Rhodes, Greece, August 0-, 008 Cycling Loading Effect on a Solid Propellant Engine Performances Part 3D CFD Study and Validation of CFD Results ADRIAN ARGHIROPOL, CONSTANTIN ROTARU, DORU SAFTA, FLORIN ZAGANESCU Aircraft Department Military Technical Academy 8-83, George Cosbuc Street, 5 th Sector, Bucharest ROMANIA arghiropol@clicknet.ro, crotaru@mta.ro, dsafta@mta.ro Abstract: - Flow inside a solid rocket engine has been studied both experimentally and numerically in the Bucharest s Military Technical Academy. The article will analyze last experimental and computational research on the 3 D and axis symmetric CFD modeling results versus same author 007 s WSEAS Athens 565-73 paper based on computational research on the D and axis symmetric CFD modeling of the flow inside a solid propellant engine with a specific axial distribution of the propellant s material temperature. The study will indicate what differences in results may provide the D or 3D modeling of the rocket engine s internal flow parameters like velocity, pressure and temperature, versus overall performances like thrust considering the variable axial temperature distribution in the solid propellant assumed to be a vascoelastic material under cycling loading. Key-Words: rocket engine, CFD simulations, vascoelastic material, flow parameters Introduction All the viscoelastic materials are dissipative in nature. They will dissipate always large amounts of mechanical energy in the form of heat when subject to high frequency loading. The solid propellant used as the fuel for the rocket engines is assumed as a viscoelastic material. Considering the high-speed of the rocket, or their own carrier i.e. supersonic aircraft - the solid fuel was proved to be a subject for cycle loading due to high speed vibrations. Heating due to vibration near a resonance frequency may lead to melting, or material failure, or change in the rocket engine performances. Tormey and Britton in [] conducted various vibration tests on various solid fuel families and revealed that the heating of the solid propellant due to vibrations increased the material temperature significantly. The scope of this article will be to continue the initial D CFD study in 3D, evaluate both D and 3D results and conclude with the validation procedures of the CFD results based on experimental data. Problem Formulation Same author s (*) 007 WSEAS Athens 565-73 [7] analyzed at that time one possible worst case scenario of the propellant s material temperature distribution. The objective of this initial study was to find the temperature distribution along the viscoelastic rod under such particular case and that was done using the governing equations (see also []): the energy balance equation, the equation of motion, the stress-strain relationship for induced vibrations from.0 khz up to 00 khz. The governing equations were next: d σ + ω ρ( J ) σ + J σ = 0 dx () d σ + ω ρ( J ) σ J σ = 0 dx () T T ω ρ C( ) = ( K ) + J ( σ + σ ) t (3) Based on that distribution temperatures along the axis of the solid propellant, an initial D CFD study employed the rocket engine s internal flow parameters study, like velocity, pressure and temperature, versus overall performances like thrust considering two cases, with variable axial temperature distribution from x=0 to x = l. And without, or T=const.. Special analyzed case As was described in [] and [7] the following equation for the thermal conductivity of the solid ISSN: 790-5095 0 ISBN: 978-960-6766-98-5
Rhodes, Greece, August 0-, 008 rocket propellant was assumed in a linear form, as follows: K = C CT (4) and that will change the form of the energy equation, as next: T T ω C C ( T ) + J( σ + σ ) (5) where ρ is the mass density, C the specific heat, J the storage modules, J the loss modules, and and σ and σ the real and imaginary parts of stress amplitudeσ, respectively. 3 Problem Solution Fig.. Picture of the rocket engine under test 3. Initial temperature data for D and 3D CFD study For our particular solid rocket engine project, the results of the initial study [7] revelead in Table from x=0 to x = l the distribution of one possible worst case scenario axial temperatures developed from an initial ambiental temperature of 38 C. These data were used for D and 3D CFD employed study. 3.. Validation of initial D and present 3D results Validation of the initial CFD results presented in the st WSEAS article [7] was one of the important issues concerning our methodology in getting fast, but accurate results in the early design stages. Six separate experimental engines were fired on the bed test platform at extreme temperatures of -50 deg.c and +50 deg.c and acquired data will be used in order to evaluate and validate both D and 3D CFD results. In the previous WSEAS article [7] the D axis-symmetric CFD was employed, using the FLUENT R software package. In order to conclude and evaluate results in this final design stage, another 3D axis-symmetrical CFD study was employed, using the same FLUENT R software package, in order to evaluate any significant differences between the D and 3D study, and check how appropriate is the D versus 3D study. Results of the 3D CFD study will show first the (F) thrust and (P) pressure maximal values possible and second how the velocity, turbulence and temperature of the rocket engine s internal flow will change, in the worst case scenarios of possible variable temperature among the axis of the solid propellant. 