Aalto University School of Engineering Degree Programme in Mechanical Engineering Matti Palin Analysis of aerodynamic stability of the MetNet Entry and Descent vehicle with FINFLO simulations Master's Thesis Espoo, October 31, 2015 Supervisor: Professor Jukka Tuhkuri, Aalto University Advisors: Professor Timo Siikonen Ari-Matti Harri, D.Sc. (Tech.) (Finnish Meteorological Institute) Aalto University School of Engineering ABSTRACT OF Degree Programme in Mechanical Engineering MASTER'S THESIS Author: Matti Palin Title: Analysis of aerodynamic stability of the MetNet Entry and Descent vehicle with FINFLO simulations Date: October 31, 2015 Pages: xiv + 86 Major: Aeronautical Engineering Code: K3004 Supervisor: Professor Jukka Tuhkuri Advisors: Professor Timo Siikonen Ari-Matti Harri, D.Sc. (Tech.) (Finnish Meteorological Institute) This Master's Thesis investigates the aerodynamic stability of the MetNet Mars atmospheric entry and descent vehicle, developed in cooperation between the Finnish Meteorological Institute (FMI) and the Lavochin Association (LA). The purpose of the study is performing Computational Fluid Dynamics (CFD) sim- ulations and obtaining the pertinent aerodynamic coefficients for the vehicle in the landing phase to Mars. The results are compared with the values obtained by LA, the most important feature being the aerodynamic stability of the vehicle. In this work, only the static stability is assessed. The simulations were performed with an inhouse FINFLO software. Before the simulations, an atmospheric model of Mars was created. Some initial trajectory calculations were made in order to have approximate values for the combinations of the Reynolds and Mach numbers that the vehicle will experience during the landing. These initial trajectory calculations also provided a condition for the mesh creation. A coarse and a dense calculation meshes were created with 1.4 and 7 million cells, respectively. The SST k-! turbulence model was used and the results were tabulated in a form of dimensionless coefficients. Apart from the lift coefficient, the values differ to some extent from the LA's results. However, the general trends lead to the same conclusions: the drag coefficient is more than sufficient to ensure the designed landing speed and the negative slope of the pitching moment coefficient indicates static stability for the vehicle. Some heat load analyses were also carried out. Unfortunately, the simulations converged only up to Ma = 1:9 and no results were obtained at larger velocities. The thermal analyses show that heating of the vehicle is highly dependent on the Mach number, and for these reasons it would be advisable to perform more simulations for the vehicle. Keywords: CFD, FINFLO, simulation, MetNet, Mars, lander, stability Language: English ii Aalto-yliopisto Insin¨o¨oritieteiden korkeakoulu DIPLOMITYON¨ Konetekniikan koulutusohjelma TIIVISTELMA¨ Tekij¨a: Matti Palin Ty¨on nimi: MetNet-laskeutujan aerodynaamisen vakavuuden analysointi FINFLO- simulaatioilla P¨aiv¨ays: 31. lokakuuta 2015 Sivum¨a¨ar¨a: xiv + 86 P¨a¨aaine: Lentotekniikka Koodi: K3004 Valvoja: Professori Jukka Tuhkuri Ohjaajat: Professori Timo Siikonen Tekniikan tohtori Ari-Matti Harri (Ilmatieteen Laitos) T¨ass¨a diplomity¨oss¨a tutkitaan Ilmatieteen Laitoksen ja Lavochin Associationin (LA) kehitt¨am¨an Mars MetNet-laskeutujan aerodynaamista vakavuutta k¨aytt¨aen laskennallista virtausmekaniikkaa. Tavoitteena on ratkaista relevantit aerodynaa- miset kertoimet laskeutumisvaiheen aikana Marsiin. Tuloksia verrataan LA:n saa- miin arvoihin ja t¨arkeimp¨an¨a tutkittavana ominaisuutena on aerodynaaminen va- kavuus. Ty¨oss¨a tutkitaan vain staattista aerodynaamista stabiiliutta. Laskentaan k¨aytettiin FINFLO-ohjelmaa. Marsin kaasukeh¨ast¨a luotiin malli en- nen simulaatioita. Lis¨aksi tehtiin ratalaskelmia laskeutujan laskeutumisvaiheen aikana kokemien Reynoldsin luvun ja Machin luvun arvojen saamiseksi. Ratalas- kelmat tuottivat my¨os ehdon, joka auttoi laskentaverkon luomisessa. Laskentaa varten luotiin harva ja tihe¨a laskentaverkko, joissa oli 1,4 ja 7 miljoonaa lasken- takoppia. Turbulenssin kuvaukseen k¨aytettiin SST k-!-mallia ja tulokset taulu- koitiin dimensiottomassa kerroinmuodossa. Nostovoimakerrointa lukuun ottamatta tulokset erosivat jonkin verran LA:n saa- mista arvoista. Tuloksista voidaan kuitenkin tehd¨a sama p¨a¨atelm¨a: laskeutu- jan vastuskerroin on enemm¨an kuin riitt¨av¨a suunniteltuun laskeutumisnopeuteen n¨ahden ja pituusmomenttikertoimen negatiivinen kulmakerroin osoittaa laskeu- tujan olevan staattisesti stabiili. Laskenta ei kuitenkaan konvergoitunut suurem- milla Machin luvuilla kuin Ma = 1; 9 eik¨a tuloksia saatu t¨at¨a suuremmilla no- peuksilla. L¨amp¨otarkastelut osoittivat lis¨aksi laskeutujan pintal¨amp¨otilan olevan voimakkaasti riippuvainen Machin luvusta ja n¨aist¨a syist¨a laskeutujalle suositel- laan suoritettavan lis¨a¨a simulaatioita. Asiasanat: CFD, FINFLO, simulaatio, MetNet, Mars, laskeutuja, vaka- vuus Kieli: Englanti iii iv Acknowledgements This thesis was created between January and September 2015 at the