Integrated Rocket Simulation of Internal and External Flow

Journal of the Korean Physical Society, Vol. 55, No. 5, November 2009, pp. 2166∼2171

Integrated Rocket Simulation of Internal and External Flow Dynamicsin an e-Science Environment

Soon-Heum Ko, Sangho Han, Jin-ho Kim and Chongam Kim∗

Department of Aerospace Engineering, Seoul National University, Seoul 151-742

Jong Bae Moon and Kum Won Cho†

Korea Institute of Science and Technology Information, Daejeon 305-333

Yoonhee Kim‡

Department of Computer Science, Sookmyung Women’s University, Seoul 140-742

(Received 2 Ocotber 2008, in final form 16 March 2009)

The internal and external flowfield variation of a launch vehicle has been simulated in an e-Scienceenvironment. To analyze the igniting process of a solid-rocket propellant, a fluid-structure interac-tion code has been developed using an ALE (arbitrary Lagrangian Eulerian) kinematical descriptionand a staggered fluid-structure interaction algorithm. Also, unsteady motion of a detached rocketbooster has been predicted by using an external flow analysis with an aerodynamic-dynamic coupledsolver. A Korean e-Science environment designed for aerospace engineering, e-AIRS [15], suppliesa user-friendly interface for such individual work and it can advance to an integrated rocket simu-lation of internal combustion and external flow variation by controlling the execution and data flowof two flow solvers. As a consequence, e-Science facilitates multi-disciplinary collaborative research,and makes individual work more convenient.

PACS numbers: 47.85.Gj, 45.40.Gj, 47.90.+aKeywords: e-Science, e-AIRS, Launch vehicle, Fluid-structure interaction, Aerodynamic-dynamic coupledanalysisDOI: 10.3938/jkps.55.2166

I. INTRODUCTION

A rocket system is a system-intensive product thateventually requires the integration of the highest tech-nologies in each discipline, such as fluid physics, struc-tures, propulsion systems, control, and so on, as depictedin Figure 1. Thus, a multi-disciplinary integrated simu-lation based on accurate computations for each disciplineis a key element for successful research and developmentof a rocket system.

However, only segregated works in each discipline havebeen tried so far, mainly because it is very hard to obtainan accurate analysis for a single work. As an example ofa propulsion system, the ignition process of a solid-rocketpropellant in the combustion chamber shows complexmulti-physical phenomena due to the non-linear visco-elasticity of a propellant grain and the hot emission gasformed during the burning process. Thus, the researchershould be well aware of the characteristics of heat and

∗E-mail: [email protected]; Fax: +82-2-887-2662†E-mail: [email protected]‡E-mail: [email protected]

high-pressure flow physics, the combustion mechanismof a propellant, and the deformation of the propellantgrain due to the structural load and combustion. As an-other example, an external flow analysis examines aero-dynamic forces and moments of a rocket system withdynamic variation, often including relative motion of de-tached bodies such as exhausted rocket boosters. Thus,in-depth knowledge of compressible flow physics, rigid

Fig. 1. Individual research for research and developmentof a launch vehicle.

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Integrated Rocket Simulation of Internal and· · · – Soon-Heum Ko et al. -2167-

body dynamics, and the interference effect of bodies isrequired.

Considering the complexity of each work, it is far moreefficient to build a virtual organization to control collab-orative works than to let an individual researcher un-dertake the whole integrative analysis. To this end, weadopted high-end e-Science technology to construct acyber workbench for integrated rocket simulations. E-Science increasingly represents the global collaborationsof people and shared resources to solve new and chal-lenging problems in science and engineering [1] on thebasis of an IT infrastructure, typically referred to as theGrid [2]. The infrastructure serves the integrated utiliza-tion of computational and experimental facilities, valu-able datasets, knowledge, and so on in a secure and trans-parent manner; thus, scientists/engineers can exploit thee-Science environment for challengeable research [3], suchas large-scale computation for complex physical mecha-nisms [4], combined works of computation and experi-ments for design-to-development processes [5], and data-intensive research [6].

