21-12-2012, 02:36 PM
Ablative Materials
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Process of Ablation
Surface ablation:
Removal of a substance from the surface only so that only volume is decreased and density
remains constant.
Four main types:
– evaporation (sublimation)
– chemical reactions with the surroundings (mainly combustion)
– melting
– thermo mechanical erosion
Volumetric ablation:
Removal of a substance from the composite caused by physical-chemical transformations such as
phase transformations or chemical reactions inside the material volume. For this case the density
decreases, but the volume remains the same.
•Two main types:
– Pyrolytic thermodecomposition (TD)
– Thermo-oxidative decomposition (TOD)
Example for ablatively cooled TPS: How does it look like
Charring and Vaporization of
material takes place in cells (due to
honeycomb structure)
• Charring depth depends on material
resistance
• Honeycomb structure prevents total
peeling of delaminated strcutures
Surface Protected Ablation Material as an Advanced Concept
“Novel“ TPS (Dornier development)
• Ablation material covered by radiation cooled C-C SiC-Layer
• Reduction of heat flux into ablation material
Reduced ablation rate
But requirement:
Drain of ablation products behind the
ceramic layer
Example for ablatively cooled TPS: Apollo
External shape consists of blunt entry face
and conical afterbody
– Minimuzation of convective heating
• Centre of gravity offset from symmetry axis
– Generation of lift
• Avco 5026-39G ablator bonded to stainless
steel structure
• Avco 5026-39G: epoxy-novalac resin
reinforced with quartz fibers and phenolic
microballoons.
Density: 496,57 kg/m3
• Ablator applied in honeycomb matrix
bonded to stainless steel sub structure (HC
first bonded to structure with HT-424
adhesive, then ablator is inserted by a
hypodermic device)
• Uncertainties in load led to pure ablator TPS
design although metal shingles in a wide
range of conical afterbody would have been
sufficient