Defence Science Journal, Vo148, No 2, Apri11998, pp. 197-204@ 1998, DESlDOC

SHORT COMMUNICATION

Application of Ultrasonic Technique for Measurement of Instantaneous

Burn Rate of Solid Propellants

Desh Deepak, R. Jeenu, P. Sridharan and M.S. Padmanabhan

Vikram Sarabhai Space Centre, Thiruvananthapuram -695 022.

ABSTRACT

The ultrasonic pulse-echo technique has been applied for the measurement of instantaneous burnrate of aluminised composite solid propellants. The tests have been carried out on end-burning 30 mmthick propellant specimens at nearly constant pressure of about 1.9 MPa. Necessary software forpost-test data processing and instantaneous burn rate computations have been developed. The burnrates measured by the ultrasonic technique have been compared with those obtained from ballisticevaluation motor tests on propellant from the same mix. An accuracy of about j: I per cent ininstantaneous burn rate measurements and reproducibility of results have been demonstrated byapplying ultrasonic technique.

NOMENCLATURE

C Acoustic velocity

K Constant

n Burn rate index

p Pressure

~t Time lapse

T Temperature

V Voltage

X Thickness

a, /3, y Constants

E Calibration constant

Subscripts

b Bumming surface

Interface

p Pressure

r Reference

T Temperature

1. INTRODUCTION

In the conventional methods for themeasurement of bum rate of solid propellants ( e.g.the strand burner, ballistic evaluation motor, etc.),the burn rate is obtained by monitoring the bumtime. of an initially known thickness of thepropellant. These methods give the average burnrate o.fthe propellant over the web at an average testpressure, but do not give the local bum rates insidethe propellant web. Hence, abnormalities in thebum rate within the grain are not brought outquantitatively by these methods. FurtHer, since eachtest gives bum rate at only one pressure (testaverage pressure ), determination of pressuredependence of bum rate necessitates conduct. ofseveral tests, each of them at a different pressure.

These drawbacks of the conventional methodscan be overcome if a measurement technique isused, wherein the instantaneous bum rate isdetermined as the propellant web bums. In thiscontext, visualisation techniques, such as high

Received 18 March 1997, revised 25 September 1997

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DEF SCI J, VOL 48, NO 2, APRIL 1998

speed photography require experimental motorswith windows. X-radiography and gamma radio-graphy may also be considered as extensions of thevisualisation technique, but these requirecumbersome setups. Moreover, these techniqueshave limitations in terms of spatial resolutionand accuracy. y abso(ptiometry also lacks accuracyin view of statistical fluctuations of the sourceradiation ( Cs : 137, Ir: 192) .Also, introduction ofradioactive source raises safety and supplyproblems that restrict its use for routine tests. Use ofelectromagnetic waves of about 10 GHz(microwave) has been successfully tried in thelaboratoryl,2. However, the complexity of themicrowave setup and the required high quality ofassociated instrumentation severely limit its use inactual motors.

x = c D. 112 (I)

The instantaneous burn rate can be calculatedby taking the time-derivative of the propellantthickness. The important elements of themeasurement system are discussed here.

2.1 Choice of Transducer

An ultrasonic transducer is characterised bythe natural frequency and damping ratio of itsceramic. For burn rate measurement, the dampingratio should be chosen to be as high as possible, sothat the return echo does not interfere with the endof the emission pulse. In the present study, a heavilydamped broadband ultrasonic transducer(Videoscan from Panametrics, USA) with peakfrequency of 2.25 MHz and of diameter 12.5 mmwas used.The ultrasonic pulse-echo technique is the

potential technique to non-intrusively measure theinstantaneous bum rate of propellants3. It also hasthe advantage of simple and portableinstrumentation for application on solid motors4,S.This technique was used in the study to measure theinstantaneous bum rate of aluminised compositesolid propellant specimens at nearly constantpressure in a laboratory setup. The objectives of thisstudy were to optimise the measurement procedureand to establish the accuracy and reproducibility ofthe measurements, which are essential before thetechnique is adopted for instantaneous bum ratemeasurements in actual motors .

2.2 Mounting of Transducer

To obtain linear operation down-to-zeropropellant thickness, an intermediary couplingmaterial between transducer and propellant isintroduced. This also brings into the measurement aslight delay that reduces the transducer dampingproblems, limits the near-field effects of thetransducer, and isolates the transducer from thesevere pressure and temperature conditions in thecombustion chamber. In the present work,polymethylmethacrylate (PMMA) was used as thecoupling material.

