Segmented Solid Rocket Motors Research Articles

Dynamical states associated with the shift in whistling frequency in aeroacoustic system

Self-sustained aeroacoustic oscillations arising from the interactions between the hydrodynamic and acoustic fields are perceived as a whistle. Such whistling can lead to large amplitude acoustic oscillations that have disastrous consequences for engineering systems such as large segmented solid rocket motors and large gas pipelines. The whistling corresponds to the state of limit cycle oscillations (LCO) in dynamical systems theory. An aeroacoustic system exhibits different dynamical states when the bulk flow velocity is varied as a control parameter. Understanding the dynamical states and the transitions between them, as the control parameter is varied, is crucial in designing control strategies for such aeroacoustic oscillations. Previous studies have shown that as the control parameter varies, in an aeroacoustic system that has a flow through orifices, the whistling frequency shifts. We show that such a change in frequency occurs via three different scenarios— (1) direct transition between the two LCOs as an abrupt transition, (2) via a state of intermittency, and (3) via a state of aperiodicity. In the current aeroacoustic system, the abrupt transition between the LCOs is manifested as a bursting behaviour where the amplitude of the acoustic pressure fluctuations abruptly switches between the high and low-amplitude LCOs. Further, we show that the dynamical state and the transition between them during the frequency shift have a correlation with the magnitude of the frequency shift. Using recurrence theory we show that there is a change in the dynamical state of the system during the frequency shift. Further, we use synchronisation theory to investigate the coupled behaviour of the velocity (u′) and the acoustic pressure (p′) fluctuations during the different dynamical states. Our findings imply that u′ and p′ exhibit phase synchronisation (PS) during the state of LCO, corresponding to whistling. In contrast, u′ and p′ are desynchronised during the state of aperiodicity, corresponding to stable operation. Furthermore, the bursts of periodic oscillations during intermittency correspond to the phase-synchronised epochs of periodic u′ and p′, and the aperiodic epochs correspond to the desynchronised aperiodic u′ and p′.

Evaluation of pressure oscillations by a laboratory motor

Aeroacoustic pressure oscillation is one of the important challenges in segmented solid rocket motors with high slenderness ratio. The reason for these oscillations can be searched in vortex shedding due to grain burning surfaces, holes and slots. In this paper, a novel sub-scaled motor was used for evaluation of aeroacoustic pressure oscillations. First, the related parameters to scale down using Buckingham’s Pi-theorem were determined and then the sub-scaled motor was designed and manufactured. After this, Strouhal number in various grain forms and vortex shedding prediction criteria have been discussed. Then, one-dimensional linear and non-linear solution approaches have been presented. To understand the motor internal flow and vortex shedding formation, steady state computational fluid dynamic performed for seven regression steps and finally, two static tests have been performed. Results show that various definitions for Strouhal number are useful only for primarily glance on vortex shedding and pressure oscillations and so CFD solution and the test program are inevitable for a correct understanding of the ballistic operational condition of the motor. Experimental results demonstrated the existence of such oscillations with frequencies nearly equal to axial modes. It seems that non-linear parameters have small effects on aeroacoustic pressure oscillation and therefore the linear solution is acceptable to obtain approximate data. Of course, it should be emphasized that linear solution represents oscillations on overall motor action time, whereas the output of non-linear solution depends on thermochemistry properties of solid propellant and combustion products. Therefore, with a non-linear solution, oscillations maybe occur in some intervals of action time. FFT (Fast Fourier Transformation) results demonstrated that although both first and second acoustic modes have been excited, the position of longitudinal oscillation has an important role on which one is dominant.

