On Tue, 18 Feb 2003, Pierce Nichols wrote:
> It's hard to beat ablative for cheap reliability. Transpiration
> might beat it, but it's going to require substantial development work.
Ablators take development work too, you know -- you can't buy a complete
reentry-rated ablative heatshield off the shelf. (Well, okay, maybe from
the Russians...)
The trick with ablators is getting them lightweight. If you can throw
mass at the problem, you can be pretty sure of building something that
will work. The desire to reduce ablator weight was what drove Gemini and
Apollo to labor-intensive honeycomb heatshields: a really lightweight
ablator was not mechanically durable enough without reinforcements.
The really bad thing about ablators is not turnaround issues, but the fact
that it brings you back to one-shot components which cannot be fully
tested before use. When the physical structure becomes complex, as it did
for the Apollo heatshield, the situation starts to look like solid-fuel
rockets -- the apparent simplicity is illusory, because small flaws in
large masses of material are potentially fatal -- and you get the same
nightmare of elaborate quality control (Apollo heatshields were X-rayed,
for example).
Better a fully reusable system that can be incrementally tested.
> Ceramic tiles are right out; metal tiles might work, but they are not only
> heavy and expensive, but will require additional development work.
As Gary Hudson put it: "People talk about tiles getting less fragile, but
they never mention that they cost $1000 per square foot." High-tech
materials -- and to my mind that includes high-tech ablators -- are bad
news for the economy-minded. Not just because they are costly to purchase
and complicated to maintain, but because you become deathly afraid of
breaking them. (Gossamer Condor took the Kremer Prize not because it was
better designed than its competitors, but because it was easy and cheap to
repair, so its developers didn't have to worry too much about breaking it.)
Henry Spencer
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