[f&s, chapter 8]. function: the spacecrafts skeleton. prinipal design driver: minimise mass...
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PH508: Spacecraft structures and
materials.[F&S, Chapter 8]
Function: the Spacecraft’s ‘skeleton’.
Prinipal design driver: minimise mass without compromising reliability.
Design aspects: ◦ Materials selection◦ Configuration design◦ Analysis◦ Verification testing (iterative process).
Spacecraft structures: I
Generalised requirements
Must accommodate payload and spacecraft systems◦ Mounting requirements etc.
Strength◦ Must support itself and its payload through all phases
of the mission. Stiffness (related to strength)
◦ Oscillation/resonance frequency of structures (e.g. booms, robotic arms, solar panels).
◦ Often more important than strength!
Spacecraft structures: II
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!
Spacecraft structures: III
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
Accessibility◦ Maintain freedom of access (docking etc.)
For OPTIMUM design require careful materials selection!
Spacecraft structures: IV
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 high specific strength).
Stiffness (deformation vs. load) Stress corrosion resistance
◦ Stress corrosion cracking (SCC).
Spacecraft structures: V
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.
Spacecraft structures: VI
Thermal parameters◦ Thermal and electrical conductivity◦ Thermal expansion/contraction (materials may
experience 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).
Spacecraft structures: VII
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
Spacecraft structures: VIIITop 18 highest melting point elements
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, boron nitride).
Spacecraft structures: VII
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.
Spacecraft structures: VII
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.
Spacecraft materials: I
Aluminium (and its alloys)ρ=2698 kg m-3, melting point=933.5 K
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).
Spacecraft materials: II
Magnesium (and its alloys)ρ=1738 kg m-3, melting point=922 K
Higher stiffness, good specific strength Less workable than aluminium. Is chemically active and requires a surface
coating (thus making is more expensive to produce).
Spacecraft materials: III
Titanium (and alloys)ρ=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.
Spacecraft materials: IV
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).
Spacecraft materials: V
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.
Spacecraft materials: VI
Beryllium (BeCu)ρ=1848 kg m-3, melting point=1551 K
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.
Spacecraft materials: VII
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). Aluminium-lithium
◦ Similar strength to aluminium but several percent lighter.
Titanium-aluminide◦ Brittle, but lightweight and high temperature
resistant.
Spacecraft materials: VIII
Refractory metals:
Main metals are W, Ta, Mo, Nb. Generally high density. Tend to be brittle/less ductile than
aluminium and steel. Specialised uses.
Spacecraft materials: IX
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.
Spacecraft materials: X
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.
Spacecraft materials: XI
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 Subsequent outgassing of water vapour can lead to
distortion of material. So have to prevent absorptions, or allow for expansion/contraction.
Spacecraft materials: XII
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.
Spacecraft materials: XIII
Films, fabrics and plastics
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’
Spacecraft materials: XIV
Films, fabrics and plastics (continued)
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.
Spacecraft materials: XV
Films, fabrics and plastics (continued)
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).
Spacecraft materials: XVI
Honeycomb sections
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).
Spacecraft materials: XVII
Honeycomb sections (continued)
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.
Spacecraft materials: XVIII
Spacecraft materials:XIX
Honeycomb schematic
Connecting honeycomb usinga L-bracket to spread the load
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.
End