Stability analysis and numerical simulation of simplified solid rocket motors

This paper investigates the Parietal Vortex Shedding (PVS) instability that significantly influences the Pressure Oscillations of the long and segmented solid rocket motors. The eigenmodes resulting from the stability analysis of a simplified configuration, namely, a cylindrical duct with sidewall injection, are presented. They are computed taking into account the presence of a wall injection defect, which is shown to induce hydrodynamic instabilities at discrete frequencies. These instabilities exhibit eigenfunctions in good agreement with the measured PVS vortical structures. They are successfully compared in terms of temporal evolution and frequencies to the unsteady hydrodynamic fluctuations computed by numerical simulations. In addition, this study has shown that the hydrodynamic instabilities associated with the PVS are the driving force of the flow dynamics, since they are responsible for the emergence of pressure waves propagating at the same frequency.

Direct numerical simulation and biglobal stability investigations of the gaseous motion in solid rocket motors

AbstractIn this article, a biglobal stability approach is used in conjunction with direct numerical simulation (DNS) to identify the instability mode coupling that may be responsible for triggering large thrust oscillations in segmented solid rocket motors (SRMs). These motors are idealized as long porous cylinders in which a Taylor–Culick type of motion may be engendered. In addition to the analytically available steady-state solution, a computed mean flow is obtained that is capable of securing all of the boundary conditions in this problem, most notably, the no-slip requirement at the chamber headwall. Two sets of unsteady simulations are performed, static and dynamic, in which the injection velocity at the chamber sidewall is either held fixed or permitted to vary with time. In these runs, both DNS and biglobal stability solutions converge in predicting the same modal dependence on the size of the domain. We find that increasing the chamber length gives rise to less stable eigenmodes. We also realize that introducing an eigenmode whose frequency is sufficiently spaced from the acoustic modes leads to a conventional linear evolution of disturbances that can be accurately predicted by the biglobal stability framework. While undergoing spatial amplification in the streamwise direction, these disturbances will tend to decay as time elapses so long as their temporal growth rate remains negative. By seeding the computations with the real part of a specific eigenfunction, the DNS outcome reproduces not only the imaginary part of the disturbance, but also the circular frequency and temporal growth rate associated with its eigenmode. For radial fluctuations in which the vorticoacoustic wave contribution is negligible in relation to the hydrodynamic stability part, excellent agreement between DNS and biglobal stability predictions is ubiquitously achieved. For axial fluctuations, however, the DNS velocity will match the corresponding stability eigenfunction only when properly augmented by the vorticoacoustic solution for axially travelling waves associated with the Taylor–Culick profile. This analytical approximation of the vorticoacoustic mode is found to be quite accurate, especially when modified using a viscous dissipation function that captures the decaying envelope of the inviscid acoustic wave amplitude. In contrast, pursuant to both static and dynamic test cases, we find that when the frequency of the introduced eigenmode falls close to (or crosses over) an acoustic mode, a nonlinear mechanism is triggered that leads to the emergence of a secondary eigenmode. Unlike the original eigenmode, the latter materializes naturally in the computed flow without being artificially seeded. This natural occurrence may be ascribed to a nonlinear modal interplay in the form of internal, eigenmode-to-eigenmode coupling instead of an external, eigenmode pairing with acoustic modes. As a result of these interactions, large amplitude oscillations are induced.

Vortex flow analysis of a large segmented solid rocket motor

It is not uncommon to find pressure oscillations in large size segmented solid rocket motors. During the static test of segmented solid rocket motor (SRM-3), unanticipated pressure oscillations were seen after some of the propellant has burnt and the oscillations sustained for certain duration of time. The purpose of this paper is to present the results of the analysis carried out on the experimental data using commercial CFD software ‘FLUENT’ and compare the results with experimental data to find out the cause of the pressure oscillations. Inhibitors are provided for the full web thickness of propellant at the segment joint interfaces to prevent end-burning of propellant at the joints. The char and erosion rate of these inhibitions are lower than the burning rate of the propellant. As burning of the propellant progresses, annular inhibition wall starts projecting above the burning surface of the propellant. These obstacles (inhibition) are acting as wall and sheared gas flow occurs in the rocket motor. CFD analysis was carried out on a quadrilateral mesh for the geometry of SRM-3 at the time of 32 s after ignition when the pressure oscillations peaked. It has been found that the vortex shedding frequency obtained by CFD analysis closely matches with the frequency of pressure oscillations occurring during static testing. Thus, it has been revealed that the obstacle vortex shedding generated by the compartmentalization of the combustion chamber by the protrusion of inhibition at the segment joints was the cause of the pressure oscillations in SRM-3. Keywords: Vortex shedding, Segmented Solid Rocket Motor, CFD analysis, Propellant, Combustion chamber

