PH508: Spacecraft structures and materials.

1. PH508: Spacecraft structures and materials. [F&S, Chapter 8]

2. 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).

3. 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!

4. 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!

5. 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!

6. 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).

7. 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.

8. 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).

9. Spacecraft structures: VIIITop 18 highest melting point elements

10. 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).

11. 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.

12. 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.

13. 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).

14. 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).

15. 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.

16. 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).

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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’

26. 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.

27. 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).

28. 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).

29. 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.

30. Spacecraft materials:XIX Honeycomb schematic Connecting honeycomb using a L-bracket to spread the load

31. 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.