Structure & Thermals
“Usually the Spacecraft Structure is the subsystem for which requirements are available last, but the physical element that is needed first when the spacecraft has to be assembled”
Structure
Requirements Flow down
- Mission Requirements: Mission life, orbit & Payload specifications
- System Requirements: Satellite bus, propulsion, avionics, deplorables
- Structural Subsystem Requirements: Interface loads, materials, alignment, mass, Center of Gravity
Types of Requirements
- Performance: Pointing, stability, orbit parameters
- Environment: Thermal, outgassing, radiation, loads (static, dynamic)
- Interface: Alignments, mechanical Interface Control Document (ICD) CoG, mass
- Programmatic: Schedule and cost
Static Loads
- Acceleration from Launch
- Thermal Loads
Dynamic Loads
- Sine Vibrations: Rocket
- Random Vibrations: Acoustic Pressure
- Shock: High frequency - pyrotechnics (separation)
Structural Calssification:
- Primary: Main load path to launch vehicle, designed for stiffness and strength. Examples include satellite bus, launch vehicle adapter.
- Secondary: Mounting for payloads, solar arrays, antennas. They behave like sub-structures when deployed in orbit. Examples include appendage booms, support trusses.
- Tertiary: Often monolithic, driven by stiffness and positional stability, and are often sensitive to base-driven vibration. Examples include brackets, component housings.
Primary Structures
Satellite bus, launch vehicle adapter
Secondary Structures
appendage booms, support trusses, platforms, solar panels, antenna dishes
Tertiary Structure
Brackets, component housing, electronic boxes, cable supports
Design Process
- Initial Configuration: Conceptual design, system-level.
- Preliminary Design: Trade studies, material selection.
- Detailed Analysis: Sizing components, creating drawings.
- Production: Manufacturing and supply chain.
- Testing: Qualification, acceptance, and workmanship tests.
Materials
- Requirements: Must be strong, lightweight, thermally stable, and resistant to space conditions.
Mechanical Properties:
- Strength-to-weight ratio
- Young’s modulus
- Fatigue resistance
Thermal Properties:
- Thermal expansion coefficient (CTE)
- Thermal conductivity
Environmental Resistance:
- Outgassing
- Radiation resistance
- Corrosion resistance
- Common Materials:
- Aluminium: Lightweight, good conductivity
- Titanium: High strength, lower CTE
- Polymer composites (CFRP, GFRP): Lightweight, customizable, used for complex shapes like sandwich panels
- Steel and Ni/Cr superalloys: High strength for bolts and high-load structures
- Ceramics: Thermal protection systems (TPS)
Testing
- Static Load Testing: Applying loads to measure structural response.
- Modal Survey: Sine sweep tests to determine natural frequencies and damping.
- Acoustic Testing: Simulating launch noise in a reverberant chamber.
- Random Vibration Testing: Using a shaker to simulate random vibrations.
- Shock Testing: Simulating pyrotechnic shocks.
Thermals
Purpose: Maintain temperatures of all spacecraft components within their designed operating ranges.
Heat Transfer Mechanisms
- Conduction: Heat transfer within a material or between materials in contact.
- Convection: Heat transfer through fluid movement, not present in space.
- Radiation: Heat transfer via electromagnetic radiation.
Thermal Balance
- Equilibrium between energy absorbed and dissipated by the spacecraft.
Radiant Energy
- Direct Solar Flux: Energy from the sun, dependent on distance and orientation.
- Albedo: Reflected solar energy from a planet.
- Planetary Infrared Energy: Infrared radiation from a planet.
- Radiated Energy: Heat emitted from the spacecraft into space.
—> Earth reflects 25-55% of the incident solar energy
Thermal Control Hardware
- Passive:
- Multi-Layer Insulation (MLI): Layers of low-emittance film with thermal spacers.
- Conductive Isolators: Thermal stand-offs made of non-metallic material.
- Thermal Fillers: Conductive materials between surfaces.
- Surface Coatings: Materials with specific absorptivity (α) and emissivity (ε) values.
- Radiators: External panels to reject waste heat, often with mirrors (OSR).
- Active:
- Heaters: Electrical resistance elements for generating heat.
- Heat Pipes: Devices that transfer heat via a two-phase process using capillary action.
—> needs to be tested in vacuum chamber to simulate space thermal environment
Memorize:
- Natural Frequency: Frequency at which structure resonates. Important to avoid matching the natural frequency of the spacecraft with frequencies of the launcher.
- Stiffness: Measure of a Structure’s resistance to deformation: Force/Displacement
Structural Subsystem
- Loads: Static, Sine Vibration, Random Vibration, Shock
- Key Structural Properties: Stiffness, Natural Frequency, Mass, CoG
- Primary Structure: Main load-bearing, stiffness, launch vehicle adapter.
- Secondary Structure: Mounting, deplorables.
- Tertiary Structure: Brackets, component housings, positional stability, high frequency base-driven vibrations.
- ICD: Interface Control Document.
- FEA: Finite Element Analysis.
- Structural Testing: Static load, modal survey (sine sweep), acoustic, random vibration, shock.
- CTE: Coefficient of Thermal Expansion.
Thermal Subsystem Summary
- Heat Transfer: Conduction, radiation (no convection in space).
- Thermal Balance: Energy absorbed = energy emitted.
- Radiant Energy Sources: Direct solar, albedo, planetary infrared, radiated to space.
- MLI: Multi-Layer Insulation.
- Absorptivity (α): Measures energy absorption from solar radiation.
- Emissivity (ε): Measures energy emitted in the infrared spectrum.
- Heat Pipes: Passive devices using two-phase flow for heat transfer.
- OSR: Optical Solar Reflector.
- Thermal Testing: Vacuum chamber tests.