Enhancement Of Regression Rate Research Articles

Effects of aluminum and aluminum hydride additives on the performance of hybrid rocket motors based on 95% hydrogen peroxide

This paper aims to study experimentally and numerically the effects of aluminum (Al) and aluminum hydride (AlH3) additives on the performance of HRMs. A steady-state numerical model of HRM is established, which adopts the 95% hydrogen peroxide (HP) as the oxidizer. A firing test is completed for the certification of the numerical simulation model, the results illustrate that the regression rate of addition of 38% AlH3 and 20% Al increases by 55.6% compared with pure hydroxyl-terminated polybutadiene (HTPB). A good agreement exists between the simulation regression rate and the experimental data. The deviations of thrust and pressure are less than 3.3% and 3.7%, respectively. Specifically, the error of the regression rate is only 0.3%, indicating that the accuracy of adopting numerical simulation method to predict the characteristics of the HRM is acceptable. Several steady-state simulations based on the different content of AlH3 and Al are performed to investigate the effects on regression rate and combustion performance of the HRM. Due to the addition of H2, the thermal conductivity increases with the decrease of the mass fraction of AlH3, which leads to the increase of the regression rate. The regression rate of the fuel with 8% AlH3 and 50% Al increases by 35.5% than the fuel with 58% AlH3, demonstrating that the effect of Al on the regression rate enhancement is more than that of AlH3. The theoretical specific impulse increases with the mass fraction of AlH3. However, it is owing that the part of H2 does not participate in the reaction with O2 and the oxidizer to fuel ratio (O/F) is far from the optimal O/F, the specific impulse is lower than the theoretical specific impulse.

Evaluation of Regression Rate Enhancing Concepts and Techniques for Hybrid Rocket Engines

The low regression rate of Hybrid Rocket Engines (HREs) is one prominent characteristic that is addressed in most abstracts concerning hybrid propulsion. Over the years, researchers developed and investigated numerous ways to tackle the low regression rate problem of HREs. This article is a collection and assessment of these diverse methods and designs. It allows for a quick overview of the different mechanisms that are being employed and can serve both as information and inspiration. The enhancement ideas are grouped together as (a) adjustments to the solid fuel chemical properties, (b) advanced injection methods and concepts and (c) improving the combustion chamber design. These different techniques are discussed and their individual impact on the regression rate is assessed both qualitatively and quantitatively. All methods that are presented come with a different set of advantages and disadvantages, making the regression rate enhancement a trade-off problem. In our view, the most promising designs and methods are those that only call for minor adjustments to the HRE design, as they can be also added to already existing engines. Above all, it is to be said that regression rate enhancing techniques that change the unique features of HREs (namely safety, simplicity and low cost) are to be employed with caution. Only if the achievable regression rate increase is justifying the implications for the HRE in the envisioned use-case, these concepts represent promising alternatives to the status quo.

Enhancing the Regression Rate of Hydroxyl-Terminated-Polybutadiene-Based Mixed Hybrid Rockets

This study has identified potential hydroxyl-terminated-polybutadiene (HTPB) based fuel compositions that provide a high-density specific impulse but do not burn like a solid propellant. Due to low solid loading in these fuel compositions, precuring the HTPB and isophorone diisocyanate mixture is necessary to get uniform density and composition across the length of the grain. Nearly 7 h of precuring gave the best results in terms of uniformity of the propellant composition. This study also discusses the effect of available burn rate modifiers for ammonium perchlorate and a type of aluminium on the enhancement of regression rate in mixed hybrid rockets. Finally, the hybrid propellants developed had an oxidizer mass flux index close to 0.5, which is desirable.

