Spacecraft structures

Report
[F&S, Chapter 8]
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Function: the Spacecraft’s ‘skeleton’.
Prinipal design driver: minimise mass without
compromising reliability.
Design aspects:
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Materials selection
Configuration design
Analysis
Verification testing (iterative process).
Generalised requirements
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Must accommodate payload and spacecraft
systems
◦ Mounting requirements etc.
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Strength
◦ Must support itself and its payload through all phases of
the mission.
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Stiffness (related to strength)
◦ Oscillation/resonance frequency of structures (e.g.
booms, robotic arms, solar panels).
◦ Often more important than strength!
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Environmental protection
◦ Radiation shielding (e.g., electromagnetic, particle)
for both electronics and humans.
◦ Incidental or dedicated
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Spacecraft alignment
◦ Pointing accuracy
◦ Rigidity and temperature stability
◦ Critical for missions like Kepler!
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Thermal and electrical paths
◦ Material conductivity (thermal and electrical)
◦ Regulate heat retention/loss along conduction
pathways (must not get too hot/cold).
◦ Spacecraft charging and its grounding philosophy
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Accessibility
◦ Maintain freedom of access (docking etc.)
For OPTIMUM design require careful materials
selection!
Materials selection
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Specific strength is defined as the yield
strength divided by density.
◦ Relates the strength of a material to its mass (lead
has a very low specific strength, titanium a high
specific strength).
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Stiffness (deformation vs. load)
Stress corrosion resistance
◦ Stress corrosion cracking (SCC).
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Fracture and fatigue resistance
◦ Materials contain microcracks (unavoidable)
◦ Crack propagation can lead to total failure of a
structure.
◦ Extensive examination and non-destructive testing
to determine that no cracks exists above a specified
(and thus safe) length.
◦ Use alternative load paths so that no one structure
is a single point failure and load is spread across
the structure.
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Thermal parameters
◦ Thermal and electrical conductivity
◦ Thermal expansion/contraction (materials may
experience extremes of temperature).
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Sublimation, outgassing and erosion of materials
(see previous lecture notes).
Ease of manufacture and modification
◦ Material homogeneity (particularly composites - are
their properties uniform throughout?).
◦ Machineability (brittleness - ceramics difficult to work
with)
◦ Toxicity (beryllium metal).
Elements (‘refractories’)
Symbol
Melting Pt. (K)
Boiling Pt. (K)
Density (kg m-3)
Carbon (diamond)
C
3820
5100 (s)
3513
Tungsten
W
3680
5930
19300
Rhenium
Re
3453
5900
21020
Osmium
Os
3327
5300
22590
Tantalum
Ta
3269
5698
16654
Molybdenum
Mo
2890
4885
10200
Niobium
Nb
2741
5015
8570
Iridium
Ir
2683
4403
22420
Ruthenium
Ru
2583
4173
12370
Boron
B
2573
3931
2340
Hafnium
Hf
2503
5470
13310
Technicium
T*
2445
5150
11500
Rhodium
Rh
2239
4000
12410
Vanadium
V
2160
3650
6110
Chromium
Cr
2130
2945
7190
Zirconium
Zr
2125
4650
6506
Protactinium
Pa
2113
4300
15370
Platinum
Pt
2045
4100
21450
Materials:
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Stainless steel used (where possible) to
1200K
Refractory elements and alloys used to 1860K
Refractory elements formed into borides,
carbides, nitrides, oxides, silicides (e.g.,
boron carbide, tungsten carbide, boron
nitride).
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Spacecraft structure design requires a very
careful selection of materials based upon
their strength, thermal properties, electrical
properties, strength, stiffness, toxicity and
shielding ability.
The overriding concern is weight! Weight =
cost and need to minimise WITHOUT
sacrificing functionality. Careful design and
construction needed.
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Most spacecraft materials are based on
conventional aerospace structural materials
(similar weight/strength requirements).
Some new ‘hi-tech’ materials are employed
where necessary (honeycombs, beryllium
alloys etc.) not found elsewhere.
Aluminium (and its alloys)
ρ=2698 kg m-3, melting point=933.5 K
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Most commonly used conventional material (used
for hydrazine and nitrous oxide propellant
tanks).
Low density, good specific strength
Weldeable, easily workable (can be extruded,
cast, machined etc).
Cheap and widely available
Doesn’t have a high absolute strength and has a
low melting point (933 K).
Magnesium (and its alloys)
ρ=1738 kg m-3, melting point=922 K
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Higher stiffness, good specific strength
Less workable than aluminium.
Is chemically active and requires a surface
coating (thus making is more expensive to
produce).
Titanium (and alloys)
ρ=4540 kg m-3, melting point=1933 K
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Light weight with high specific strength
Stiff than aluminium (but not as stiff as steel)
Corrosion resistant
High temperature capability
Are more brittle (less ductile) than
aluminium/steel.
Lower availability, less workable than aluminium
(6 times more expensive than stainless steel).
