Abstract: Ablative composites are fibre-reinforced matrix composites capable of withstanding high temperatures, pressures and particle impingement. They find wide ranging applications in solid rocket motor nozzles, liquid engine throats, control thrusters, re-entry nose caps, leading edges etc. These components encounter extremely hostile service conditions. The high temperature, pressure, velocity of hot exhaust gases, heat flux, particle impingement of the solid propellant particles etc contribute to harsh environment inside the nozzles. Similarly, during re-retry of space vehicles, the vehicle is to be protected from the frictional aerodynamic heating. High temperature resistant metals or alloys cannot survive these extreme operating conditions. While the metallic structure provides the necessary structural capability, high performance composite materials are required for thermal protection. Ablative composites generally use Carbon or Silica as the reinforcement and phenolic resin as the matrix resin. These are processed using a complex processing cycle starting from impregnating the fibres with phenolic resin followed by moulding or winding prepare tapes over metallic mandrels before polymerization under pressure in Autoclave/Hydro claves. The property of ablation of these high performance composites is utilised to protect the metallic backup structure from thermal degradation during the firing of the solid rocket motors and during re-entry manoeuvres of space capsules. The mechanism of ablation and the desirable properties of ablative composites are explained. This paper describes the synthesis of carbon phenolics and silica phenolics, the most important and widely used ablatives in rocket launch vehicles and re-entry missions. The state-of-the-art technologies in the field of high temperature resistant composite processing are addressed in detail. Various processing techniques like hand layup, tape winding, mounding and curing and their relative importance are clearly enumerated. Special processing techniques, innovative and improvised processes and characterization methods are explained. The common defects and non-conformances encountered during processing too are mentioned. Finally, the qualification and evaluation schemes being followed for ablative composites are addressed.
Keywords: Composites, fiber-reinforced plastics, ablatives, carbon-phenolic, silica-phenolic, hydro lave, curing