Lecture 5 - Materials and Structural design

AE1110-II Introduction to Aerospace Engineering - Materials

Structural performance

Function of material properties and geometrical properties

e.g. Equal material and variation in geometry could have the same effect as variation in material and equal geometry

Characteristic Material Values

Specific material strength: σ_u/ρ

Specific bending material 3rt(σ_y²)/ρ

Column stability: sqrt(E)/ρ

Sheet stability: 3rt(E)/ρ

Minimising weight for given stiffness:

Vertical load: 3rt(E)/ρ

Horizontal load: 3rt(E)/ρ

Torsion/pressure: E/ρ

Hoop pressure: E/(1-v)ρ

Minimizing weight for given strength:

Horizontal load: sqrt(σ)/ρ

Torsion: σ/ρ

Structural design

Design change related to geometry (to lower weight at same strength)

Design change related to improved materials (e.g. wood, fabric, steel tube truss to load bearing aluminium skin structure

Typical spacecraft structures

Spacecraft:

• Struts

• Polar platform

Launch vehicle:

• Fairings

• Stage structure

• Thrust structure

• Adaptors

Characteristics of the polar platform

Central thrust-load-bearing member (cone/cylinder)

All systems attached at strong points directly, or by combination of struts/platforms/shear

Mission requirements (space)

Minimum mass

High stiffness

High strength (to withstand the loads)

Accomodate payload and equipment

High reliability

Low cost

Accessibility, Manufacturability

Material selection criteria: natural frequency (spacecraft)

Limiting the natural frequencies of spacecraft is essential to avoid resonance between launch vehicle and spacecraft

Low dynamic coupling results in lower loads for spacecraft

Oscillations can be damped or excited (damped is needed)

Dimensioning the primary spacecraft structure

First: Lowest natural frequencies → minimum required natural frequencies

Then: Design for quasi static loads