Regression Rate Enhancement of Hybrid Rockets by Introducing Novel Distributed Tube Injector

Low fuel regression rate is one of the major drawbacks of hybrid rocket motors. In this study, a novel injector concept is proposed to provide a substantial enhancement in the fuel regression rate. A tubular injector, Distributed Tube Injector (DTI), is inserted in the center of the cylindrical fuel port in order to tailor the oxidizer flow introduced into the combustion chamber with the desired combination of radial, tangential, or axial components of velocity. This concept has been tested using a 500 N thrust class hybrid rocket motor, which uses a paraffin-based fuel and supercharged (L) as the oxidizer. As a result of 30 hot firings conducted in the test program, it is determined that the DTI configuration provides regression rates up to 3.9 times higher than the regression rates obtained using fore-end injector commonly employed in conventional hybrid rockets. Using the motor test data, a comprehensive nondimensional and scalable regression rate relation has been established. This nondimensional regression rate equation can be used to design the internal ballistic configuration of hybrid rocket motors using the novel injector.

Nickel acetylacetonate as decomposition catalyst for HTPB-based fuels: Regression rate enhancement effects

The slow regression rate induced by the difficulty of pyrolysis has limited the practical application of hydroxyl-terminated polybutadiene (HTPB)-based fuels for hybrid rocket propulsion. A possible strategy is the use of suitable transition metal elements promoting polymer matrix thermal decomposition of fuels. This paper investigates the effects of nickel acetylacetonate, Ni(acac)2, on the thermal stability and combustion of HTPB-based fuel formulations. The presented experimental results show that the addition of Ni(acac)2 can intensely decrease the thermal stability of HTPB thus enhancing the solid fuel regression rate even at a small additive mass fraction: under an oxygen mass flux of 50 kg/m2s, HTPB + 5 wt% Ni(acac)2 shows a 25.5% increase over the non-loaded baseline. Kinetics analyses reveal that the catalytic effect is mainly induced by the Ni2+ in Ni(acac)2 at the early stage of decomposition, and by the NiO produced from the oxidative decomposition of Ni(acac)2 in the fuel final degradation stage. On the other hand, the addition of Ni(acac)2 decreases the combustion heat of HTPB-based fuels significantly and implies the accumulation of its decomposition products (much metal Ni, moderate elemental C, and a little NiO.) at the fuel regressing surface. Eventually, when the content of Ni(acac)2 exceeds 5 wt%, the growth of regression rate slows down rapidly, and a performance detriment occurs at 40 wt%. This study verifies the catalytic effect of Ni(acac)2 on polymer matrix for HTPB based fuels showing the attractive regression rate performance of this additive.

Experimental evaluation of HTPB/paraffin fuel blends for hybrid rocket applications

Hybrid rockets have many advantages over pure solid or liquid propellant rockets, but low solid fuel regression rates and correspondingly low thrust have hindered their application to operational systems. Paraffin-based fuels regress significantly faster than traditional polymeric formulations, such as HTPB, and paraffin inclusion in HTPB represents a potential tool for performance augmentation in hybrid rockets. A survey of the available literature indicated disparities regarding the utility of this approach which are resolved herein. Fuel specimen consisting of plain HTPB; plain paraffin; and HTPB loaded with molten macrocrystalline paraffin wax (10–75%) or solid microcrystalline paraffin particles (10–60%) were manufactured and evaluated for their thermal decomposition and ballistic properties. Fuel samples were heated (10 K/min) in an argon atmosphere in simultaneous TGA/DTA experiments. The inclusion of macrocrystalline paraffin enhanced the low-temperature decomposition of HTPB, while the inclusion of microcrystalline paraffin had the opposite effect. The prepared fuel grains were burned in gaseous oxygen on one of two lab-scale hybrid rockets over a range of oxidizer mass fluxes (5–430 kg/m2-s) and pressures (0.5–1.0 MPa). The plain macrocrystalline paraffin fuel exhibited a 300% increase in regression rate over plain HTPB. However, none of the mixed-fuel formulations exhibited notable, if any, regression rate enhancement at the evaluated operating conditions. First principles modeling was completed for the combustion of plain HTPB, plain paraffin, and mixed-fuel systems comprised of HTPB containing molten liquid paraffin or solid paraffin particles. The combustion of mixed-fuel systems is dominated by the pyrolysis of HTPB which does not allow for the formation of a melt layer at the fuel surface, such that any enhancement is due to an increase in the vaporization rate of the fuel and not entrainment effects. This study was the first to concurrently evaluate the inclusion of both molten liquid paraffin and solid paraffin particles in HTPB and demonstrated a lack of performance augmentation with either strategy in two separate laboratories. The results presented herein resolve the disparities in the literature and indicate that paraffin inclusion in HTPB is not a viable means for tailoring the combustion behavior of hybrid rocket systems.

