Structures and Mechanisms Subsystems

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 spacecrafts 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

Met Requirements?
No
Iterate
Yes
Detailed Design
7
Requirement Sources
Mission Phase Source 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
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
Step Description References
1 Select a structural approach by identifying the type of structure Chapters 9, 10, Section 11.6.2
2 Estimate mass distribution for all equipment and the structure, including the booster adapter Section 11.6.8
3 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 11.6.8
4 Combine loads (axial, lateral, and bending) to determine distribution of load concentration. Section 11.6.7
5 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 11.6.7, 11.6.8
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
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

12
Tools

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