See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/323295503 Numerical Analysis on an aerodynamically thrust-vectored Aerospike Nozzle Conference Paper · September 2014 CITATIONS READS 13 1,171 4 authors: Christian Bach Jan Sieder-Katzmann Technische Universität Dresden Technische Universität Dresden 32 PUBLICATIONS 76 CITATIONS 26 PUBLICATIONS 74 CITATIONS SEE PROFILE SEE PROFILE Martin Propst Martin Tajmar Technische Universität Dresden Technische Universität Dresden 15 PUBLICATIONS 46 CITATIONS 291 PUBLICATIONS 2,460 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Advanced Electric and Breakthrough Space-Propulsion View project SMART Rockets View project All content following this page was uploaded by Martin Tajmar on 12 February 2019. The user has requested enhancement of the downloaded file. SEE PROFILE NUMERICAL ANALYSIS ON AN AERODYNAMICALLY THRUSTVECTORED AEROSPIKE NOZZLE M. Propst1, J. Sieder2, C. Bach3 and M. Tajmar4 Institute of Aerospace Engineering TU Dresden, 01069 Dresden, Germany Abstract The present study shows results from numerical investigations of aerodynamic thrust vectoring on a small scale aerospike thruster. The thruster will generate 3 kN of axial thrust at an expansion ratio of ε = 2,941. The full isentropic spike contour was designed using the open source FORTRAN code by C. C. Lee on pure external flow expansion plug nozzles. All numerical analyses shown in the study were performed on two-dimensional spike geometry models using ANSYS FLUENT® to solve the governing flow equations. Investigations were carried out on flow characteristics for spike truncations from 0 % to 75 %. For secondary injection analyses an aerospike nozzle, truncated at 75 % of its full theoretical length, was chosen. To generate thrust vectoring a secondary fluid was injected into the primary flow field normal to the nozzle axis. Two different injection sites located at 60 % and 90 % of the truncated spike length as well as varying mass flow rates were applied to examine effects of secondary injection on the thruster main flow. Finally, an estimation was made regarding the achievable side forces for the investigated configurations due to aerodynamic thrust vectoring. The thesis shows that secondary fluid injection is a feasible method for active thrust vectoring on small scale aerospike thrusters and gives an objective for future applications. and their flow characteristics. In the context of a parameter study, concerning variable mass flow and injection sites, flow properties are examined in chapter five. An estimate of the achievable side forces under the predetermined conditions completes chapter five. 1. INTRODUCTION As an alternative to classical bell nozzle designs, aerospike or plug nozzle propulsion systems have been developed and tested since the 1950s [1-12]. They have been in discussion for the upper stage of Saturn V and later for the Space Shuttle Main Engine. In form of a linear plug construction, the nozzles were the single-stage-toorbit propulsion system of choice of the X-33 Mission at NASA during the 1990s, which is also known as Venture Star. Today, well known for their altitude adaptive characteristics up to the designed pressure ratio, the engines experience a renewed growing interest in recent years because of further advantageous performance characteristics towards comparable bell nozzles and the additional capability of active aerodynamic thrust vectoring [13, 14]. 2. AEROSPIKE CONCEPTS In general, aerospike nozzles can be devided into linear and annular plug nozzle designs. Both configurations offer the possibility to either have only one nozzle or a clustered arrangement. The advantage of a clustered configuration is that it already offers an option for aerodynamic thrust vectoring by applying the method of differential throttling of single nozzle clusters. Moreover, there is the possibility to expand the hot exhaust gases either completely externally or partially internally and externally, by adding an additional divergent nozzle structure behind the nozzle throat [3, 4]. In terms of the progression the objective of this paper is to present a model which provides the possibility to evaluate flow characteristics of expansion nozzles and aerodynamic steering in the context of Computational Fluid Dynamics (CFD) analyses using the commercial solver ANSYS FLUENT®. Therefore chapter two will give a brief introduction in aerospike concepts. Section three shows the modeling of the CFD analyses including the design of the nozzle contour, the computing strategy and specific physical models which aim to achieve a realistic depiction of the fluid flow. In the subsequent chapter four, a suitable nozzle configuration to investigate aerodynamic thrust vectoring is selected by comparing different designs 1 2 3 4 Because of the high thermal load at the nozzle exit as well as aspects of weight optimization and the fact that the ideal plug is very difficult to manufacture, it is usually truncated. Truncation itself is always bound to losses according to the nozzle performance [3]. Furthermore, plug nozzles offer various advantages for exoatmospheric flight scenarios. Because of their shape, the nozzles can reach greater expansion ratios by having the same size of the structures area compared to bell nozzles as pictured in figure 1. Graduate student Martin Propst is a volunteer research assistant at the Institute of Aerospace Engineering TU Dresden Dipl.