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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 os-

cillations 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 con�gurations tested in our laboratory over the last 20 years.

We present the phenomena we search to reproduce and the successes and failures of the

di�erent 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 modi�cations 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 segmentedgrains in which pressure and thrust oscillationsmay appear. The coupling with the chamberacoustics leads to unstable �uctuations whose fre-quencies oscillate around the frequencies of thelongitudinal modes. Because of these oscillationfrequencies, structural vibrations are generated inthe launch vehicle. Since 1990 and under the sci-enti�c coordination of ONERA, several Europeanand French laboratories as well as aerospace in-dustries have been involved in the Aerodynam-ics of Segmented Solid Motors (ASSM) programand the Pressure Oscillation Program (POP) sup-ported by the CNES.

As a part of these programs, we have de-�ned and tested a non-metallised propellant (non-metallised because it turned out to be impossi-ble to down-scale the aluminium size distributionwhile conserving the over all behaviour of the pro-pellant) in a large amount of di�erent con�gura-tions of down scaled axisymmetric set-ups, repre-sentative of the Ariane 5 P230 boosters, in or-der to obtain an experimental database. Thisdatabase is used for the validation of stability pre-dictions and to improve the knowledge of vortexshedding driven pressure oscillation phenomena inlarge segmented solid rocket motors. The de�-

nition of these experiments leans heavily on nu-merous numerical2,4··· and theoretical1,5··· stud-ies carried out in the ASSM and POP framework.This research has given us a better understandingof the development of unwanted and potentiallyhazardous pressure and thrust oscillations.

The paper is organised as follows. The �rstsection describes why and how we use these down-scaled motors. Section 2 gives a bit more detailson the data resulting from these experiments andthe way we use it. Section 3 details several se-ries of experiments, each designed to address onegiven physical phenomenon that in�uences the de-velopment of pressure oscillations. We end with aconclusion in section 4.

I. Why do we use reduced scale

models?

Thrust oscillations are produced by the cou-pling of vortex shedding induced pressure oscilla-tions with the motors chamber acoustics. Glob-ally, 3 types of vortex shedding have been identi-�ed:8

• Obstacle vortex-shedding (VSO) created bya protruding obstacle such as an inhibitoror 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‡Scienti�c Deputy Director of the Aerodynamics and Energetics Modelling Department at ONERA

2 II MEASUREMENTS AND DATA ACQUISITION.

Figure 1. Obstacle vortex-shedding

• Angle vortex-shedding (VSA) created by ashear layer coming from an angle such asfound around inter-segment cavities.

Figure 2. Angle vortex shedding

• Parietal vortex-shedding (VSP). This typeof shedding results from an intrinsic insta-bility of the internal �ow.

Figure 3. Parietal vortex-shedding

Full size experiments on the scale of the P230are horrendously expensive, complicated and timeconsuming and are therefore not really suited forresearch. Downscaled models, on the other hand,are relatively cheap, their geometry is simpler andmodular. Scaling problems (turbulence, heat �ux,particle sizes) do arise though and need to be con-sidered but all in all, downscaled rocket motorsallow for precise and well delimited experimentsand they are good tools to deepen our understand-ing of complicated phenomena such as vortex-shedding induced pressure oscillations. Our labo-ratory uses 3 di�erent families of these models:

LP6 Realistic, scale 1/15th representation of theP230.

Figure 4. LP6

Figure 5. LP6 in action

• 12 pressure transducers

• Ultra sound propellant surface regressionmeasurements

• 40 �rings at this date

LP9 Simpli�ed, modular, scale 1/35th represen-tation of the P230. Meant for parametricstudies of separate phenomena.

Figure 6. LP9

• 4 pressure transducers

• Ultra sound propellant surface regressionmeasurements

• 35 �rings at this date

LP10 Realistic, 1/35th representation of theP230.

Figure 7. LP10

• 4 pressure transducers

• Ultra sound propellant surface regressionmeasurements

• 32 �rings at this date

II. Measurements and data

acquisition.

In order to study thrust oscillations, onemight think that direct thrust measurements,both steady and unsteady, would be the �rst thingthese experimental set-ups should deliver. Theoscillations we are looking for represent a rippleon the overall thrust signal with an amplitudeof about 2% of the average thrust. Given thescale of these oscillations , their frequency andthe mass of both engine and rig, direct measure-ment of the thrust oscillations is di�cult. Since itis far easier and more precise to measure chamberpressure and since relative thrust oscillations ∆F

Fare thought to be proportional to relative pres-sure oscillations ∆P

P , our model rocket enginesare equipped with a certain number of pressuretransducers. Data acquisition is assured at 20kHz per channel. All channels are doubled and a200Hz<F<4800Hz pass-band �lter on each dupli-cate delivers the pressure oscillation signal. Fig-ure 8 shows a typical measurement on an LP6, inblue we �nd the un�ltered, steady, pressure while

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the red graph represents the �ltered, unsteady,component.