3.. Last 3D FLUENT R simulations and results The 3D axis-symmetrical single precision modeling and simulation employed some compromise between a large hardware versus real life design time constraints. There was necessary a significant amount of single processor computer time due up to 5k iterations for convergence, as can be seen in the Figures to 4. Comparing with the initial D axis-symmetrical double precision previous computational task [7], there was a difference between 5 to 0 times in more computer machine and modeling effort. The difference in (F) thrust and (P) inside pressure maximal values results are below 5% in all of the cases, including values for velocity distribution and turbulence, under the same temperature initial data (see Figures 5 to 9). To summarize up to this point, the D study was efficient and accurate enough for a fast check-up and performance evaluation inside the engine flow. The 3D additional time effort were paying results in order to discover any of the possible hidden hot spots, just to counteract further mechanical issues of the final product from this early design stages. 3..3 Experimental results and further validation procedure basics of CFD modeling As was proved in the early stages of authors experience, failure to conduct CFD validation based on technology testing could result in the absence of reliability enhancing improvements in the rocket engine configuration and could cause subtle failure modes or performance limits to be unnoticed until critical points in the engine development or flight schedule. In a short overview of [4], any initial CFD design sessions versus highly instrumented experimental engine bed tests should be able to provide: () engine system level validation of advanced propulsion technology concepts prior to incorporation of these concepts into further development or production units; () an opportunity for greater understanding and fine-tuning of analytical and CFD tools that characterize any rocket engine performance; (3) results in the development and improvement of diagnostic methods; ISSN: 790-5095 03 ISBN: 978-960-6766-98-5
Rhodes, Greece, August 0-, 008 (4) increases the depth of available knowledge about the inner workings, sensitivities, and detailed performance characteristics of solid and liquid rocket engine systems. The overall benefit are the validation of most appropriate CFD model from the early design stage, before expensive experimental tests to final validate technology, improved system performance, high system reliability, and mission safety of the research program. It s well known that CFD modeling in the early stages of the design will help developers to reduce the cost of the project. However, most likely only experimental test will validate the CFD Model used for early design stages. Before final approval, any rocket engine technology candidate should be judged by its technical merit and potential benefit, the risk of testing the item on the Technology Test Bed engine, and the cost of integrating the item into the engine or facility. Key milestones in the process should be: () - Technology Item Screening and CFD Model(s) Validation; () - Technology Item Final Design Review; (3) - Technology Item Integration Design Review; (4) - Hot Fire Testing. Fig. 4 History of residuals Fig. 5 Initial flow temperatures because of the initial temperature distribution inside the rocket engine Fig. Drag convergence history Fig. 6 Nozzle area flow temperatures like results of the initial temperature distribution inside the rocket engine Fig. 3 Convergence history of mass flow The () Technology Item Final Design Review it is based on a model developed by NASA [4], involving a decisional chain sense: once the technology item is accepted as an output of () - Technology Item Screening, technology item development proceeds with the conduct of analytical ISSN: 790-5095 04 ISBN: 978-960-6766-98-5
Rhodes, Greece, August 0-, 008 studies, CFD modeling, component testing, and the incorporation of design revisions, if required. Fig. 7 Nozzle area velocity distribution like results of the initial temperature distribution inside the rocket engine Then a () - Technology Item Final Design Review is conducted in which four subject areas are presented and discussed: (a) Technology Item Design Description; (b) Technology Item Design Verification; (c) System Issues; (d) Safety/Quality Issues. Such provisions should also be compliant with ISO standards, like any European country. When the technology item final design review is successfully completed, fabrication can proceed and readiness certified upon acceptance accordingly with EU-ESA and NASA standards.. The (3) Technology Item Integration Design Review is conducted to verify that the technology item can be accommodated safely and effectively into the Technology Test Bed. It consists of: (e) an Integration Design Description; (f) Integrated Design Verification and CFD modeling results; (g) System Issues; (h) Safety/Quality Issues. The subject areas covered in the Technology Item Integration Design Review are similar to those for the Technology Item Final Design Review except that all factors are viewed from the standpoint of interaction of the technology item with the Test Bed and its related subsystems, facilities, instrumentation, software and data. Fig. 8 Convergent nozzle area turbulence issues like results of the initial temperature distribution inside the rocket engine Fig. 9 Divergent nozzle area turbulence issues like results of the initial temperature distribution inside the rocket engine The (4) Hot Fire Testing and Results are always based on a test plan. A test plan should be prepared for each test series, and it is reviewed at a pretest readiness review before each test. Instrumentation