded with Al-based energetic additives. Tested metal-based fillers span from micron- to nano-sized powders and include oxidizer-containing fuel-rich composites. The latter are obtained by chemical [...] Read more.
This work provides a lab-scale investigation of the ballistics of solid fuel formulations based on hydroxyl-terminated polybutadiene and loaded with Al-based energetic additives. Tested metal-based fillers span from micron- to nano-sized powders and include oxidizer-containing fuel-rich composites. The latter are obtained by chemical and mechanical processes providing reduced diffusion distance between Al and the oxidizing species source. A thorough pre-burning characterization of the additives is performed. The combustion behaviors of the tested formulations are analyzed considering the solid fuel regression rate and the mass burning rate as the main parameters of interest. A non-metallized formulation is taken as baseline for the relative grading of the tested fuels. Instantaneous and time-average regression rate data are determined by an optical time-resolved technique. The ballistic responses of the fuels are analyzed together with high-speed visualizations of the regressing surface. The fuel formulation loaded with 10 wt.% nano-sized aluminum (ALEX-100) shows a mass burning rate enhancement over the baseline of 55% ± 11% for an oxygen mass flux of 325 ± 20 kg/(m2∙s), but this performance increase nearly disappears as combustion proceeds. Captured high-speed images of the regressing surface show the critical issue of aggregation affecting the ALEX-100-loaded formulation and hindering the metal combustion. The oxidizer-containing composite additives promote metal ignition and (partial) burning in the oxidizer-lean region of the reacting boundary layer. Fuels loaded with 10 wt.% fluoropolymer-coated nano-Al show mass burning rate enhancement over the baseline >40% for oxygen mass flux in the range 325 to 155 kg/(m2∙s). The regression rate data of the fuel composition loaded with nano-sized Al-ammonium perchlorate composite show similar results. In these formulations, the oxidizer content in the fuel grain is <2 wt.%, but it plays a key role in performance enhancement thanks to the reduced metal–oxidizer diffusion distance. Formulations loaded with mechanically activated ALEX-100–polytetrafluoroethylene composites show mass burning rate increases up to 140% ± 20% with metal mass fractions of 30%. This performance is achieved with the fluoropolymer mass fraction in the additive of 45%. Full article
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Open AccessArticle
Design and Testing of a Paraffin-Based 1000 N HRE Breadboard
Aerospace 2019, 6(8), 89; https://doi.org/10.3390/aerospace6080089 - 12 Aug 2019
Cited by 1
Abstract
The paper presents some relevant achievements in hybrid rocket propulsion carried out by the Italian Aerospace Research Centre. On the basis of the experimental results obtained on a 200 N thrust class engine, a 1000 N class breadboard, fed with gaseous oxygen coupled [...] Read more.
The paper presents some relevant achievements in hybrid rocket propulsion carried out by the Italian Aerospace Research Centre. On the basis of the experimental results obtained on a 200 N thrust class engine, a 1000 N class breadboard, fed with gaseous oxygen coupled with a paraffin-based fuel grain, was designed and experimentally tested in different conditions. The breadboard exhibited a stable combustion in all the firing test conditions; the testing campaign allowed the acquisition of different experimental data, as pre and post-combustion chamber pressure, throat material temperature, pre-combustion chamber temperature. The new breadboard was characterized by higher measured regression rate values with respect to corresponding data obtained with the smaller scale one, highlighting that the oxidizer mass flux is not the only operating quantity affecting the fuel consumption behavior, which could be also influenced by scale parameters, such as the grain port diameter, and other operating conditions, such as the mixture ratio. Full article
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Open AccessArticle
Modeling of High Density Polyethylene Regression Rate in the Simulation of Hybrid Rocket Flowfields
Aerospace 2019, 6(8), 88; https://doi.org/10.3390/aerospace6080088 - 09 Aug 2019
Cited by 1
Abstract
Numerical analysis of hybrid rocket internal ballistics is carried out with a Reynolds-averaged Navier–Stokes solver integrated with a customized gas–surface interaction wall boundary condition and coupled with a radiation code based on the discrete transfer method. The fuel grain wall boundary condition is [...] Read more.
