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[Excerpt from a letter to Allen Meece]

[Updated 2 September 2002]

I've discovered a new favorite book. :) It's Jack Lambie's Composite Construction for Homebuilt Aircraft from Aviation Publishers at Markowski International, c. 1995. The ISBN is 0-938716-26-3, and it's a must read. I've currently got a copy from the library, but I intend to order one at my earliest convenience. The most important parts are simple how-to instructions for applying fiberglass construction techniques to Kevlar composites.

I'm pleasantly surprised at how simple the techniques are, particularly for laminates. This could be done in a suitably equipped home workshop. I believe Kevlar will in fact prove easier to work with than fiberglass. And all fiberglass laminating techniques will work with it, including the use of foam and wood cores.

I had previously disregarded fiberglass because it's relatively heavy and brittle, and graphite composites, though lighter and stronger, are even more brittle. But Kevlar is quite light and is not as prone to the catastrophic cracking you see when fiberglass and graphite composites fail. Similarly, it won't produce the sort of irritating fibrous dust that fiberglass and graphite will. It has strength equivalent to graphite.

It's also available as an automotive body supply, which is very useful.

Kevlar's useful temperatures range is from -195C to 170C, which makes pure, non-composite Kevlar cloth suited to the VBP platform conditions. Unfortunately, the resins used to make laminates are not all so versatile. Fiberglass is made using polyester or epoxy resins as filler between the fibers. (I've also seen good old fashioned polyurethane wood sealant used, but only as an outer seal or rubberizer. It is very soft and rubbery compared to heat-cured epoxy, and lacks thermal tolerance.) Some of these are not suitable for the temperature extremes the platform will encounter, and are subject to shrinking and cracking at low temperatures. However, according to studies done by Lockheed, many ployester resins are adequate at high temperatures and actually become stronger at cryogenically low temperatures for the purposes of use in composites. (Re: Lockheed Missiles & Space Co. Space Materials Handbook c. 1978) The polyester resins still become brittle at those temperatures, but unlike other epoxies they don't shrink, their tensile strengths do not decrease and their compressive strengths almost double. So long as they remain bound within the fiber matrix of a kevlar composite, this means that there is no significant reduction of any measure of strength (compressive, impact, shear, etc.) of the material.

Using Kevlar composites opens up quite a range of possibilities for simplifying construction. For example, we could use plywood laminates in the hab construction. Four layers of #285 Kevlar cloth properly epoxied around a 2x1/4” plywood strip would give it lateral strength similar to an unlaminated 2x6” of the same length, though it could take less torsion. We could custom make our own tank. Laminates require no welding and are easy to patch. The laminates can be made to accept heat sealing in places where outgassing is a problem for patches -- polypro sticks to epoxy.

Spectra could also be employed for composites, though it doesn't have the durability of Kevlar and would probably bond well with fewer resins. A sheet of PET can be heat laminated onto Kevlar cloth for the hab's outer pressure wall. And any piece of T-900 rope is a potential solid beam if you just dip it in resin. The Technora outer covering alone would be quite strong if wet through and cured. Technora, though softer, is chemically Kevlar in the same way that Coke-bottle PET is chemically Spectra, so it should work OK with most resins that serve for Kevlar.

Composites still have problems. Outgassing is one -- many resins are toxic, and they're all nasty. Composites also favor single piece construction, which makes patches easy but replacements difficult. And there are a limited number of resins suitable for near vacuum conditions with temparature extremes between the boiling points of water and nitrous oxide. However, Mr. Lambie's book has inspired me with new hope that we may be able to use them. I'll look into it.

CME