Characterisation of paraffin-based hybrid rocket fuels loaded with nano-additives

ABSTRACTIn this work, a new composition based on Paraffin wax and HTPB fuel, loaded with nanoparticles has been proposed for hybrid propulsion system. Lithium aluminium hydride (LiAlH4) and Magnesium hydride (MgH2) nanoparticles have been used as additives. A detailed rheological, thermal and ballistic characterisation has been carried out. The Magnesium hydride doped hybrid fuel exhibits lower viscosity as compared to the Lithium aluminium hydride doped one, leading to comparatively enhanced entrainment-aided combustion. LiAlH4 doped hybrid fuels also exhibit solid-like behaviour and thus greater stability in the solid phase in contrast to the MgH2 doped fuel. LiAlH4 doped fuel is thermally more stable and produces relatively greater residual-mass. The loading of nanoparticles significantly improves the fuel regression performance during ballistic firing. This can be attributed to the release of nascent hydrogen and metal nanoparticles during dehydrogenation of metal hydrides. Regression rate enhancement in the range of 350%–475% is observed in comparison to the conventional HTPB hybrid fuels. A power law governing regression rate has been proposed for the tested hybrid fuels.

Three-Dimensional Printing of “Green” Fuels for Low-Cost Small Spacecraft Propulsion Systems

This paper details the advantages of employing modern additive manufacturing methods to fabricate hybrid rocket fuels for intrinsically safe and green small spacecraft propulsion systems. Using additive manufacturing overcomes multiple issues frequently associated with hybrid propulsion, including poor volumetric efficiency, system ignitability, and low fuel regression rates. When certain three-dimensionally printed thermoplastics are subjected to a high-voltage low-wattage charge, electrostatic arcing along the surface pyrolizes a small amount of material that, with the introduction of an oxidizer, “seeds” combustion and produces immediate and reliable ignition. Thermoplastic fuel grains can be printed with port shapes that enhance burn properties and increase volumetric efficiencies. Embedded helical fuel ports significantly increase regression rates. The presented test results from several prototype systems using gaseous oxygen and printed acrylonitrile butadiene styrene demonstrate the various advantages of additive manufacturing, including low-power ignition, regression rate enhancement, and system scalability. The test results from both ambient and vacuum tests of a 25 N flight-weight small spacecraft thruster are presented. Multiple burn tests allowed statistical characterization of ignition timing and burn-to-burn thrust, as well as total impulse consistency. The test results demonstrating specific impulse values exceeding 295 s are presented. When fully developed, this propulsion technology has the potential for “drop-in” replacement of many hydrazine-based propulsion applications.

A numerical investigation of oxidizer mixed hybrid rocket motors

In this work, the effect of solid oxidizer (AP) mixed fuel grain (HTPB) on regression rate and performance in GOX hybrid rocket motors is numerically studied. Here the pyrolysis products of AP and the fuel forms a premixed flame very close to the grain surface in addition to the fuel–oxygen diffusion flame located further away from the grain surface. The presence of premixed flame increases heat feedback to the grain and consequently increases the local regression rate throughout the grain length. This is contrary to the locally limited influence observed while using other regression rate enhancement techniques like swirling oxidizer, or transverse diaphragms/protrusions. Scaling of motors with AP mixed grain of a given composition follows that of a pure fuel grain where the grain geometry and the inlet oxidizer mass flux need to be maintained.

