1 On the importance of reduced scale Ariane 5 P230 solid rocket motor models in the comprehension and prevention of thrust oscillations. J. Hijlkema,M. Prevost, G. Casalis Down-scaled solid propellant motors are a valuable tool in the study of thrust oscillations and the underlaying, vortex-shedding-induced, pressure instabilities. These uctuations, 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 congurations tested in our laboratory over the last 20 years. We present the phenomena we search to reproduce and the successes and failures of the dierent approaches we 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 modications 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. Introduction Like all very large solid rocket motors (SRM), the Ariane 5 P230 boosters require segmented grains in which pressure and thrust oscillations may appear. The coupling with the chamber acoustics leads to unstable uctuations whose frequencies oscillate around the frequencies of the longitudinal modes. Because of these oscillation frequencies, structural vibrations are generated in the launch vehicle. Since 1990 and under the scientic coordination of ONERA, several European and French laboratories as well as aerospace industries have been involved in the Aerodynamics of Segmented Solid Motors (ASSM) program and the Pressure Oscillation Program (POP) supported by the CNES. As a part of these programs, we have de- ned and tested a non-metallised propellant (nonmetallised because it turned out to be impossible to down-scale the aluminium size distribution while conserving the over all behaviour of the propellant) in a large amount of dierent congurations of down scaled axisymmetric set-ups, representative of the Ariane 5 P230 boosters, in order to obtain an experimental database. This database is used for the validation of stability predictions and to improve the knowledge of vortex shedding driven pressure oscillation phenomena in large segmented solid rocket motors. The denition of these experiments leans heavily on numerous numerical 2, 4 and theoretical 1, 5 studies carried out in the ASSM and POP framework. This research has given us a better understanding of the development of unwanted and potentially hazardous pressure and thrust oscillations. The paper is organised as follows. The rst section describes why and how we use these downscaled motors. Section 2 gives a bit more details on the data resulting from these experiments and the way we use it. Section 3 details several series of experiments, each designed to address one given physical phenomenon that inuences the development of pressure oscillations. We end with a conclusion in section 4. I. Why do we use reduced scale models? Thrust oscillations are produced by the coupling of vortex shedding induced pressure oscillations with the motors chamber acoustics. Globally, 3 types of vortex shedding have been identi- ed: 8 Obstacle vortex-shedding (VSO) created by a protruding obstacle such as an inhibitor or a thermal protection. Research Engineer in the Propulsion Laboratory at ONERA. email: jouke.hijlkema@onera.fr Research Engineer and head of the Propulsion Laboratory at ONERA Scientic Deputy Director of the Aerodynamics and Energetics Modelling Department at ONERA
2 II MEASUREMENTS AND DATA ACQUISITION. 40 rings at this date Figure 1. Obstacle vortex-shedding LP9 Simplied, modular, scale 1/35 th representation of the P230. Meant for parametric studies of separate phenomena. Angle vortex-shedding (VSA) created by a shear layer coming from an angle such as found around inter-segment cavities. Figure 6. LP9 Figure 2. Angle vortex shedding Parietal vortex-shedding (VSP). This type of shedding results from an intrinsic instability of the internal ow. 4 pressure transducers Ultra sound propellant surface regression measurements 35 rings at this date LP10 Realistic, 1/35 th representation of the P230. Figure 3. Parietal vortex-shedding Full size experiments on the scale of the P230 are horrendously expensive, complicated and time consuming and are therefore not really suited for research. Downscaled models, on the other hand, are relatively cheap, their geometry is simpler and modular. Scaling problems (turbulence, heat ux, particle sizes) do arise though and need to be considered but all in all, downscaled rocket motors allow for precise and well delimited experiments and they are good tools to deepen our understanding of complicated phenomena such as vortexshedding induced pressure oscillations. Our laboratory uses 3 dierent families of these models: LP6 Realistic, scale 1/15 th representation of the P230. Figure 5. Figure 4. 