Wednesday, March 16, 2005

Composites in Space

Update Below

For the most part, I agree with Rand Simberg regarding the obsolescence of the materials technology on the Space Shuttle. I also believe that he is on the right track with the parallels he draws in respect to polymer/carbon fiber composites used on Airbuses. There are a few other considerations that I think that he missed, however.

First is the characterization that space is a "benign environment" with respect to structural stress. That is true within that frame, but reusability will have to account for the radiation that will impinge on the atomic bonds of the epoxy resin matrix. As a general rule, polymers do not handle ultra-violet and higher energy radiation very well; think about your vinyl dashboard after several years. Under prolonged exposure, polymers tend to become brittle. In materials terms its fracture toughness, the key parameter in fatigue resistance, decreases with exposure. Potentially, a spacecraft could be coming down with parts substantially more prone to fatigue than when it went up. This will put an absolute upper limit on reusability.

From the quoted article, it is foolish to think that one could find internal voids through external examination. Kind of like surgeons' knocks on internists, that internists try to guess what is going on inside the patient, only that polymers can't say precisely where they feel gassy. If there is an occurrence of voids of vapor appearing inside the matrix, it could be one of two things, neither of which would be good for a spacecraft. One is that there are flaws in the lay-up process. Failure to properly lay down the laminates could leave air pockets like bubbles under linoleum. Second is that there is gas being released from the matrix that is coalescing into bubbles, a process known as out-gassing.

Both of these processes yield flaws that create internal stresses in the composite. The important thing to remember is that the stress would be partly determined by pressure, so you can see that traveling through vacuum could have a serious downside.

I would be interested in looking at research being done in ceramic-matrix composites. While not as light as polymers, the chemical inertness of ceramics would give it longer life. The use of metallic whiskers in the ceramic would check the growth of cracks and provide improved tensile properties. I'm sure that there are downsides to these materials, but I'll leave that to another know it all.

Link via Instapundit.

Update: I spent a little more time thinking about this, and I want to cover a couple of points, on pro-composite and one anti-composite.

Pro-composite: My thought regarding trapped gasses in composites during lay-up. Vacuum molding, where the plies of composite are pressed together after layup by a vacuum between the mold and a membrane, would take care of any trapped air beteen the plies. While the cured state would be more resistant to the development of gas pockets due to its stiffness, it still does not completely answer the issue of out-gassed vapors.

Anti-composite: In an orbitting vehicle, the temperature differences between day and night would be huge. Polymer matrices, by and large, have considerable coefficients of thermal expansion. This is a differnce with carbon fibers, and a difference would between the two would be bad. While the carbon fibers would prevent excess expansion of the resin matrix at first, over time (and hence, a reusability issue) the matrix would break its bond with the fibers. With no connection to particular fibers, those fibers might as well not be there.

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