-Ing. Christian Bach is Ph.D. student and administrator of the SMART Rockets Project (SRP) Dipl.-Ing. Jan Sieder is Ph.D. student and administrator of the SRP Prof. Dr. Martin Tajmar is head of Space Systems Chair and leader of the research group “Space propulsion and new concepts” 1 The greater expansion ratio of the aerospike nozzle results in a higher generated thrust by having a small form factor compared to a conventional nozzle. FIG 1. predetermined conditions. To reduce the computational effort of the CFD analysis, the structure of the combustion chamber is truncated. The resulting line bodies of the fully isentropic spike and the minimized combustion chamber are merged as pictured in figure 2. F1 engine and proposed J-2T-250K aerospike (left), engine of Altair Lander with proposed plug nozzle design (right) [13] 3. CFD-MODELLING FIG 2. Based on the design principles stated in the previous section, the center of chapter three is made up by generating a suitable nozzle geometry, defining the physical conditions under which the considerations are made and explaining the simulation process itself. 3.1. Coordinates of the initial nozzle geometry All investigations are performed using two-dimensional CFD analyses. Therefore all line bodies used for the simulation need to form one structure that encloses the later flow field. The flow field geometry is depicted in figure 3. Geometry There are different approaches to designing the spike contour of a plug nozzle. Besides the already mentioned method lately shown by Kraiko et al [6], there are the classical approaches by Lee [1] and Angelino [2], which use the one-dimensional isentropic flow theory. In the present work, the plug contour was calculated using the open source FORTRAN code for purely external flow expansion of ideal gases presented by Lee. The considerations done by Sieder [15] in a previous thesis according to an ideal aerospike nozzle, which generates a total thrust of 3 kN, serve as a basis for the calculation of the spike geometry. His research provides the required input data for the FORTRAN program shown in table 1. Parameter FIG 3. Value Mach number Geometry of the flow field 2,3 Expansion ratio 2.941 -1 -1 Specific gas constant RS in [J kg K ] 374.8 Temperature at nozzle exit Te in [K] 2024 Pressure ratio pnozzle exit/pchamber 1/15 Isentropic exponent 1.4 3.2. Physical model The purpose of the present paper is to simulate an onground engine test at sea level under idealized conditions. In the context of these numerical analyses, the ideal gas model is used to describe the mixture of 70% Vol. Ethanol as fuel and liquid Oxygen (LOX) as the oxidizer at an applied ratio of 1:1. Besides there is no directed fluid flow imprinted for the surrounding flow field. The boundary conditions for the combustion chamber defined in table 1 are valid only for the idle state of the fluid. So the actual conditions given at the truncated end of the combustion TAB 1. Input data for plug nozzle program Sieder also developed the MATLAB® program for calculating the contour of the combustion chamber for the 2 FIG 4. chamber were calculated dimensional flow theory. using Sequence of performed CDF analyses isentropic one- Furthermore, there are no turbulent models added to the simulation yet. The idealization is appropriate for flows with high Reynolds numbers where inertial forces outweigh those of viscosity. Because of the absence of molecular diffusion, the conservation equations of momentum and energy are simplified, whereas the conservation of mass is identical to laminar behavior. Future CFD analyses could use the Spalart-Allmaras model to model turbulence. This approach was especially conceived for aerospace applications. In correspondence with the Sutherland model to describe temperature dependent fluid viscosity, the accuracy of the analysis could be increased [16]. 3.3. FIG 5. Simulation Process Initial mesh (left), adapted final mesh (right) Gradient adaption based on the gradient of the total pressure provides a possibility to refine the mesh only in the region where it is needed. The solution is thereby significantly refined by a minimum of computational expense. The procedure of gradient adaption is repeated until there are only infinitesimal changes in the flow characteristics of the flow field between the last two converged solutions. To define a converged solution, an approach of double monitoring is used. It consists of a minimization of the residuals while having a converged and constant primary mass flow. By applying the preliminary considerations concerning modeling, the following section describes the simulation