12 pressure transducers LP6 LP6 in action Ultra sound propellant surface regression measurements Figure 7. 4 pressure transducers LP10 Ultra sound propellant surface regression measurements 32 rings at this date II. Measurements and data acquisition. In order to study thrust oscillations, one might think that direct thrust measurements, both steady and unsteady, would be the rst thing these experimental set-ups should deliver. The oscillations we are looking for represent a ripple on the overall thrust signal with an amplitude of about 2% of the average thrust. Given the scale of these oscillations, their frequency and the mass of both engine and rig, direct measurement of the thrust oscillations is dicult. Since it is far easier and more precise to measure chamber pressure and since relative thrust oscillations F F are thought to be proportional to relative pressure oscillations P P, our model rocket engines are equipped with a certain number of pressure transducers. Data acquisition is assured at 20 khz per channel. All channels are doubled and a 200Hz<F<4800Hz pass-band lter on each duplicate delivers the pressure oscillation signal. Figure 8 shows a typical measurement on an LP6, in blue we nd the unltered, steady, pressure while
3 the red graph represents the ltered, unsteady, component. Figure 8. Typical measurement on an LP6 Figure 9. Examples of automatic analysis Besides pressure transducers we use Ultrasound pads to measure the radial position of the surface of the propellant at every instant of the ring. This allows us to determine the surface regression rate as a function of time and hence of engine pressure. Several attempts to equip model rocket motors to allow for visual access to its interior during operation have failed or were not satisfactory due to smoke and residue deposition obscuring the window. We have given up on this sort of measurements for the time being. Punctual temperature measurements in a largely nonhomogeneous temperature eld are of limited interest. This, and the complexity of adding cabling holes to an installation that needs to withstand high pressure and high temperatures has made us decide against temperature measurements inside our model rocket motors. The data that gets collected during our experiments serves multiple purposes. Primarily we are interested in the reproduction or the repression of pressure instabilities in order to better understand the underlying phenomena in the hope to nd a cure for (or at least a way to reduce) these unwanted, and hazardous oscillations. For that propose a modular setup such as the LP9 family is well adapted. It allows for parametric studies by isolating and varying the most relevant parameters one by one. A signal processing toolbox has been developed to detail, analyse and compare the dierent rings. Amongst other things this toolbox allows us to easily create Hilbert analysis graphs (left) and Power Density Spectra (right) directly from the measurements database as shown in gure 9. These results can be normalised by a characteristic time and pressure, making it possible to easily compare data from dierent families of experiments and ight results. III. III.A. Phenomena thought to be contributing to thrust oscillations. Cavities. Cavities such as inter-bloc slots and aft-end cavities resulting from the submerged nozzle design of the P230 are generating noise (mainly resulting from VSA cf. gure 2) that can provoke pressure oscillations when a coupling with the chamber acoustics occurs. Cavities are conned by walls that are either passive or generating a mass ow (propellant). In the series of experiments presented here, the LP9 24, 67 is the most representative of the P230 with an inter block cavity, an aft-end cavity and a submerged nozzle. The P230 suers from 4 bursts at several stages of the ight (See gure 10) and we see in gure 11 that the LP9 24 reproduces the rst 3 nicely (a medium peak at 0.7 and 2 stronger peaks at 0.8 and 0.9 which compares well with the timing of the bursts in the table of gure 10) and hints the fourth burst.
