Coupled Aero-Structural Modelling and Optimisation of Deployable Mars Aero-Decelerators Lisa Peacocke, Paul Bruce and Matthew Santer International Planetary Probe Workshop 11-15 June 2018 Boulder, CO, USA
Deployable Aero-Decelerators Enable large masses to be delivered to Mars surface Also enable higher elevation landing sites and more precise landing Other advantages Can be deployed and restowed Resilient to micrometeoroid impact Can withstand dual heat pulse Could enable guidance by individual control of ribs Could use ribs as landing gear Cassell et al. (2017) Savino et al. (2015) Wiegand & Konigsmann (1996) Akin (1990) Venkatapathy et al. (2011)
Mass Estimation Widely varying mass assessments for all concepts 8% - 46% of entry vehicle mass Different margin assumptions Hard to compare against inflatables and rigid bodies Robust mass estimates are key for determining performance A coupled aero-structural tool will improve deployable rib mass estimation process Enables assessment of different architectures/concepts Aero-Decelerator Mass Fraction 50% 45% 40% 35% 30% 25% 20% 15% 10% 5% 0% Ø6 m, Yount Ø2 m, Skidbladnir Ø5.7 m, Underwood Ø8.5 m, Trabandt Ø16 m, Cassell 0 20,000 40,000 60,000 80,000 100,000 120,000 Entry Mass [kg] Ø23 m, Venkatapathy Ø22 m, Braun
Coupled Aero-Structural Model 6DOF entry trajectory simulator Geometry mesh of any shape/size European Mars Climate Database Modified Newtonian method Equations of motion integrated Aerodynamic forces & coefficients updated at each timestep + Structural model of deployable ribs Aerodynamic forces across TPS summed and applied to rib nodes Euler-Bernoulli beam model Numerical integration method Individual ribs deform separately Updated shape passed back Undeformed Mesh Mesh at Peak Deformation
Correlation and Validation Trajectory Simulator Correlated against results from internal Airbus tool BL43 Schiaparelli-based rigid entry vehicle Validated against published NASA flight data Structural Model Correlated against deflection results from Abaqus FEA model 5% deflection error with mesh points > 15 along rib length Industry Tool MER Spirit
Reference Mission Mission Surface Payload Stowed Diameter Entry Strategy Entry Velocity Human Cargo 20 tonnes 4.5 m Direct entry from transfer trajectory 6 km/s Descent Strategy Supersonic retropropulsion at Mach 3.5 above 3 km altitude Landing Site Elevation 0 km MOLA 16 m diameter 70 rib angle Credit: NASA 4.5 m rigid nosecone 6.1 m deployable rib
Deformation Animations Reference Mission 16 m Diameter with Realistic Rib Design Unbalanced Forces with Highly Flexible Rib Design Variable parameters include: All 6DOF trajectory initial conditions Entry vehicle size and shape Number of ribs Rib cross-section, dimensions and material properties Support strut location Payload centre of gravity
120,000 Rib Stiffness Variation 100,000 80,000 Rigid Case 2 Case 4 Case 6 Case 7 Case 8 Varied bending stiffness of ribs!" range: 4-84 10 6 Nm 2 Reference Human Cargo mission assumed Clear effect on drag coefficient Only very flexible ribs show significant effect on trajectory!" 7 10 6 Nm 2 25% higher velocity at 10 km 7% increase in peak heat flux 13% decrease in peak g-load Altitude [m] Altitude [m] 60,000 40,000 20,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000 0 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Drag Coefficient Rigid Case 2 Case 4 Case 6 Case 7 Case 8 Most Flexible Case Rigid Case Most Flexible Case Rigid Case 0 0 1000 2000 3000 4000 5000 6000 7000 Velocity [m/s]
Rib Stiffness Variation Increasing rib flexibility damps attitude oscillations more effectively New deformed shape is more stable e.g. similar to 45 spherecone having greater stability Flexibility alone does not lead to beneficial effects on trajectory Angle of Attack [degrees] 1 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6 Case 2 Case 4 Case 6 Case 8 0 50 100 150 200 250 Time [s]
Rib Tapering Effect Mass savings from flexible tapered ribs => increase entry vehicle diameter Maintained entry vehicle mass Balanced decreased rib mass with increased TPS mass Beneficial trajectory effect Larger diameters decelerate more effectively at higher altitudes Lowers peak heat flux significantly (42 => 30 W/cm 2 ) Reallocating the mass gained from flexibility is very beneficial Altitude [m] 70,000 60,000 50,000 40,000 30,000 20,000 10,000 0 Diameter 16 m, No Taper Diameter 17 m, Taper Ratio 0.75 Diameter 19 m, Taper Ratio 0.5 Diameter 21 m, Taper Ratio 0.25 Peak G-Load Peak Heat Flux 21 m diameter, 0.25 taper ratio 16 m diameter, no taper 0 1000 2000 3000 4000 5000 6000 7000 Velocity [m/s]
120,000 Number of Ribs 100,000 80,000 8 Ribs 10 Ribs 12 Ribs 14 Ribs 16 Ribs 18 Ribs Maintained total rib mass by balancing rib size/stiffness with number of ribs Very large effect on trajectory Drag coefficient varies significantly Fewer stiffer ribs deform less but give lower drag coefficient initially Prefer larger number of more flexible ribs to a limit e.g. 16 ribs in this case Optimise number of ribs for each specific mission more flexible ribs generally preferred Altitude [m] Minimum Drag Coefficient 8 Ribs 18 Ribs 60,000 40,000 20,000 0 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7 Drag Coefficient 1.54 1.52 1.5 1.48 1.46 1.44 1.42 1.4 0 2 4 6 8 10 12 14 16 18 20 Number of Ribs
Support Strut Location Strut can be located at any point along deployed element Investigated for one rib design case Improvement in drag coefficient with strut distance from hinge Minor (< 3%) change in peak heat flux, 700 g-load, velocity at 10 km 600 => Strut location should be based on 500 maximum principal stress 400 Ensure material yield strength including 300 safety factor is not exceeded 200 Optimise with rib flexibility for lowest 100 0 mass design Minimum Drag Coefficient Maximum Principal Stress [MPa] 1.7 1.65 1.6 1.55 1.5 1.45 1.4 1.35 1.3 Decreasing deformation 0 1 2 3 4 5 6 Support Strut Radius [m] 0 1 2 3 4 5 6 Support Strut Radius [m]
Conclusions and Next Steps Aero-structural simulator tool developed to assess deployable aerodecelerator concepts and improve mass estimates Continue using tool to investigate variables and optimise designs Flexible deployable ribs are beneficial if resulting mass savings are reallocated to increase vehicle diameter Decreases peak heat flux significantly Attitude damping increases with flexibility Number of ribs has a large effect on the drag properties and must be optimised for each mission Next steps: validation of aero-structural effects via experiment Lab-scale test to investigate TPS flexure/wrinkling as ribs deform High-speed wind tunnel test to investigate stability
Backup Slides
Mesh Convergence 0.085 Matlab 0.08 Abaqus Converged Solution Maximum Deflection [m] 0.075 0.07 0.065 0.06 0 20 40 60 80 100 120 140 No. of Mesh Elements