This paper describes the utilization of an e-Scienceenvironment to launch vehicle simulations. As valuablecomponent-wise research, individual studies on the igni-tion process of a solid-rocket propellant and the separa-tion motion of a strap-on booster due to external flowvariation are addressed in Sections 2 and 3. A coupledsimulation of internal and external flowfields with the aidof an e-Science environment is then presented in Section4. Finally, individual and integrated rocket simulationsare summarized in Section 5.

II. INTERNAL FLOW SIMULATION OF ASOLID-ROCKET PROPELLANT

1. Motivation

Solid propellant rockets produce a thrust force byburning contained propellant grain in the combustionchamber and ejecting hot combustion gas through asupersonic nozzle. During the ignition process, com-plex multi-physical phenomena develop in the interiorof the combustion chamber due to the non-linear visco-elasticity of a propellant grain and the hot exhaustion gasformed during the burning process. The main factorsthat govern the interior phenomena of the solid-rocketare the flow of hot and high-pressure gases, and the de-formation of the propellant grain due to the structuralload and combustion. Also, each principal element (fluid,structure, combustion) governing complex physical phe-nomena in the combustion chamber induces a feedbackcycle, thus influencing one another.

Because of the physical complexity and strong inter-action of multiple disciplines, many previous research ef-forts have been based on instrumental experiments, andnumerical analyses have been conducted just to show

the detailed features of a specific local physical domain.However, experimental research is very dangerous andcostly to perform, and it is not easy to measure all nec-essary physical quantities. Therefore, coupled simula-tion, by integrating the fluid, structure, and combustionparts, is required to address the highly complex unsteadyphenomena in a combustion chamber.

2. Governing Equations and Numerical Algo-rithms

The arbitrary Lagrangian Eulerian (ALE) kinemat-ical description [7, 8] has been incorporated into thefluid/solid formulation to accurately capture the defor-mation of the solid propellant grain. The ALE descrip-tion is a hybrid approach that combines the advantagesof the classical Lagrangian and Eulerian formulations.In the Lagrangian approach, mesh nodes are attachedto continuum particles and move with them. In the Eu-lerian approach, however, nodes of the computationalmesh remain fixed, and continuum particles pass throughthem. Thus, it is clear that neither the Lagrangiannor the Eulerian approach may efficiently describe theprogress of burning a solid propellant because some par-ticles are eroded due to the regression of the propellantsurface. In the ALE approach, numerical simulations arefirst conducted in an Eulerian manner, and deformationof the computational mesh is described in the Lagrangianpattern. It is, thus, suitable for simulating the regressionof the solid propellant.

Governing equations for fluid simulation are the two-dimensional axisymmetric Euler equations in ALE form.Inviscid governing equations can provide an accurate so-lution as mass flux at exposed grain surface will sweepaway the boundary layer. For the structural analysis,the two-dimensional dynamic finite element formulationusing the principle of virtual work is transformed intoan ALE kinematical description. Also, the mechanicalresponse of the grain is simulated using the Arruda-Boyce nonlinearly elastic constitutive model [9], whichhas been shown to be quite successful in capturing thesmall- and large-strain response of filled elastomers. Asa combustion modeling, the one-dimensional transientburning model is used to efficiently simulate the burningprocess at the interface between the fluid domain and thepropellant grain surface. It is assumed that the grain isheated by the hot gas in a chamber, and that the igni-tion process takes place on its exposed surface when thesurface temperature exceeds a specific value. Finally, forthe adjustment of interchanging information between thefluid and the solid domains, we adopted the loosely cou-pled conventional sequential staggered scheme suggestedby Park & Felippa [10].

The resulting solver is then validated by conductinga numerical simulation for the problem of the solid-propellant rocket interior and by comparing the com-

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Fig. 2. Burning and flow development process as in theforward areas of the combustion chamber.

putational results with the experimental data. Detailson the validation process are given in Ref. 11.