Effect of protrusion on the enhancement of regression rate

It has been reported in literature that the use of protrusion in the combustion chamber of a hybrid rocket motor enhances the regression rate. A detailed study was carried out by using two types of protrusion (a) rubber (which burns along with fuel but at different rate, hence its shape changes with burn time), and (b) graphite (does not burns along with fuel and hence its shape does not changes with burn time). The results showed that the improvement in regression rate with protrusions was only up to a certain fraction of the overall burn time. From the results obtained, it was observed that an X/L of 0.5 was the best location for both types of protrusion. The maximum improvement in regression rate was around 45%. The combustion efficiency was observed to improve from 58% without protrusion to 69% with the use of protrusion, when placed at an X/L of 0.8. It was observed that the use of bluff body and protrusion improves combustion efficiency, when used individually. Hence, combination of the two was used and the combustion efficiency was determined. The combustion efficiency was around 90%, when bluff body was used at the head end and the protrusion at an X/L of 0.8 from head end.

Trade-off between paraffin-based and aluminium-loaded HTPB fuels to improve performance of hybrid rocket fed with N2O

In the effort of raising the slow fuel-regression rate of hybrid rockets, two approaches focused on the fuel nature, i.e. the inclusion of metal particles into classical HTPB grains and the use of paraffin-wax based fuels, are investigated and contrasted with each other in this paper. A series of static tests on two hybrid rocket engines of different lab scales have been carried out by burning pure HTPB, aluminium-loaded HTPB or a paraffinic fuel with gaseous nitrous oxide. Fuel-regression rate and combustion efficiency outcomes are discussed. Aluminized HTPB yields regression-rate improvement whose magnitude decreases at high mass fluxes. Regression-rate increase of about 80% over aluminized HTPB is featured by paraffin-wax based fuel, but with lower efficiency and at the cost of poorer mechanical stiffness. A comparison with some other data obtained from pure HTPB and metal-loaded HTPB burned with gaseous oxygen shows that, in the tested operating conditions, regression rate enhancement achieved with the addition of aluminium powder is slightly larger with nitrous oxide than with oxygen. A trade-off between the two classes of fuels is made based on overall engine performance.

Combined Effect of Diaphragm and Oxidizer Swirl on Regression Rate in Hybrid Rocket Motors

The present numerical work investigates the effect of a combination of two known regression rate enhancement techniques, namely diaphragm located in the port and swirling oxidizer flow at the inlet. Diaphragm enhances regression rate by heat transfer enhancement due to recirculation zone whereas swirling flow enhances regression rate by enhancement in radial diffusive transport. The use of diaphragm in combination with swirl was found to be more effective in enhancing regression rate compared with the individual enhancements by the two techniques. However, the net enhancement in regression rate using combined techniques was less than the arithmetic sum of enhancement from two individual techniques. Interestingly, the characteristic exhaust velocity C* is enhanced only if diaphragms are sufficiently tall. Defence Science Journal, Vol. 64, No. 1, January 2014, DOI:10.14429/dsj.64.3770

Combustion Experiments of HTPB/RFNA Mixed Hybrid Propellants

A series of combustion experiments have been conducted for investigating the regression rates of propellants using Red Fuming Nitrous Acid (RFNA) as liquid oxidizer and Hydroxyl-Terminated Polybutadiene (HTPB) with the addition of Ammonium Perchlorate (AP) upto 20 percent as the solid fuel. HTPB is the state-of-theart binder used in solid propellant rocket motors and is considered to be a potential candidate fuel for rocket applications due to its higher regression rate characteristics, greater fuel value, higher carbon/hydrogen ratio and solid loading capability. One of the effective ways of increasing the regression rates in hybrids is the addition of solid oxidizer such as AP in the fuel in small percentages. This type of configuration is called mixed hybrid and regression rate enhancement upto 100% is obtained. A series of static tests are conducted to establish the ignition and combustion in the mode. It is seen that the propellant has burnt smoothly. The ignition pressure could also be controlled and kept to a reasonable value. Stable combustion within a variation of chamber pressure ±2% and combustion efficiency above 0.95 was achieved. The characteristics of the combustion products are calculated using the NASA CEA code. Regression rate correlations for the different combinations are obtained using the experimental data and ballistic code developed for predicting the performance of system.

