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Structures and Mechanisms Subsystems

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

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Structures and Mechanisms Subsystems

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  1. Structures and Mechanisms Subsystems AERSP 401A

  2. 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

  3. 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

  4. 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)

  5. Cassini Spacecraft(image courtesy of JPL)

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

  7. Requirement Sources

  8. 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

  9. Estimating Size and Mass of Spacecraft Structure

  10. 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

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

  12. Tools • Example problem in Section 11.6.7 • Structural modeling • Beam analysis • Frequency analysis • Load analysis • Material selection

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