Structural Design and Analysis Principles of Space Systems Design U N I V E R S I T Y O F MARYLAND Structural Design • Loads and Load Sources – Designing or Critical Loads – Load Information / Estimation • Piece Parts Analysis – Margin of Safety Definition – Factors of Safety to use – Summary Table • Important Structural Concepts – Primary/Secondary Structure – Failsafe & Fracture Critical Structure – Aerospace Materials – Structural Failure
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Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
MARYLAND
Structural Design• Loads and Load Sources
– Designing or Critical Loads– Load Information / Estimation
• Piece Parts Analysis– Margin of Safety Definition– Factors of Safety to use– Summary Table
Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
MARYLAND
Loads• "Designing Load" is the load that determines one or
more structural characteristic of the part:– shape, thickness, strength, stiffness, material...
• Critical Load (somewhat synonymous) is more exactlythe load that gives the minimum margin of safety (MS)for a part
– MS represents the amount of extra structural capability youhave over the applied load (elbow room)
• Examples of Critical Loads– pressurization loads for a rocket casing– launch loads for a spacecraft– thermal loads for a propulsion subsystem– crash loads for a car
Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
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Load Sources• Where do these loads come from?• For every part (subsystem) in your design, you should
review every phase of its life and identify all loads thathave the potential to be critical:
– manufacturing & assembly– test (qualification, proof test)– transportation (truck or launch)– operation– contingencies (crash landing)
• Obtain or estimate loads– look up loads in reference books– ask other groups to determine loads– guestimate for the purposes of starting analysis
• Calculate all margins of safety
Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
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Launch Vehicle Loads• Max Q - Aerodynamic Loads
– Q = ρ V2 / 2– maximum pressure and bending on vehicle
• Max g's– usually occurs at stage burnout– maximum axial load on vehicle and payload
– tight turn– driving on an incline– loosing traction / support on one wheel
• Crash loads– driving into a big boulder– rolling vehicle in unstable soil– safety is primary consideration
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Piece Parts Analysis• Structural analysis of a system consists of at least
the following three tasks– Load Cycle Modeling (system-level) - iterative process– Piece-Part Analysis (static) - minimum margins of safety– Fracture and Fatigue Analysis (dynamic) - safe life
analysis
• Piece Parts Analysis– Identify all loads on each part / subsystem– Calculate margins of safety– Tabulate minimum margins of safety
• Example: OTD Boom Piece Parts Analysis
Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
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• Limit Loads: maximum loads expected (applied loads)
• Yield Load and Ultimate Load
• Factors of Safety : numbers imposed by the Customer(or your own good sense) that reflect� how uncertain you are of the load or structure� how safe you want to be� examples: 10 for bridges, 5 for ground handling equip, 2 for a/c
• Margins of Safety are calculated as follows:
• Beware: There are other definitions of these terms in engineering, but theabove approach is the most common in Aerospace
MSAllowable Load
Applied Load x FS
.= − 1 0
Factors & Margins of Safety
Structural Design and AnalysisPrinciples of Space Systems Design
– Catastrophic failure is generally defined by customer– Failsafe structure can take redistributed loads after failure (ie, not
single point failures); shall release no hazardous mass; shall not changedynamics significantly; shall have no fatigue problems
– Low-risk structure is not primary structure; has only a remotepossibility of failure; will not propagate a crack in 4 lifetimesσmax < Ftu / [ 4 (1-0.5 R) Kt ]
– Fracture critical parts must be labeled and analyzed as such, theninspected, treated, and tracked more carefully than conventional parts
• Crack Growth Analysis (FLAGRO)– All FC parts must be shown good for four lifetimes of load cycles with an
initial flaw (determined by NDI)
Structural Design and AnalysisPrinciples of Space Systems Design
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Aerospace Materials• Comparison of specific stiffness, specific