Numerical analysis of hybrid rocket internal ballistics is carried out with a Reynolds-averaged Navier–Stokes solver integrated with a customized gas–surface interaction wall boundary condition and coupled with a radiation code based on the discrete transfer method. The fuel grain wall boundary condition is based on species, mass, and energy conservation equations coupled with thermal radiation exchange and finite-rate kinetics for fuel pyrolysis modeling. Fuel pyrolysis is governed by the convective and radiative heat flux reaching the surface and by the energy required for the propellant grain to heat up and pyrolyze. Attention is focused here on a set of static firings performed with a lab-scale GOX/HDPE motor working at relatively low oxidizer mass fluxes. A sensitivity analysis was carried out on the literature pyrolysis models for HDPE, to evaluate the possible role of the uncertainty of such models on the actual prediction of the regression rate. A reasonable agreement between the measured and computed averaged regression rate and chamber pressure was obtained, with a noticeable improvement with respect to solutions without including radiative energy exchange. Full article
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Open AccessArticle
Small-Scale Static Fire Tests of 3D Printing Hybrid Rocket Fuel Grains Produced from Different Materials
Aerospace 2019, 6(7), 81; https://doi.org/10.3390/aerospace6070081 - 15 Jul 2019
Cited by 2
Abstract
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 [...] Read more.
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. Full article
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Open AccessArticle
Theoretical Investigation on Feedback Control of Hybrid Rocket Engines
Aerospace 2019, 6(6), 65; https://doi.org/10.3390/aerospace6060065 - 03 Jun 2019
Cited by 1
Abstract
Despite the fact that hybrid propulsion offers significant benefits, it still suffers from some limitations such as the natural oxidizer to fuel ratio shift which induces variations of the engines’ performances while operating. To overcome that issue, Japan Aerospace Exploration Agency (JAXA) has [...] Read more.
Despite the fact that hybrid propulsion offers significant benefits, it still suffers from some limitations such as the natural oxidizer to fuel ratio shift which induces variations of the engines’ performances while operating. To overcome that issue, Japan Aerospace Exploration Agency (JAXA) has been studying an innovative concept for several years based on the combination of controlled axial and radial oxidizer injections, called altering-intensity swirling-oxidizer-flow-type engine. This type of motor is theoretically capable of managing both the thrust and the oxidizer to fuel ratio independently and instantaneously by using a feedback control loop. To be effective, such engines would require in-flight instantaneous and precise thrust and an oxidizer to fuel ratio measurements as well as an adapted feedback control law. The purpose of this study is to investigate the effect of measurement errors on the engine control and to propose a regulation law suitable for these motors. Error propagation analysis and regulation law are developed from fundamental equations of hybrid motors and applied in a case where resistor-based sensors are used for fuel regression rate measurement. This study proves the theoretical feasibility of hybrid engines feedback control while providing some methods to design the engine and regression rate sensors depending on the mission requirements. Full article
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Open AccessArticle
Comprehensive Data Reduction for N2O/HDPE Hybrid Rocket Motor Performance Evaluation
Aerospace 2019, 6(4), 45; https://doi.org/10.3390/aerospace6040045 - 17 Apr 2019
Cited by 5
Abstract
Static firing tests of a hybrid rocket motor using liquid nitrous oxide (N2O) as the oxidizer and high-density polyethylene (HPDE) as the fuel are analyzed using a novel approach to data reduction that allows histories for fuel mass consumption, nozzle throat [...] Read more.
