Structures and Mechanisms Subsystems AERSP 401A

Introduction to Structural Estimation Primary Structure: load-bearing structure of the spacecraft Secondary Structure: brackets, panels, deployables, and other non-load bearing components The Primary Structure is generally the most massive and is sized to handle launch loads

Introduction to Structural Estimation Structure size is driven by the larger of: –Strength: capacity of structure to withstand forces without breaking –Stiffness: capacity of the structure to resist flexible motion (e.g. vibration frequency) –Stability: capacity to resist buckling, permanent bending, or deformation

Introduction to Structural Estimation As a first estimate, the spacecraft’s structural weight is between 8% and 12% of the spacecraft injected weight (dry weight plus propellant)

Cassini Spacecraft (image courtesy of JPL)

Preliminary Design for Structures and Mechanisms Mission Launch Vehicles Environment Subsystem Requirements Envelope Accessibility Producibility Define Load Paths Construction Options Material Options Test Criteria Design Criteria Met Requirements? Iterate Detailed Design Identify Requirements Develop Packaging Configuration Consider Design Options Choose Test/Analysis Criteria Size Members No Yes

Requirement Sources Mission PhaseSource of Requirements Manufacturing and assembly Handling fixture or container reactions Stresses induced by manufacturing processes (welding) Transportation and handling Crane or dolly reactions Land, sea, or air transport environments Testing Environments from vibration or acoustic tests Prelaunch Handling during stacking sequence and pre-flight checks Launch and Ascent Steady-state booster accelerations Vibro-acoustic noise during launch and transonic phase Propulsion system engine vibrations Transient loads during booster ignition and burn-out, stage separations, vehicle maneuvers, propellant slosh, and payload fairing separation Pyrotechnic shock from separation events Mission Operations Steady-state thruster accelerations Transient loads during pointing maneuvers and attitude control burns or docking events Pyrotechnic shock from separation events, deployments Thermal environments Reentry and landing (if applicable) Aerodynamic heating Transient wind and landing loads

Launch Load Terms Terms: –Load factor: maximum acceleration in specified direction –Limit load: maximum expected acceleration –Yield load: load at which structural member suffers permanent deformation –Ultimate load: load at which structural member fails –Safety factor: ratio of ultimate load to limit load –Safety margin: safety factor minus one –Yield factor: ratio of yield load to limit load –Yield margin: yield factor minus one –Uncertainty factor: ratio of design load to limit load ELVs: –Ultimate load = 1.25 x limit load –Yield load = 1.0 x limit load or sometimes 1.1 x limit load Space Shuttle –Ultimate load = 1.4 x limit load

Estimating Size and Mass of Spacecraft Structure StepDescriptionReferences 1Select a structural approach by identifying the type of structure Chapters 9, 10, Section Estimate mass distribution for all equipment and the structure, including the booster adapter Section Estimate size and mass of structural members using information from steps (1) and (2) and the axial and bending frequencies for the selected booster. Iterate as required Chapter 18, Section Combine loads (axial, lateral, and bending) to determine distribution of load concentration. Section Compute the structural capability and compare with the applied loads to determine the margin of safety. Iterate the design as required to obtain the necessary margin of safety. Sections ,

Estimating Mass of the Structure Obtain fundamental axial and lateral vibration frequencies from launch system Compute area moments of inertia Determine thickness required to meet load requirements Check stability Select minimum thickness that meets stiffness, strength, and stability criteria Strength Stiffness

Choose the Materials Strength Stiffness Density (weight) Thermal conductivity Thermal expansion Corrosion resistance Cost Ductility Fracture toughness Ease of fabrication Versatility of attachment options Availability

Tools Example problem in Section Structural modeling Beam analysis Frequency analysis Load analysis Material selection