process. Figure 4 elucidates the detailed sequence of the CFD analyses. All sequencing calculation methods are oriented towards solution strategies which are documented in detail in [16, 20, 21]. Once the two-dimensional geometry of the flow field is established, an initial discretization can be performed on it. Subsequently, the boundary conditions as well as the basic solver settings are defined. To give every node of the mesh a start value regarding its flow characteristics, the flow field has to be initialized. This is done by using the full multigrid initialization (FMG). In general, this method virtually fuses mesh elements and thereby generates a coarse solution of the flow field. The initialization completes the pre-processing and the calculation of the flow can be started. To compensate potential large non-linearities which may emerge from compression shocks and other flow characteristics which are typical for expanding flows at supersonic conditions, the analysis is performed stepwise by increasing the accuracy of the solution. This procedure provides a good numerical stability during calculation. Having reached a converged solution of the flow field using the most accurate depicted calculation methods, the solution can be refined by adapting the mesh as pictured in figure 5. 4. SELECTING THE NOZZLE SETUP The following section compares three different spike truncations and the ideal isentropic spike configuration regarding their fluid flow characteristics. Upon these flow fields a configuration is selected to investigate aerodynamic thrust vectoring. The truncations under consideration range between 0 % and 75 % in equidistant steps of 25 %, so that we have setups with 25 % to 100 % of full isentropic spike length. 3 4 FIG 6. Distribution of the Mach number for varying spike truncations FIG 7. Calculated flow characteristics for a spike truncation of 25 % Figure 6 shows all four setups and the computed flow fields with increasing spike truncation from a) to d) using the mentioned equidistant steps. The depicted distribution functions of the Mach number show that the nozzle flow is in general slightly over-expanded. The reason for this flow behavior is the idealized shape of the spike, which is computed using the theory of one-dimensional isentropic fluid flow. A fully adapted flow at the design pressure ratio is described by a free-jet boundary parallel to the spike axis. The depicted flow field corresponds to an aerospike nozzle which is designed for a high flight altitude and in that case tested on ground (sea level). This behavior is advantageous for the case investigated in our paper, because the aim of the simulation was to show such a scenario, which allows to compare the computed data to real test data. Nevertheless, it is still necessary to find a design algorithm for the spike contour based on a predefined pressure ratio that provides the flow-optimized nozzle structure for the regarded conditions. The method pointed out by Kraiko et al [6] could be one approach for that. 4 The displayed fluid flow analyses show that for bigger spike truncations there are more and especially more distinct compression shocks. That leads to areas with higher static pressure downstream of the shock waves. The aerodynamic spike, which is already fully developed at a spike truncation of 25 %, widens up for shorter spike lengths. At the same time, the size of the region of closed wake recirculation at the plug base increases. Furthermore, the shock waves, generated at the lip of the truncated spike, are getting more pronounced for greater spike truncations. In general, one can see that shorter spike configurations lead to more complex flow behavior. The following investigations require a homogeneous, almost undistorted flow field, so that the effect of aerodynamic thrust vectoring can be shown and studied. For that reason the subsequent considerations are made using a nozzle configuration with 75 % of the full isentropic spike length. The flow characteristics for the chosen setup are determined according to Hagemann et al[3] and are pictured in figure 7. 5. PARAMETER VECTORING STUDY ON FIG 8. Injection site at 60 % (left) and 90 % (rigth) of the spike legth For the smaller thrust classes considered here, thrust vectoring can be performed by injecting a secondary fluid flow into the expanding primary flow of the thruster along the spike contour. As pictured in figure 8, the present study investigates two injection sites: one close to the base at 90 % of the spike length and one situated nearer to the nozzle throat at 60 % of the truncated plug. Furthermore, the injected mass flows are varied for both setups. In the context of the parameter study injected mass flows of 0.78 % up to 3.92 % of the primary mass flow are investigated. THRUST After having found a suitable nozzle configuration for the subsequent studies in the previous section, chapter five presents a parameter study according to aerodynamic thrust vectoring. 5.2. 