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PH508: Spacecraft structures and materials. [F&S, Chapter 8]. Spacecraft structures: I. Function: the Spacecraft’s ‘skeleton’. Prinipal design driver: minimise mass without compromising reliability. Design aspects: Materials selection Configuration design Analysis

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spacecraft structures i
Spacecraft structures: I
  • 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 ii
Spacecraft structures: II

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 iii
Spacecraft structures: III
  • 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 iv
Spacecraft structures: IV
  • 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 v
Spacecraft structures: V

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 vi
Spacecraft structures: VI
  • 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 vii
Spacecraft structures: VII
  • 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 vii1
Spacecraft structures: VII

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 vii2
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 materials i
Spacecraft materials: I
  • 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 ii
Spacecraft materials: II

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 iii
Spacecraft materials: III

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 iv
Spacecraft materials: IV

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 v
Spacecraft materials: V

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 vi
Spacecraft materials: VI

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 vii
Spacecraft materials: VII

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 viii
Spacecraft materials: VIII

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 ix
Spacecraft materials: IX

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 x
Spacecraft materials: X

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 xi
Spacecraft materials: XI

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 xii
Spacecraft materials: XII

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 xiii
Spacecraft materials: XIII

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 xiv
Spacecraft materials: XIV

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 xv
Spacecraft materials: XV

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 xvi
Spacecraft materials: XVI

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 xvii
Spacecraft materials: XVII

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 xviii
Spacecraft materials: XVIII

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 xix
Spacecraft materials:XIX

Honeycomb schematic

Connecting honeycomb using

a L-bracket to spread the load

slide31
End
  • 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.