Figure 8. Typical measurement on an LP6

Besides pressure transducers we use Ultra-sound pads to measure the radial position of thesurface of the propellant at every instant of the�ring. This allows us to determine the surface re-gression rate as a function of time and hence ofengine pressure.

Several attempts to equip model rocket mo-tors to allow for visual access to its interior dur-ing operation have failed or were not satisfac-tory due to smoke and residue deposition ob-scuring the window. We have given up on thissort of measurements for the time being. Punc-tual temperature measurements in a largely non-homogeneous temperature �eld are of limited in-terest. This, and the complexity of adding cablingholes to an installation that needs to withstandhigh pressure and high temperatures has made usdecide against temperature measurements insideour model rocket motors.

The data that gets collected during our experi-ments serves multiple purposes. Primarily we areinterested in the reproduction or the repressionof pressure instabilities in order to better under-stand the underlying phenomena in the hope to�nd a cure for (or at least a way to reduce) theseunwanted, and hazardous oscillations. For thatpropose a modular setup such as the LP9 familyis well adapted. It allows for parametric studiesby isolating and varying the most relevant param-eters one by one. A signal processing toolbox hasbeen developed to detail, analyse and compare thedi�erent �rings.

Figure 9. Examples of automatic analysis

Amongst other things this toolbox allows usto easily create Hilbert analysis graphs (left) andPower Density Spectra (right) directly from themeasurements database as shown in �gure 9.These results can be normalised by a character-istic time and pressure, making it possible to eas-ily compare data from di�erent families of exper-iments and �ight results.

III. Phenomena thought to be

contributing to thrust

oscillations.

III.A. Cavities.

Cavities such as inter-bloc slots and aft-end cav-ities resulting from the submerged nozzle designof the P230 are generating noise (mainly result-ing from VSA cf. �gure 2) that can provoke pres-sure oscillations when a coupling with the cham-ber acoustics occurs. Cavities are con�ned bywalls that are either passive or generating a mass�ow (propellant). In the series of experimentspresented here, the LP9 24,67 is the most rep-resentative of the P230 with an inter block cavity,an aft-end cavity and a submerged nozzle. TheP230 su�ers from 4 �bursts� at several stages ofthe �ight (See �gure 10) and we see in �gure 11that the LP9 24 reproduces the �rst 3 nicely (amedium peak at 0.7 and 2 stronger peaks at 0.8and 0.9 which compares well with the timing ofthe bursts in the table of �gure 10) and hints thefourth 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

Figure 10. P230 Flight 510. Power Spectrum Density

When we use an external nozzle on the LP9 23,the third burst disappears while the �rst an sec-ond burst stay unaltered (�gure 11). For the LP922 experiment we �lled the aft-end cavity whilekeeping an external nozzle. This completely erad-icated the pressure oscillations. To quantify thee�ect of the inter-segment slot we designed theLP9 25 experiment. One single block with a sub-merged nozzle. Strangely this resulted in one sin-gle burst about half way between the original sec-ond and third. It hence is clear that cavities, bothinter-segment and aft-end, play an important rolein provoking and sustaining pressure oscillations.

Figure 11. Cavities. LP9 number 22,23,24 and 25. First longitudinal mode.

III.B. Propellant composition.

We know that the aluminium and alumina par-ticles resulting from the combustion process andpresent in the �ow have an in�uence on the ampli-�cation of pressure instabilities. Numerical sim-ulations done by the SNPE3 have provided thegraph presented in �gure 12 and have shown theimportance of the in�uence of particles, both inertor reactive. 2 series of experiments have been car-ried out at our laboratory to try to con�rm these�ndings. The �rst series consists of experimentLP10 9 and 10 and is aimed at �nding the in�u-ence of each class of reactive particle sizes foundin the full-scale motors.