5.1. After having discussed the approach of thrust vectoring on aerospike nozzles, the following section evaluates the calculated results from the parameter study. Aerodynamic thrust vectoring The main difference between aerodynamic and mechanical thrust vectoring lies in the absence of heavy mechanical elements. The thrust vector of conventional bell nozzles is usually influenced by mechanical structures such as movable gimbals or flaps. One approach for mechanical thrust vectoring on an aerospike nozzle could be a moveable mounted plug, for example. However, it is obvious that this configuration would lead to an additional, heavy structure because of the fully gimbaled suspension. Moreover, possible losses in nozzle performance need to be taken into account because of the continuously changing divergent part of the nozzle. 5.2.1. Evaluation of the parameter study Injection site at 90 % of the spike length The distribution functions of the Mach number in figure 9 expose the formation of a bow shock at the position of injection at 90 % of the spike length. With an increasing injected fluid flow the intensity of the shockwave increases as well. From the injection point on the generated shockwave extends to the free-stream boundary. Eventually, it gets reflected at this borderline and is being directed forward to the centralized compression shock behind the spike. The shockwave interferes with the already compressed area and is no longer verifiable at the downstream located region. In total, the entire exhaust gas flow gets stretched with an increasing secondary mass flow. Starting from the reflection point of the bow shock on the free-stream boundary, the necking of the over-expanded primary flow is unilaterally longitudinally extending. Considering the strongest secondary flow, the opposing shear layers of the exhaust gas flow run almost parallel behind the trailing shock. The already mentioned method of differential throttling provides an opportunity for the alternative aerodynamic thrust vectoring. The approach, being applicable only on clustered nozzle configurations, could be a suitable technique for medium or high thrust classes. 5 With an increasing mass injection, the flow pattern changes only by the intensity of the shape. Thereby a new expansion region is formed, starting from the specified necking point, which is caused by the injection bow shock. This region reaches the high pressure zone behind the trailing shock and proceeds downstream as a new shockwave to the opposing boundary of the free-stream. Here, the shockwave gets reflected again and propagates within the exhaust gas flow until it is no longer noticeable. Overall, the flow field at 60 % of the spike length, as it is depicted in figure 10, gets much more influenced by the injection compared to the one considered previously. 5.2.3. FIG 9. Distribution of Mach number for an injected mass flow of ca. 2 % (top) and ca. 4 % (bottom) of the primary mass flow 5.2.2. Injection site at 60 % of the spike length Comparison of the results In direct comparison of the flow patterns, one cannot identify a visible extension of the free-stream in case of an injection at 90 % of the spike length. Admittedly, the flow field shows stronger characteristics of shock- and expansion waves, but the trail is less affected. However, both configurations show a visible deflection of the freestream in positive y-direction, when applying great secondary mass flows. Transferred to an infinitesimal section of an axisymmetric spike, it is likely that this reaction has no strong effect on the complete free-stream because of the surrounding annular flow and thus threedimensional effects. Though having a linear spike structure, a deflection of the whole exhaust gas flow would occur. This reaction would definitely affect the direction of thrust vector. Originating from an injection site at 60 % of the spike length, one can observe the same forming of a bow shock as in the previous case. Because of the injection position, which is now closer to the narrowest nozzle cross section, the shockwave now interacts directly with the expanding primary flow. As a result, a visible unilateral necking is formed at the position where the bow shock meets the shear layer of the main flow, even at a minimal secondary mass flow. Furthermore, the data shows that the secondary mass flow has a cooling effect on the spike surface downstream of the injection point. The cooled flow then propagates centrally downstream within the free-stream. In direct vicinity of the exhaust gas flow, the simulation showed small areas with higher total temperatures. This is caused by circulation flows. The primary free-stream sucks gas particles from its surrounding, which get accelerated towards the exhaust gas stream. When these particles meet the boundary layer of the primary stream, stagnation points in the form of localized turbulences can occur. These stagnation points naturally feature a slightly increased temperature and density. The same phenomena can be observed directly above and below of the nozzle exit where the surrounding flow is directed to the primary one. In this stern region a wake is formed, which is characterized by a circulation as well. This effect would get enhanced for the given rear structure, if there would be an imprinted surrounding flow in positive x-direction. This simulated flight scenario would be able to specify the influence of the surrounding stream on the exhaust gas flow. In any case, the wake generates an unwanted aerodynamic drag, which decreases the nozzle performance. In this case a suitable structural design could provide relief. 