Loads, Structures, and Mechanisms Design Project Fall 2012 Stephanie Bilyk Leah Krombach Josh Sloane Michelle Sultzman
Mission Specifications Design vehicle for lunar exploration mission 10 day mission with 3 contingency days Choose design from power, propulsion and thermal design project Perform structural analysis of crew vehicle [1] Analyze buckling Analyze displacements Analyze shear flow Finite element analysis Support all significant load sources Earth launch Pressurization loads Docking loads Lunar landing loads Earth EDL 1
Mission Specifications Cont. Perform structural analysis of lunar landing vehicle Tank size Engine size Gross mass of propulsion stage is 12,110 kg Crew vehicle mass of 4,795 kg Detailed analysis of landing gears Touchdown velocity of 3 m/s vertically Touchdown velocity of 1.5 m/s horizontally Impact attenuation to prevent bounce Design of deployment articulation and actuators 2
Outline Selecting a design Crew cabin analysis Crew vehicle's dimensions Finite element analysis cases 1-3 Docking Loads Launch Loads Pressurization Loads Summary of Cases FEM cabin summary Case 2: Lunar landing loads Case 2: Re-entry Loads Landing Gear Design Landing gear 3
Outline Cont. Material trade study Boundary conditions trade study Aluminum-Fixed Titanium-Fixed Aluminum-Pinned Titanium-Pinned FEM landing gear table FEM landing gear summary Landing gear dimensions Spring mass damper analysis Deployment of landing gear Landing vehicle summary CAD References 4
Selecting a Design Crew vehicle from team B5 (Michelle's group) selected The vehicle was made in autodesk inventor Able to do structures analysis on crew vehicle Michelle and Leah are both familiar with this design 5
Crew Cabin Analysis 6
Crew Vehicle's Dimensions 7
Cabin Outer Shell Design uses outer shell from 8 B5 Window and door are left open Examining worst case scenario displacements and stresses
Finite Element Analysis Cases Case 1 Material: Aluminum 6061 Yield strength: 55 MPa Minimum wall thickness: 45 mm Maximum wall thickness: 100 mm No fillet for the door or window Case 2 Material: Aluminum 6061 3 cm thinner wall than case 1 to compensate for addition of honeycomb Fillet door and window 9
Finite Element Analysis Cases Cont. Case 3 Material: Titanium Yield strength: 275.6 MPa 3 cm thinner wall than case 1 to compensate for addition of honeycomb Fillet door and window 10
Case 1: Docking Loads Pressure of 10 MPa applied at tip of cone to simulate docking loads Heat shield is a fixed constraint Max stress: 8 MPa MOS: 3.58 Max displacement:.14 mm 11
Case 1: Launch Loads Ring between heat shield and cone is constraint 5 g gravity load Max stress: 8.2 MPa MOS: 5.43 Max displacement: 0.1 mm 12
Case 1: Pressurization Loads Ring between heat shield and cone is constraint 10 psi pressure inside cabin Max stress: 7.7 MPa MOS: 3.76 Max displacement: 0.14 mm 13
Case 1 Summary Margin of Safety > 3 for each case Overdesigned 10 cm wall of solid aluminum is unrealistic Next case uses honeycomb wall interior design As an approximation, make the walls all 3cm thinner Fillet Sharp edges on window and door will have high stress Add fillets to the window and door corners Window: 50 mm fillet Door: 100 mm fillet 14
Case 2: Docking Loads Pressure of 10 MPa applied at tip of cone Heat shield is a fixed constraint Max stress: 19.3 MPa MOS:.9 Max displacement: 0.12 mm 15
Case 2: Launch Loads 5 g gravity load which only considers weight of the outer shell Ring between heat shield and cone is constraint Max stress: 11.5 MPa MOS: 2.19 Max displacement: 0.19 mm 16
Case 2: Pressurization Loads 10 psi pressure inside cabin Ring between heat shield and cone is constant Max stress: 29 MPa MOS:.26 Max displacement: 0.66 mm 17
