Download - Spacecraft Structures Lecture 9
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[F&S, Chapter 8]
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Function: the Spacecrafts skeleton.
Prinipal design driver: minimise mass without
compromising reliability.
Design aspects: Materials selection
Configuration design Analysis
Verification testing (iterative process).
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Generalised requirements
Must accommodate payload and spacecraftsystems
Mounting requirements etc. Strength
Must support itself and its payload through all phases ofthe mission.
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
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 conductionpathways (must not get too hot/cold).
Spacecraft charging and its grounding philosophy
Accessibility Maintain freedom of access (docking etc.)
For OPTIMUM design require careful materialsselection!
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Materials selection
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 highspecific strength).
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 astructure.
Extensive examination and non-destructive testingto 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 acrossthe structure.
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Thermal parameters Thermal and electrical conductivity
Thermal expansion/contraction (materials mayexperience extremes of temperature).
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).
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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
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Materials:
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, boronnitride).
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Spacecraft structure design requires a verycareful selection of materials based upontheir strength, thermal properties, electricalproperties, strength, stiffness, toxicity and
shielding ability.
The overriding concern is weight! Weight =cost and need to minimise WITHOUTsacrificing functionality. Careful design andconstruction needed.
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Most spacecraft materials are based onconventional aerospace structural materials(similar weight/strength requirements).
Some new hi-tech materials are employedwhere necessary (honeycombs, berylliumalloys etc.) not found elsewhere.
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Aluminium (and its alloys)=2698 kg m-3,melting point=933.5 K
Most commonly used conventional material (usedfor hydrazine and nitrous oxide propellanttanks).
Low density, good specific strength Weldeable, easily workable (can be extruded,
cast, machined etc).
Cheap and widely available Doesnt have a high absolute strength and has alow melting point (933 K).
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Magnesium (and its alloys)=1738 kg m-3,melting point=922 K
Higher stiffness, good specific strength Less workable than aluminium.
Is chemically active and requires a surfacecoating (thus making is more expensive to
produce).
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Titanium (and alloys)=4540 kg m-3,melting point=1933 K
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.
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Ferrous alloys (particularly stainless steel)=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 highdensity (screws, bolts are all mostly steel).
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Austenitic steels (high temperature formation)
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.
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Beryllium (BeCu)=1848 kg m-3,melting point=1551 K
Stiffest naturally occurring material (berylliummetal doesnt occur naturally but its compoundsdo).
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.
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Other alloys Inconel (An alloy of Ni and Co)
High temperature applications such as heat shieldsand rocket nozzles.
High density (>steel, 8200 km m-3).
Aluminium-lithium Similar strength to aluminium but several percent
lighter.
Titanium-aluminide Brittle, but lightweight and high temperature
resistant.
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Refractory metals:
Main metals are W, Ta, Mo, Nb.
Generally high density. Tend to be brittle/less ductile than aluminium
and steel.
Specialised uses.
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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 apolymer.
Very lightweight
Can be moulded into complex shapes
Can tailor the strength and stiffness via materialchoice, fibre density and orientation and compositelaminar structures.
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Carbon and boron reinforced plastics
High strength and stiffness
Excellent thermal properties Low expansivity
High temperature stability
Used for load bearing structures
E.g. spacecraft struts Titanium end fittings.
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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 andleading wing edges of the space shuttle.
Hygroscopic absorption upto 2% by weight Subsequent outgassing of water vapour can lead to
distortion of material. So have to prevent absorptions,or allow for expansion/contraction.
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Metal-matrix composites: Metals can overcome limits of epoxy resin
(GFRP etc have to be stuck together, orbonded 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.
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Films, fabrics and plastics (continued)
Kapton Polyimide (e.g. Vespel)
High strength and temperature resistance (alsoused for thermal blankets)
Low outgassing
Susceptible (like most polymers) to atomic oxygen
erosion and is thus coated with metal film (normallygold or aluminium) or teflon.
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Films, fabrics and plastics (continued)
Teflon (PTFE polytetraflouroethylene) andpolyethylene Smooth and inert
Good specific strength
Can be used as bearings, rub rings etc. without theneed for lubricants (which can freeze and outgas).
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Honeycomb sections
Low weight, high stiffness panels (fromaerospace aircraft flooring).
Various combinations of materials can beused.
Outgassing and thermal stability can be
problematic and must be considered (thehoneycomb is glued together).
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Honeycomb schematic
Connecting honeycomb usinga L-bracket to spread the load
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Summary The basics of spacecraft structures
Balancing the requirements of the spacecraftagainst material selection
A brief overview of some of the materials used inspacecraft engineering
Advantages and disadvantages of each
A spacecraft designer must consider all these
against the cost (i.e. weight) of the spacecraftwithout compromising safety or missionrequirements.