5.3. Estimation of side forces The recently presented studies by Shannon et al [13, 22] and Erni et al [14] concerning thrust vectoring on plug nozzles, give some indication of achievable side forces, response qualities and amplification factors caused by FIG 10. Distribution of Mach number for an injected mass flow of ca. 2 % (top) and ca. 4 % (bottom) of the primary mass flow 6 FIG 11. Distribution function of the applied force in y-direction for an injection of ca. 4 % of the primary mass flow at 60 % of the spike length FIG 12. Distribution function of the applied force in y-direction for an injection of ca. 4 % of the primary mass flow at 90 % of the spike length superposition of primary and secondary flow. However, as yet, there does not exist an analytical method that provides reliable prognoses according to achievable side forces and amplification factors. The values for the injected mass flow minj , the averaged flow velocity at the nozzle exit v e , as well as the averaged static pressures at the nozzle exit pe and in the ambient flow field p are provided by the CFD analyses. Together An early approach introduced by Zukoski and Spaid [23], considers the injection of gases into a supersonic flow and supplies an approximation of the generated side force Fy, applied on an overflowed flat plate by equation (1) (1) with the known nozzle exit area A e , the thrust generated by the injection is computed to 70.88 N for the injection site at 60 % and to 68.16 N for the injection site at 90 % of the Spike length. For the second addend of equation (1) the evaluation of the analysis results could give at least a qualitative statement. According to Zukoski [23] the induced side force Find is calculated from the integral of the pressure distribution over the regarded area. The CFD analyses provide the applied forces in y-direction resulting from the pressure distribution. The gained results are depicted in figure 11 and 12. Fy Tinj Find where Tinj is the thrust generated by secondary injection and Find is the additional force induced by superposition of the two flows. By applying equation (2) agreeable to [24] a quantitative estimation of Tinj is possible. (2) Finj minj ve ( pe p ) Ae 7 The evaluation of the data gives a value range from -27.88 N to 26.56 N for the injection site at 60 % and from -28.95 N to 27.35 N for the injection site at 90 % of the spike. The induced forces over the spike are thus not symmetrically distributed. Figure 11 shows an almost homogenous force distribution over the upper and lower spike area, for the more upstream situated injection site. Additionally to the fact that the forces induced on the upper spike area, are of a greater amount than on the lower one it should be noted that in the trail of the injection a significant force vector is formed. This vector is caused by the high pressure region in the trail of the shockwave resulting from the flow injection. The lower spike surface, however, experiences a continuously decreasing pressure, starting with the greatest applied force directly behind the smallest cross section of the nozzle. Due to the low technical readiness level, the system provides a variety of possible research and development tasks, which should be of interest for student education and to expedite the technology. 6. OUTLOOK The gained results serve as a basis for further calculations and tests. One first approach is to determine an ideal spike geometry, which provides an adapted nozzle flow for the considered conditions. The advancement of the simulation model should be made with regard to three-dimensional analyses. This would allow to study thrust vectoring especially for axisymmetric nozzle configurations in the future, which are affected by three-dimensional flow effects. For this purpose considerations should be given to the use of alternative solver programs. The Viscous Upwind Algorithm for Complex Flow Analysis (VULCAN) developed by NASA could offer one option to do so. For further optimization of the nozzle, structural and thermal numerical analyses should be considered. With the injection site closer to the base, a clearly enlarged area with an applied force in negative y-direction can be noticed on the upper spike surface, directly behind the nozzle throat. The lower surface again experiences a continuously decreasing pressure towards the base. Qualitatively, it is noticeable that the injection at 90 % plug length has a greater influence regarding the generated force distribution over the