At 04:33 PM 2/18/2003 -0500, Henry Spencer wrote:
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...)
True, but ablators are a well-understood, proven technology. Therefore, the necessary development work is much reduced.
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.
IIRC, the ablator material used is still in production and available from the old supplier. An Apollo-style honeycomb heat shield wouldn't be nearly as labor-intensive to produce today -- I have an image in my mind of a little squirt thing on a 3-axis gantry arrangement over the heat shield filling in each cell automatically.
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).
I was thinking of something more like the SERV heatshield, which was a sectional, replaceable ablator. Making it in manageable, bolt-on section eases the quality control and structural issues. If the sections are a reasonable size, it's even possible to x-ray them on the production line in real time. I agree with you that they are serious QC and testability issues with ablators, but it appears to me that every possible TPS option has one or more serious development and/or operational issues that must be resolved.
Better a fully reusable system that can be incrementally tested.
Agreed in principle -- I really like transpiration cooling, for all kinds of reasons, many of which I've beaten to death on this list. Lately, I've been trying to focus my thinking on the problems and possible solutions for first-gen craft, and I like ablators because they are a well-proven technology.
As Gary Hudson put it: "People talk about tiles getting less fragile, but they never mention that they cost $1000 per square foot."
I did say 'heavy and expensive' :).
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.)
Ok, maybe I'm completely off-base, but I thought most common ablators were variants of rubber or phenolic plastic materials, and therefore likely to be relatively robust. What's the catch I missed?
-p
Mars or Bust!
www.marssociety.com
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