3. Numerical Results

The computational domain and the initial burningprocess near the igniter are shown in Figure 2. A two-dimensional unstructured mesh system consisting of asolid propellant and an internal flow region is generatedto represent an axisymmetric solid-rocket interior witha pyrogen igniter attached to it. Also, contours at thebottom right show variations of temperature just afterthe igniter has spouted out the flame. As shown in Fig-ure 2(a), the hot gas ejected from the igniter starts todevelop a flow in the combustion chamber. This com-bustion gas first heats the exposed grain surface nearthe trimmed slot surface, so the solid grain in this regionstarts to ignite and send out a mass flux (Figure 2(b)).As the combustion proceeds, the flame propagates veryrapidly to the exit direction along the grain surface whilethis propagation is rather delayed inside the slot whereinitial cold air is congested (Figure 2(c)). Finally, hotemission gas fills the entire chamber and the whole grainsurface produces a mass flux, as in Figure 2(d).

During the combustion process, the propellant graindecreases and is deformed because of the burning mech-anism and the structural load from the high-pressure flowin the combustion chamber. Figure 3(a) shows the geo-metric change of a solid propellant grain 0.786 secondsafter the pyrogen igniter ignited. At that time, about 14% of the contained propellant was consumed for propul-sion. Figure 3(b) shows the pressure history near theforehead of the rocket interior. The pressure in the com-bustion chamber increases instantly and reaches a peakpoint. At that time, the nozzle membrane, which is de-signed to obstruct the outflow until the pressure loadingreaches some specific value, is broken, and the confined

Fig. 3. (a) Temperature contours and change of fluid do-main at t = 0.786 sec, compared to initial state, (b) Pressurehistory at 0.15 m below the exit of an igniter.

flow can spout out through the nozzle. Thus, the pres-sure in the combustion chamber decreases rapidly untilthe incoming mass flow from the burned grain and theoutgoing mass flow through the nozzle are balanced. Af-ter that, the internal gas pressure shows a gradual de-crease caused by the volumetric expansion of the fluidicdomain due to the consumption of the propellant.

III. EXTERNAL FLOW VARIATIONAROUND A ROCKET SYSTEM

1. Motivation

Strap-on boosters have traditionally been used to in-crease the payload of launch vehicles. From an aerody-namic point of view, the optimal shape is the one thathas minimal drag during flight, and the designed rocketengine is equipped in the booster. However, an optimalbooster shape for steady flight does not guarantee a safeseparation of the strap-on booster. That is, the detachedempty booster after completion of the combustion pro-cess may sometimes have a catastrophic collision againstthe core rocket during free-fall. Hence, it is important toinvestigate in details of the separation behavior of strap-on boosters and to reflect on their aerodynamic-dynamiccharacteristics during the design process.

Many valuable research projects have been conductedto exploit the aerodynamic-dynamic behavior of launchvehicle configurations during the separation stage [12,13]. However, most previous numerical simulations wereinsufficient in accurately showing the unsteady flow pat-terns around the vehicle and the dynamics of detachedboosters. Most studies simulated quasi-steady flowfieldsat predefined positions by using wind tunnel experimentor, when an aerodynamic-dynamic-coupled analysis wasconducted, by using inviscid flow simulations conductedwithout considering the base flow effect. Thus, the cur-rent study focuses on accurately predicting the dynamicbehavior of detached boosters in an unsteady flowfieldby adopting an aerodynamic-dynamic-coupled approachand by conducting a turbulent flow analysis in the broadspatial region around the launch vehicle.


2. Governing Equations and Numerical Tech-niques

Three-dimensional compressible full Navier-Stokesequations are adopted to accurately resolve the massiveflow separation in the base region. For an adequate de-scription of the turbulent flowfield within the frameworkof the RANS (Reynolds-averaged Navier-Stokes) formu-lation, the k-ω SST (shear stress transport) two-equationturbulence model is implemented as flow solver. As fornumerical techniques, the AUSMPW+ (modified AUSMusing pressure-based weight functions) flux scheme withthe MUSCL (monotone upstream-centered schemes forconservation laws) approach and the LU-SGS (lower-upper symmetric Gauss-Seidel) method are used for spa-tial discretization and implicit time integration, and dualtime stepping is employed to obtain a second-order ac-curacy at the time domain.

The relative motion of bodies is predicted by solvingsix-degree-of-freedom rigid-body equations of motion. Ina coupled solver, the result of the aerodynamic force fromflow simulation at a former physical time is summed withgravity and returns an acceleration and an angular ac-celeration by solving Eulers equations of motion. Linearand angular displacements are easily acquired by inte-grating the former results. Then, the positions of bodiesare updated, and fluid analysis is conducted again. All ofthe details on the above techniques are given in Ref. 14.

