Simulation of pyrolysis gas in a charring ablative thermal protection material using gas injection through a porous graphite sample
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When a spacecraft traveling at orbital speeds enters a planetary atmosphere, shock waves and friction heat the surrounding air to temperatures in excess of 6,000 K. Often the only way to protect the spacecraft structure is to insulate it with a layer of sacrificial material that absorbs the heat by thermally degrading and ablating away. As the interior of the material heats up, it pyrolyzes into gaseous byproducts which are expelled through the charred surface of the material and provide an extra buffer between the hot free stream and the spacecraft. The performance of these charring ablative materials depends on poorly understood chemical, thermal, and mass flow interactions of the material with the superheated free stream fluid. The current work characterizes the flow and operating range of a new inductively coupled plasma based high enthalpy flow facility for testing high-temperature thermal protection materials. The facility uses magnetic induction to heat up to 1.5 g/s of air to temperatures of between 5000 K and 6500 K. The flow uniformity, total enthalpy and heat flux of the plasma stream are measured. Raman spectroscopy is used to measure the gas temperature within the flow and calculate an enthalpy distribution. The facility is used to investigate the effect that injection of ablation products into the boundary layer has on the heat flux into the material and mass loss rate of the char surface. The ablation process is simulated by injecting a gas mixture through a FiberForm test sample so that the flow and composition of pyrolysis products, which usually depend on the specific material composition, can be varied independently. It was found that all gas compositions tested reduced the rate of surface recession and chemically reactive mixtures reduced it the most. Emission spectroscopy in the boundary layer showed the mechanism to be reduced oxygen diffusion to the surface. However, if the heat of combustion of the pyrolysis products was too high, the heat flux to the surface was not reduced. For materials with above a 60% char yield, pyrolysis gases have little effect.