Vortex Flow Analysis of a Large Segmented Solid Rocket Motor

It is not uncommon to find pressure oscillations in large size segmented solid rocket motors. During the static test of segmented solid rocket motor (SRM-3), unanticipated pressure oscillations were seen after some of the propellant has burnt and the oscillations sustained for certain duration of time. The purpose of this paper is to present the results of the analysis carried out on the experimental data using commercial CFD software 'FLUENT' and compare the results with experimental data to find out the cause of the pressure oscillations. Inhibitors are provided for the full web thickness of propellant at the segment joint interfaces to prevent end-burning of propellant at the joints. The char and erosion rate of these inhibitions are lower than the burning rate of the propellant. As burning of the propellant progresses, annular inhibition wall starts projecting above the burning surface of the propellant. These obstacles (inhibition) are acting as wall and sheared gas flow occurs in the rocket motor. CFD analysis was carried out on a quadrilateral mesh for the geometry of SRM-3 at the time of 32 s after ignition when the pressure oscillations peaked. It has been found that the vortex shedding frequency obtained by CFD analysis closely matches with the frequency of pressure oscillations occurring during static testing. Thus, it has been revealed that the obstacle vortex shedding generated by the compartmentalization of the combustion chamber by the protrusion of inhibition at the segment joints was the cause of the pressure oscillations in SRM-3.

Coupled Analysis the Segmented SRM Internal Flow Field with Influence of Inhibitor Deformation

The coupled influence between structure and internal flow field will make the pressure oscillation during working process of the solid rocket motor. This coupled effect will bring the dynamic press on the payload and extremely destroyed the payload. For researching the influence of internal flow field by the deformation of inhibitor, the parallel fluid structure interaction method with the large eddy simulation model was used to analyze the solid rocket motor with segments. The results show that the deformation of inhibitor will influence the internal flow field parameter’s distribution and enhance the pressure frequency and amplitude remarkably. The partitioned method could solution the fluid structure interaction problems in the segmented solid rocket motor properly.

Coupled Analysis the Segmented SRM Internal Flow Field with Influence of Inhibitor Deformation

The coupled influence between structure and internal flow field will make the pressure oscillation during working process of the solid rocket motor. This coupled effect will bring the dynamic press on the payload and extremely destroyed the payload. For researching the influence of internal flow field by the deformation of inhibitor, the parallel fluid structure interaction method with the large eddy simulation model was used to analyze the solid rocket motor with segments. The results show that the deformation of inhibitor will influence the internal flow field parameter’s distribution and enhance the pressure frequency and amplitude remarkably. The partitioned method could solution the fluid structure interaction problems in the segmented solid rocket motor properly.

On the importance of reduced scale Ariane 5 P230 solid rocket motor models in the comprehension and prevention of thrust oscillations

Down-scaled solid propellant motors are a valuable tool in the study of thrust oscillations and the underlying, vortex-shedding-induced, pressure instabilities. These fluctuations, observed in large segmented solid rocket motors such as the Ariane 5 P230, impose a serious constraint on both structure and payload. This paper contains a survey of the numerous configurations tested at ONERA over the last 20 years. Presented are the phenomena searched to reproduce and the successes and failures of the different approaches tried. The results of over 130 experiments have contributed to numerous studies aimed at understanding the complicated physics behind this thorny problem, in order to pave the way to pressure instability reduction measures. Slowly but surely our understanding of what makes large segmented solid boosters exhibit this type of instabilities will lead to realistic modifications that will allow for a reduction of pressure oscillations. A “quieter” launcher will be an important advantage in an ever more competitive market, giving a easier ride to payload and designers alike.

