The ablation, mechanical and thermal properties of vapor grown carbon fiber/phenolic resin composites were evaluated to determine the potential of using this material in solid rocket motor nozzles. The plasma torch testing indicates that this class of composites is a good candidate for further development of rocket nozzles and heat shielding materials. The tiny dimensions of the VGCF causes major changes in the heat transfer rates and this, in turn, affects the resulting combustion/decomposition chemistry. The tiny fiber size reduces the rate of heat transfer perpendicular to the surface into the composite compared with the rate of heat transfer in the MX-4926 material, which used continuous carbon fibers.
Table top left: VGCF/phenolic composite flexural strength and flexural modulus. Significant modulus increases occurred for the ball milled and high shear mixed samples 9 and 10, which had lower void volumes compared to samples 1, 2, 5, and 6.
Table top right: VGCF/phenolic composite thermal conductivities. All of the composite samples exhibited similar thermal conductivities with values being twice that of the cured phenolic resin without fiber. When good resin infusion is achieved, each fiber has been coated with resin and is located in a continuous resin phase. Since the resin is an insulator, and the fiber lengths are short, the net composite is not highly heat conducting.
Table bottom left: Three-dimensional thermal conductivity testing. The thermal conductivities in three dimensions were obtained on a 40/60 VGCF/phenolic ablation test component. The results indicate that the material is slightly anisotropic, differing by less than +/- 9% from the average in all three directions.
Bottom right table: NASA/MSFC plasma torch ablation test results (plasma torch test temperature 1649oC and heat flux 16.5 MW/m2. MX-4926 is NASA’s standard nozzle material. The VGCF composites experienced higher erosion rates, lower weight losses, and lower load changes. More weight was being lost at or near the surface of the specimen due to higher erosion rates versus weight loss from subsurface thermal decomposition. Load change is an indirect measure of heat penetration and char depth; increase in load change indicate an increase in heat penetration and char depth. The VGCF specimens had less char formation and heat penetration than MX-4926. The VGCF composites appear to be far better insulators than the MX-4926.
To improve the ablation, mechanical and thermal properties of phenolic resin composites, PPI recommends the following products PR-19-XT-HHT and PR-24-XT-HHT.