Static firing tests of a hybrid rocket motor using liquid nitrous oxide (N2O) as the oxidizer and high-density polyethylene (HPDE) as the fuel are analyzed using a novel approach to data reduction that allows histories for fuel mass consumption, nozzle throat erosion, characteristic exhaust velocity (c) efficiency, and nozzle throat wall temperature to be determined experimentally. This is done by firing a motor under the same conditions six times, varying only the burn time. Results show that fuel mass consumption was nearly perfectly repeatable, whereas the magnitude and timing of nozzle throat erosion was not. Correlations of the fuel regression rate result in oxidizer port mass flux exponents of 0.62 and 0.76. There is a transient time in the c efficiency histories of around 2.5 s, after which c efficiency remains relatively constant, even in the case of excessive nozzle throat erosion. Although nozzle erosion was not repeatable, the erosion onset factors were similar between tests, and greater than values in previous research in which oxygen was used as the oxidizer. Lastly, nozzle erosion rates exceed 0.15 mm/s for chamber pressures of 4 to 5 MPa. Full article
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Open AccessArticle
Viability of an Electrically Driven Pump-Fed Hybrid Rocket for Small Launcher Upper Stages
Aerospace 2019, 6(3), 36; https://doi.org/10.3390/aerospace6030036 - 14 Mar 2019
Cited by 1
Abstract
An electrically driven pump-fed cycle for a hybrid rocket engine is proposed and compared to a simpler gas-pressurized feed system. A liquid-oxygen/paraffin-based fuel hybrid rocket engine which powers the third stage of a Vega-like launcher is considered. Third-stage ignition conditions are assigned, and [...] Read more.
An electrically driven pump-fed cycle for a hybrid rocket engine is proposed and compared to a simpler gas-pressurized feed system. A liquid-oxygen/paraffin-based fuel hybrid rocket engine which powers the third stage of a Vega-like launcher is considered. Third-stage ignition conditions are assigned, and engine design and payload mass are defined by a proper set of parameters. Uncertainties in the classical regression rate correlation coefficients are taken into account and robust design optimization is carried out with an approach based on an epsilon-constrained evolutionary algorithm. A mission-specific objective function, which takes into account both the payload mass and the ability of the rocket to reach the required final orbit despite uncertainties, is determined by an indirect trajectory optimization approach. The target orbit is a 700 km altitude polar orbit. Results show that electrically driven pump-fed cycle is a viable option for the replacement of the conventional gas-pressurized feed system. Robustness in the design is granted and a remarkable payload gain is achieved, using both present and advanced technologies for electrical systems. Full article
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Open AccessArticle
Hybrid Rocket Underwater Propulsion: A Preliminary Assessment
Aerospace 2019, 6(3), 28; https://doi.org/10.3390/aerospace6030028 - 06 Mar 2019
Cited by 3
Abstract
This paper presents an attempt to use the hybrid rocket for marine applications with a 500 N class hybrid motor. A 5-port high density polyethylene (HDPE) fuel grain was used as a test-bed for the preliminary assessment of the underwater boosting device. A [...] Read more.
This paper presents an attempt to use the hybrid rocket for marine applications with a 500 N class hybrid motor. A 5-port high density polyethylene (HDPE) fuel grain was used as a test-bed for the preliminary assessment of the underwater boosting device. A rupture disc preset to burst at a given pressure was attached to the nozzle exit to prevent water intrusion where a careful hot-firing sequence was unconditionally required to avoid the wet environment within the chamber. The average thrust level around 450 N was delivered by both a ground test and an underwater test using a water-proof load cell. However, it was found that instantaneous underwater thrusts were prone to vibration, which was due in part to the wake structure downstream of the nozzle exit. Distinctive ignition curves depending on the rupture disc bursting pressure and oxidizer mass flow rate were also investigated. To assess the soft-start capability of the hybrid motor, the minimum power thrust, viewed as the idle test case, was evaluated by modulating the flow controlling valve. It was found that an optimum valve angle, delivering 16.3% of the full throttle test case, sustained the minimum thrust level. This preliminary study suggests that the throttable hybrid propulsion system can be a justifiable candidate for a short-duration, high-speed marine boosting system as an alternative to the solid underwater propulsion system. Full article
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Review

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Open AccessReview
Review of Classical Diffusion-Limited Regression Rate Models in Hybrid Rockets
Aerospace 2019, 6(6), 75; https://doi.org/10.3390/aerospace6060075 - 20 Jun 2019
Cited by 2
Abstract
In this article, we undertake a concise review of several milestone developments in classical regression rate models of hybrid rocket motors. After a brief description of the physical processes entailed in hybrid rocket combustion, Marxman’s diffusion-limited theory is re-constructed and discussed. Considerations beyond [...] Read more.