Case 2 Summary Walls are thinner than case 1 Area load is applied on is less making Total force is less Mass is less than in case 1 Margin of Safety = 0.26 (due to pressurization) Window: 50 mm fillet Door: 100 mm fillet 18
Case 3: Docking Loads Pressure of 10 MPa applied at the tip of the cone Heat shield is a fixed constraint Max stress: 19 MPa MOS: 8.67 Max displacement: 0.08 mm 19
Case 3: Launch Loads 5 g gravity load Ring between heat shield and cone is constraint Max stress: 19.1 MPa MOS: 8.62 Max displacement:.21 mm 20
Case 3: Pressurization Loads 10 psi pressure inside cabin Ring between heat shield and cone is constraint Max stress: 29.2 MPa MOS: 5.29 Max displacement: 0.44 mm 21
Case 3 Summary The margins of safety are all very high using titanium There is not much benefit of max displacement using titanium instead of aluminum Window: 50 mm fillet Door: 100 mm fillet 22
FEM Cabin Summary Case 1 Case 2 Case 3 Max Displacement (mm) Max Stress (MPa) Safety Factor (Yield stress/max applied stress) Margin of Safety (with a design factor of 1.5) [2] Docking 0.14 8 6.88 3.58 Launch 0.1 5.7 9.65 5.43 Pressurization 0.14 7.7 7.14 3.76 Docking 0.12 19.3 2.85 0.90 Launch 0.19 11.5 4.78 2.19 Pressurization 0.66 29 1.90 0.26 Docking 0.08 19 14.51 8.67 Launch 0.21 19.1 14.43 8.62 Pressurization 0.44 29.2 9.44 5.29 23
Crew Cabin Conclusions The crew vehicle is designed using case 2 Lowest margin of safety Closest to optimally designed Titanium is expensive No justification for using case 3 to increase the margin of safety further than using aluminum The maximum displacement of 0.66 mm is an over-approximation In reality, the door will help to maintain the vehicle's shape Additional analysis will be performed for Lunar landing Earth descent 24
Case 2: Lunar Landing Loads Lunar gravity 60 kn vertical force and 10 kn horizontal force Door frame fixed Max stress: 23.7 MPa Max displacement: 1.71 mm 25
Case 2: Re-entry Loads 5 g load pointing up Ring between heat shield and cone is constraint Max displacement:.22 mm Max stress: 19.3 MPa 26
Landing Gear Design 27
Landing Gear Truss Structure Chosen to isolate shock load from landing vehicle [8] Tetrahedron shape Cylindrical rod containing MSD system Model as mass spring damper system attached to vertex of the tetrahedron Analyzed transient response of MSD [7] Honeycomb used as damper to absorb impact energy [3] 28
Material Trade Study Aluminum Titanium Steel Critical Force for Buckling (N) 3.42 x 107 5.46 x 107 9.57 x 107 Stiffness K (N/m) 6.12 x 107 9.77 x 107 1.72 x 108 Viscous Damping Constant C (N-s/m) 6.86 x 104 1.11 x 105 2.90 x105 Mass (kg) 19.19 31.42 56.87 29
Boundary Conditions Trade Study Considered boundary conditions for Aluminum 6061 and Titanium materials Aluminum-Fixed Titanium-Fixed Aluminum-Pinned Titanium-Pinned 30
Aluminum - Fixed 20 kn force applied at the foot Fixed connections to spacecraft Normal Stress: 28.9 MPa Max displacement: 1.9mm 31
Titanium - Fixed 20 kn force applied at the foot Fixed connections to spacecraft Normal Stress: 28.9 MPa Max displacement: 1.3 mm 32
Aluminum - Pinned 20 kn force applied at the foot Pinned connections to the spacecraft Max normal stress: 28.9 MPa Max displacement: 1.6 mm 33
Titanium - Pinned 20 kn force applied at the foot Pinned connections to the spacecraft Max normal stress: 28.9 MPa Max displacement: 1.1 mm 34
FEM Landing Gear Table Max Displacement (mm) Max Stress (MPa) Safety Factor (Yield stress/max applied stress) Margin of Safety (with a design factor of 1.5) Aluminum Fixed 1.9 28.9 1.90 0.27 Aluminum Pinned 1.6 28.9 1.90 0.27 Titanium Fixed 1.3 28.9 9.54 5.36 Titanium Pinned 1.1 28.9 9.54 5.37 35