2. BURN RATE MEASUREMENT SYSTEM2.3 Pulse-Echo Interface Measurement System

The pulse-echo interface measurement (PIM)system, developed by Prins Maurits 1..ab- TNO, TheNetherlands, was used6 to measure theinstantaneous burn rate of the solid propellant. Thesystem has an ultrasonic transmitter and receiverwhich measures the time lapse between the emittedand the received echoes from the propellant. Fromthe output of the PIM system, which is in the formof voltage corresponding to the time lapse, webthickness of the burning propellant at any instant oftime can be calculated by:

The ultrasonic technique for burn ratemeasurement is based on the property of ultrasoundthat an acoustic wave. reflects (partly) onencountering an interface where the local acousticimpedance changes. Using this property, ultrasoundis employed to determine the instantaneousthickness, x of the propellant using the 'ultrasonicpulse-echo technique', in which the same probe(transducer) acts alternatively as an emitter and areceiver of sound waves. The thickness of thepropellant at any instant of time during combustionis determined as: x = c E (Vb -Vi /2 (2)

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DESH DEEPAK, et al: MEASUREMENT TECHNIQUE FOR INSTANTANEOUS ~URN RATE OF SOLID PROPELLANTS

where

c = Cr [l+Kp (P-P r)] [l+KT (T -T r)] (3)

and 8 is the calibration constant of the PIM system,which is determined using a calibrator simulatingthe time lapses ranging from 0 to 90 Jls. The

instantaneous bum rate of the propellant can beobtained by differentiating the instantaneous webthickness (Eqn 2) with respect to time .

propellant thickness was -35 mm with diameter-30 mm.

3.2 Experimental Setup

The schematic of the experimental set-up isshown in Fig. 2. It consists of a combustionchamber (inner diameter 70 mm, length 170 mm)connected to a N 2 gas reservoir of 100 V to maintainnearly constant pressure during propellant burning.An end-burning propellant specimen (Fig.1) withPMMA as the coupling material, was mounted inthe combustion chamber. The ultrasonic tran~ducerwas attached to the propellant specimen on theouter face of PMMA. The pressure transducer wasconnected to the combustion chamber. Forpropellant ignition, a pyrotechnic charge was fixedon the propellant surface.

3. EXPERIMENTAL DETAILS

The instantaneous bum rates of aluminisedcomposite solid propellant specimens at nearlyconstant pressure were measured by ultrasonictechnique using the experimental setup describedhere. For this, the acoustic velocity in the propellantwas also measured (Eqns 2 ~d 3) to experimentlydetermine Kp and KT.

To determine Kp, the acoustic velocity in thepropellant specimen (Fig. I) was measured at anambient temperature of 30 °C for differentpressures. Kp for test prQpellant was 3.3 x 10-3/MPa.To determine KT, the propellant specimen cast inPMMA cup was soaked for 4 hr at a constanttemperature in an oven. Thermocouples(chromel-alumel) fixed at three different locationson the propellant specimen confirmed homogeneityof temperature in the specimen. The acousticvelocity in the propellant specimen at differenttemperatures (at atmospheric pressure) wasobtained. KT for the test propellant was

-2.76 x 10-31 °C.

The bum rates of the propellant obtained by theultrasonic technique w~re compared with the bumrates from ballistic evaluation motor test for the

same batch mix of the propellant.

3.3 Instan,.tneous Burn Rate Measurements

Before the test, the PIM system and thepressure transducer were calibrated. The ultrasonictransducer was connected to the PIM systemthrough a cable. The entire experimental setup waspressurised by N2 gas at a pressure of- 1.8 MPa.Ignition of the propellant specimen in the chamber

was initiated from remote by a pyrotechnic chargeattached to tbe propellant surface. The output of thePIM system \ and the pressure transducer wererecorded at the rate of 200 data/s for the entire bumtime of the propellant specimen (-8s) in a computer.

3.1 Propellant Specimens

The specimens (Fig. 1) were prepared using

aluminised composite propellant (AP-HTPB-Al)with 18 per cent metal and 86 per cent solidloading). Since PMMA was used as the couplingmaterial, PMMA cups were fabricated and. thepropellant was cast directly in these cups. The

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DEF SCI J, VOL 48, NO 2, APRIL 1998

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Figure 2. Experimental setup for burn rate measurement by PIMsystem

less than an acceptable value. From the fittedequation of web thickness agai\lst time, theinstantaneous bum rate was evaluated by taking thetime derivative.