나선형 홈에 의한 하이브리드 로켓 고체연료의 연소율 증가 특성

하이브리드 로켓 연료의 연소율 향상을 위한 나선형 홈이 유동 특성에 미치는 영향을 규명하기 위하여 난류 증가와 선회 유동 발생에 대한 수치계산을 수행하였다. 나선형 튜브의 열전달 관계식을 이용하여 여러 형상의 나선형 홈이 있는 연료의 열전달 향상을 예측하였으며 연소율 측정값과 비교하여 상호 연관성을 확인하였다. 실험 결과와 열전달 향상 예측 값은 동일한 경향을 나타내더라도 수치는 큰 차이를 나타냈는데 이것은 연료가 연소하기 위하여 기화할 때 발생하는 분출속도가 존재하기 때문이다. 분출효과를 고려하여 RANS 계산을 수행한 결과, 난류에너지는 상당히 증가한 반면 선회 수는 조금 증가하는 결과를 보여 주었다. 그럼에도 불구하고 나선형 홈에 의한 난류 증가 또는 선회유동 생성은 연소율을 증가시키는 중요한 메커니즘임이 밝혀졌다. 그러나 분출로 인하여 난류에너지가 증가되어도 열전달 향상에 기여하지 못하는 것으로 확인되었는데 이것은 분출 속도가 표면의 경계층을 밀어내어 대류 열전달이 증가되는 것을 차단하기 때문인 것으로 밝혀졌다. To understand the role of helical geometry on the regression rate enhancement, two competing underlying mechanisms such as turbulence enhancement and swirling motion production were studied by numerical calculations. Experimental results showed that the enhancement of heat transfer rate has the very close relation to the increase in regression rate even though the percentage of increase in heat transfer rate is different from that in regression rate. This discrepancy is presumably due to the change of turbulent flow feature caused by so-called "blowing mass flux" from the fuel surface. In this regard, the results of RANS calculation show that the blowing velocity is responsible for the reduction of the swirl generation and the increase in the turbulent kinetic energy. And the dominancy of one of the mechanisms causes the increase in the regression rate. Meanwhile, the increase in turbulent kinetic energy due to the mixing of blowing flow and free stream flow does not contribute for the enhancement of the heat transfer rate to the surface because the blowing flow pushes boundary layer away from the solid surface.

Effect of induced swirl flow on regression rate of hybrid rocket fuel by helical grain configuration

Induced swirl flow has proved to be an effective way of increasing the residence time of oxidizer in the solid fuel of hybrid rocket motor. For the present study, both experimental and numerical studies have been conducted in order to investigate the effect of both swirl injector and helical configuration of the grain (represented by a pitch number) on the enhancement of regression rate. The results showed that a higher regression rate can be achieved in the case of pitch 6 than in the case of pitch 100 when either no swirl or substantially strong swirl is imposed at the inlet. However, the regression rate in the case of pitch 100 showed even further increase than as observed in pitch 6 if a moderate strength of swirl is imposed. This is believed not only due to an increase in residence time of oxidizer flow but also due to modified flow structures resulted from the interaction of the inlet swirl and the geometric configuration of pitch 100. Thus, it was concluded that a proper combination of helical configuration with large pitch number and a moderate inlet swirl can produce the condition for achieving the maximum regression rate. Numerical calculations with RANS type turbulence model supported the experimental findings that the pitch 100 is more effective in maintaining the rotational component of momentum flux all the way up to the port exit than the pitch 6 when a swirl is imposed. Also calculation result suggests that a less pressure drop is involved in the pitch 100. Nevertheless, more detailed flow information such as turbulence statistics and residence time will be required for the better understanding of the role of swirl and helical configuration and, thus, it would be desirable to perform a DNS or LES type computation.