3. Numerical Results

The aerodynamic characteristics of the KSR-III (Ko-rean sounding rocket) and the detachment motion of astrap-on during the separation process are examined.KSR-III is a rocket system researched by the KoreanAerospace Research Institute. The application configu-ration has a core rocket with two strap-on boosters at-tached to it, as seen in Figure 4 of the overset mesh sys-tem, which has about 3.5 million mesh points in total.The freestream Mach number and the Reynolds numberare 1.7 and 1.431 × 107, respectively. The flight anglewith respect to the ground is 90 degrees, and the angleof attack is zero. The flow properties of the plume gasare obtained by using the chamber condition and coldgas assumption.

Figure 5 shows the trajectory of a separated rocketbooster and the pressure field at free separation. The re-sult shows various compressible flow physics that cause acomplex dynamic pattern for the detached booster. Thefirst principal factor is the bow shock from the boosternose. After hitting the core rocket, a shock wave turnsback to the booster and acts to increase the repulsiveforce and to generate a positive pitching moment. Thenext factor is propagated oblique shocks from the finand flare skirt of the core rocket, which cause a nega-tive pitching moment and an increment in the normal

Fig. 4. Overset mesh system around a launch vehicle con-figuration.

Fig. 5. Separation motion analysis of a launch vehicle(from 0.00 to 0.40 sec, with a time interval of 0.20 sec).

force. Finally, expansion waves from the baseline of thecore rocket change the aerodynamic characteristics of thestrap-on in the form of a counterbalancing force with theeffect of oblique shocks from the core rocket. All thingscombined, the strap-on initially experiences a negativepitching moment and starts a counterclockwise rotation.However, as the nose of the booster gets closer to thecore rocket, the reflected bow shock becomes strongerand generates a strong repulsive force. At the same time,the oblique shock from the core rocket to the bottom ofthe booster gradually becomes weaker as the travelingdistance of the oblique shock increases. These processesmake the booster begin a reverse rotation. When thebooster eventually gains a positive inclination angle, ittends to accelerate in a positive pitching motion mainlybecause the asymmetric bow shock of the nose producesa strong positive pitching moment. In addition, the neg-ative pitching moment induced by the oblique shock fromthe core rocket is sufficiently diminished because thedownward motion of the booster reduces the effectivelength of the moment arm with respect to the boosterscenter of gravity. Thus, the current configuration finallyshows a safe separation.

IV. INTEGRATED ROCKET SIMULATIONIN THE E-SCIENCE ENVIRONMENT

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1. Motivation

Even though internal and external flow analyses canbe successfully conducted using individual solvers, it isstill hard to do integrative simulations for the accuratedesign and development of a rocket system. In a conven-tional way, a multi-disciplinary analysis by collaborationof researchers includes regular offline meetings, coordi-nation of their ideas, and the sharing and unificationof their numerical tools. First, unification of differentcodes requires such time-consuming and tedious jobs asthe unification of variables, I/O formats, etc., and fre-quent offline meetings. Moreover, opening researcherssource codes to other collaborators means that develop-ers face the risk of leaking or abuse of their intellectualproperty by other researchers or illegal intruders.

This restrictions of the conventional research environ-ment have motivated current researchers to implementan integrative rocket-simulation workbench on e-AIRS(e-Science Aerospace Integrated Research System). E-AIRS has been showing its usefulness in fluid dynamiclectures and academic research, on the basis of stabilizedinfrastructure and convenient user interfaces. To developa secure integrative rocket-simulation system, current re-search fully utilizes e-AIRS middleware and minimizesdomain scientist’s efforts.

2. Construction of an e-Science Environmentfor Rocket Simulation

The procedure of integrated rocket simulation is brieflyschematized in Figure 6. As this figure shows, an inte-grated rocket simulation can be conducted by exploit-ing the outgoing flow through the nozzle of a combus-tion chamber as the plume condition for an external flowanalysis. In the simulation process, the internal combus-tion solver first starts until a solid propellant producesa stable thrust. Then, the flow properties at the exit ofthe nozzle are transferred to an external solver, and anexternal flow solver concurrently starts by using theseproperties as plume conditions. In an unsteady simu-lation, the internal solver advances independently fromthe external solution procedure, while the external solvergets the plume condition at a given time from an accu-mulated intermediate dataset of an internal analysis orwaits for the result to come.