Me- chanical Engineering Department of the Aalto University in Otaniemi. This research was initiated and funded by the Finnish Meteorological Institute in order to answer their need for aerodynamic research for the MetNet lander. I wish to thank first and foremost professor Timo Siikonen for giving me the great occasion to work on this project. I am thankful for all the support and guidance he has given me throughout the project. I am also grateful to my supervisor Jukka Tuhkuri for the possibility to carry out my research in the Department of Mechanical Engineering. Furthermore, I would like to thank Dr. Ari-Matti Harri and Harri Haukka for providing all the necessary material for the work and for help along the way. Moreover, I wish to thank Lucien Vienne (Universit´ede Toulon) for his conducive observations and cooperation during his internship in the department. My gratitude goes also to the Department and staff of Mechanical Engi- neering of the Aalto University for providing me with the necessary facilities, know-how, and hardware for the demanding project. Espoo, October 31, 2015 Matti Palin v vi Abbreviations and Acronyms CAD Computer-aided Design DV Descent vehicle FMI Finnish Meteorological Institute EDLS Entry, Descent and Landing System HEART High Energy Atmospheric Reentry Test LA Lavochkin Association CFD Computational Fluid Dynamics RITD Re-entry: inflatable technology development in Rus- sian collaboration NASA National Aeronautics and Space Administration MIBD Main Inflatable Braking Device AIBD Additional Inflatable Braking Device MCD Mars Climate Database vii viii List of symbols Symbol description a1 constant in the turbulence model A (reference) area c speed of sound c¯ reference length cf friction coefficient cp heat capacity at constant pressure CA axial force coefficient CD drag coefficient Cm pitching moment coefficient Cmα pitching moment coefficient slope with respect to the angle of attack Cmq pitching moment coefficient slope with respect to the pitch rate CN normal force coefficient CNα normal force coefficient slope with respect to the angle of attack Cn Courant number CL lift coefficient CLα lift coefficient slope with respect to the angle of attack D diameter E total internal energy Fi flux in the ith cell F^ flux through a cell face F1, F2 blending functions FA axial force FN normal force g acceleration of gravity m gm ≈ 3:71 s , acceleration of gravity on the surface of Mars ix h enthalpy h altitude ~i, ~j, ~k direction (unit) vectors Iz mass moment of inertia k kinetic energy of turbulence Kn Knudsen number L D lift-to-drag ratio Ls solar longitude, the angle between Mars and the Sun M moment Ma Mach number N number of values nj surface unit normal on a computational cell p pressure P production of kinetic energy q pitch rate 1 2 q1 = 2 ρU , the dynamic pressure Q arbitrary quantity Qs, Qi source term R specific gas constant Re Reynolds number referred to the main diameter Rex Reynolds number referred to the x-coordinate Rm ≈ 3386 km, the average radius of Mars si;j mean strain-rate tensor S reference area S norm of strain-rate tensor Scell face area t time T temperature T1 free stream temperature ui velocity component U, U~ free stream velocity; vehicle velocity magnitude Uτ friction velocity Vi volume of a computation cell W vehicle weight xi; x; y; z; coordinates y+ dimensionless distance from the wall α angle of attack α_ time derivative of the angle of attack α¨ second time derivative of the angle of attack βw ballistic coefficient, calculated using the vehicle weight x βm traditional ballistic coefficient, calculated using the vehicle mass ∗ β,β1, β2, β constants in the turbulence model ∆ difference δi;j Kronecker delta. δi;j = 1 if i = j, and 0 otherwise. ∆s height of the first mesh cell ∆t time step turbulent dissipation γ specific heat ratio γ trajectory angle in radians γ1, γ2 coefficients in the turbulence model λ1;2 roots of the characteristic equation µ dynamic viscosity µT turbulent eddy viscosity ! specific rate of dissipation of the turbulence kinetic energy ρ medium density σk1, σk2, σ!1, σ!2 constants in the turbulence model τw wall shear stress τ, τi;j viscous stress tensor xi xii Contents 1 Introduction and background 1 2 The MetNet Entry and Descent System 5 2.1 Background and history . .5 2.2 Mars atmospheric entry and descent conditions . .6 2.2.1 Dependence of time and space . .7 2.2.2 Gas state equation . .7 2.2.3 The speed of sound on Mars . .9 2.2.4 Atmospheric density . 14 2.2.5 Atmospheric viscosity and Sutherland coefficients . 15 2.3 The vehicle & landing phases . 15 2.3.1 MetNet DV in the transport configuration . 16 2.3.2 MetNet DV with inflated Main Inflatable Braking De- vice (MIBD) . 17 2.3.3 MetNet DV after the inflation of Additional Inflatable Braking Device (AIBD) . 18 2.4 Potential problems . 18 2.5 Previous work . 20 2.5.1 Wind tunnel tests and their results . 21 2.5.2 Heat transfer tests . 25 2.5.3 Drop tests . 26 3 Theoretical background 27 3.1 Aerodynamics of the vehicle . 27 3.2 Aerodynamic stability of the vehicle . 31 3.3 Stability criterion for the vehicle .