4. DATA PROCESSING SOFTWARE

PIM outp\lt. (voltage) can be converte.d intotime lapse for the ultrasonic pulse by using thecalibration constant of the equipment. Since theacoustic velocity of the propellant is pre.determined,the data for the measured propellant web thicknessvs time can be obtained (Eqn 2). However, beforeevaluating the instantaneous bum rate from theslope of this curve, the data needs filtering andsmoothening due to scatter of data points, noise andbase line shift, if any. The required software wasdeveloped by this team for data processinginvolving filtering, base line shifting, smoothening,and instantaneous bum rate evaluation. The detailsof this software are given in the flow chart (Fig. 3).First of all, raw data for web thickness vs time wasprocessed to identify various regions of base lineshift. To each of these regions, a second-ordercurve (x = af + ~t + y) was fitted by keeping a and ~

same for all the regions. Data points most deviatedfrom the fitted curves were removed one by one andthe curves were refitted till all the remaining pointsgot fitted in the curve with a scatter in data points

5. RESULTS

Instantaneous bum rates of end-bumingalum,inised compo.site solid propellant specimens ofthe same batch mix were measured by ultrasonictechnique in an experimental setup for the bum timeof -8 s and the corresponding pressure in the

.chamber was also monitored. In spite of the largevolume reservoir connected to the combustionchamber, chamber pressure varied from 1.8-2.0MPa; the average pressure being 1.9 MPa due tocombustion. Hence, to compare the instantaneousbum rate results in the tests, the bum rate valueswere normalised to a reference pressure of 3.24MPa, (33 kgf/cm1. For normalising the bum rates,the value of n for the propellant (n = 0.3334)

obtained earlier from several ballistic evaluationmotor tests was used .

200

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DESH DEEPAK, et al: MEASUREMENT TECHNIQUE FOR INSTANTANEOUS B,URN RATE OF SOLID PROPELLANTS

START

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Figure 3. Flow chart of software for burn rate evaluation

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DEF SCI VOL 48, NO 2, APRIL 1998

Table I. Results or burn rate measurements obtained usingultrasonic technique

Propellant' AP-HTPB-Al(18% metal & 86% solid loading)-35 mm-8s1.8-2.0 MPa

Propellant web thicknessBum timePressure range in test

Test No, Average of instantaneous Maximum variationbum rates measured by (:t) of normalised burnultrasonic technique rate in a test(normalised to 3.24 MPa) (%)

(mmls)

US-l 4.92 0.04

US-2 4.94 0.90

1.06US-3 4.96

US-4 4.95 0.66

* For this batch mix of propellant. burn rate values obtained

by the conventional method using ballistic evaluation motortests gave the following burn rates:

(i) 4.96 mm/s at 3.24 MPa (motor I)

Figure 4. Thicknesss or web burnt measured by the PIM systemand the chamber pressure.

The thickness of web burnt vs time measured

by ultrasonic.technique in a typical test (No. US-l)

is shown in Fig. 4. The instantaneous burn rates

measured in this test and normalised bum rate

values are plotted in Fig. 5. Figures 4 and 5 also

give pressure variation during the tests. Since the

pressure change during the test is quite small, the

measured instantaneous burn rates do not show

much variation. The burn rates at each instant,

normalised to 3.24 MPa , are also shown in Fig.5.

The average of these normalised burn rates in this

test is 4.92 mm/s, with maximum variation of :!: 0.04

per cent (Table 1) .Results from the other tests using

ultrasonic technique are given in Table 1 along with

the burn rates obtained from ballistic evaluation

motor tests with propellant of the same batch mix.

(ii) 4.97 mm/s at 3.24 MPa (motor 2)

the instantaneous burn rate results obtained byultrasonic technique.

6. CONCLUSIONS

Measurements of instantaneous bum rates ofthe solid propellant using ultrasonic pulse-echotechnique at specimen level have given consistentresults with an accuracy of about:!: I per cent. Thetechnique offers simple and portableinstrumentation, which is advantageous for itsapplication to measure instantaneous bum rate inballistic evaluation motor tests. Towards this end,measurement procedures have been optimised andnecessary software for post-test data processing andinstantaneous bum rate computations have been

developed.The averages of the normalised burn rates for

the propellant in the four tests are 4.92,4.94,4.96,and 4.95 mm/s (Table I), which are measured withan accuracy of about :I: I per cent. The average

burn rate of the propellant of this batch evaluatedusing the conventional method in two ballisticevaluation motor tests were 4.96 and 4.97 mm/s.This confirmed the accuracy and reproducibility of

