bobkrech
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Nice piece of work, Eric. As you discovered the epoxy pyrolizes and generates a reducing gas that sucks up a lot of thermal energy, and the molten glass flows and covers the downstream surface preventing oxidation. I'm curious if you measured the recession of the glass composite.I realize the OP is looking for comments on structural ablatives but I figured I'd mention my experience with ablatives used in combustion chambers for those interested. The comments below are targeting a purely amateur project.
I've had real good luck with ablatives used in my liquid engine over the years. I've used everything from simple E-Glass/resin forms to more complex cured S-Glass/phenolic structures. In the end, the application determined the best option which also turned out to be the easiest to produce. For "my" application, I needed an ablative liner that would operate in the combustor of my LOX/Ethanol engine and last for 15 seconds minimum without anything melting.
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My ablative needed to perform three basic functions. First it has to act as an insulator, second it needs to move heat by ablation, and finally it needs to maintain some sort of structural integrity. The first engine liners I produced used standard store bought fiberglass as the structural component bonded with an ablating epoxy resin. These liners would go through a cycle of ablating the resin while also melting the glass. Molten glass would flow down walls and coat the nozzle which provided some protection against oxidation of the graphite throat.
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I later switched to a silicia glass which wouldn't melt but had other problems. First was the ablative would vaporize leaving behind an unsupported silica cloth that would tear and the motor would then "cough" out fur balls of silica as it ran. I then switched resins, changed the layup structure to no avail. In the end I went back to the simple E-glass/epoxy system. I only needed 15 seconds and the liner always provided me with that.
In this photo you can see the molten glass deposited on the throat of the engine. The burn mark on the outer ring is just burned electrical tape. The fellow in the background is Tom Mueller (spacex).
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Eric
In some professional nozzles, overlapping tape is used instead of cloth. This permits an angled, lengthened thermal path to increase the strength and thermal resistance of the nozzle. If carbon fiber is used instead of glass, the phenolic epoxy pyrolizes in depth and the gases flow through the carbon fibers and prevent oxygen from reaching the carbon surface. Carbon fiber does not melt, and provides a radiative cooling mechanism for the surface, and becomes a true ablative, vaporizing into C, C2 and C3 molecules and in the process, carrying off a lot more heat.
You might be interested in googling "epoxy intumescent fire protection coating" for commercial thermal protection coatings that behave similarly.
Bob