Response of Graphite/Epoxy Structures to the Reentry Environment

Recovered debris from the Space Shuttle Columbia accident has been examined as part of an effort to understand the thermal response of structural composite materials to the reentry environment. This paper discusses why accurate reentry breakup models are important and gives an overview of the analysis of a graphite/epoxy payload bay door fragment and four composite overwrapped pressure vessels carried within the Shuttle. Included are details on the reconstruction of the trajectories flown by each piece after its release during the breakup and conclusions from laboratory analyses relative to the damage caused by aerodynamic heating after release. These results were used to develop an initial understanding of how reentry heating affects these materials. It was concluded that thermally induced reentry break-up of graphite/epoxy structures is probably controlled by the loss of the epoxy "glue" as the reentry progresses. Preliminary results from a simplified graphite/epoxy ablation model are used to demonstrate how this failure criterion could be used to predict breakup. Nomenclature w k = thermal conductivity of pyrolysis gas at wall (W/m/K) py h = enthalpy of pyrolysis gas at wall (J/kg) b py h , = base (cold) enthalpy of the epoxy resin (J/kg) air h = enthalpy of air (J/kg) w h = enthalpy of the carbon-air mixture at the wall (J/kg) b C h , = bulk enthalpy of carbon cw q & = cold wall heat flux (J/m 2 )