ACKNOWLEDGEMENTS

The authors are thankful to Dr B.C. Pillai,Group Director, Propulsion Group, VikramSarabhai Space Centre, Thiruvananthapuram, forconstant encouragement and support for thisresearch programme. Thanks are also due to

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DESH DEEPAK, et al: MEASUREMENT TECHNIQUE FOR INSTANTANEOUS BURN RATE OF SOLID PROPELLANTS

1 2 3 4 5 6 7 8 9 10

TIE: (5)

Figure 5. Instantaneous and normalised burn rates or propellant

authorities of the Rocket Propellant Plant for burning rate. La Recherche Aerospatiale, 1979,providing propellant specimens for all the tests and 1, 71-90.~o ~gniter and Pyrotechnic Division for supplying 4. traineau, J .cl. & Kuentzman, P. Some

IgnIters. measurements of solid propellant burning rates in

REFERENCES ~ozzleless motors. AIAA/SAE/ASME 2Oth JointPropulsion Conference/ American Institute of

I. Strand, L.D.; Schultz, A.L. & Reedy, G.K. Aeronautics and Astronautics, Cincinnati, Ohio,Microwave doppler shift technique for 1984. Paper No. AIAA-84-1469. .

determining solid propellant transient regression .rates. J Spacecraft Rockets, 1974,11(2),75-83. 5. Traineau, J.cl. & .Kuentzman, P. .Ultrason~c

measurement of solid propellant bummg rates 102. Strand, L.D. & McNamara, R.P. A variable"' nozzlelessrocket motors. J Propul. Power, 1986,

frequency driver-rnicrowave transient regression 2(3), 215-22.rate measurement system. AIAA 14th Aerospace ...Sciences Meeting, American Institute of 6. ThjkStra, F.; KOrt1Og, P. & van den Berg, R.Aeronautics and Astronautics, Washington, 1976. Ultrasonic regression rate measurement in solidPaper No. AIAA- 76-105, fuel ramjets. AIAA/SAE/ ASME/ ASEE 26th Joint

.Propulsion Conference, American Institute of3. Kuentzman, P.; Demafls, J.C. & Cauty, F. A t . d Ast t " O 1 d FI .

d..eronau ICS an ronau ICS, r an 0, on aUltrasornc measurement of solid propellant 1990. Paper No. AIAA -90-1963.

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DEF SCI VqL 48, NO 2, APRIL 199&

Contributors

Dr Desh Deepak is Section Head, Combustion R&D Section of Propulsion Group at Vikram

Sarabhai Space Centre (VSSC) Thiruvananthapuram. He obtained his PhD from GorakhpurUniversity in 1977. During 1979-85, he worked at Southampton University, Edinburgh Universityand Imperial College of Science & Technology, London, Atomic Energy Research Establishment,Harwell, UK, for post-doctoral research. He has published a number of research papers. His areas ofresearch include solid propulsion, solid propellant ignition/combustion, hybrid propulsion,air-breathing propulsion and laser-based diagnostic techniques for fluid flow measurements andcombustion research.

Dr R Jeenu obtained his BTech (Chemical Engineering) from Calicut University in 1984 and PhDfrom Indian Institute of Technology, Chennai, in 1992. Hyderabad. Hejoined the Combustion R & DSection of VSSC, Thiruvananthapuram in 1992. Currently, he is on post-doctoral assignment at theSchool of Aerospace Engineering at the Georgia Institute of Technology, USA. His areas of researchinclude solid propulsion and solid propellant combustion.

Mr p Sridharan obtained his BE (Mechanical Engineering) from Anna University in 1986 and

MTech (Refrigeration & Cryogenic Engineering) from Kerala University in 1989. He joined VSSC,Thiruvananthapuram in 1990. At present, he is working at the Combustion R & D Section on solidpropellant combustion. .

Dr M s Padmanabhan received his BE ( Mechanical Engineering) from Bangalore University'1969, ME (Aeronautics) from Indian Institute of Science 1971 and PhD (Aerospace Engineering)

from Georgia Institute of Technology, USA 1975. After post-doctoral research at Princeton

University, he joined VSSC in 1979, His areas of research are solid motor performance, propulsion,combustion and fluid mechanics. At VSSC, he has been involved in the design and development ofvarious solid rocket motors being used in ISRO's satellite launch vehicles. Currently, he is the Head,

Fluid Mechanics and Internal Ballistics Division in the Propulsion Group at VSSC.

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