[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). 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! 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! 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! 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). 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. 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). 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: 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 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. 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 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 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 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) 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 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). Aluminium-lithium ◦ Similar strength to aluminium but several percent lighter. Titanium-aluminide ◦ Brittle, but lightweight and high temperature resistant. Refractory metals: 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 High strength and stiffness Excellent thermal properties ◦ Low expansivity ◦ High temperature stability 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 ◦ ◦ ◦ ◦ 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 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) 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) 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 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) 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 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.