Analysis of Injecting Wall Inclination on Segmented Solid Rocket Motor Instability

The influence of a solid propellant inclination angle was analyzed through small-scale setup simulations. Two injected blocks separated by thermal protection reproduced the main geometric solid rocket motor features, and a dual phenomenon driving interactions between the shear layer and injecting wall vortex-shedding phenomena was clearly identified at any inclination angle. A threshold angle level was also pinpointed, and it literally controls either the decrease of oscillation levels for small angles or the increase of the coupling energy involved. Under similar dynamic conditions, a decreasing factor of two was observed between a = 0 and -1.5 deg, whereas an even higher amplification factor was noted for a = -2 deg. Correlation coefficients and phase-averaged analysis have allowed the authors to clearly explain these opposed trends by depicting the wall and shear-layer interactions along the chamber.

Influence of adaptive control on vortex-driven instabilities in a scaled model of solid propellant motors

Aeroacoustic instabilities occur in many applications of technological interest and have undesirable effects on the steady operation of the system. Passive and active means are sought to reduce the level of oscillation and eliminate the instability. In the case of segmented solid rocket motors, observations indicate that low-frequency oscillations are generated by a coupling between vortex shedding in shear regions established in the flow and the acoustic eigenmodes of the system. This process is investigated in this article on a model-scale configuration representing the geometry of the motor. An active control loop is exploited to obtain resonant and non-resonant conditions for the same operating point. Adaptive techniques are used to stabilize the flow and the experiment serves as a testbed for active control. It is shown that an adaptive system may be applied to essentially suppress the pressure oscillations. The instability mechanism is then studied by analyzing the flow field with particle image velocimetry. It is found that control noticeably modifies the mean flow structure. Detailed studies of the vortex pattern in the shedding region indicate that the concentrated vorticity and the corresponding circulation values remain in the same range but that vorticity is shed more randomly when the resonance is eliminated by the controller. This indicates that control is achieved by reducing the level of organization in the vortex pattern. Under resonant conditions the level of pressure fluctuations results from coherent interactions between vortices and the downstream nozzle. This process feeds energy in one of the acoustic modes of the system enhancing the pressure level. It is made less effective by the control loop.

Instabilities and pressure oscillations in solid rocket motors

The purpose of this paper is to give an overview of the main results obtained on instabilities and pressure oscillations in segmented solid rocket motors. A major part of this work was carried out in the framework of the ASSM and POP R&T programs supported by the French national space agency CNES during the last decade. ASSM is related to Aerodynamics of Segmented Solid Motors and POP for Pressure Oscillations Program for the Ariane 5 solid booster (P230). Due to the use of segmented technology for the P230 motor and the possible acoustic oscillations inside the motor chamber, anticipated at the beginning of the programs and confirmed later on static firing tests, the main scientific objective of the ASSM program was oriented towards the comprehension and the modeling of the vortex shedding phenomena that are supposed to be responsible of the pressure and thrust oscillations observed in the P230. POP program was started in order to obtain an experimental and numerical data base using subscale tests of 1/15th of the P230. After the description of the instabilities observed in the P230 solid rocket booster, the scientific approach of the ASSM program is detailed insisting on the validation of numerical tools in order to predict oscillation frequencies and amplitudes. The logic of work regarding POP program is also presented. The main section of this paper provides an overview of different results obtained in ASSM and POP programs to understand the mechanisms driving to the instabilities in solid rocket chamber. The most important recent result, inside ASSM and POP programs, was the discovery of the parietal vortex shedding and the role of aluminum combustion on instabilities. Together, these two mechanisms seem to be an important potential source of instabilities and provide a new vision of the P230 stability.

ADAPTIVE CONTROL OF AEROACOUSTIC INSTABILITIES

This paper describes an experimental investigation of adaptive control algorithms applied to aeroacoustic instabilities. The study is carried out on a cold flow experimental rig, designed to reproduce the essential features of acoustically coupled vortex shedding. This mechanism is the source of thrust oscillations in large segmented solid rocket motors. It is also found in a wide variety of combustion instabilities. Two adaptive control strategies are investigated and selected experimental results are reported. These results show the feasibility of control. The effect of the controller on the instability mechanism is analyzed and improvements to the control strategy are proposed.

