spacecraft structures lecture 9

8/10/2019 Spacecraft Structures Lecture 9

1/31

[F&S, Chapter 8]


2/31

Function: the Spacecrafts skeleton.

Prinipal design driver: minimise mass without

compromising reliability.

Design aspects: Materials selection

Configuration design Analysis

Verification testing (iterative process).


3/31

Generalised requirements

Must accommodate payload and spacecraftsystems

Mounting requirements etc. Strength

Must support itself and its payload through all phases ofthe mission.

Stiffness (related to strength) Oscillation/resonance frequency of structures (e.g.

booms, robotic arms, solar panels).

Often more important than strength!


4/31

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!


5/31

Thermal and electrical paths Material conductivity (thermal and electrical)

Regulate heat retention/loss along conductionpathways (must not get too hot/cold).

Spacecraft charging and its grounding philosophy

Accessibility Maintain freedom of access (docking etc.)

For OPTIMUM design require careful materialsselection!


6/31

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 highspecific strength).

Stiffness (deformation vs. load)

Stress corrosion resistance Stress corrosion cracking (SCC).


7/31

Fracture and fatigue resistance Materials contain microcracks (unavoidable)

Crack propagation can lead to total failure of astructure.

Extensive examination and non-destructive testingto 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 acrossthe structure.


8/31

Thermal parameters Thermal and electrical conductivity

Thermal expansion/contraction (materials mayexperience 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).


9/31

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


10/31

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, boronnitride).


11/31

Spacecraft structure design requires a verycareful selection of materials based upontheir strength, thermal properties, electricalproperties, strength, stiffness, toxicity and

shielding ability.

The overriding concern is weight! Weight =cost and need to minimise WITHOUTsacrificing functionality. Careful design andconstruction needed.


12/31

Most spacecraft materials are based onconventional aerospace structural materials(similar weight/strength requirements).

Some new hi-tech materials are employedwhere necessary (honeycombs, berylliumalloys etc.) not found elsewhere.


13/31

Aluminium (and its alloys)=2698 kg m-3,melting point=933.5 K

Most commonly used conventional material (usedfor hydrazine and nitrous oxide propellanttanks).

Low density, good specific strength Weldeable, easily workable (can be extruded,

cast, machined etc).

Cheap and widely available Doesnt have a high absolute strength and has alow melting point (933 K).


14/31

Magnesium (and its alloys)=1738 kg m-3,melting point=922 K

Higher stiffness, good specific strength Less workable than aluminium.

Is chemically active and requires a surfacecoating (thus making is more expensive to

produce).


15/31

Titanium (and alloys)=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.


16/31

Ferrous alloys (particularly stainless steel)=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 highdensity (screws, bolts are all mostly steel).


17/31

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.


18/31

Beryllium (BeCu)=1848 kg m-3,melting point=1551 K

Stiffest naturally occurring material (berylliummetal doesnt occur naturally but its compoundsdo).

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.


19/31

Other alloys Inconel (An alloy of Ni and Co)

High temperature applications such as heat shieldsand 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.


20/31

Refractory metals:

Main metals are W, Ta, Mo, Nb.

Generally high density. Tend to be brittle/less ductile than aluminium

and steel.

Specialised uses.


21/31

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 apolymer.

Very lightweight

Can be moulded into complex shapes

Can tailor the strength and stiffness via materialchoice, fibre density and orientation and compositelaminar structures.


22/31

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.


23/31

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 andleading 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.


24/31

Metal-matrix composites: Metals can overcome limits of epoxy resin

(GFRP etc have to be stuck together, orbonded 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.


25/31


26/31

Films, fabrics and plastics (continued)

Kapton Polyimide (e.g. Vespel)

High strength and temperature resistance (alsoused for thermal blankets)

Low outgassing

Susceptible (like most polymers) to atomic oxygen

erosion and is thus coated with metal film (normallygold or aluminium) or teflon.


27/31

Films, fabrics and plastics (continued)

Teflon (PTFE polytetraflouroethylene) andpolyethylene Smooth and inert

Good specific strength

Can be used as bearings, rub rings etc. without theneed for lubricants (which can freeze and outgas).


28/31

Honeycomb sections

Low weight, high stiffness panels (fromaerospace aircraft flooring).

Various combinations of materials can beused.

Outgassing and thermal stability can be

problematic and must be considered (thehoneycomb is glued together).


29/31


30/31

Honeycomb schematic

Connecting honeycomb usinga L-bracket to spread the load


31/31

Summary The basics of spacecraft structures

Balancing the requirements of the spacecraftagainst material selection

A brief overview of some of the materials used inspacecraft engineering

Advantages and disadvantages of each

A spacecraft designer must consider all these

against the cost (i.e. weight) of the spacecraftwithout compromising safety or missionrequirements.

spacecraft structures lecture 9

Documents