4 III PHENOMENA THOUGHT TO BE CONTRIBUTING TO THRUST OSCILLATIONS. burst Time Normalised time 1 80 ~0.7 2 100 ~0.8 3 115 ~0.9 4 125 ~1.0 When we use an external nozzle on the LP9 23, the third burst disappears while the rst an second burst stay unaltered (gure 11). For the LP9 22 experiment we lled the aft-end cavity while keeping an external nozzle. This completely eradicated the pressure oscillations. To quantify the eect of the inter-segment slot we designed the LP9 25 experiment. One single block with a submerged nozzle. Strangely this resulted in one single burst about half way between the original second and third. It hence is clear that cavities, both inter-segment and aft-end, play an important role in provoking and sustaining pressure oscillations. Figure 10. P230 Flight 510. Power Spectrum Density Figure 11. Cavities. LP9 number 22,23,24 and 25. First longitudinal mode. III.B. Propellant composition. We know that the aluminium and alumina particles resulting from the combustion process and present in the ow have an inuence on the ampli- cation of pressure instabilities. Numerical simulations done by the SNPE 3 have provided the graph presented in gure 12 and have shown the importance of the inuence of particles, both inert or reactive. 2 series of experiments have been carried out at our laboratory to try to conrm these ndings. The rst series consists of experiment LP10 9 and 10 and is aimed at nding the inuence of each class of reactive particle sizes found in the full-scale motors. Figure 12. Inuence of the Stokes number on pressure oscillations For this series, 2 grains of propellant have been doped with aluminium particles. A rst block
III.B Propellant composition. 5 with particles with a diameter of 5µm representing 4% of the total mass and a second block with particles with a diameter of 30µm also representing 4% of the total mass. Because we can't respect the down-scaled aluminium size distribution, the aluminium mass fraction and the overall behaviour of the propellant, these blocks are not conform with a down-scaled Ariane 5 P230 propellant but are considered to have the highest possible concentration of down-scaled aluminium particles while conserving the overall behaviour of the propellant. The particle size distribution of the solid phase in the gas ow inside the P230 is not well known. Basically we have 3 classes; smoke particles < 3µm that we normally ignore, alumina residues 30µm resulting from the combustion of the aluminium present in the propellant and nally agglomerates > 100µm resulting from the coalescence of liquid alumina droplets. Scaled down, the biggest 2 classes give particles of 5µm and 30µm. LP10 9 and 10 are to be compared to LP10 17 which has the same geometry and the same propellant without the added aluminium particles and presents 3 blasts similar to the once found in ight. Figure 13 shows the results of all 3 experiments. As predicted by the SNPE numerical simulations 3 we conrm that small particles have a tendency to amplify the instabilities while bigger particles, on the contrary, reduce the pressure oscillations, especially for the second and third blast. Figure 13. Propellant composition. LP10 number 9,10 and 17. First longitudinal mode. The numerical simulations results in gure 12 show that for both reactive and inert particles with a Stokes number St 1 there is a strong amplication tendency. With St = ωτ u where τ u = ρpd2 18µ. LP10 30 was devised to conrm this. A fuel grain identically to the one used in LP10 17 has been seeded with 7% ZrO 2 particles with a diameter of 8.07µm which corresponds to St = 1.27. ZrO 2 is capable of withstanding the high temperatures inside a motor, therefore these particles can be considered inert. The results for both LP10 17 and 30 are shown in gure 14.