Recent progress in the prediction and analysis of the operation of solid rocket motors

The development of big segmented solid rocket motors during the nineties has been supported by an important progress in science and computational tools. This progress is largely related to the solution of various technical problems in the design and processing of the propellant grains and their effect on the motor operation. It has been possible because of the enormous increase in the available computing power. After a review of the main problems encountered (grain-flow coupling, motor trace shape prediction, pressure oscillations, slag deposit etc...) recent developments in modeling are described in two areas related to the analysis and prediction of motors operation. The first development couples the propellant surface burnback and pressure field computations during the whole operation of the motor: real tune geometry, pressure field and local burning rates are calculated in order to achieve a much more precise prediction of the motor trace. Pressure oscillations and instabilities are also predicted. The second development is related to the influence of the process, through the rheology of the propellant on the combustion of the propellant. Rheology and particle segregation under shear may explain such effects as the so-called ⪡hump⪢ and ⪡knitlines⪢ effects. Experimental and modeling work in the field are described.

Analysis of energy transfers of a sheared flow generated by wall injection

We propose in this work to characterize the unsteady behavior of a flow generated by wall injection and encountering an obstacle. This sutdy concerns the prediction of the stability of segmented solid propellant rocket motors. The simulation of such a system is studied in cold flow, which makes it possible to analyze the basic phenomena and the energy transfer mechanisms of the flow. The results obtained allow the identification of the vortex structures by visualization inside a shear layer created at the top of an obstacle. The analysis of the pressure field shows that the dynamic parameters (mass flow rate or flow velocity) generate a phenomenon of selective excitation and of longitudinal acoustic modes amplification, which is accompanied by an energy transfer between modes.

Numerical simulation of vortex-shedding phenomenon in a channel with flow induced through porous wall

Abstract Segmented solid propellant rocket motors tend to develop unpredicted pressure and thrust oscillations that could be attributed to a periodic phenomenon. An experimental and numerical assessment of the stability of a solid propellant motor was conducted. This numerical computation was based on an experimental study carried out on a cold-flow, reduced-scale, bidimensional duct with a complex internal geometry (obstacles, outflowing cavities, submerged nozzle). The objectives of our study are to characterize the internal flow (mean, fluctuating induced by injection through a porous wall and the shear layer created at the top of the second obstacle. The code employed solves the unsteady, compressible two-dimensional (2-D) Navier-Stokes equations in a laminar regime by a predictor-corrector MacCormack scheme. Three grids are developed for the evaluation while respecting the complexity of the internal geometry. The results obtained allow us to separate the flow into two zones: one, laminar, upstream of the obstacle; the other, disturbed, downstrea, in which vortex structures develop. The phenomena of instabilities (vortex-shedding, pairing) are in accordance with the first two acoustic longitudinal modes of the chamber. Last, the numerical computations are discussed and systematically compared with the experimental results.

ACOUSTIC-MEAN FLOW INTERACTION AND VORTEX SHEDDING IN SOLID ROCKET MOTORS

The present work is devoted to the numerical simulation of two important phenomena in the field of solid propellant rocket motors: the first is acoustic boundary layers that develop above the burning propellant; the other is a periodic vortex-shedding phenomenon which is the result of a strong coupling between the instability of mean flow shear layers and acoustic motions in the chamber. To predict the acoustic boundary layer, computations were performed for the lower half of a rectangular chamber with bottom-side injection. The outflow pressure is sinusoidally perturbed at a given frequency. For the highest CFL numbers the implicit scheme is not able to compute the unsteadiness in the acoustic boundary layer. With very low CFL numbers or with the explicit scheme the main features of the acoustic field are captured. To simulate the vortex-shedding mechanismin a segmented solid rocket motor, the explicit version is used. This computation shows a mechanism for ‘self-excited’ vortex shedding close to the second axial mode frequency. The use of the flux-splitting technique reduces substantially the amplitude of the oscillations. A few iterations are done with flux splitting, then the computation is performed without this technique. In this case both the frequency and the intensity are well predicted. A geometry more representative of the solid rocket motor is also computed. In this case the vortex-shedding process is more complex and pairing is observed.