FEM Landing Gear Summary Minimal difference in normal stress and displacement for different configurations Aluminum has a lower margin of safety that titanium Aluminum chosen to prevent over-designing the system More detailed design iterations can be used to choose between pinned or fixed connections 36
Landing Gear Dimensions Landing gear will be made of 6061 aluminum Landing vehicle will have three legs, placed 120o apart Lander leg radii results in shorter settling time but more massive structure Radius of 0.2 m chosen to try and minimize mass and settling time Thickness of walls is 4 mm Length of each leg is 2.83 m Leg length comes from requirement to fold leg into truss structure for storage 37
Landing Gear - Spring Mass Damper Analysis Position of spacecraft during landing driven by spring constant of system Effective stiffness of each landing gear leg determined by size and material [5] Effective spring constant of landing system is three times each leg's effective stiffness With effective spring constant, analysis of system can be performed [4] Assume system is critically damped [6] The value for the effective damping constant of the system is determined, which will affect how much honeycomb structure will be required for impact attenuation 38
Landing Gear - Spring Mass Damper Analysis Position of landing vehicle plotted vs time The spacecraft settles after a very short period of time (under 0.1 seconds) Less than 1.5 cm (negligible) bounce in either direction 39
Deployment of Landing Gear Landing gear designed so bottom lander leg folds in at vertex of tetrahedron and does not infringe on the engine Lander leg is equal in length to the horizontal difference between the vertex and side of the craft 40
Landing Vehicle Summary Tank size N2O4/MMH has oxidizer/fuel ratio of 2.16 by mass MN2O4: 6777 kg MMMH: 3137 kg VN2O4: 4.674 m3 VMMH : 3.565 m3 Assuming cylindrical propellant tanks with height 1.5 m rn2o4: 1.0 m rmmh: 0.87 m Engine size Maximum nozzle diameter is 3.57 m Maximum height of nozzle 1.785 m 41
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References 1) Akin, Dave. Lecture 18 Space Architecture, 2012. 2) Akin, Dave. Lecture 19 Structural Design and Analysis, 2012. 3) Akin, Dave. Lecture 20 Structural Design Practices, 2012. 4) Wereley, Norman. Lecture 2 Transient Response of a Mass-Spring System, 2012. 5) Bowden, Mary. Buckling. Structures Lecture Notes, 2012. 6) Lee, Sung. Lecture 2 Steady State Response, 2012. 7) Mulville, Daniel R. "Load Analyses of Spacecraft and Payloads Nasa Technical Standard." National Aeronautics and Space Administration, 21 June 1996. Web. 28 Nov. 2012. <https://docs.google. com/viewer?a=v&q=cache:baji2g27y9sj:https://standards.nasa. gov/documents/viewdoc/3314904/3314904+&hl=en&gl=us&pid=bl&srcid=adgeesi5o_fja5sa8euqrqw J0L-SyNje2AdpWDrI8yn4gBfIC_7HL9F0AQ-xHBVkZ98ejIEtFApOu3-zdwBYKpbkgvR9ShoGmc7Y5Fqu5nAk0Iy8Ud6qC7CLRlm6Ialsb1pNuzJerIS&sig=AHIEtbSbOdVJ3lMryc4pG8pfTa5Efu4k5Q>. 8) Griffin, Michael D., and James R. French. "Configuration and Structural Design." Space Vehicle Design. 2nd ed. Washington, DC: American Institute of Aeronautics and Astronautics, 1991. 395. Print. 9) "Encyclopedia Astronautica N2O4/MMH." N2O4/MMH. N.p., n.d. Web. 29 Nov. 2012. <http://www. astronautix.com/props /n2o4mmh.htm>. 44
References 10) "Grade 5 (6Al-4V, 3.7165, R56400) Titanium." - Material Properties Data. N.p., n.d. Web. 29 Nov. 2012. <http://www.makeitfrom.com/material-data/?for=grade-5-6al-4v-3.7165-r56400-titanium>. 11) "6061 (AlMg1SiCu) Aluminum." - Material Properties Data. N.p., n.d. Web. 29 Nov. 2012. <http: //www.makeitfrom.com/material-data/?for=6061-almg1sicu-aluminum>. 45