To construct this research workbench, applicationcodes need only to include the I/O routine and tunenon-dimensionalization for the exchange of plume con-ditions. On the other hand, some additional computerscientific modules are to be added to the current e-AIRSsystem. Basically, this work space uses core technologiesimplemented on the e-AIRS CFD Service, such as jobsubmission and metadata management systems, sched-ulers, monitoring tools, etc., that are described in detailin Ref. 15. The one exception is that application solvers

Fig. 6. Procedure of the integrated rocket simulation.

Fig. 7. Portal interface of integrative and individual rocketsimulations.

are maintained by code developers, and this makes thecurrent system more complex. As application solversof internal combustion and external flow are repositedand executed at owners′ sites, a middleware module thattransmits intermediate data between the two sites is de-veloped using Grid FTP, and another module, which con-trols the execution process of binary codes, is developedusing GT4 technologies. Also, a portal interface for in-tegrated rocket simulation is newly developed.

3. Utilization

Figure 7 shows the portal interface of integrated andindividual rocket analyses. By choosing a target applica-tion, the user can either conduct an individual simulationof internal combustion or external flow analysis, or try anintegrated simulation of full flowfields. To apply variousflow/numerical conditions, users can manually change aninput text file and upload it. As for the mesh system,the current interface only permits use of the default meshsystem, due to the generalization problem in the I/O for-mat of application codes. When a job is requested, a jobexecution signal is sent to a fixed site where an applica-tion code is stored, and e-Science controls the executionand data transmission of this job.

Figure 8 shows the steady-state pressure contours ofintegrated simulations for two rocket geometries, arewith and are without fins. A quasi-steady solution from


Fig. 8. Steady-state pressure contours of integrated simu-lations for slender and finned geometries.

the internal analysis is used as the exit condition of therocket nozzle in the external flow analysis. Overall, thetwo simulations show a stronger plume emission thanwas seen in the previous external flow analysis, wherethe plume was assumed to be a cold gas. As a re-sult, higher pressure is seen at the bottom of a booster,and it increases the negative pitching moment, whichbrings about a counterclockwise rotation of the strap-onbooster. Consequently, the booster is presumed to showa more unsafe motion during separation than is seen withthe cold-plume gas simulation.

V. CONCLUSIONS

Integrated and individual rocket simulations of inter-nal combustion and external flow variation are conductedin an e-Science environment. An internal combustionsolver is used to analyze the ignition process of a solid-rocket propellant by integrating fluid and structure anal-ysis tools, and combustion modeling. The result de-scribes the details of the initial burning and flame prop-agation process of the exposed propellant grain surfaceand the structural deformation of a solid propellant graindue to the effect of the hot, and high-pressure gas insidethe chamber. For an external flow analysis with a sepa-ration motion simulation of rocket boosters, the aerody-namic solution procedure is coupled with rigid body dy-namics. The result shows complex flow physics around alaunch vehicle and the influence of various factors on un-steady motion of strap-on boosters. Finally, a workbenchfor rocket simulations was developed in the e-Science en-vironment. Additional computer scientific modules wereimplemented on an existing e-AIRS system, and inte-grative simulations were successfully conducted in an e-Science environment.

ACKNOWLEDGMENTS

The current work is a product of the Korea Nationale-Science project. The authors are grateful to the Ko-rea Institute of Science and Technology Information fortheir financial support. Also, the authors appreciatethe financial supports provided by NSL(National SpaceLab.) program throgh the National Research Founda-tion of Korea funded by the Ministry of Education, Sci-ence and Technology (Grant 20090091724) and the au-thors are grateful to the Agency for Defence Develop-ment for financial support on solid-rocket propellant re-search. Additional support from the Institute of Ad-vanced Aerospace Technology at Seoul National Univer-sity is also appreciated.

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Integrated Rocket Simulation of Internal and External Flow