In this article, we undertake a concise review of several milestone developments in classical regression rate models of hybrid rocket motors. After a brief description of the physical processes entailed in hybrid rocket combustion, Marxman’s diffusion-limited theory is re-constructed and discussed. Considerations beyond the scope of basic convection-driven models, which address disparate forms of the blowing correction, variable fluid properties, and pressure and radiation effects, are also given. Finally, a selection of kinetically-limited models is presented, with the aim of comparing the characteristics of several competing theories that become applicable under particular circumstances. Full article
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Open AccessFeature PaperReview
The Application of Computational Thermo-Fluid-Dynamics to the Simulation of Hybrid Rocket Internal Ballistics with Classical or Liquefying Fuels: A Review
Aerospace 2019, 6(5), 56; https://doi.org/10.3390/aerospace6050056 - 16 May 2019
Cited by 1
Abstract
The computational fluid dynamics of hybrid rocket internal ballistics is becoming a key tool for reducing the engine operation uncertainties and development cost as well as for improving experimental data analysis. Nevertheless, its application still presents numerous challenges for the complexity of modeling [...] Read more.
The computational fluid dynamics of hybrid rocket internal ballistics is becoming a key tool for reducing the engine operation uncertainties and development cost as well as for improving experimental data analysis. Nevertheless, its application still presents numerous challenges for the complexity of modeling the phenomena involved in the fuel consumption mechanism and its coupling with the chemically reacting flowfield. This paper presents a review of the computational thermo-fluid-dynamic models developed for the internal ballistics of hybrid rockets burning gaseous oxygen with classical polymeric or paraffin-based fuels, with a special focus on the interaction between the fluid and the solid fuel surface. With the purpose of predicting the local fuel regression rate, which is the main parameter needed for the hybrid rocket design, the model is coupled with an improved gas/surface interface treatment based on local mass, energy and mean mixture-fraction balances, combined to either a pyrolysis-rate equation in the case of classical polymers, or to an additional equation for the liquid paraffin entrainment fraction of the total fuel consumption rate. A number of experimental test cases obtained from the static firing of two different laboratory-scale rockets are simulated to determine the models’ capabilities, showing very good agreement between the calculated and measured fuel regression rates with both standard pyrolyzing and liquefying fuels. The prediction of the chamber pressure measured with paraffin fuel resulted in it being more cumbersome for the single-phase flow assumption. The advantages and limitations of the models are discussed. Full article
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Open AccessReview
Innovative Methods to Enhance the Combustion Properties of Solid Fuels for Hybrid Rocket Propulsion
Aerospace 2019, 6(4), 47; https://doi.org/10.3390/aerospace6040047 - 22 Apr 2019
Cited by 3
Abstract
The low regression rates for hydroxyl-terminated polybutadiene (HTPB)-based solid fuels and poor mechanical properties for the alternative paraffin-based liquefying fuels make today hybrid rocket engines far from the outstanding accomplishments of solid motors and liquid engines. In this paper, a survey is conducted [...] Read more.
The low regression rates for hydroxyl-terminated polybutadiene (HTPB)-based solid fuels and poor mechanical properties for the alternative paraffin-based liquefying fuels make today hybrid rocket engines far from the outstanding accomplishments of solid motors and liquid engines. In this paper, a survey is conducted of several innovative methods under test to improve solid fuel properties, which include self-disintegration fuel structure (SDFS)/paraffin fuels, paraffin fuels with better mechanical properties, high thermal conductivity fuels and porous layer combustion fuels. In particular, concerning HTPB, new results about diverse insert and low-energy polymer particles enhancing the combustion properties of HTPB are presented. Compared to pure HTPB, regression rate can be increased up to 21% by adding particles of polymers such as 5% polyethylene or 10% oleamide. Concerning paraffin, new results about self-disintegrating composite fuels incorporating Magnesium particles (MgP) point out that 15% 1 μm- or 100 μm-MgP formulations increase regression rates by 163.2% or 82.1% respectively, at 335 kg/m2·s oxygen flux, compared to pure paraffin. Overall, composite solid fuels featuring self-disintegration structure appear the most promising innovative technique, since they allow separating the matrix regression from the combustion of the filler grains. Yet, the investigated methods are at their initial stage. Substantial work of refinement in this paper is for producing solid fuels to fulfill the needs of hybrid rocket propulsion. Full article
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