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re: Delicate heatshield?
Gary — (archived)

Jacob Samorodin asked some questions:

What would you replace a shuttle's silicon-based tiled heatshielding with?

A "hot metal" thermal protection system (TPS). This would be titanium shingles on a titanium structure. The leading edges would be either reinforced carbon-carbon (RCC) or an exotic material like hafnium diboride. The current Space Shuttle uses silica foam tiles glued to a stress isolation material that was glued directly to the Shuttle's aluminum structure. The failed X-33 would have used titanium shingles with RCC leading edges. The SR-71 also used a titanium structure and had titanium leading edges.

Ablative materials?...

Not on a reusable vehicle . Ablators work by converting TPS material from virgin material into char while out-gassing. The out-gassing from the TPS blows hot boundary layer gases away from the TPS surface. The ablator used on MER and Pathfinder was SLA-561V. SLA-561V doesn't start to ablate until the heat flux gets to 80 watts/cm^2. If the heat flux is below that value then the TPS acts purely as an insulator. The peak heating for the Space Shuttle was signficantly below 80 watts/cm^2.

Wouldn't the depletion of the ablative materials screw-up the aerodynamics of a shuttlecraft?

Ablation on the Galileo Probe significantly effected its aerodynamics (made the nose more blunter). However the Probe was designed for that. Ablation had no significant effect upon the aerodynamics of Mars Pathfinder. The peak heat flux for MER was about 41 watts/cm^2 so there was no ablation.

Gary

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