Transcript 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
• 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
Identify
Requirements
• Mission
•Launch
Vehicles
•Environment
Develop
Packaging
Configuration
• Subsystem
Requirements
Consider
Design
Options
• Construction
Options
Choose
Test/Analysis
Criteria
• Test
Criteria
•Envelope
•Material
Options
•Design
Criteria
•Accessibility
Size
Members
•Producibility
•Define Load
Paths
Iterate
No
Met
Requirements?
Yes
Detailed
Design
Requirement Sources
Mission Phase
Source of Requirements
Testing
• Handling fixture or container reactions
• Stresses induced by manufacturing processes (welding)
• Crane or dolly reactions
•Land, sea, or air transport environments
• 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
• Steady-state thruster accelerations
• Transient loads during pointing maneuvers and attitude control burns or docking
events
• Pyrotechnic shock from separation events, deployments
• Thermal environments
• Aerodynamic heating
• Transient wind and landing loads
Manufacturing
and assembly
Transportation
and handling
Mission
Operations
Reentry and
landing (if
applicable)
Launch Load Terms
• Terms:
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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
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
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
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Strength
Stiffness
Density (weight)
Thermal conductivity
Thermal expansion
Corrosion resistance
Cost
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Ductility
Fracture toughness
Ease of fabrication
Versatility of
attachment options
• Availability
Tools
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Example problem in Section 11.6.7
Structural modeling
Beam analysis
Frequency analysis
Load analysis
Material selection