Used for pressure tanks, fuels tanks, high speed
vehicle skins.
Ferrous alloys (particularly stainless steel)
ρ =7874 kg m-3, melting point (Fe)=1808 K
 Have high strength
 High rigidity and hardness
 Corrosion resistant
 High temperature resistance (1200K)
 Cheap
 Many applications in spacecraft despite high
density (screws, bolts are all mostly steel).
Austenitic steels (high temperature formation)
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Non-magnetic.
No brittle transition temperature.
Weldable, easily machined.
Cheap and widely available.
Susceptible to hydrogen embrittlement
(hydrogen adsorbed into the lattice make the
alloy brittle).
Used in propulsion and cryogenic systems.
Beryllium (BeCu)
ρ=1848 kg m-3, melting point=1551 K
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Stiffest naturally occurring material (beryllium
metal doesn’t occur naturally but its compounds
do).
Low density, high specific strength
High temperature tolerance
Expensive and difficult to work
Toxic (corrosive to tissue and carcinogenic)
Low atomic number and transparent to X-rays
Pure metal has been used to make rocket
nozzles.
Other alloys
 ‘Inconel’ (An alloy of Ni and Co)
◦ High temperature applications such as heat shields
and rocket nozzles.
◦ High density (>steel, 8200 km m-3).
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Aluminium-lithium
◦ Similar strength to aluminium but several percent
lighter.
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Titanium-aluminide
◦ Brittle, but lightweight and high temperature
resistant.
Refractory metals:
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Main metals are W, Ta, Mo, Nb.
Generally high density.
Tend to be brittle/less ductile than aluminium
and steel.
Specialised uses.
Composite materials (fibre reinforced)
 Glass fibre reinforced plastics (‘GFRP’) –
‘fibreglass’.
◦ Earliest composite material and still most common.
◦ Glass fibres bonded in a matrix of epoxy resin or a
polymer.
◦ Very lightweight
◦ Can be moulded into complex shapes
◦ Can tailor the strength and stiffness via material
choice, fibre density and orientation and composite
laminar structures.
Carbon and boron reinforced plastics
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High strength and stiffness
Excellent thermal properties
◦ Low expansivity
◦ High temperature stability
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Used for load bearing structures
◦ E.g. spacecraft struts
◦ Titanium end fittings.
Carbon-carbon composites
 Carbon fibres in a carbon matrix
Excellent thermal resistance
Very lightweight
Little structural strength
Uses confined to extreme heating environments
with minimal load bearing e.g. nose cap and
leading wing edges of the space shuttle.
◦ Hygroscopic absorption – upto 2% by weight
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 Subsequent outgassing of water vapour can lead to
distortion of material. So have to prevent absorptions,
or allow for expansion/contraction.
Metal-matrix composites:
 Metals can overcome limits of epoxy resin
(‘GFRP’ etc have to be stuck together, or
bonded inside a resin).
 E.g. aluminium matrix containing boron,
carbon or silicon-carbide fibres.
 Problem: the molten aluminium can react
with fibres (e.g. graphite) and coatings.
 Boron stiffened aluminium used as a tubular
truss structure.
Films, fabrics and plastics
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Mylar
◦ Most commonly used plastic
◦ Strong transparent polymer
◦ Can be formed into long sheets from 1μm thick and
upwards
◦ Can be coated with a few angstroms of aluminium
to make thermally reflective ‘thermal blankets’
Films, fabrics and plastics (continued)
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Kapton
◦ Polyimide (e.g. ‘Vespel’)
◦ High strength and temperature resistance (also
used for thermal blankets)
◦ Low outgassing
◦ Susceptible (like most polymers) to atomic oxygen
erosion and is thus coated with metal film (normally
gold or aluminium) or teflon.
Films, fabrics and plastics (continued)
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Teflon (PTFE – polytetraflouroethylene) and
polyethylene
◦ Smooth and inert
◦ Good specific strength
◦ Can be used as bearings, rub rings etc. without the
need for lubricants (which can freeze and outgas).
Honeycomb sections
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Low weight, high stiffness panels (from
aerospace – aircraft flooring).
Various combinations of materials can be
used.
Outgassing and thermal stability can be
problematic and must be considered (the
honeycomb is glued together).
Honeycomb sections (continued)
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Design generally customised for individual
cases
◦ Calculate required stiffness
◦ Select skin and core thickness combinations (thick
skin for load bearing)
◦ Select core section for maximum shear stress
requirement
◦ Load attachment points can be a problem as forces
must be spread across the skin. Good for load
spreading, not localised loads.
Honeycomb schematic
Connecting honeycomb using
a L-bracket to spread the load
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Summary
◦ The basics of spacecraft structures
◦ Balancing the requirements of the spacecraft
against material selection
◦ A brief overview of some of the materials used in
spacecraft engineering
◦ Advantages and disadvantages of each
◦ A spacecraft designer must consider all these
against the cost (i.e. weight) of the spacecraft
without compromising safety or mission
requirements.

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