should be configured in accordance with an existing procedure described in an Instrumentation Program and Command List (IP&CL). A test results review is held after each test and a test report is prepared. When two or more tests are combined into a test series, a test series report is prepared. This is the place in the decisional chain were CFD modeling results can be confirmed or not by experimental tests. Based on previous experience of the authors there was identified a need of systematic and methodical procedure for planning, CFD modeling, rocket engine test bed testing, data analysis, and reporting the results of various solid rocket engine models, also applicable for liquid rocket engines. As seen from Figure to Figure 9, the CFD modeling has ranged from evaluation of new engine performances, to experimental evaluation of the results in a quick ISSN: 790-5095 05 ISBN: 978-960-6766-98-5
Rhodes, Greece, August 0-, 008 but enough accurate time for a decision (see Figure 0 to Figure 5). The imposed procedure means that an engine technology item who has proceeded through the concept evaluation process - including CFD modeling - up to a point where a decision is made to pursue test bed evaluation for validation of the CFD results, it is presented by the principal investigator to the Test Bed project manager for prescreening review and then a technology item screening review. The six experimental rocket engines acquired data are next: Fig. Individual measured pressure (P) of the experimental engines at [-50 C] Fig.0 Average measured pressure (P) inside the experimental engines at -50 C and + 50 C Fig. 3 Individual measured pressure (P) of the experimental engines at [+50 C] Fig. Average measured thrust (F) of the experimental engines at -50 C and + 50 C Fig. 4 Individual measured thrust (F) of the experimental engines at [-50 C] ISSN: 790-5095 06 ISBN: 978-960-6766-98-5
Rhodes, Greece, August 0-, 008 the ground tests will confirm the requested amount of detailed knowledge of the initial estimated CFD performance of the rocket engines under evaluation. Fig. 5 Individual measured thrust (F) of the experimental engines at [+50 C] 4 Conclusions 4.. Validation of various CFD results The D study was efficient for a fast check-up and performance evaluation inside the engine flow. The 3D study highlighted the possible hidden hot spots, helping to conclude the mechanical design. The difference in (F) thrust and (P) inside pressure maximal values results are below 5% in all of the cases, including values for velocity distribution and turbulence, under the same initial data. The experimental results will validate the both D and 3D CFD results in a maximum 5% maximum relative error envelope and the validation procedures of the project will tell if there is a need for more accuracy. 4.. Validation procedures review The authors will strongly advice any Engine Test Program Manager(s) to employ such CFD calculation during the early design stages and also start such development of a procedure program like a way to achieve an indispensable tool in the validation of propulsion technology advances for any reactive propulsion system. Such procedure may offer a fast and cheap approach using in early stages CFD modeling based on further validation on measurement and diagnostic methods that are continuing to be used in any rocket engine research program and are applicable to other similar test and evaluation scenarios. Accordingly with NASA standards, their Technology Test Bed highly instrumented engine employs over five times the number of measurements used for an acceptance test of a flight engine. Only under such circumstances References: [] Tormey, J.F. and Britton, S.C., Effect of Cyclic Loading on Solid Propellant Grain Structure, AIAA Journal, Vol., 963, pp.763-770; [] Pourrazady, M. and Harish Krishnamurty, Thermal Response of A Dynamically Loaded Viscoelastic Rod with Variable Proprieties, University of Toledo, Toledo Ohio, USA [3] FLUENT R help files and related documentation. FLUENT R (ANSYS) [4]. NASA/MSFC, "Technology Test Bed Test Report: Engine #300," NASA/MSFC Report # EP5(9TR-033), August 99, Huntsville, AL. [5]. Arghiropol, A Test Bed Reliability Practices & Procedures Applied to Rocket Engine Technology, The 3 st Internationally Attended Scientifically Conference of Bucharest s Military Technical Academy, November 005, ISBN 973-640-074-3 [6] Arghiropol, A and Rotaru, C and Zaganescu, F CFD Procedures Applied to Rocket Engine Technology, FLUENT R Conference Romania, May 008, [7] Arghiropol, A and Boscoianu, M and Coman, A Cycling Loading Effect on a Solid Propellant Engine Performances, Fluid Mecanics & Aerodynamics The 5 th IASME / WSEAS International Conference of Fluid Mechanics and Aerodynamics Athens, August 007, pg..0 05, ISBN 978-960-8457-99-7 About authors: (*) Adrian Arghiropol, Ph.D student at Department of Aviation Integrated Systems, Military Technical Academy, 8-83 George Cosbuc Avenue, Sector 8, Bucharest, Romania consultant, tel: +40-44-59485, cell:+40-73- 653987, e-mail: arghiropol@clicknet.ro. (**) Constantin Rotaru, professor, Department of Aviation Integrated Systems, Military Technical Academy, 8-83 George Cosbuc Avenue, Sector 8, Bucharest, Romania, cell: +40-745-974480, e- mail: crotaru@clicknet.ro (***) Doru Safta, professor, Department of Aviation Integrated Systems, Military Technical Academy, 8-83 George Cosbuc Avenue, Sector 8, Bucharest, Romania, tel: +40--3354460, e- mail: dsafta@mta.ro. (****) - Florin Zaganescu, professor at Military Technical Academy in Bucharest, Romania, ISSN: 790-5095 07 ISBN: 978-960-6766-98-5