Investigative studies on the critical prepreg variables influencing the functional performance of ablative composites for extreme environments in launch vehicles and re-entry missions

Abstract

Ablative composites are an elite class of polymer composites used for thermal protection in aerospace applications like solid rocket motor nozzles, liquid propellant engines, nose caps and external surfaces of atmospheric re-entry space vehicles to protect them from the extremely severe operating conditions of high temperature, pressure and particle impingement. Carbon or Silica fibred impregnated with phenolic resin is the most widely used material for synthesizing ablative composites. These composites are processed from carbon-phenolic/silica-phenolic prepregs using a complex processing cycle starting from impregnating the fibres with phenolic resin followed by moulding or winding prepreg tapes over metallic mandrels before polymerization under pressure in Autoclave/Hydroclaves. The functional performance of these ablatives are very critical for the space missions as any failure in the thermal protection system of a re-entry manned mission or a satellite launch vehicle can lead to loss of precious human life and expensive payload in satellites. The performance of an ablative is influenced largely by the critical raw material and process parameters. The basic characteristic parameters of the pre-impregnated Carbon/Silica phenolic have the most significant influence on the thermal and mechanical properties of the ablative composite. Though there had been many efforts to improve the ablative characteristics of polymer composites by introducing nano-particles and by using alternate materials, no significant results have been reported in the optimization of prepreg parameters like resin content on the thermo-mechanical properties and hence the functional performance of ablative composites used in aerospace applications like satellite launch vehicles. From the different prepreg parameters like volatile content, resin content, fibre volume fraction and degree of advancement of the resin, fibre volume fraction and resin content are the most critical. Hence these are selected for detailed investigations. This paper is a critical study of the influence of the resin content in the prepreg which is the most critical process variable which significantly influences the thermal and mechanical properties of the composite and in turn the functional performance of ablatives. Both Carbon phenolic and Silica phenolic laminates were processed with different resin contents and fibre contents. For each of these laminates the functionally critical mechanical and thermostructural properties were evaluated and compared. The most optimal process parameters of resin or fiber content for both Carbon and Silica phenolic composite were identified. With these optimal prepreg parameters, excellent thermo-mechanical properties can be achieved for the ablative composites.