Advanced Ablative composites for Aerospace applications

Abstract

The external surface of a re-entry space vehicle experiences high heat flux and temperatures of the order of 2500°C during re-entry into the atmosphere and the internal contour of a rocket motor nozzle has to encounter temperatures above 2000°C during its operation. Special Thermal Protection Systems (TPS) are employed to protect the rocket hardware and re-entry capsule from such extreme thermal environments. Ablative composites are cost-effective and well-proven candidates for such applications. However, processing, characterisation and qualification of ablatives are quite complex and involves long cycle times. Ablation is a heat and mass transfer process in which a large amount of heat is dissipated in a very short period of time with sacrificial loss of material. Ablative composites are polymeric composites where the reinforcement is generally a fibre with high melting point like Carbon or Silica and the matrix is a high char yielding resin like phenolics. In this paper, the process of ablation, the physical and chemical changes and the process of synthesis of ablatives are clearly elaborated. The recent advances in design, development, characterisation and testing of carbon and silica phenolics are explained with examples. The process sequence and customised equipments used for the synthesis and characterisation are described. The raw materials, critical process parameters, quality control mechanisms, non-destructive testing methods etc are covered. The defects which were observed during the realisation and the corrective measures taken to solve them are explained. Different laminate and subscale level tests have been done to qualify the process and verify the design margins. Finally a full scale rocket engine test is carried out for qualification before induction into an operational launch vehicle.