6 III PHENOMENA THOUGHT TO BE CONTRIBUTING TO THRUST OSCILLATIONS. Figure 14. Propellant composition. LP10 number 17 and 30. First longitudinal mode. We notice a strong amplication of the second burst as well as a slight phase shift. The latter might be due to the fact that the ZrO 2 particles change the combustion temperature and hence the acoustic velocity. Two more experiments are planned with particles of St 0.1 and St 10 to see if the amplication disappears. The ratio 0.016 0.012 = 1.33 of the peak amplitudes of the LP10 30 (inert particles) and the LP10 9 (reactive particles) compares relatively well with 2.1 1.4 = 1.5, the ratio found by the numerical simulations shown in gure 12. III.C. 3D eects. The margins for change in the P230 are rather small. At least two possibilities to reduce the thrust oscillations are: 1. Reducing the thickness of the thermal protections. This way they will erode faster and hence be less protuberant. 2. Provoking a 3D eect in the internal gas ow in order to try to de-organise the vortex shedding. This way the coupling with the acoustic modes should be lessened. To address the second point, numerical simulation were insucient to dene a suitable 3D geometry. The LP6 ARTA 1, 2, 3 and 4 have been used to gain insight in the inuence of the geometry of a 3D thermal protector ring on the pressure instabilities. To assure the integrity of the 3D motif during the ring, a metallic thermal protection ring was used, placed between blocs S2 and S3 (see gure 15). 4 tests have been carried out with the same shape, basically a ring with 7 teeth (cf gure 15 for images). These test are identical to the LP6 27 experiment with a metallic thermal protection ring added. The only dierence between the different test is the height of the teeth. ARTA Table 1. Height of the teeth 1 0 mm (2D) 2 8 mm 3 5.3 mm 4 14.3 mm 3D thermal protection ring Figure 15 shows the results of these experiments. It is clear that the 3D motif has a big impact on the pressure oscillations, ARTA 2 and 4 have practically eradicated the uctuations where as ARTA 3 exhibits a surprisingly small eect. Full size applications of 3D thermal protection rings have never allowed to conrm such an important reduction in instability levels. However in this type of experiments, where the size of the motor is considerably bigger than for the LP9 and LP10 series, the main problem lies in the uncertainty we have on the instantaneous geometry of the thermal protection ring. Even though it is metallic it is not excluded that it erodes partially during the ring. This might explain the surprising behaviour of the ARTA 3.
7 Figure 15. 3D eects. LP6 ARTA number 1,2,3 and 4. First longitudinal mode. IV. Conclusion. 20 years of testing with our down-scaled solid propellant motors have given us a valuable insight in the phenomena triggering or amplifying pressure oscillations. The modularity, cost and versatility of these experimental set-ups have allowed for a large spectrum of congurations and possible reduction measures to be explored. Far from being perfect since some physical aspects or phenomena just can't be respected at a smaller scale, they will never replace full-scale experiments. However, the valuable experiences we have gathered over the years have proven that this type of installation has it's own, important place, next to numerical simulations and full-scale tests. The past of the LP family was rich and fruitful, the future remains bright and promising. 2 References 1 G. Casalis, G. Avalon, and J. P. Pineau. Spatial instability of planar channel ow with uid injection through porous wall. Physics of Fluids, 10(10):25582568, 1998. 2 S. Gallier, F. Godfroy, and F. Plourde. Computational study of turbulence in a subscale solid rocket motors. AIAA Paper, 2004-4052, July 2004. 3 J. Guery, F. Godfroy, S. Ballereau, S. Gallier, P. Della Pieta, O. Orlandi, Eric Robert, and Nathalie Cesco. Thrust oscillations in solid rocket motors. AIAA Paper 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Hartford, CT, 2008-4979, July 2008. 4 N. Lupoglazo and F. Vuillot. Parietal vortex shedding as a cause of instability for long solid propellant motors. AIAA Paper, 96-0761, January 1996. 5 J. Perraud and F. Chedevergne. Etude des oscilations de poussée - calculs d'instabilités biglobales à partir de champs moyens de type RANS pour des maquettes de propulseurs LP6 et LP9. Technical report, ONERA, Décembre 2006. 6 M. Prévost, J.C. Godon, and O. Innegraeve. Thrust oscillations in reduced scale solid rocket motors part i : Experimental investigations. AIAA Paper, 2005-4003, July 2005. 7 M. Prévost, A. Le Quellec, and J.C. Godon. Thrust oscillations in reduced scale solid rocket motors, a new conguration for the MPS of Ariane 5. AIAA Paper, 2006-4418, July 2006. 8 M. Prévost, F. Vuillot, and J. C. Traineau. Vortex shedding driven oscillations in subscale motors for ariane 5 mps p230. AIAA Paper 96-3247, July 1-3 1996.