upper spike surface compared to the second configuration. To validate the CFD analyses, several experiments are suitable. Besides a full scale test setup of an aerospike nozzle including a combustion chamber, some more easily realizable tests could be performed. Shallow water analyses and cold gas engine tests, for example, offer such opportunities. Overall, the evaluation of the CFD analyses verifies an induced force according to secondary fluid injection. The force reaches a maximum, when the injection site is close to the base. Admittedly, the generated thrust is slightly smaller for the injection at 90 % of the plug length, but it should be noted that the procreated steering moment is directly dependent on the distance between the force transmission point and the center of gravity of the launcher system. Since the differences in generated thrust are marginal for the studied setups and a maximum of the induced force is verifiable for the injection at 90 % of the spike length, it can be concluded that this injection configuration generates the largest side force under the regarded conditions. Further tests and analyses have to show to what extent the injected secondary mass flows could be optimized. 5.4. We will start our validation experiments with a shallow water test bench at the Institute of Fluid Mechanics (ISM). The needed nozzle model is shown in figure 13 and currently manufactured. First results are expected within the next quarter. Utilizing the analogy of shallow water and one-dimensional gas flows, the flow patterns of the water test can be compared directly to the simulation results. A gas flow analysis using Schlieren photography will be realised in a second step, which also can be conducted in cooperation with the ISM. Possible Applications The presented propulsion system with aerodynamic thrust vectoring is especially suitable for applications in the field of small and medium thrust classes. In Erni et al [14] the capability of a CubeSat propulsion system using an aerospike engine is discussed. The advantage of higher expansion ratios by having the same size of area of the system compared to conventional bell nozzles results in a more compact structure. At the same time the specific impulse increases and the fuel consumption is reduced. Furthermore, the secondary injection could be used as a stand-alone attitude control system which is not possible for bell nozzles [13]. In summary, thrust vectored aerospike engines offer the possibility to release a swarm of small satellites by only one launcher system. Each satellite is then able to reach its own individual orbit and to perform attitude control, everything done by one system [14]. Of course the engines are also conceivable for deorbiting maneuvers. Last but not least, the described propulsion system appears to be an attractive scope for the student rockets project conducted at the TU Dresden. FIG 13. Shallow water test model While the above mentioned methods are for optical validation and analyses of the flow behaviour for linear aero spike nozzles or two dimensional models, a cold gas engine setup can be used for three dimensional analyses of forces and turning moments. Furthermore, we intend to build a test bench with a variable nozzle setup for various parameter studies, such as spike lengths, injection sites, mass flow rates etc. With these experiments and 8 numerical analyses we want to contribute to the establishment of a larger database for aerospike nozzles using secondary fluid injection for thrust vectoring, so that an analytical description might be derived. H. Immich, P. Sacher, and P. Reijasse, Plug nozzles: Summary of flow features and engine performance, AIAA, no. 2002-0584, 2002. [9] 7. CONCLUSION [10] M. Nazarinia, A. Naghib-Lahouti, and E. Tolouei, Design and numerical analysis of aerospike nozzles with different plug shapes to compare their performance with a conventional nozzle, vol. 11. , AIAC, 2005. In the context of the presented paper a CFD model was developed which enables the determination of fluid dynamic characteristics of nozzle flows using numerical flow analyses. By applying the one-dimensional isentropic flow theory, an initial aerospike nozzle geometry for further studies has successfully been generated. In the course of subsequent CFD analyses, a nozzle configuration was determined which allows the optimal investigation of aerodynamic thrust vectoring. The plug nozzle was then submitted to a parameter study regarding different injection sites and secondary mass flow rates. The results show the emergence of a significant force vector by injecting a secondary fluid flow. A final estimation showed that the maximum steering force is generated for a nozzle configuration with an injection site at 90 % of the spike length. [11] A. Naghib-Lahouti and E. Tolouei, Investigation of the effect of base bleed on thrust performance of a truncated aerospike nozzle in off-design conditions. , ECCOMAS CFD, 2006. [12] T. Zebbiche and Z. 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