스월 유동과 나선형 그레인에 의한 하이브리드 로켓 연료의 연소율 향상

하이브리드 로켓 연료의 연소율 향상을 위해서 스월 유동을 이용한 방법과 나선형 그레인 방법의 두 가지 방법을 사용하여 연소율 증가에 대한 실험 연구를 수행하였다. 스월 유동을 적용하기 위해서 두 가지의 인젝터를 사용하였으며 스월 수 3.61인 인젝터를(Type II) 사용할 때 더 높은 연소율 증가를 나타내었다. 그러나 두 가지 인젝터 모두에서 공통으로 연료 앞부분에서 연소가 집중적으로 발생하는 현상이 발생하였다. 나선형 그레인에 의한 연소율 증가를 실험하기 위하여 피치 6과 피치 100인 두 종류 연료를 사용하였다. 스월 유동이 없는 인젝터를 사용할 경우 피치 6 그레인에서는 강한 난류의 발생에 따른 연소율 향상이 더 크게 일어나는 것을 확인할 수 있었다. 그러나 스월 인젝터와 나선형 그레인을 동시에 적용하여 실험한 결과에 의하면, Type II 인젝터와 피치 100 그레인를 적용할 경우에 가장 높은 연소율 증가를 이루었다. 이것은 인젝터에서 발생한 스월 유동이 나선형 그레인을 통해 연료의 출구까지 유지되었기 때문인 것으로 판단된다. Experimental tests have been done with swirl injector and helical grain configuration to increase the regression rate of hybrid rocket solid fuel. Two types of injector were designed to evaluate the swirl effect of oxidizer stream on the increase in the regression rate. Results showed Type II injector with swirl number of 3.61 induced the better regression rate than Type I injector. Meanwhile, fuels with two different pitch number of 6 and 100 were used to analyzes the flow characteristics on the enhancement of regression rate. Test with fuels of pitch 6 showed better increase in the regression rate than in the pitch 100 when no swirler was imposed. This is due to the generation of strong turbulences in the oxidizer stream along the pitch 6 configuration. However, the regression rate could be increased further in the fuel with pitch 100 than with pitch 6 when swirl flow was imposed by Type II injector. This result implied that the fuel with pitch 100 could take a role of sustainer of the imposed swirl by swirler II instead of turbulence generator.

The experiments for the enhancement of regression rate of hybrid rocket fuel

Many studies have been conducted to increase regression rate of solid fuel in hybrid rocket. One of them resorts to swirl flow since it can extend the residence time of oxidizer in the fuel grain. Also, metal wires may lead to increase the regression rate of solid fuel as shown in solid propellants. In this study, a series of experiments was designed to investigate the enhancement of regression rate of solid fuel by embedded metal wires and by fuel port grain. And fuel port was designed with a helical configuration to attempt to induce swirl flow. PMMA with gaseous oxygen is the solid fuel used for investigation. Test results showed that embedded metal wires turned out to be ineffective method because only 3-4% increases in regression rate were observed. However, fuel port grain configuration yields higher burning performance of up to 50% increase in regression rate. Also pitch number as well as total impulse was found to be a design variable.

Scale-Up Tests of High Regression Rate Paraffin-Based Hybrid Rocket Fuels

Recent research at Stanford University has led to the identification of a class of paraffin-based fuels that burn at surface regression rates that are three to four times that of conventional hybrid fuels. The approach involves the use of materials that form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid-gas interface substantially increases the rate of fuel mass transfer, leading to much higher surface regression rates than can be achieved with conventional polymeric fuels. Thus, high regression rate is a natural attribute of the fuel material, and the use of oxidizing additives or other regression rate enhancement schemes is not required. The high regression rate hybrid removes the need for a complex multiport grain, and most applications up to large boosters can be designed with a single port configuration. The fuel contains no toxic or hazardous components and can be shipped by commercial freight as a nonhazardous commodity. At the present time, grains up to 0.19 m [19.1 cm (7.5 in.)] in diameter and 1.14 m [114.8 cm (45.2 in.)] long are fabricated in a general-purpose laboratory at Stanford University. To further demonstrate the feasibility of this approach, a series of scale-up tests with gaseous oxygen have been carried out using a new Hybrid Combustion Facility (HCF) at NASA Ames Research Center. Data from these tests are in agreement with the small-scale, low-pressure, and low mass flux laboratory tests at Stanford University and confirm the high regression rate behavior of the fuels at chamber pressures and mass fluxes representative of commercial applications.