Experimental Investigation of Pyrolysis Pressure in Ablators and Its Effect on Spallation

Experiments are presented measuring the in-depth pressure in the low-density carbon–phenolic ablator HARLEM (HEFDIG Ablation Research Laboratory Experiment Material). Two different setups with respect to the connection of the pressure transducer with the ablator samples are investigated. The tested materials are the carbon–phenolic ablator HARLEM, as well as a dense HARLEM version and the carbon preform Calcarb. The setup features a pressure transducer placed immediately behind the ablator sample, which is connected to the measurement position at 22 mm behind the sample surface via a ceramic tube. The low-density carbon–phenolic reaches an in-depth pressure of up to 276 hPa, which is more than 10 times higher than the stagnation pressure. In the case of the dense HARLEM sample, a recurring mechanical failure through char layer ejection is observed as a result of high in-depth pressure. The pressure measurements are supplemented by measurements of the number of spalled particles and it is shown that pyrolysis pressure is not causing spallation of single fibers and fiber bundles.