Vortex shedding in segmented solid rocket motors

This article is devoted to the numerical simulation of vortex shedding in the field of solid-propellant rocket motors resulting from the strong coupling between the shear-layer instability and acoustic waves in the chamber. Segmented solid rocket motors tend to develop thrust and pressure oscillations, linked to a periodic vortex shedding. An axisymmetric geometry close to the realistic configuration is computed. The computer code solves the unsteady Navier-Stokes equations by means of an explicit predictor/ corrector MacCormack scheme. Mesh dependence is studied by using three grid levels. In all cases, the vortex shedding process is observed. This mechanism is complex and vortex pairing occurs. The viscosity effect is analyzed by using three different viscosity values. The effect of the shape of diaphragms oh vortex shedding is also studied by comparing sharp and smooth diaphragms. HIS work is part of the overall combustion stability assessment of the Ariane 5 P230 MPS solid motor and has been supported by Centre National d'Etudes Spatiales (CNES) within a research program managed by ONERA. For this motor it is believed that there exists a severe risk for instability. Occurrence of low-amplitude , sustained oscillations pulsating at a frequency associated with one or more acoustic modes of the combustion cavity affects motor performances.13 One mechanism that drives pressure oscillations in rocket motors is the vortex shedding,1'4 which can interact with the chamber acoustics to generate pressure oscillations.5 Because of the segmented design of solid-propell ant rocket motors, shear layers induced by surface discontinuities appear and produce this vortex shedding. The dipole mechanism involving the interaction of the vortices with an impingement surface can be invoked as a typical source of energy transfer from the vortex fluctuations to the acoustic field. The observed periodic vortex shedding in rocket motors2'4 is the result of a strong coupling between the instability of mean shear flow and organ-pipe acoustic modes in the chamber. The feedback from the acoustic waves provides the control signal for the aerodynamic instability. Because of the complexity of the problem an analytical solution to the governing equations with complex boundary conditions does not exist. Numerical simulations may provide important help for understanding the complex physics. Such simulations would naturally couple mean-flow shear layer and acoustic waves. Numerical methods have been performed to isolate and study the interaction between acoustic waves and vortex structures.610 The present work is also concerned by a numerical simulation. The aim of this article is to present the ability of numerical codes to predict the unsteady behavior inside the combustion chambers of solid-propellant rocket motors and to analyze the acoustic and aerodynamic instability interaction. The computational domain corresponds to a 1/15 axisymmetric subscale motor representative of the Ariane 5 P230 solid rocket booster for which experimental results exist.1113 For this subscale motor, the propellant is an unmetallized analog of Ariane 5 propellant and the inhibitors are

Comment on 'Method for reducing stagnation pressure losses in segmented solid rocket motors'

Comment on 'Method for reducing stagnation pressure losses in segmented solid rocket motors'

Reply by the Author to R. L. Glick and L. H. Caveny

T HE author wishes to thank Glick and Caveny for their interest in his Note, and to comment briefly on the issues which they raise. In their Comment, Glick and Caveny make two points. First, and most important, they emphasize that an internal ballistic analysis of a solid rocket motor (SRM) ought to be coupled to a grain structural analysis to properly analyze SRM performance, since grain deformation can significantly alter the internal geometry. Second, they suggest that it may be possible to create more efficient segmented SRM designs than the simple modification proposed in Ref. 1, where alternate segments are installed in the motor upside down (relative to the usual orientation of segments in SRMs). Their prime candidate appears to be a design in which all of the segments are installed upside down (see Fig. Ic of Glick and Caveny's Comment), and the individual segments are tailored so that the resulting port flow area increase across slots is filled by the flow which emerges from the slots.