strength, and buckling parameter for a variety ofaerospace metals and composites
• Definition of Structural Failure– Detrimental Yield vs Textbook Yield
• deformation that detrimentally affects functionality ofsystem
• 0.2% Tresca yield condition (assumes system linear infirst place)
– Ultimate Failure• any material rupture or loss of functionality
Structural Design and AnalysisPrinciples of Space Systems Design
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Material Strength & Stiffness• Typical Yield & Ultimate Strengths
Conclusion: for aerospace structures - titanium andaluminum
Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
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Structural Analysis
• Some key structural formulas that are handy tohave for early (back-of-the-envelope) designanalyses:
– Spring & Beam Stiffnesses– Beam Natural Frequencies– Euler Buckling Loads– Stresses in Simple Pressurized Shell
σhoop = p R / t ; σlong = p R / 2 t
– Random Vibe and Acoustic Equivalent g's
Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
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Structural Analysis
• Definition of Example Problem• Definition of Load Cases• Analysis of Stresses• Tabulation of Margins of Safety• Identification of Critical Load Case
Structural Design and AnalysisPrinciples of Space Systems Design
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Structural Example
• Storage canister forISS solar arraydeployment system
• 200 lb tip mass• Cantilever launch
configuration• Thin-wall aluminum shell
structure
Structural Design and AnalysisPrinciples of Space Systems Design
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Loads Sources
• Launch– Accelerations– Pressurization– Acoustics– Random Vibration– Thermal
• Crash Landing• On-Orbit
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Structural Parameters
E psi= ×1 107 α = ×⋅°
−13 10 6 inin F
ρ = 0 10 3.lbsinI R R R t ino i= −( ) ≅ =
ππ
448004 4 3 4
R in= 25 l =100 in t in= 0 10.
W tR lbscanister = =2 157πρ l W lbstip = 200
σ Ty ksi= 37 σ Tu ksi= 42
A Rt in= =2 15 71 2π .g
in= 386 4 2.
sec
Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
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Launch Accelerations±4.85 g
±5.8 g
±8.5 g
FOS =1 4.
σ LAtip
x
MRI
W
Ag= +
M g W h W htransverse canister CG tip tip= +( )g gtransverse = + =5 8 8 5 10 32 2. . .
M in lb= ⋅ + ⋅( ) = ⋅10 3 157 50 200 100 286 900. ,
σ LA
inlb inin
lbin
= +( )( )
..
286900 254800
20015 71
4 854 2
σ LA psi= + =1494 61 75 1556.
Structural Design and AnalysisPrinciples of Space Systems Design
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Pressurization LoadsFOS = 3 0.
σ Hoop
PRt
psi inin
psi= = =( . )( )
.14 7 25
0 13675
σ Longitudinal
PRt
psi= =2
1838
Structural Design and AnalysisPrinciples of Space Systems Design
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Launch Vehicle Vibration Environment
0.001
0.01
0.1
1
1 1 0 100 1000 10000
Frequency (Hz)
Power Spectral Density(g2/Hz)
Structural Design and AnalysisPrinciples of Space Systems Design
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Random Vibration LoadsFOS = 3 0.
RLFf PSD
nn=
πξ4
f
EIgW W
Hztip canister
1 3 3
1 7322 0 236
80=+
=.
.π l lfn ξ
<150 Hz .045
150-300 Hz .020
>300 Hz .005
RLF g= 7 93.
(repeat for each axis)
M in lbs= ⋅220 700,
σ RV
MRI
psi= =1150
Structural Design and AnalysisPrinciples of Space Systems Design
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Thermal LoadsFOS =1 4.
∆ ∆l l= ⋅ ⋅α T
σ εThermal E psi= = ⋅×
=105 13100
65007 . .
∆l = × ⋅− ° ⋅ =−13 10 100 100 136 F in.
Assume support structure shrinks only half asmuch as canister
Structural Design and AnalysisPrinciples of Space Systems Design
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Launch Loads SummaryLoad Source Limit
StressesFOS Design
StressLaunch
Accelerations1556 1.4 2178
Pressurization 3675 3.0 11,025
RandomVibration
1150 3.0 3450
Thermal 6500 1.4 9100
Total 25,750 psi
MSAllowable Load
Design Load= − = − =1
37 00025 750
1 43 7,,
. %
Structural Design and AnalysisPrinciples of Space Systems Design
U N I V E R S I T Y O F
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Observations about Launch Loads
• Individual loads could be applied to sameposition on canister at same times -conservative approach is to usesuperposition to define worst case
• 43% margin indicates that canister issubstantially overbuilt - if launch loadsturn out to be critical load case, redesignto lighten structure and reduce mass.