MEASUREMENTS OF SPECIFIC IMPULSE OF SOLID ROCKET PROPELLANTS

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1 MEASUREMENTS OF SPECIFIC IMPULSE OF SOLID ROCKET PROPELLANTS A.Korotkikh 1,V.Arkhipov 2,A.Kiskin 3,V.Zarko 3, ands.yankovskiy 1 1 InstituteofPowerEngineering Tomsk Polytechnic University Tomsk, Russia korotkikh@tpu.ru 2 ResearchInstituteofAppliedMathematicsandMechanics Tomsk State University Tomsk, Russia 3 InstituteofChemicalKineticsandCombustion Siberian Branch of the Russian Academy of Sciences Novosibirsk, Russia kiskin@kinetics.nsc.ru A new laboratory technique is presented for fast determination of the speci c impulse of solid rocket propellants(srps), based on the measurement of recoil force of gasi cation products owing from the burning surface of solid propellants. The values of speci c impulse for the model composite solid propellant are obtained while varying the pressure in the combustion chamber. Keywords: speci c impulse; solid rocket propellant; gasi cation products; burning surface Introduction The main energy characteristics of SRPs is the speci c impulse of force J 1 whichisequaltotheincrementofthrust,beingrealizedby the combustion of a unit mass of propellant. When considering the thermodynamicvalueof J 1,itsspeci cimpulseisdeterminedinthe absence of losses and complete chemical transformations. The thermodynamic value of the speci c impulse with the pressure at the nozzle exitequaltotheexternalpressure p a isde nedbytherelationship[1]: A. Korotkikh et al. 179

2 ADVANCES IN NONEQUILIBRIUM PROCESSES J 1 = 2kRT c k 1 ( 1 p ) (k 1)/k a (1) p c where Rand karetheuniversalgasconstantandtheratioofspeci c heatsofthecombustionproducts;and p c and T c arethepressureand temperature in the engine combustion chamber. The theoretical calculation method for determining the thermodynamic speci c impulse is implemented in various software codes(see, for example, Astra-4 [2]). To implement this method, one should specify the composition of SRPs or its equivalent chemical formula, as well as the enthalpies of formation for fuel components and phase transitions which, in some cases, are unknown for new solid compositions. Furthermore, the calculated values of the speci c impulse ignore incomplete combustion of the components of solid propellants and provide the upperestimatefor J 1. To model combustion of SRP test samples, the direct determination methodbasedonthediagramsofthrust P(t)andpressure p k (t)inthe combustion chamber of rocket engine is usually used. The magnitude of J 1 iscalculatedastheratioofthetotalthrustimpulseduringengine operation period and the mass of consumed propellant during this time[1]: J 1 = τ 0 τ 0 P(t)dt G(t) dt where G(t) is the mass ow rate of combustion products. Another known technique for measurement of speci c impulse is the ballistic pendulum[3]. Impulse of the force acting on the pendulum upon ringansrpchargeinarocketengineisproportionaltothe lengthofthechordde ectioncenterofmassofthependulumfrom its equilibrium position. The disadvantages of direct methods for the measurements of the speci c impulse is the need for model engines with solidpropellantchargetoweighnotlessthan0.20.5kgandtheneed for a special bench equipment placed in the explosion-proof boxes. 180 A. Korotkikh et al.

3 Measurement Technique Figure 1 Scheme of the laboratory setup for measuring the speci c impulse:1 samplesrp;2 quartz glass; 3 bombofconstantvolume; 4 base;5 reactiveforcegage;6 signal converter; 7 analog-to-digital converter; 8 personal computer; 9 battery of gas tanks; 10 manometer; and11 valve For determining the speci c impulse of the investigated SRPs, the laboratory setup was developed (Fig. 1) [4]. Cylindrical sampleofstudiedsrp1placed intoquartzglasscup2 wasinstalled horizontally on the reactive force gage 5 [5]. Capacitance type gage for measuring the reactive force of combustion products owing from the burning surface of the sample 1 of combustion products was housed inaconstantvolumebomb3installedonthebase4. Theelectrical signal from the sensor 5 via the electronic signal converter 6 into a voltage and analog-to-digital converter voltage7intoadigitalcodewasfed to the recording personal computer 8. Before the experiment, aconstantvolumebomb3was lled with inert gas(nitrogen or argon)fromthebatteryofgas tanks 9 to a predetermined pressure which was recorded with a manometer 10. After experiment, the SPR combustion products discharge was carried out through the valve 11. Before the conducting of experiment, a sample SRP was weighed onananalyticalbalancewithanaccuracy ±0.01ganditsheight h pr anddiameter d pr weremeasured. Basedonthesedata, density ρ pr andtheareaofthecombustionsurface S pr werecalculated.ata xed pressure p c,asamplesrpwasignitedbytheignitingcomposition.in the combustion process, the reactive force F of the gasi cation products A. Korotkikh et al. 181

4 ADVANCES IN NONEQUILIBRIUM PROCESSES owing from the front surface of the burning sample was measured bysensor5. Inaddition,theburningtimeofthesampleSRP τ was measuredonthepressurediagram p c (t). Pressurewasmeasuredwith astraingageoflh-412type(infig.1,thepressuregageandigniter are not shown). The value of reactive force of the gasi cation products is determined by[3]: F= ρu 2 S pr (2) where uand ρarethedensityandout owrateofgasi cationproducts. From the law of conservation of mass, follows: ρ pr u pr = ρu (3) where u pr isthelinearburningrateofsrp. FromEqs.(2)and(3)andtheequationofstateforanidealgas p c = ρrt c, theexpressionforreactiveforcecanbegotintheform F= (ρ pru pr ) 2 p c RT c S pr. Fromhere,onecande nethecomplex(rt c ) thepowderforce [3], which is included in the equation for the calculation of the speci c impulse Fp c RT c = S pr (ρ pr u pr ) 2. (4) All parameters in expression(4) are directly measured in the experiment(f, p c, ρ pr, S pr ).LinearburningrateofSRPisalsodetermined experimentally through the combustion time τ of a sample of the known height h pr : u pr = h pr τ. (5) Substituting(4) and(5) in(1), one obtains the formula for calculating the value of speci c impulse of SRP: J 1 = τ ( Fp c 2k 1 p ) (k 1)/k a. ρ pr h pr S pr k 1 p c 182 A. Korotkikh et al.

5 Thevalueoftheadiabaticindex kistakenfromthethermodynamic calculation or its average value is selected for similar composition of SRPs(k 1.25). This value varies slightly when varying the initial component of SRPs[1]. Thus, it is shown experimentally that the proposed method[4] allows for the laboratory determination of the speci c impulse of SRPs usingcompactequipmentandsamplesoflowmass(notmorethan5g). References 1. Sorkin, R. E Gas thermodynamics of solid propellant rocket engine. Moscow: Nauka. 368 p. 2. Sinyarev,G.B., N.A. Vatolin, B.G. Trusov, and G.K. Moiseev Computer application for thermodynamic calculations of metallurgical processes. Moscow: Nauka. 264 p. 3. Zel dovich, Ya. B., M. A. Rivin, and D. A. Frank-Kamenetsky Impulse of reactive force of gunpowder rockets. Moscow: Oborongiz. 190 p. 4. Arkhipov, V.A., V.E. Zarko, A.B. Kiskin, and A.G. Korotkikh. The method for determining the speci c impulse of solid propellants. Patent RFNo Cl.MPKG01N33/ Kiskin, A. B., and V. N. Simonenko Special features of implementation of the recoil-force registration method for burning-rate measurement. Combust. Explo. Shock Waves 36(1):4853. A. Korotkikh et al. 183

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