Figure 12. In�uence of the Stokes number on pressure

oscillations

For this series, 2 grains of propellant have beendoped with aluminium particles. A �rst block

III.B Propellant composition. 5

with particles with a diameter of 5µm represent-ing 4% of the total mass and a second block withparticles with a diameter of 30µm also represent-ing 4% of the total mass. Because we can't re-spect the down-scaled aluminium size distribu-tion, the aluminium mass fraction and the over-all behaviour of the propellant, these blocks arenot conform with a down-scaled Ariane 5 P230propellant but are considered to have the highestpossible concentration of down-scaled aluminiumparticles while conserving the overall behaviourof the propellant. The particle size distributionof the solid phase in the gas �ow inside the P230is not well known. Basically we have 3 classes;smoke particles < 3µm that we normally ignore,alumina residues ≈ 30µm resulting from the com-

bustion of the aluminium present in the propel-lant and �nally agglomerates > 100µm resultingfrom the coalescence of liquid alumina droplets.Scaled down, the biggest 2 classes give particlesof ≈ 5µm and ≈ 30µm. LP10 9 and 10 are to becompared to LP10 17 which has the same geom-etry and the same propellant without the addedaluminium particles and presents 3 blasts simi-lar to the once found in �ight. Figure 13 showsthe results of all 3 experiments. As predicted bythe SNPE numerical simulations3 we con�rm thatsmall particles have a tendency to amplify the in-stabilities while bigger particles, on the contrary,reduce the pressure oscillations, especially for thesecond and third blast.

Figure 13. Propellant composition. LP10 number 9,10 and 17. First longitudinal mode.

The numerical simulations results in �gure 12show that for both reactive and inert particleswith a Stokes number St ≈ 1 there is a strongampli�cation tendency. With St = ωτu where

τu = ρpd2

18µ . LP10 30 was devised to con�rm this.A fuel grain identically to the one used in LP10 17

has been seeded with 7% ZrO2 particles with a di-ameter of 8.07µm which corresponds to St = 1.27.ZrO2 is capable of withstanding the high temper-atures inside a motor, therefore these particles canbe considered inert. The results for both LP10 17and 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 ampli�cation of the secondburst as well as a slight phase shift. The lattermight be due to the fact that the ZrO2 particleschange the combustion temperature and hencethe acoustic velocity. Two more experiments areplanned with particles of St ≈ 0.1 and St ≈ 10to see if the ampli�cation disappears. The ratio0.0160.012 = 1.33 of the peak amplitudes of the LP1030 (inert particles) and the LP10 9 (reactive par-ticles) compares relatively well with 2.1

1.4 = 1.5, theratio found by the numerical simulations shown in�gure 12.

III.C. 3D e�ects.

The margins for change in the P230 are rathersmall. At least two possibilities to reduce thethrust oscillations are:

1. Reducing the thickness of the thermal pro-tections. This way they will erode faster andhence be less protuberant.

2. Provoking a 3D e�ect in the internal gas�ow in order to try to de-organise the vor-tex shedding. This way the coupling withthe acoustic modes should be lessened.

To address the second point, numerical simulationwere insu�cient to de�ne a suitable 3D geometry.The LP6 ARTA 1, 2, 3 and 4 have been used togain insight in the in�uence of the geometry of a3D thermal protector ring on the pressure insta-bilities.

To assure the integrity of the 3D motif duringthe �ring, a metallic thermal protection ring was

used, placed between blocs S2 and S3 (see �gure15). 4 tests have been carried out with the sameshape, basically a ring with 7 �teeth� (cf �gure 15for images). These test are identical to the LP627 experiment with a metallic thermal protectionring added. The only di�erence between the dif-ferent test is the height of the �teeth�.

ARTA Height of the �teeth�

1 0 mm (2D)

2 8 mm

3 5.3 mm

4 14.3 mm

Table 1. 3D thermal protection ring

Figure 15 shows the results of these experi-ments. It is clear that the 3D motif has a big im-pact on the pressure oscillations, ARTA 2 and 4have practically eradicated the �uctuations whereas ARTA 3 exhibits a surprisingly small e�ect.Full size applications of 3D thermal protectionrings have never allowed to con�rm such an im-portant reduction in instability levels. Howeverin this type of experiments, where the size of themotor is considerably bigger than for the LP9 andLP10 series, the main problem lies in the uncer-tainty we have on the instantaneous geometry ofthe thermal protection ring. Even though it ismetallic it is not excluded that it erodes partiallyduring the �ring. This might explain the surpris-ing behaviour of the ARTA 3.

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Figure 15. 3D e�ects. LP6 ARTA number 1,2,3 and 4. First longitudinal mode.

IV. Conclusion.

20 years of testing with our down-scaled solidpropellant motors have given us a valuable insightin the phenomena triggering or amplifying pres-sure oscillations. The modularity, cost and versa-tility of these experimental set-ups have allowedfor a large spectrum of con�gurations and possiblereduction measures to be explored. Far from be-

ing perfect since some physical aspects or phenom-ena just can't be respected at a smaller scale, theywill never replace full-scale experiments. How-ever, the valuable experiences we have gatheredover the years have proven that this type of in-stallation has it's own, important place, next tonumerical simulations and full-scale tests. Thepast of the LP family was rich and fruitful, thefuture remains bright and promising.

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