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