Structure

The mechanical structure is the backbone of the spacecraft. It touches every other subsystem, and has a lot of interfaces.

Typical Requirements

• Performance
  • Pointing and stability, orbit parametes
• Enviroment
  • Thermal
  • Outgassing and radiation
  • Loads
• Interface / subsystems
  • Alignments
  • Mechanical ICD
  • GoC (vs. launch vehicle), mass
• Programmatic

Loads

There are different kinds of loads:

• static
• dynamic
• external
• self-contained

On Earth: there is the manufacturing, assembly, ground operations, testing, transportation.
During Launch: flight limit loads, engine thrusts, sound pressure, gusts of wind
In Space: Pulsed thrust to reposition, micro vibrations

Launch Loads

Launch Loads

• (Quasi) static acceleration load
• Sine vibration load
• Random vibration load
• Shock load

Classification

Primary structures

• Determines whether the mechanical design is compliant with the mechanical requirements of the launcher
• Primary load path between spacecraft and launch vehicle
• Usually designed for stiffness or structural frequency
• Acceleration and transient loading during launch

Secondary structures

• Provide mounting provisions for the payload, units, solar arrays, antennas, etc.
• Often behave like independent substructures when deployed in orbit
• In-orbit sometimes high thermal loads, not as good protected as primary structure
• It transfers load to the primary structure

Tertiary structures

• Often a monolithic design
• High frequency base driven vibration causes the most severe loading
• Stiffness and positional stability are the main driving requirements
• Thermal or damping spacers sometimes mounted underneath

How to design a structural subsystem

Driven by following properties:

• Stiffness
• Dynamic response
• Strength (Load-cases)

Testing

Static Load Testing

• No permanent deformation and failure in strength
• Assess stiffness to correlate with FEA
• Qualifies the primary structure, structural connections, joints, interfaces
• Actuators apply loads and displacements and strains are measured

Sinusoidal modal survey testing (Low level sine sweep)

• Sine sweep (2 octave/min) at constant low acceleration from 20 to 2000 Hz
• Accelerometers capture structural responses vs. input and track any resonance.
• To assess natural frequencies but also damping
• Assess health of the test article by comparing response plots from pre-test and post-test sine sweeps.

Random vibration testing

• ensuring structures can withstand the mechanical stresses induced by acoustic vibration
• Shaker inject vibration at multiple frequencies simultaneously, with randomly varying acceleration
• Random acceleration is not predictable at any point in time.
• Defines the environment by its frequency content
• acceleration spectral density (ASD) or Power spectral density (PSD) as input

Shock testing

• Specified by Shock Respond Spectrum (SRS)
• The shock load environment is dependent on both the total mass of the space vehicle and the connection between the payload and the launch vehicle

Acoustic Testing

• Acoustic loads simulated in a reverberant acoustic chamber
• Frequency range between 20 and 10'000 Hz.

Materials

Factors that influence material selection:

• Mechanical Properties
  • Strength-to-Weight Ratio
  • E-Modul
  • Fatigue Resistance
• Thermal Properties
  • Thermal Expansion
  • Conductivity
• Environmental Resistance
  • Outgassing
  • Radiation resistance
  • Corrosion Resistance
• Manufacturability
  • Ease of processing
  • Ability to form complex shapes
  • Availability
  • Cost-effectiveness
• Lifecycle considerations
  • Sustainability
  • Recyclability