Structural Design Practices ENAE 483/788D - Principles of Space Systems Design U N I V E R S I T Y O F MARYLAND Structural Methods and Materials • Excel Solver Routine to Minimize Margins • Load Paths • Octave Rule • Aerospace Structural Materials – Strength Design – Stiffness Design – Buckling Design 1
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Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
Structural Methods and Materials• Excel Solver Routine to Minimize Margins• Load Paths• Octave Rule • Aerospace Structural Materials
The structure is over-designed by 51%… it’s 51% heavier than necessary
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
Observations about Launch Loads• Individual loads could be applied to same position
on canister at same times - conservative approach is to use superposition to define worst case
• 51% margin indicates that canister is substantially overbuilt - if launch loads turn out to be critical load case, redesign to lighten structure and reduce mass.
• Did we consider all the cases? What about buckling?
13
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
Load Path
14
• The stiffer structural member carries the greater part of the loading1. Honeycomb penthouse deck
attached to outer structure via flat plate, supporting instrument electronics
2. Flat plat offers little resistance to bending
3. Thrust load path is predominantly through the center structure (vertical plate)
• Control load paths by controlling stiffness
V.L. Pisacane, Fundamentals of Space Systems, 2nd ed., Oxford University Press, 2005
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
Dynamic Interactions
15
• Every structure has a fundamental resonant frequency
• Use this to control load paths by controlling stiffness
R. Stengel, Space System Design, Princeton University
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
Transmissibility
16
R. Stengel, Space System Design, Princeton University
Transmissibility or “Q Factor” is the response to sinusoidal oscillation at different frequencies, 𝑇𝑇 = 𝑓𝑓 𝜔𝜔
𝜔𝜔𝑛𝑛
Assuming component frequency 𝜔𝜔 = 𝜔𝜔𝑖𝑖, and structural damping 𝜁𝜁 = 0.05
𝑇𝑇 =𝑥𝑥𝑛𝑛𝑇𝑇𝑓𝑓𝑥𝑥𝑖𝑖𝑖𝑖
= 1 −𝜔𝜔𝜔𝜔𝑖𝑖
2
+ 2𝜁𝜁𝜔𝜔𝜔𝜔𝑖𝑖
2
→12𝜁𝜁
= 10
At twice the natural frequency, 𝜔𝜔𝜔𝜔𝑛𝑛
= 2 , T=0.33
Keeping the component frequency an octave ( 𝜔𝜔𝜔𝜔𝑛𝑛
= 2)above the mounting structure’s 𝜔𝜔𝑖𝑖 reduces transmissibility by 66%... Enough to assume input won’t be amplified
𝜔𝜔𝑖𝑖 =𝑘𝑘𝑚𝑚
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
The Octave Rule
17
• “Rule” applied to ensure the interaction between spacecraft components and their mounting structure is minimized.
• General loads estimate applicable to components and secondary structures
V.L. Pisacane, Fundamentals of Space Systems, 2nd ed., Oxford University Press, 2005
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
What changed between the Falcon 9 v1.0 and v1.1?
18
The Falcon 9 v1.1 uses the vehicle’s skin to resolve the vertical thrust loads, avoiding the need for specialized thrust structures (like in the tic-tac-toe of v1.0).
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
S/C stiffness requirements for ELVs (minimum spacecraft fundamental frequency to avoid resonance w/ launch vehicle)
19
V.L. Pisacane, Fundamentals of Space Systems, 2nd ed., Oxford University Press, 2005
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLAND
Creep in Composite Structures
26
An important factor when choosing the type of reinforced composite for a structural application is understanding the limits of a fiber to resist long term loading.
Continuous and cyclic loading on a fiber reinforced polymer in excess of its ability to resist those loads may induce long-term deflection, fatigue failure, or creep-rupture in the structural component.
To eliminate the deflections caused by creep, the stresses in FRP reinforcement in structural members must be less than the creep-rupture stress limit.
Carbon FRPs have a much greater useable strength after the application of the reduction factor, equating to less material and less mass.
•American Concrete Institute (ACI) Committee 440, 440.6-08 "Specification for Carbon and Glass Fiber-Reinforced Polymer Bar Materials for Concrete Reinforcement," 2008•Prince Engineering, PLC, "Characteristics and Behaviors of Fiber Reinforced Polymers (FRPs) Used for Reinforcement and Strengthening of Structures," 2011
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLANDR. Stengel, Space System Design, Princeton University
Structural Design PracticesENAE 483/788D - Principles of Space Systems Design
U N I V E R S I T Y O FMARYLANDR. Stengel, Space System Design, Princeton University